17 Medical Parasitology (81)

Question 1

What is a characteristic of the microbial agent causing her symptoms?

1 Flagellated protozoan
2 Produces cystic form under unfavorable conditions
3 Able to survive on fomite for 2-3 days
4 Undulating membrane extends the full length of the parasite
5 Ideal pH for growth is 3.8-4

Answer 1

Flagellated protozoan

The microorganism causing vaginitis in the patient is Trichomonas vaginalis, a flagellate protozoan parasite. For T. vaginalis, no cyst form is known. The parasite has four anterior flagella and a fifth flagellum along the undulating membrane. Flagella are the organs of motility. Cilia are absent. The undulating membrane of T. vaginalis is short and reaches up to the middle of the body, a differentiating feature from other trichomonads of humans in which the undulating membrane extends the full length of the parasite. The organism grows best under anaerobic conditions at 35-37°C, with an optimal pH 5.5-6. Normal acidic pH of 3.8-4 is detrimental to the growth of T. vaginalis.
The trophozoite, a protozoan in the metabolically active growth stage, cannot survive outside the body for long, so transmission has to be from person to person by close contact. Humans are the only natural hosts of the parasite. T. vaginalis lives mainly in the vagina and cervix in women and the anterior urethra of men. Sexual transmission is the typical mode of infection. The parasite divides by longitudinal binary fission. Infection is most common in sexually active women of reproductive age. The incubation period ranges from 4 days to 4 weeks. Infection may be asymptomatic or cause acute inflammatory disease of the vagina and cervix.
Laboratory diagnostic methods include microscopy, culture, antigen detection tests by enzyme-linked immunosorbent assay (ELISA), and molecular tests based on polymerase chain reaction (PCR). Microscopy for motile trichomonas in wet preparation of vaginal discharge should be performed within 10-20 minutes of collection to prevent the organisms from losing their viability. Nucleus, flagella, undulating membrane, and axostyle are prominent structures in stained smears.
The CDC recommends metronidazole 2 g orally as a single dose for treatment of T. vaginalis infection. Treatment of sexual partners is also recommended. Metronidazole-resistant infections have been reported, and Tinidazole, a 5-nitroimidazole, is useful for treatment of such cases.

Question 2

What species of malarial parasite is the most likely cause of this patient’s illness?

1 Plasmodium falciparum
2 Plasmodium knowlesi
3 Plasmodium malariae
4 Plasmodium ovale
5 Plasmodium vivax

Answer 2

Plasmodium falciparum

The developmental forms of malarial parasite observed in the peripheral blood smear are characteristic of Plasmodium falciparum, which causes the most severe type of malaria. Small, delicate multiple rings and crescent-shape of the gametocytes are typical features of P. falciparum. Observation of gametocyte crescents helps with easy identification of this species, as gametocytes of all other species are round or oval. In P. falciparum infection, only ring forms and gametocytes are generally seen in the peripheral smear. Late trophozoites and schizonts are not ordinarily seen and the presence of falciparum schizonts in peripheral smear indicates grave prognosis of the disease. In infections by the other species of plasmodia, all forms of the asexual cycle (schizogony) as well as gametocytes are seen in peripheral blood smear. These differentiating points help to identify P. falciparum as the cause of malaria in the patient referred to in the question.
P. falciparum invades RBCs of all ages, including the erythropoetic stem cells of the bone marrow, resulting in high level of parasitemia. The parasitized cells develop numerous projecting knobs, which cause their adherence to blood vessel endothelium resulting in obstruction, thrombosis, and ischemia. P. falciparum infections may lead to severe and fatal complications like cerebral or algid malaria, hyper pyrexia, and black water fever. Renal damage causes appearance of protein, casts, and RBCs in urine.
Laboratory diagnosis is mainly by examination of thick and thin blood smears stained by Giemsa. For detection of malarial parasites, thick smear is helpful as the concentration of the parasites is higher. Characteristic changes in infected RBCs cannot be observed because of dehemoglobinization before staining. Thin smear helps species differentiation by observing appearance of the infected RBCs and different intraerythrocytic forms of the parasite.
Plasmodium knowlesi is a rare but emerging malarial infection, which more commonly affects children. It typically presents as a nonspecific febrile illness with associated thrombocytopenia.
In Plasmodium malariae, infected RBCs are not enlarged and no specific stipplings are observed. Ring forms are large with one chromatin dot and occupy 1/3 diameter of the infected cell. Older trophozoites may show band forms, mature schizonts contain merozoites arranged like rosettes, and gametocytes are round or oval.
In Plasmodium ovale, parasitized red cells are enlarged and pale and often oval-shaped with fimbriated or crenated appearance. Conspicuous stipplings are seen (Schuffner’s dots). Rings are large with 1 chromatin granule and older trophozoites are round. Mature schizonts and gametocytes show rosette forms and gametocytes are round or oval.
With Plasmodium vivax, infected RBCs are enlarged and pale and contain Schuffner’s dots. Ring trophozoites are large with single chromatin. Older trophozoites are very pleomorphic and mature schizonts contain a large number of merozoites (14-24). Gametocytes are round or oval.

Question 3

What is the most likely causative agent of the patient’s illness?

1 Isospora belli
2 Cryptosporidium hominis
3 Giardia lamblia
4 Dientameba fragilis
5 Cyclospora cayetanensis

Answer 3

Cryptosporidium hominis

The correct choice is Cryptosporidium hominis.
All of the listed parasites are diarrheal agents. The description of small oocysts observed in the patient’s stool sample is typical of that of Cryptosporidium, a coccidian parasite that causes cryptosporidiosis.
Cryptosporidium hominis is a minute coccidian parasite of worldwide distribution. Besides humans, natural infection occurs in many species of animals and birds. The major reservoir is domestic livestock, predominantly cattle. Human infection is acquired mainly by ingestion of oocysts present in contaminated food or drink. The incubation period is 1-7 days. The parasite completes both sexual and asexual phases of its life-cycle in a single host. After ingestion, the 4 sporozoites in the oocyst are released in the small intestine and they infect the epithelial cells. They develop into trophozoites, multiply asexually (schizogony/merogony), and release merozoites. Merozoites infect the neighboring epithelial cells and repeat schizogony. Some of the merozoites develop into micro and macrogametes. The zygote formed by fertilization develops into oocyst, which is then shed in feces. The oocyst in freshly passed feces is fully mature and is infective. It is very resistant to disinfectants and temperatures up to 60°C, and it can remain viable in the environment. Waterborne outbreaks of cryptosporidiosis have occurred in many countries. In the US, a large outbreak involving >400,000 people occurred in 1993 in Milwaukee, Wisconsin.
Infection in immunocompetent persons may be asymptomatic or may result in self-limited febrile illness with watery diarrhea. In HIV-infected and other immunosuppressed individuals, the parasite may cause severe life-threatening illness. Extraintestinal infections with pulmonary and biliary tract involvement have been reported in patients with AIDS.
Diagnosis of cryptosporidium infection is made by microscopy of both unstained and stained preparations of stools. Wet mount with iodine is useful for screening fecal samples and the sporozoites can be visualized inside the oocysts. In addition to modified acid-fast stain, safranin also can be used to stain the oocysts. With trichrome stain the oocysts remain unstained. C. hominis oocysts can be demonstrated by fluorescent stains such as Auramine O and auramine- rhodamine. Definitive identification of the oocysts can be made by indirect immunofluorescence using specific antibody. An ELISA using monoclonal antibody is highly specific and sensitive for detection of cryptosporidium in stools.
Isospora belli oocyst usually contains a single sporoblast. It matures outside the body and develops 2 sporocysts containing 4 sporozoites each. Human infection occurs by ingestion of mature oocysts in food or drink. Schizogony and sporogony take place in the epithelial cells of the small intestine. In immunocompetent persons, the infection is asymptomatic or self-limited. HIV-infected individuals and other immunocompromised persons develop protracted diarrhea. Treatment with cotrimoxazole is effective.
Giardia lamblia, an intestinal flagellate, causes giardiasis. It is the only common pathogenic protozoan living in the duodenum and upper jejunum of humans. Infection can occur in immunocompetent and immunosuppressed persons. Giardiasis in immunosuppressed individuals is more prolonged, with severe clinical manifestations.
Dientameba fragilis is a protozoan parasite of humans that infects large intestinal mucosa and causes chronic or recurrent diarrhea. There is no evidence of natural hosts other than humans. It is now considered an ameboflagellate because of the similarities to trichomonads.
Cyclospora cayetanensis is a coccidian parasite and causes cyclosporiasis. The infection is found worldwide. Outbreaks of cyclosporiasis have been reported in the U.S. and Canada. A prodrome of flu-like symptoms may precede diarrhea. Oocysts in freshly passed stools are not infective. Sporulation and maturation occurs in the environment and mature oocyst is the infective form. Infection is confirmed by stool examination.

Question 4

A resident of Egypt complained of painless terminal hematuria. His urine sample was received in the microbiology laboratory. Microscopic examination of centrifuged deposit of urine detected presence of embryonated eggs of a Trematode that is known to be endemic in most parts of Africa. What is likely to be a feature of this trematode?

1 It is a hermaphrodite
2 Eggs are operculated
3 Sheep are natural definitive hosts
4 A stage known as radia develops in snail
5 Infectious stage for humans is cercaria
6 Requires 2 intermediate hosts

Answer 4

Infectious stage for humans is cercaria

Infectious stage for humans is cercaria.
The embryonated eggs in the patient’s urine are of Schistosoma hematobium, a trematode that is endemic in most parts of Africa and Middle East.
Schistosomes (blood flukes) do not possess the other listed features.
Schistosomes are dioecious (sexes are separate). The adult male and female worms live in copula in the vesical and pelvic venous plexuses. The female worm releases numerous eggs per day. The egg of S.hematobium is ovoid, 150x50 microns, with transparent shell and having a terminal spine at one pole. Eggs pass into the lumen of the urinary bladder with some extravasated blood and are excreted in urine causing the typical manifestation of terminal hematuria. When released into fresh water, the eggs hatch out ciliated miracidia, which enter the intermediate host snail. Further development takes place in the snail and large numbers of cercariae are produced. The cercariae swarm out of the snail and, on coming in contact with human skin, enter through penetration of the unbroken skin. They get transformed into schistosomules, enter the peripheral venules, and after a long migration and sexual differentiation ultimately reach the vesical and pelvic venous plexuses where they mature, mate, and begin to lay eggs.
Chronic infection by S.hematobium can cause persistent cystitis, pyelonephritis, and obstructive renal disease and is found to be associated with increased incidence of bladder carcinoma. Neuroschistosomiasis, Katayama fever, and female genital schistosomiasis are other manifestations associated with S.hematobium infection. In women with chronic cervical schistosomiasis, the lesions may mimic cervical carcinoma and could become co-factors for viral infections such as HIV and HPV.
Laboratory diagnosis depends on microscopy for eggs in urine. Occasionally eggs can be demonstrated in stool samples of the infected person. In cases where urine and stool are negative, bladder and rectal biopsy specimens will be helpful for demonstration of ova.
Antibody detection is useful in persons who have traveled to endemic areas and in whom the microscopy is negative. Crude and purified antigens prepared from adult worm, cercaria, and eggs have been used for serological tests. Anti-schistosomal antibodies can be detected by FAST-ELISA (Falcon Assay Screening Test-Enzyme lnked immunosorbent assay). Species identification is not possible by this test. Immunoblot assay using adult worm microsomal antigen detects species-specific antibodies. Positive serology indicates only exposure to the parasite and cannot be correlated with active disease.
ELISA test developed for detection of circulating schistosomal antigen (CSA) using anti-S.hematobium monoclonal antibodies is reported to be specific and sensitive for diagnosing active infection.
Praziquantel is the drug of choice for treatment.
There is ongoing research in the field of S.hematobium vaccine development.
[Microscopic appearance of schistosome eggs and diagram of life cycle of the parasite is available in reference 2].

Question 5

What is most likely causing this patient’s symptoms?

1 Cryptosporidiosis
2 Cyclosporiasis
3 Giardiasis
4 Isosporiasis
5 Microsporidiosis

Answer 5

Giardiasis

Diarrhea can manifest in all five diseases. Giardiasis is the only one diagnosed by microscopic detection of motile trophozoites of the etiological agent in stool.
Giardiasis is a parasitic infection caused by intestinal flagellate, Giardia lamblia, the most common intestinal protozoan pathogen of humans. Giardiasis is common in daycares and institutions. It is transmitted by ingestion of food or water contaminated with cysts of the organism. Most initially present with acute watery diarrhea associated with abdominal discomfort, bloating, and foul-smelling stools. Watery stools may alternate with fatty stools that tend to float. Giardia cysts can be detected in stool samples of symptomatic and asymptomatic infections. Immunosuppressed persons are susceptible to massive infection and severe clinical manifestations.
Cryptosporidiosis is a highly infectious disease caused by Cryptosporidium hominis. Outbreaks of diarrhea due to cryptosporidiosis are common in daycares, transmitted by ingestion of food or water contaminated with oocysts of the parasite. Diagnosis is established by demonstration of sporulated oocysts in the feces by modified acid-fast or other staining methods.
Cyclosporiasis is an infection caused by Cyclospora cayetanensis that manifests with diarrhea 2-11 days after consuming food or water contaminated with oocyst-laden feces. Diagnosis is by identification of oocysts in stool samples stained by modified safranin, acid-fast, or autofluorescence with UV-light microscopy.
Isosporiasis, caused by Isospora belli, is common in daycares and mental institutions. After incubation of 7-11 days, watery diarrhea can persist for several months. Diagnosis is by detection of oocysts in freshly passed stools.
Microsporidiosis is caused by microsporidia, minute intracellular parasites that reproduce by spores. They can cause a wide range of illness from diarrhea to involvement of the CNS, eyes, viscera, muscles, and disseminated disease. Diagnosis is established by visualization of spores of microsporidia in stools, body fluids, or tissues after appropriate staining or electron microscopy.

Question 6

What is the most likely causative agent in this patient?

1 Sporothrix schenckii
2 Ancylostoma braziliense
3 Enterobius vermicularis
4 Bartonella henselae
5 Ixodes

Answer 6

Ancylostoma braziliense

The correct answer choice is Ancylostoma braziliense. This patient has cutaneous larva migrans, which most commonly occurs in tropical environments. This patient has a predisposition to this condition, as he has common contact with the sand. Other people who have contact with warm, moist, sandy soil are also at risk. The classic rash is described as an erythematous, serpiginous, pruritic, cutaneous eruption. The presentation of the rash, which expands a few mm per day, is also highly suggestive of Ancylostoma braziliense.
Sporothrix schenckii is commonly found in gardeners. This is a fungal disease and usually progresses slowly. The first symptom may appear 1 to 12 weeks (average 3 weeks) after the initial exposure to the fungus. The lesion starts off small and painless and ranges in color from pink to purple.
Enterobius vermicularis presents with anal pruritus.
Cat scratch disease (CSD) is a bacterial disease caused by Bartonella henselae. Most people with CSD have been bitten or scratched by a cat and develop a mild cutaneous infection at the point of injury.
Ixodes tick is responsible for Lyme disease. The classic lesion of Lyme disease is bulls-eye shaped with a red center, an area of central clearing, and a periphery of redness around the clearing.

Question 7

What is the treatment of choice for this patient?

1 Vancomycin
2 Albendazole
3 Doxycycline
4 Metronidazole
5 Ceftriaxone

Answer 7

Metronidazole

Metronidazole is the drug of choice in the treatment of giardiasis. Giardiasis, an illness that affects the digestive tract, is caused by a microscopic parasite called Giardia lamblia. The parasite attaches itself to the lining of the small intestines in humans, where it frequently causes diarrhea and malabsorption. The patient’s history is suggestive, as she was likely exposed to contaminated water. Giardiasis is usually treated with metronidazole.
Vancomycin is indicated for the treatment of serious, life-threatening infections by Gram-positive bacteria that are unresponsive to other antibiotics. Oral vancomycin may also be used to treat Clostridioides difficile infection.
Albendazole is not first-line in the treatment of giardiasis. It is an antihelminthic drug used to treat hookworm, pinworm, and whipworm.
Doxycycline can be used to treat a variety of infections. Doxycycline is frequently used to treat chronic syphilis, chlamydia, acne, prostatitis, as well as many other infections.
Ceftriaxone is more commonly utilized in treating infections such as gonorrhea.

Question 8

An outbreak of diarrhea occurred in a child day care center situated in Dhaka, Bangladesh. Freshly passed stool samples collected from the affected children were subjected to microscopy. Diagnosis was made based on detection of trophozoite and cystic forms of an intestinal flagellate protozoan parasite. What is most likely a feature of this infection?

1 Natural infection occurs only in humans
2 The disease is confined to developing countries
3 Trophozoites of the parasite are seen predominantly in the colon
4 Freshly passed cysts in feces are infective
5 Detection of specific antibodies is a routine diagnostic method
6 Trophozoites are excreted by asymptomatic carriers

Answer 8

Freshly passed cysts in feces are infective

Of the clinically important intestinal flagellate protozoan parasites, Giardia lamblia is the only one that affects gastrointestinal tract of humans. Infection may be asymptomatic or may result in diarrhea. Clinical disease caused by G.lamblia is known as Giardiasis. Outbreaks of giardiasis occur especially in children. G.lamblia has trophozoite and cystic forms, and the detection of these forms in stools helps diagnosis of giardiasis. Therefore, it is clear that the outbreak of diarrheal illness referred to in the question is Giardiasis.
Cyst is the infective form of G.lamblia. Freshly passed cysts in feces are infective. This characteristic helps direct person-to-person transmission of the infection by fecal-oral route. Direct transmission is more often seen in children, the mentally ill, and male homosexuals. Most common mode of acquiring infection is by ingestion of cysts in contaminated water or food. Cysts passed in stools remain viable in soil and water for several weeks.
Other features listed are not applicable to giardiasis.
Cysts of Giardia lamblia are oval 8-12 micronsx7-10 microns in size and surrounded by a tough hyaline cyst wall. The mature cyst contains 4 nuclei situated at 1 end. As few as 10 cysts are capable of initiating infection. Infection may result in asymptomatic carriage or clinical illness ranging from diarrhea with abdominal symptoms to fulminant diarrhea and malabsorption. Asymptomatic carriers excrete only cysts.They are important reservoirs of infection.
After being ingested, excystation of the parasite occurs in the duodenum. 2 trophozoites hatch out of a single cyst. The trophozoites multiply by binary fission and remain in the lumen of the small intestine, where they get attached to the mucosal epithelium. Encystation occurs as the trophozoites pass down the colon.
Giardia trophozoite is 9-21 microns long and 5-15 microns wide. It is pear-shaped, bilaterally symmetrical, and has 4 pairs of flagella. There are 2 nuclei, 1 on either side of the midline and have prominent central karyosomes. In iron hemotoxylin or trichrome-stained preparations the trophozoite creates a face-like image. The trophozoite firmly attaches to the epithelial surface of the duodenum and jejunum by means of a large concave sucking disc present on the ventral surface of its anterior portion.
A large number of parasites adhering to the mucosal cells may cause low-grade inflammation, resulting in clinical symptoms of acute diarrhea, abdominal pain, and flatulence. Usually the parasite does not invade or produce necrosis of the mucosal epithelium. In chronic giardiasis, flattening or atrophy of villi, epithelial cell damage, and loss of brush border enzyme activity occur. Resulting lactose intolerance and malabsorption cause a sprue-like syndrome. Occasionally Giardia trophozoites may colonize in the biliary tree and gall bladder, causing biliary colic. Children are more susceptible to giardiasis. Giardiasis is one of the common causes of traveler’s diarrhea.
Reactive arthritis has been reported as a rare sequel of giardiasis.
It has been shown that the immunodominant cysteine rich surface proteins of Giardia, known as variant surface proteins (VSPs), can undergo antigenic variation in the host and may play a role in the immune evasion by the parasite.
Laboratory diagnosis: microscopy of wet mounts of diarrheal stool samples is used to detect the trophozoite and cysts of the parasite. Samples should be freshly passed, as the trophozoites easily die outside the body. Trophozoites show typical dancing or swaying motility. Microscopy of stained preparations help to identify the structural details of the parasite. Only cysts are seen in asymptomatic carriers. Stool samples are tested after concentration if cysts are sparse. Direct Fluorescent Antibody (DFA) test is considered very specific and sensitive and is used in many laboratories for detection of the giardia cysts in stool samples (microscopic appearances of iron-hemotoxylin/trichrome stained trophozoite and cyst forms, and giardia cysts stained by DFA are available in Ref 5).
Detection of giardia antigens in stools by enzyme-linked immunosorbent assay (ELISA) and immunochromatographic-cartridge assay are sensitive and rapid diagnostic methods.
Sample collected by duodenal aspiration (Entero test) may be required for demonstration of the parasite, especially when the biliary symptoms predominate.
Biopsy of small intestine may be required for establishing the diagnosis in some cases.
Giardia can be cultured in vitro; cultures are used mainly for research purposes.
Since giardia trophozoites rarely invade the tissues, stimulation of systemic immune response is variable. Testing for serum antibodies for diagnosis is reported to be unreliable.
Giardiasis affects various animal species, including domestic animals like dogs, cats, and cattle, and wild animals, including beavers and bears.
Molecular typing of Giardia by PCR-based methods has helped to identify the genotypes associated with human infections. Some genotypes have been shown to cause giardiasis in both humans and animals, suggesting the zoonotic potential of giardia. Genotype determination can be helpful in epidemiological studies.
Giardiasishas worldwide distribution affecting developing and developed countries. The infection is endemic in most of the developing countries. In the U.S. and other industrialized countries, giardiasis is recognized as a re-emerging disease. Water borne epidemics and sporadic infections occur. In the US, giardiasis is a national notifiable disease since 2002.
Metronidazole and tinidazole are drugs of choice for treatment of giardiasis. Nitazoxanide is useful in children. Paromomycin can be used for treating giardiasis in pregnant women.

Question 9

Which trophozoite ingests RBCs?

1 Entamoeba histolytica
2 Endolimax nana
3 Blastocystis hominis
4 Entamoeba coli
5 Iodamoeba butschlii

Answer 9

Entamoeba histolytica

Except Entamoeba histolytica, all the amebae mentioned above are nonpathogenic commensals found in humans.
Entamoeba histolytica is the major pathogen found in the large intestine and is associated with amebic dysentery in humans. The organism secretes proteolytic enzymes that aid in the disruption of the intestinal barrier. It invades the epithelial cells and causes lysis of the cells. The trophozoites feed on the red cells that appear either as a whole or on partially digested red cells in the cytoplasm of the trophozoites.
Entamoeba histolytica causes flask-shaped ulcers in the intestinal mucosa of the host. The organism shows resistance to phagocytosis and complement-mediated cell lysis.

Question 10

The protozoa associated with primary amebic meningoencephalitis is:

1 Iodamoeba bütschlii
2 Naegleria fowleri
3 Giardia lamblia
4 Trichomonas vaginalis
5 Entamoeba coli

Answer 10

Naegleria fowleri

Naegleria fowleri is found in contaminated water environments and soil. Individuals acquire infection during summertime while swimming in contaminated water. The organism gains entry through the nasal passage. It invades the nasal mucosa and can reach the brain and cause destruction of brain tissue. This results in primary amebic meningoencephalitis, which is characterized by a rapid and often fatal course. The clinical symptoms are intense headache, sore throat, fever, blocked nose, and stiff neck. These symptoms are followed by irrational behavior, coma, and death.
Iodamoeba bütschlii and Entamoeba coli are nonpathogenic commensals.
Giardia lamblia is a flagellate associated with traveler’s diarrhea.
Trichomonas vaginalis is implicated in urogenital infections.

Question 11

Which of the following infections is caused by Acanthamoeba in immunocompromised individuals?

1 Amebic dysentery
2 Malabsorption syndrome
3 Granulomatous amebic encephalitis
4 Chagas disease
5 Kala azar

Answer 11

Granulomatous amebic encephalitis

Acanthamoeba is associated with granulomatous amebic encephalitis, primarily in an immunocompromised host. The amebae spread hematogenously to the brain from primary infection sites (nasal mucosa, lungs, or skin). The infection may be characterized by brain abscesses, granulomas, thrombosis, or hemorrhage. Patients experience headaches, drowsiness, stiff neck, hemiparesis, and seizures. Death may result in 2-3 days if infection occurs directly through nasal mucosa. Brain biopsy may reveal cyst, as well as trophozoites stages of the organism. Acanthamoeba is also implicated in keratitis in contact lens users.
The following table shows the etiological agents associated with the clinical conditions given in the question.
Clinical Conditions Etiological Agents
Amebic dysentery Entamoeba histolytica
Malabsorption syndrome Giardia lamblia
Chagas disease Trypanosoma brucei
Kala azar Leishmania donovani

Question 12

What trophozoite demonstrates “falling leaf” motility in the wet mount?

1 Entamoeba histolytica
2 Endolimax nana
3 Giardia lamblia
4 Dientamoeba fragilis
5 Trichomonas vaginalis

Answer 12

Giardia lamblia

Giardia lamblia is an intestinal flagellate that has both a trophozoite and cyst form. The pear shaped trophozoite of Giardia lamblia exhibits characteristic “falling leaf” motility on a wet mount. Trophozoites are bilaterally symmetrical with 2 oval nuclei containing large central karyosomes on each side of midline. It has 4 pair of flagella, midline axonemes, 2 median bodies, and a large ventral sucking disk for attachment.
Trophozoites of Entamoeba histolytica have progressive directional motility while Endolimax nana and Dientamoeba fragilis trophozoites have non-directional motility. Trophozoites of Trichomonas vaginalis exhibit a non-directional, jerky motility.

Question 13

Which of the following is an example of a pathogenic intestinal flagellate?

1 Dientamoeba fragilis
2 Trichomonas hominis
3 Chilomastix mesnili
4 Trichomonas vaginalis

Answer 13

Dientamoeba fragilis

Trichomonas hominis and Chilomastix mesnili are non-pathogenic intestinal flagellates.
Dientamoeba fragilis is pathogenic flagellate associated with gastrointestinal illness. Trichomonas vaginalis is a non-intestinal, pathogenic flagellate transmitted by sexual contact.

Question 14

The etiological agent of malaria is

1 Babesia sp
2 Plasmodium sp
3 Leishmania sp
4 Trypanosoma sp
5 Plesiomonas sp

Answer 14

Plasmodium sp

Plasmodium sp. infects erythrocytes and causes malaria. It is the major cause of mortality in people in underdeveloped countries. Four organisms which are responsible for this disease are: Plasmodium vivax, Plasmodium falciparum, Plasmodium malariae, and Plasmodium ovale. Once established, Malaria can be transmitted via the Anopheles mosquito, blood transfusion, infected needles and across the placenta. Symptoms include fever, chills and the presence of parasites in the blood smear.

Question 15

Plasmodium vivax has

1 quatrain life cycle
2 Crescent shaped gametocytes
3 The widest geographic distribution
4 Schizont with 6-12 merozites

Answer 15

The widest geographic distribution

Plasmodium vivax has the widest geographic distribution and is the major cause (48%) of malaria cases reported in the United States. Additional characteristics include the following: Tertian life cycle, (i.e. reproductive cycle of 48 hours), infects young red blood cells, schizont has 12-24 merozoites, and Schüffner stipplings are present on the cell. A peripheral blood smear may show a wide range of developmental stages. Plasmodium malariae has a quatrain life cycle (72 hours) and schizont with 6-12 merozoites. Crescent shaped gametocytes are characteristics of Plasmodium falciparum.

Question 16

Which of the following Plasmodium sp. is/are characterized by presence of multiple ring forms and crescent shaped gametocytes in the peripheral blood smear?

1 Plasmodium vivax
2 Plasmodium ovale
3 Plasmodium falciparum
4 Plasmodium malariae

Answer 16

Plasmodium falciparum

Plasmodium falciparum has an asynchronous life cycle and red cell rupture takes place between 36 to 48 hours. It infects erythrocytes of all ages resulting in massive erythrolysis. This parasite causes electron-dense knobs on the surface of erythrocytes leading to alteration in their surface membrane. Peripheral blood smears demonstrate ring-form trophozoites and crescent-shaped gametocytes. Maturation of other stages takes place in the venules and capillaries of major organs.
Plasmodium vivax and Plasmodium ovale mainly target immature erythrocytes. Peripheral blood smears are characterized by the presence of enlarged red blood cells with Schüffner stipplings. Plasmodium malariae infects mature erythrocytes. Peripheral smears may show normal size red cells and compact trophozoites with a characteristic “band” appearance.

Question 17

Which of the following is a vector for malaria?

1 Anopheles mosquito
2 Tsetse fly
3 Reduviid bug
4 Sandfly
5 Ixodes tick

Answer 17

Anopheles mosquito

The female Anopheles mosquito is a biological vector and definitive host for malaria.
The vector ingests the microgametocytes and macrogametocytes when she takes the blood meal from a human host. Sporogony takes place in the vector and results in the production of sporozoites, which are infective for human. The mosquito bites the human and injects the sporozoites present in the salivary gland. Once the sporozoites enter the human body, a primary exoerythrocytic cycle of asexual reproduction takes place in the liver, leading to production of merozoites. In the erythrocytic phase of asexual reproduction, merozoites invade red blood cells. The parasite feeds on the hemoglobin and matures into a trophozoite. The trophozoites develop into a schizont containing merozoites. Upon lysis of the red cells, merozoites are released and can then invade other red cells. Merozoites can develop into a gametocyte form, which is infective for mosquito.
The following table shows the association of the vector with the infections mentioned in the question.
Vector Parasitic Infection
Tsetse fly Sleeping sickness
Reduviid bug Chagas disease
Sandfly Leishmaniasis
Ixodes tick Babesiosis

Question 18

Trypanosoma brucei gambiense is the etiological agent of

1 Rocky mountain spotted fever
2 Sleeping sickness
3 Babesiosis
4 American trypanosomiasis
5 Elephantiasis

Answer 18

Sleeping sickness

Trypanosoma brucei gambiense is a flagellate that causes west African sleeping sickness. Trypomastigote is ingested by the tsetse fly when it takes a blood meal from an infected human. Trypomastigote stage develops into an epimastigote stage in the insect’s gut and migrates to the salivary gland. This develops into infective metacyclic trypomastigote and is transmitted to humans through insect bite. This enters the blood and lymphatics of the host. The patient develops headache, fever, muscle pain and enlarged lymph nodes. The parasite can invade the central nervous system causing severe headaches, mental dullness and apathy, altered reflexes, and paralysis. This can result in convulsion, coma and death of the patient.

Question 19

Chagas disease is caused by

1 Trypanosoma brucei
2 Trypanosoma cruzi
3 Leishmania donovani
4 Leishmania braziliensis
5 Babesia microti

Answer 19

Trypanosoma cruzi

Trypanosoma cruzi is the etiological agent of Chagas disease which is transmitted by the Reduviid bug. This parasite infects cells of the lymph system, macrophages, cardiac muscle and skeletal muscles. It causes severe forms of the disease in children and can result in death. Symptoms of acute Chagas disease includes fever, unilateral conjunctivitis, ulcerative skin lesion, lymphadenitis, hepatosplenomegaly and muscular pain. It can also cause acute myocarditis leading to coronary heart failure. Chronic disease results in the development of the organism in visceral organs. The amastigotes multiplies and destroys the host cells and can lead to megacolon, megastomata or megaesophagus.
In acute infections, Trypanosoma cruzi appears in peripheral blood as C or U shaped trypomastigotes with a single large nucleus midbody and posterior kinetoplast with an undulating membrane attached to it. An Enzyme Linked Immunoassay can be used to diagnose chronic cases of Chagas disease. Nifurtimox can be used to eliminate trypomastigotes from circulation and improve the patient’s clinical condition.

Question 20

Sandflies are vectors for

1 Malaria
2 Sleeping Sickness
3 Leishmaniasis
4 Chagas disease
5 Lyme disease

Answer 20

Leishmaniasis

Leishmaniasis is transmitted by the bite of the Sandfly among animal and human reservoirs. When sandflies take a blood meal, the organism is ingested as an amastigote. This develops into a promastigote in the insect gut and migrates to its salivary glands. When the sandfly subsequently bites another reservoir, the promastigote is ingested by the reservoir’s macrophages and is transformed to the amastigote stage and multiplies within the cell.
The following table shows the association of the vector with the diseases mentioned in the question.
Disease Vectors
Malaria Anopheles Mosquito
Sleeping Sickness Tsetse fly
Chagas Disease Reduviid bug
Lyme Disease Tick

Question 21

Which parasite is acquired by ingestion of raw or improperly cooked fish?

1 Diphyllobothrium latum
2 Taenia solium
3 Taenia saginata
4 Hymenolepis diminuta
5 Dipylidium caninum

Answer 21

Diphyllobothrium latum

Diphyllobothrium latum is acquired by ingesting raw or improperly cooked fish. It is the largest of the human tapeworms and is also known as the broad or fish tapeworm. It can cause abdominal discomfort, nausea, vomiting, diarrhea, and weight loss. Large numbers of this parasite can cause blockage of the intestine. It also can compete with the host for vitamin B12 and cause pernicious anemia.
The following table shows the association of parasites with the diseases mentioned above.
Parasites Acquired by ingestion of
Taenia solium Undercooked Pork
Taenia saginata Undercooked Beef
Hymenolepis diminuta Flea containing cysticercoid
Dipylidium caninum Flea containing larval stage

Question 22

Which of the following is the most common human tapeworm in the United States?

1 Diphyllobothrium latum
2 Taenia solium
3 Taenia saginata
4 Hymenolepis diminuta
5 Hymenolepis nana

Answer 22

Hymenolepis nana

Hymenolepis nana is the most common tapeworm in the United States and is prevalent in the Southeastern United States. It is most commonly seen in children younger than 8 years of age. Hymenolepis nana does not require an intermediate host and can cause direct infection upon ingestion of eggs. It can be transmitted hand to mouth or by contaminated foods or fluids. Mice, fleas and beetles can also act as a transport host and produce cysticercoid larvae that can infect humans and rodents. Adult worms live in the intestine and can cause abdominal discomfort, diarrhea, and headaches if present in large numbers. Infection can be diagnosed by recovery of the parasite’s eggs.

Question 23

Cysticercosis is caused by the tissue stage of

1 Taenia saginata
2 Taenia solium
3 Diphyllobothrium latum
4 Ascaris lumbricoides
5 Hymenolepis nana

Answer 23

Taenia solium

Cysticercosis is a serious disease caused by the larvae or tissue stage of Taenia solium. Humans accidentally become the intermediate host and harbor the larvae in their tissues. The egg hatches in the small intestine and enters the circulation developing into a cysticercus in the tissue or organ. This larva has predilection for skeletal tissue and the nervous system. It can lead to ocular cysticercosis or cerebral cysticercosis. The organism elicits host-tissue reaction and produces a fibrous capsule. Dead larvae can cause increased inflammatory reaction and can lead to calcification.
Cysticercus are oval, translucent and contain an invaginated scolex with 4 suckers and a circle of hooklets on the rostellum.

Question 24

Which parasitic infection in humans is acquired by ingestion of beef?

1 Taenia saginata
2 Taenia solium
3 Diphyllobothrium latum
4 Leishmania donovani
5 Trypanosoma cruzi

Answer 24

Taenia saginata

Taenia saginata is a beef tapeworm. It is prevalent in Ethiopia, Iran, Taiwan, Kenya and other beef eating countries in the world. Human infection results from the ingestion of larvae in the beef. Larva attach to the ileum and mature into an adult worm. It attaches by means of the scolex to the mucosa of the small intestine. The infection can cause abdominal discomfort, hunger pain or loss of appetite, weight loss, etc.
The infection is detected when a person passes the proglottids in the stool or detection of eggs through the microscopic examination of stool. The infection can be treated by niclosamide or praziquantel.
Taenia solium infection results when humans ingest insufficiently cooked pork containing cysticercus. Diphyllobothrium latum infection is acquired from ingestion of fish containing plerocercoid larva. Sandflies are vectors for Leishmania donovani. Reduviid bug is a vector for Trypanosoma cruzi.

Question 25

Which Trematode, or giant intestinal fluke, is associated with the ingestion of water chestnuts?

1 Fasciolopsis buski
2 Opisthorchis sinensis
3 Paragonimus westermani
4 Schistosoma mansoni

Answer 25

Fasciolopsis buski

Fasciolopsis buski is an intestinal fluke that is common in Vietnam, India, and China. Humans become infected by ingesting metacercaria on freshwater vegetation such as water chestnuts and bamboo shoots. The adult fluke lives in the duodenum and causes mechanical and toxic damage leading to inflammation and ulceration. Clinical symptoms of heavy infection include anorexia, nausea, vomiting, edema, and ascites. Recovery of adult flukes or eggs of the organism in feces is diagnostic of infection.

Question 26

Paragonimus westermani is a

1 Liver fluke
2 Lung fluke
3 Intestinal fluke
4 Bladder fluke
5 Blood fluke

Answer 26

Lung fluke

Paragonimus westermani is lung fluke primarily found in the Far East, India and parts of Africa. Ingestion of crabs or crayfish containing metacercaria can result in human infection. The metacercaria penetrates the intestinal wall and diaphragm on its way to the lung. Adult flukes live within capsules in the bronchioles which can lead to an inflammatory response, persistent cough, chest pain and hemoptysis. A wet mount of sputum can reveal presence of eggs in the infected individual.
Fasciola hepatica - Sheep liver fluke
Opisthorchis sinensis - Chinese liver fluke
Fasciolopsis buski - Large intestine fluke
Schistosoma japonicum - Oriental blood fluke
Schistosoma mansoni - Mansion’s blood fluke
Schistosoma haematobium - Bladder fluke

Question 27

The adult fluke of Opisthorchis sinensis lives in the

1 Bladder
2 Heart
3 Stomach
4 Distal bile ducts
5 Lymphatics

Answer 27

Distal bile ducts

Adult Opisthorchis sinensis or Chinese liver fluke live in the distal bile duct of the liver. Ingestion of metacercaria in raw undercooked or pickled fish can cause infection in humans. Heavy infection can lead to inflammation, fever, diarrhea, pain, fibrosis and bile duct obstruction. Demonstration of eggs in stool specimen can be used for diagnosis. The embryonated eggs are vase shaped with a domed operculation with prominent shoulder, and a knob at the end opposite to operculum.

Question 28

The etiological agent of Elephantiasis is

1 Enterobius vermicularis
2 Wuchereria bancrofti
3 Trichinella spiralis
4 Trypanosoma cruzi
5 Leishmania braziliensis

Answer 28

Wuchereria bancrofti

Wuchereria bancrofti causes bancroftian filariasis and elephantiasis. Elephantiasis is a debilitating and deforming condition occurring in patients with many years of continuous filarial infection. It is transmitted by mosquitoes of Culex , Aedes and Anopheles spp. Infective filariform larval stages are introduced into human circulation through insect bites. Wuchereria bancrofti invades the lymphatics and causes granulomatous lesions, chills fever, and eventually elephantiasis. Lymphatics and lymph nodes of the lower extremities are mostly involved. The adult filarial worm initiates the cellular reaction; edema and hyperplasia. Presence of a dead worm causes granulomatous reaction leading to formation of fibrous tissue. This causes blockage of lymphatics with development of collateral lymphatics. Many years of filarial infection can lead to elephantiasis.

Question 29

Onchocerca volvulus is the etiological agent of

1 Rectal prolapse
2 Kala azar
3 Sleeping sickness
4 Black water fever
5 River blindness

Answer 29

River blindness

Onchocerca volvulus is a roundworm and a blood parasite associated with river blindness. The parasite is transmitted by blackflies. Adult worms live in subcutaneous tissues encapsulated within fibrous nodules. The nodule forms as a result of an inflammatory and granulomatous reaction around the worm. Microfilariae can migrate to other parts of the body including the eye. Blindness is the serious complication of infection with Onchocerca volvulus . It can be lodged in the cornea and iris leading to keratitis and atrophy of the iris. Sclerotic keratitis can lead to following conditions:
Blindness
Cataracts
Iridocyclitis
Secondary glaucoma

Question 30

The etiological agent of cutaneous larval migrans is

1 Fasciola hepatica
2 Fasciolopsis buski
3 Trichuris trichiura
4 Ancylostoma braziliense
5 Toxocara canis

Answer 30

Ancylostoma braziliense

Dog and cat hookworms like Ancylostoma braziliense and Ancylostoma caninum are associated with cutaneous larval migrans. When Ancylostoma braziliense penetrates the human skin, it can not enter the circulation to complete the life cycle. It causes pruritic eruption by creating long winding tunnels when it moves through subcutaneous tissue. The clinical symptoms include itching, reddish papule at the point of entry, edema and inflammatory trait with crusty opening and eosinophilia. The infection resolves on its own when the larva dies. Toxocara canis is a dog roundworm and is associated with visceral larval migrans.

Question 31

Toxocara canis is associated with

1 Visceral larval migrans
2 Cutaneous larval migrans
3 Primary amebic encephalitis
4 Granulomatous amebic encephalitis
5 Toxoplasmosis

Answer 31

Visceral larval migrans

Visceral larval migrans is caused by Toxocara canis and Toxocara cati. Toxocara canis is a dog roundworm. Toxocara cati is a cat roundworm. When the human accidentally ingests the eggs, the larva hatches in the intestine. It penetrates the gut and wanders through the abdominal cavity to other organs of the body like liver, lungs, eyes and brain. Infection is common in children between 1 to 4 years of age. Fever, pneumonitis, hepatomegaly and eosinophilia are the symptoms associated with the infection. Complications of central nervous system are seen when the larva invades the brain.
Ancylostoma braziliense and Ancylostoma caninum are associated with cutaneous larval migrans.
Naegleria fowleri is the etiological agent of primary amebic encephalitis.
Acanthamoeba spp. causes granulomatous amebic encephalitis primarily in immunocompromised patients.
Toxoplasmosis is caused by Toxoplasma gondii .

Question 32

Which parasite is associated with anal pruritus?

1 Trichuris trichiura
2 Enterobius vermicularis
3 Wuchereria bancrofti
4 Paragonimus westermani
5 Schistosoma haematobium

Answer 32

Enterobius vermicularis

Enterobius vermicularis is commonly known as “pinworm” and is mostly found in children or families in crowded condition. The eggs of Enterobius vermicularis are resistant to drying, and direct feco-oral transmission is common in children. The adult worm lives in the large intestine. Adult females migrate outside the body to lay eggs in the perianal area. The crawling of the female adult worm and the eggs causes intense itching resulting in anal pruritus. Enterobius vermicularis can be asymptomatic or can cause nausea, vomiting, abdominal pain, insomnia and restlessness. Treatment includes pyrantel pamoate, mebendazole, or piperazine citrate.

Question 33

Rectal prolapse is the result of heavy infection by

1 Trichuris trichiura
2 Enterobius vermicularis
3 Loa loa
4 Schistosoma haematobium
5 Trichinella spiralis

Answer 33

Trichuris trichiura

is found in moist and warm climates. Eggs are embryonated in the warm and moist soil. Following ingestion of infective eggs, the larva is released in the small intestine and matures into an adult form in the cecum. The whip shaped worms thread themselves through the mucosa, and cause inflammation. Heavy infection causes abdominal pain, vomiting, chronic diarrhea, and bleeding. In undernourished children, it can cause rectal prolapse, and can also cause hypochromic anemia.
Enterobius vermicularis is commonly known as “pinworm”. It is associated with anal pruritus
Loa loa is the eyeworm associated with calabar swelling
Schistosoma haematobium is a blood fluke that lives in bladder and causes hematuria
Trichinella spiralis is the round worm that infects muscle tissue and causes trichinosis

Question 34

Which of the following stages of Trichinella spiralis is associated with trichinosis?

1 Microfilaria
2 Cercaria
3 Eggs
4 Larvae

Answer 34

Larvae

The larval form of Trichinella spiralis causes trichinosis which is an infection of muscle tissue. When humans ingest undercooked meat containing the larva, it releases from the tissue capsule in the intestine and matures into adults. The larvae produced by adult females penetrate the intestinal wall and enter the circulation and migrate through the body. The larvae can stimulate an inflammatory reaction in different organs. It is encapsulated in striated muscle leading to calcification within 18 months. Diagnosis can be done by identification of encapsulated coiled larva in a muscle tissue biopsy.

Question 35

Humans acquire strongyloidiasis by

1 Penetration of skin by filariform larva
2 Penetration of skin by microfilaria
3 Ingestion of fish containing metacercaria
4 Ingestion of beef containing eggs

Answer 35

Penetration of skin by filariform larva

Strongyloides stercoralis is commonly known as thread worm and can have three different forms of life cycles.
Direct life cycle
Indirect life cycle
Auto infection
Direct life cycle
The female filarial form worms live in the small intestine and lays eggs. The fertile eggs hatch in the intestine and develop into the rhabditiform larva that is passed in the feces. Rhabditiform larva develops into filariform larva which is infective for human. This larva penetrates the skin directly and enters the circulation. It breaks out from capillaries in the lung and breaks through alveoli. It migrates up the bronchial tree and over the epiglottis and enters the intestine and matures into adult form.
Indirect life cycle
This cycle is common in the tropics. The free-living adult form develops from the rhabditiform larvae in the feces and produces eggs. Infective filariform larva form when the free living cycle reverts.
Autoinfection
Autoinfection is also known as hyperinfection. It is common in immunocompromised patients. Filariform larva can develop in the intestine in some patients. It penetrates the mucosa and enters the circulation resulting in disseminated strongyloidiasis. These larvae can invade various tissues causing considerable damage. Some of the larvae can migrate to lungs or can returns to the intestine to develop into adult form.

Question 36

Which of the following parasites is associated with heavy autoinfection and death in immunocompromised patients?

1 Opisthorchis sinensis
2 Strongyloides stercoralis
3 Heterophyes heterophyes
4 Diphyllobothrium latum

Answer 36

Strongyloides stercoralis

Adult females of Strongyloides stercoralis are parthenogenetic (i.e. capable of unisexual reproduction without fertilization); therefore, it is possible to have multiplication and autoinfection in the same host. In immunocompromised patients heavy infection and disseminated strongyloidiasis can cause serious damage to the body and can lead to deterioration and death of the patient.
Humans acquire Heterophyes heterophyes through ingestion of uncooked fish containing encysted metacercariae. The adult worm of Heterophyes heterophyes lives in the small intestine.
Ingestion of freshwater fish containing plerocercoid larva can cause Diphyllobothrium latum in humans. Heavy infection with this broad and largest fish tapeworm can cause intestinal obstruction and macrocytic anemia due to vitamin B12 deficiency.
Opisthorchis sinensis infection occurs when fish with encysted metacercariae is ingested. This parasite lives in bile ducts and can cause functional impairment of the liver in cases of heavy infection.

Question 37

A filarial infection in humans is transmitted by the bite of black flies of genus Simulium. This filariasis is endemic in Equatorial Africa and in Central and South America. The infection is traditionally diagnosed by microscopic detection of microfilariae of the parasite in clinical samples. What is the most appropriate specimen to be collected for microscopic examination in order to assess the prevalence of this filarial infection in an endemic community?
Answer Choices

1 Night blood smear
2 Skin snip
3 Lymph node biopsy
4 Urine
5 Tears

Answer 37

Skin snip

The filarial infection transmitted by black flies of Simulium genus is Onchocerciasis (River blindness), caused by Onchocerca volvulus. The infective larvae that are deposited in the skin by the vector bite develop at the site to male and female adult worms. Adult worms are seen singly, in pairs, or in tangled masses in subcutaneous tissues. As a result of fibroblastic reaction around the worms, circumscribed painless subcutaneous nodules develop (Onchocercomata). The microfilariae released by the gravid female worms migrate out of the nodule and concentrate in the dermis. So during surveys to assess the prevalence of the infection, skin snip is the appropriate specimen for microscopic examination for detection of microfilariae.
Skin snips are placed in buffered saline for 2 to 4 hours. Microfilariae emerging from the skin snip can be visualized by low power microscopy. Smears can be stained with Giemsa to confirm the species. Microfilaria of Onchocerca volvulus is unsheathed.
Other samples listed are not used for microscopic screening for microfilariae of O.volvulus. Microfilariae of O.volvulus are very seldom seen in blood and urine. Urine, serum, and tears have been used for detection of O.volvulus specific antigens. Microfilariae have been demonstrated in lymph node biopsies, but the procedure is not generally done as part of prevalence surveys.
Onchocerciasis primarily affects the skin and eyes. Most of the symptoms are due to migration of the larvae. Skin lesions are characterized by dermatitis with pruritus, vitiligo-like hypopigmentation, and atrophy. Microfilariae may enter the eye by direct invasion from the conjunctiva and lead to ocular manifestations, such as gradual blurring of vision progressing to total blindness. Onchocerciasis is a frequent cause of blindness in regions where the infection is endemic. Ocular lesions include punctate or sclerosing keratitis, iridocyclitis, chorioretinitis, and optic atrophy. Slit lamp examination may show free-floating microfilaria in the anterior chamber of the eye.
Highly sensitive and specific PCR assays for detection of onchocercal DNA in skin snips, and assays to detect specific antibodies are available in specialized laboratories.

Question 38

A nosocomial outbreak of an invasive pulmonary infection caused by a ubiquitous saprophytic mycelial fungus occurred in the hematology/oncology unit of a cancer center, following renovation work. This fungal species is recognized as one of the most common causes of mycotic nosocomial invasive infections, especially in patients with hematological malignancies. Laboratory tests that established the fungal infection included microscopy, culture of respiratory tract specimens, lung biopsies from patients, and detection of galactomannan antigenemia. What best describes the microscopic appearance of this fungus in tissue suggestive of invasive infection?

1 Hyaline septate hyphae showing acute angle branching
2 Thick-walled spherules containing endospores
3 Fruiting heads with conidiophores and chains of conidia
4 Budding yeast cells with pseudomycelia
5 Broad non-septate hyphae with irregular branching
6 Irregular dark sclerotic cells with septae within giant cells

Answer 38

Hyaline septate hyphae showing acute angle branching

The most common saprophytic mycelial fungus that is associated with invasive nosocomial pulmonary infections is Aspergillus species. Construction work-related outbreaks have been reported and invasive pulmonary disease is the most common manifestation. High-risk categories include patients with hematological malignancies with prolonged granulocytopenia and solid organ transplant recipients.
In addition to microscopy and culture of clinical specimens, detection of galactomannan antigenaemia is reported to be of help in early diagnosis of invasive pulmonary aspergillosis. It can be inferred that the mycelial fungus responsible for the nosocomial outbreak is Aspergillus species.
Microscopic appearance of fungal elements in tissue suggestive of Invasive Aspergillosis is the presence of hyaline septate hyphae of uniform width (about 4 microns) showing acutely angled branching. The fungal hyphae can be demonstrated by direct microscopic examination of respiratory specimens with KOH or calcofluor white and in lung biopsy specimens stained with histochemical stains like Gomori’s silver methanamine (GMS).
Of the other choices, fruiting heads with conidiophores and conidia, a feature of Aspergillus spp, is not commonly seen in clinical specimens from invasive pulmonary infections. These may be detected in colonized sites, as in wounds or lung cavities, and presence of fruiting heads alone is not suggestive of invasive infection.
Thick-walled spherules are characteristic of Coccidioidomycosis. Yeast cells with pseudomycelia are seen in Candidiasis, and presence of broad non-septate hyphae is characteristic of mucormycosis (zygomycosis). Dark brown sclerotic cells within giant cells are seen in Chromoblasomycosis, a subcutaneous mycosis.
Dust or aerosols containing aspergillus conidia are the most common sources of pulmonary aspergillosis. Following inhalation, conidia germinate, and the hyphae actively invade the lung tissue. Invasive disease develops as an acute pneumonic process with or without dissemination. The fungal hyphae may invade the pulmonary vasculature, causing thrombosis, infarction, and necrosis.
Several species of Aspergillus have been associated with opportunistic invasive infections, the most common species being A.fumigatus, A.flavus, and A.terreus, and have also been reported to be common causes of invasive infections. Species like A.niger, which have large conidia, are uncommon agents, as the conidia may not reach deep in the lung to produce invasive pulmonary infections.
Galactomannan (GM), a cell wall antigen, is released from Aspergillus hyphae while they invade the host tissue. A sandwich enzyme immunoassay (EIA) that detects Aspergillus galactomannan antigenemia is considered useful for diagnosing invasive aspergillosis (IA). GM has been detected in bronchioalveolar lavage of patients with invasive aspergillosis, but the utility of the test in diagnosis has not been established.
Molecular methods have been developed for diagnosis of IA. Due to the ubiquitous nature of aspergillus conidia, false positives can occur, so the tests are not standardized. Utility of a combination of Aspergillus GM -ELISA and real time PCR has been suggested for improved diagnosis of IA (Ref: 8).
Other opportunistic molds with septate mycelia like Fusarium and Scedosporium also occasionally cause invasive nosocomial infections. Cultural isolation of the fungus from clinical samples helps to confirm diagnosis.

Question 39

Xenodiagnosis is conventionally used as a laboratory technique for diagnosis of which of the parasitic infections listed below?

1 Kala azar
2 Chagas’ disease
3 Filariasis
4 African sleeping sickness
5 Babesiosis

Answer 39

Chagas’ disease

All choices listed are parasitic diseases transmitted by insect vectors. Kala-azar is transmited by sandflies, Chagas’ disease by reduviid bugs, filariasis by mosquitoes, African sleeping sickness by tsetse flies, and Babesiosis by ticks. Conventionally, xenodiagnosis is done only for diagnosis of Chagas’ disease.
Chagas’ disease (American trypanosomiasis) is endemic in Latin America and seen mainly among the poor. It is a zoonosis involving domestic and sylvatic mammalian reservoirs and is caused by the hemoflagellate Trypanosoma cruzi. Insect vector for human infection is Triatomine species of reduviid bug. The triatomine vectors are found in the Americas, mostly in the rural areas of Latin America (Ref: 2, 3). Infection occurs when breaks in the skin, mucus membrane, or conjunctivae become contaminated with bug feces containing infective parasites. At the site of entry, there may develop subcutaneous inflammatory nodule called Chagoma. Acute and chronic forms of Chagas’ disease are seen.
Infection can also occur by eating food contaminated with infected bug’s feces, transplacentally from infected mother, by blood transfusion, organ transplantation, and by laboratory exposure.
In Chagas’ disease, diagnosis may be established by xenodiagnosis by letting laboratory-reared (parasite-free) reduviid bug feed on the patient with suspected infection. The bug feces are microscopically examined periodically for flagellate forms of the parasite. It may require 4-5 weeks or more for completion of the test. Xenodiagnosis is highly specific, and a positive result is diagnostic of the infection.
The test is done during the early phase of the disease onset, especially if other tests are negative. Xenodiagnosis has the disadvantages of being labor intensive and time-consuming. It shows low sensitivity when used during the chronic phase of Chagas’ disease due to low parasitemia.
Xenodiagnosis has been used also for follow up studies on patients after trypanocidal drug therapy to evaluate effectiveness of the drug on parasite clearance. It is reported that the sensitivity of this test can be increased by incorporating molecular methods to detect the parasite DNA in the bugs’ intestinal contents in place of microscopic demonstration of the flagellates.
The technique has been used for detection of animal reservoirs of T.cruzi as well.
Other laboratory diagnostic methods for Chagas disease include microscopy, culture, and serology.
Polymerase chain reaction (PCR) assays developed for detection of parasite DNA in blood are not available for routine use.

Question 40

In malaria, the form of plasmodia that is transmitted from mosquito to human is

1 Hypnozoite
2 Sporozoite
3 Merozoite
4 Gametozoite
5 Trophozoite

Answer 40

Sporozoite

The life cycle of the malarial parasite in humans begins with the introduction of sporozoite into the blood from the saliva of the biting mosquito.

Question 41

Individuals with which one of the following are protected against malaria?

1 Sickle cell trait
2 Down’s syndrome
3 Philadelphia chromosome
4 Pernicious Anemia
5 Leukopenia

Answer 41

Sickle cell trait

Sickle cell gene is caused by a single amino acid mutation (valine instead of glutamate at the 6th position) in the beta chain of hemoglobin gene. The exact cause for the immunity against malaria is not known. It could be due to the varied morphology of sickle cells. Another theory suggested is that individuals with sickle cell trait are protected against malaria because their red cells have too little ATPase activity and cannot produce sufficient energy to support the growth of the parasite.

Question 42

Which of the following causes the most severe form of malaria?

1 Plasmodium falciparum
2 Plasmodium vivax
3 Plasmodium ovale
4 Plasmodium malariae

Answer 42

Plasmodium falciparum

Malaria caused by P.falciparum is more severe than that caused by other plasmodia. It is characterized by infection of far more red cells than the other species and by occlusion of the capillaries with aggregates of parasitized red cells.

Question 43

Which one of the following protozoa infects primarily the macrophages?

1 Trichomonas vaginalis
2 Plasmodium ovale
3 Leishmania donovani
4 Trypanosoma cruzi
5 Trypanosoma gambiense

Answer 43

Leishmania donovani

Leishmania donovani is the cause of kala-azar. The life cycle involves the female sandfly, which takes blood meals. When the sand fly sucks blood from an infected host, it ingests macrophages containing amastigotes. After dissolution in the macrophages, the freed amastigotes differentiate into promastigotes in the gut. They multiply and travel to the pharynx. The promastigotes are engulfed by human macrophages when an infected sandfly bites a human. They are then transformed into amastigotes.

Question 44

Which of the following agents is used to treat malaria?

1 Mebendazole
2 Chloroquine
3 Inactivated vaccine
4 Praziquantel
5 Fansidar

Answer 44

Chloroquine

Chloroquine is the drug of choice for acute malaria. It kills the merozoites, thereby reducing the parasitemia.

Question 45

Chagas disease is caused by

1 Trypanosoma gambiense
2 Trypanosoma rhodesiense
3 Trypanosoma cruzi
4 Leishmania donovani
5 Toxoplasma gondii

Answer 45

Trypanosoma cruzi

Chagas disease occurs primarily in rural Central, and South America and is caused by Trypanosoma cruzi . The life cycle involves the reduviid bug as the vector and both humans and animals as reservoir hosts.
Leishmania donovani is the cause of kala-azar. The life cycle involves the female sandfly which takes blood meals. When the sand fly sucks blood from an infected host, it ingests macrophages containing amastigotes. After dissolution in the macrophages, the freed amastigotes differentiate into promastigotes in the gut. They multiply and travel to the pharynx. The promastigotes are engulfed by human macrophages when an infected sandfly bites a human. They are then transformed into amastigotes.

Question 46

The most frequently and severely affected tissue in Chagas disease is

1 Lung tissue
2 Liver tissue
3 Cardiac muscle
4 Red blood cells
5 Skin

Answer 46

Cardiac muscle

The trypanosoma amastigotes can kill and cause inflammation consisting mainly of mononuclear cells. Cardiac muscle is the most frequently and severely affected tissue. Neural damage also leads to cardiac arrhythmias and loss of tone in the colon and esophagus.

Question 47

The drug of choice for Chagas disease is

1 Stibogluconate
2 Suramin
3 Sulfamethoxazole
4 Trimethoprim
5 Nifurtimox

Answer 47

Nifurtimox

The drug of choice for acute phase of Chagas disease is Nifurtimox , which kills trypomastigotes in the blood but is much less effective against amastigotes in tissue.

Question 48

The property that allows the trypanosomes to evade the host response is

1 Short life cycle
2 They attack T-cells
3 Antigenic variation of their surface glycoprotein
4 Lipid coat on the surface of the organism
5 Their resistance to various drugs

Answer 48

Antigenic variation of their surface glycoprotein

The trypanosomes, (Trypanosoma gambiense and Trypanosoma rhodosiense ) exhibit remarkable antigenic variation of their surface glycoproteins with hundreds of antigenic types found. The variation is due to sequential movements of the glycoprotein genes to a preferential location on the chromosome, where only that specific gene is transcribed into mRNA. These antigenic variations allow the organism to continually evade the host’s immune response.

Question 49

Which one of the following is used for the diagnosis of acute and congenital toxoplasma infection?

1 Microscopic examination of blood
2 Immunofluorescence assay for IgM antibody
3 Microscopic examination of fecal smears
4 Microscopic examination of intestinal epithelium
5 Chest X-rays

Answer 49

Immunofluorescence assay for IgM antibody

For diagnosis of acute and congenital infections, an immunofluorescence assay for IgM antibody is used. IgM is used to diagnose congenital infection, because IgG can be maternal in origin. Tests of IgM antibody can be used to diagnose acute infections if a significant rise in antibody titer in paired sera is observed.

Question 50

A 34-year-old male immigrant from Mexico presents with a 4 day history of headache, fever, chills, nausea, vomiting, loose stools, gas, and mucus production. He also has had right upper quadrant pain and tenderness for the past 7 days. He told you that several weeks ago, in his native village, he experienced bouts of intermittent diarrhea and gas, which he relieved with a 2-day course of home-made tea. None of his family or neighbors had similar symptoms. He has no pets. On examination, he is febrile (temperature of 40°C), his liver cannot be examined because of the extreme tenderness, and he has no jaundice. These signs and symptoms are highly suggestive for the presence of a liver abscess. What test should be performed to confirm the diagnosis of liver abscess due to Entamoeba histolytica?

1 Spinal fluid examination
2 Stool examination and indirect hemagglutination test
3 CBC and liver function tests
4 Three way abdominal x-ray and string test
5 Stool examination and colonoscopy

Answer 50

Stool examination and indirect hemagglutination test

Entamoeba histolytica is the only ameba responsible for liver abscesses due to its ability to cause tissue invasion. The parasite exists either as a trophozoite or a cyst, in its infective form. An amebic liver abscess is the most frequent extraintestinal manifestation of Entamoeba histolytica and typically presents in a patient from developing country with a fever, right upper quadrant pain, and tenderness of less than 10 days duration. Diagnosis of E. histolytica can be established by the examination of stool samples for ova and parasites that may reveal the presence of trophozoites, cysts or both. Serologic testing by indirect hemagglutination or ELIZA may detect elevated ameba serologies consistent with invasive disease.
Spinal fluid examination is not indicated for a patient in whom you suspect a liver abscess and who has no neurological deficit.
A three way abdominal x-ray is indicated in the case of swelling, pain, or a mass in the abdomen, in patients complaining of severe constipation, diarrhea, or vomiting. It will not prove the presence of Entamoeba histolytica in the liver. A string test (swallowing a string to obtain a sample from the upper part of the small intestine) is rarely used in the United States. It will eventually prove the presence of intestinal parasites, but not the presence of Entamoeba histolytica in the liver.
CBC and liver function tests may show mild anemia, elevated erythrocyte sedimentation rate, elevated bilirubin, leukocytosis without eosinophilia, elevated alkaline phosphatase, mildly elevated transaminases – all those signs of liver disease are not specific for the presence of Entamoeba histolytica in the liver.
Stool examination and colonoscopy may show the presence of parasite in stool and colon, but, again, those tests are not specific for the presence of Entamoeba histolytica in the liver.

Question 51

Which one of the following can be diagnosed by observing trophozoites in a wet mount?

1 Cryptosporidiosis
2 Trichomoniasis
3 Trypanosomiasis
4 Toxoplasmosis
5 Giardiasis

Answer 51

Trichomoniasis

Trichomonas vaginalis causes trichomoniasis. It exists only as a trophozoite and there is no cyst form. In wet mount of vaginal or prostatic secretions, the pear-shaped trophozoites have a typical jerky motion.

Question 52

Which one of the following can be diagnosed by thick and thin Giemsa stained blood smears?

1 Malaria
2 Giardiasis
3 Toxoplasmosis
4 Pneumonia
5 Kala-azar

Answer 52

Malaria

Diagnosis of malaria depends upon microscopic examination of blood, using both thick and thin Giemsa stained smears. The thick stain is used to screen for presence of organisms, and the thin smear is used for species identification.

Question 53

Which one of the following can be diagnosed by observation of cysts on acid-fast stain?

1 Giardiasis
2 Cryptosporidiosis
3 Malaria
4 Amebiasis
5 Kala-azar

Answer 53

Cryptosporidiosis

Diagnosis of cryptosporidiosis is made by finding oocysts in fecal smears when using a modified Kinyoun acid-fast stain.

Question 54

Only ONE of the following is transmitted by ingestion of cope pods in water. Which one?

1 Loiasis
2 Guinea worm
3 Onchocerciasis
4 Filariasis
5 Trichinosis

Answer 54

Guinea worm

Dracunculus medinensis (guinea fire worm) causes dracunculiasis. Humans are infected when tiny crustaceans (copepods) containing infective larvae are swallowed in drinking water.

Question 55

Only ONE of the following infections can be diagnosed by skin biopsy. Which one?

1 Loiasis
2 Onchocerciasis
3 Filariasis
4 Ascariasis
5 Hookworm

Answer 55

Onchocerciasis

Onchocerciasis is caused by Onchocerca volvulus . Humans are infected when the female blackfly simulium deposits infective larvae while biting. Skin biopsy of the affected skin reveals microfilariae.

Question 56

Only ONE of the following is transmitted though ingestion of raw crab. Which one?

1 Schistosoma mansoni
2 Schistosoma japonicum
3 Schistosoma haematcbium
4 Clonorchis sinensis
5 Paragonimus westermani

Answer 56

Paragonimus westermani

Paragonimiasis is caused by Paragonimus westermani . Humans are infected by eating raw or undercooked crab meat containing the encysted larvae.

Question 57

Only ONE of the following diseases can be treated with stibogluconate. Which one?

1 Giardiasis
2 Amebiasis
3 Malaria
4 Leishmaniasis
5 Toxoplasmosis

Answer 57

Leishmaniasis

The treatment of Leishmaniasis is sodium stibogluconate which is a pentavalent antimony compound.

Question 58

Only ONE of the following is transmitted by the Deer fly bite. Which one?

1 Filariasis
2 Loiasis
3 Onchocerciasis
4 Guinea worm
5 Trichinosis

Answer 58

Loiasis

Loa loa causes loiasis. Humans are infected by the bite of the deer fly (mango fly), chrysops, which deposits infective larvae on the skin. The larvae enter the bite wound, wander in the body and develop into adults.

Question 59

Only ONE of the following is transmitted by the Blackfly bite. Which one?

1 Filariasis
2 Trichinosis
3 Ascariasis
4 Hookworm
5 Onchocerciasis

Answer 59

Onchocerciasis

Onchocerca volvulus causes onchocerciasis. Humans are infected when the female blackfly, Simulium, deposits infective larvae while biting.

Question 60

Which one of the following is the only common protozoan found in the duodenum and jejunum of humans?

1 Giardia lamblia
2 Trichomonas vaginalis
3 Retortamonas intestinalis
4 Enteromonas hominis
5 Chilomastix mesnil

Answer 60

Giardia lamblia

Giardia lamblia is the only protozoan that is found in the duodenum and jejunum of humans and not Trichomonas vaginalis, Retortamonas intestinalis, Enteromonas hominis, or Chilomastix mesnili.

Question 61

The trophozoite of which one of the following is a heart shaped symmetric organism that is 10-15 μm in length organism?

1 Giardia lamblia
2 Trichomonas vaginalis
3 Chilomastix mesnili
4 Trypanosoma brucei
5 Leishmania donovani

Answer 61

Giardia lamblia

The trophozoite of Giardia lamblia is a heart shaped symmetric organism 10-15 μm in length. Trichomonas is pear shaped with a short undulating membrane lined with four anterior flagella. Chilomastix mesnili is also pear shaped, but the spiral motion of trophozoite is unlike that of Trichomonas. Trypanosoma brucei appears as elongated bodies supporting a lateral undulating membrane and a flagella that borders the free edge of the membrane and emerges at the anterior end as a whip-like extension.

Question 62

The promastigote stage in Leishmania is restricted to

1 Anopheles
2 Sandfly
3 Mouse
4 Louse
5 Tick

Answer 62

Sandfly

The promastigote stage in Leishmania is restricted to the sandfly and not the mouse, louse, tick, and Anopheles.

Question 63

Kala-azar is caused by

1 Leishmania donovani
2 Leishmania braziliensis
3 Tinea rubrum
4 Microsporum canis
5 Epidermophyton floccosum

Answer 63

Leishmania donovani

Kala-azar is caused by Leishmania donovani. Leishmania braziliensis causes mucocutaneous or nasopharyngeal leishmaniasis. Tinea rubrum causes athlete’s foot. Microsporum canis causes ringworm and Epidermophyton floccosum causes jock itch.

Question 64

The number of cysts formed in Dientamoeba is

1 0
2 2
3 4
4 6
5 8

Answer 64

0

Dientamoeba forms no cysts and not 2, 4,6, or 8. Absence of cysts in this protozoan is used to distinguish it from others.

Question 65

Which one of the following statements best describes the appearance of red blood cells parasitized by Plasmodium falciparum when stained with the Romanowsky stain?

1 Not enlarged with coarse stippling, invades cells regardless of their age
2 Enlarged pale fine stippling, primarily invades reticulocytes
3 Not enlarged, no stippling, invades older cells
4 Enlarged pale Schuffner’s dots, conspicuous cells often oval, fimbriated or crenated
5 Appearance same as that of normal cells

Answer 65

Not enlarged with coarse stippling, invades cells regardless of their age

When red blood cells infected by Plasmodium falciparum are stained with the Romanowsky’s stain, they appear not enlarged with coarse stippling and invade cells regardless of their age. Cells infected by Plasmodium vivax appear enlarged and pale with fine stippling and primarily invade reticulocytes. Cells infected by Plasmodium malariae appear not enlarged, with no stippling, and invade older cells. Those infected with Plasmodium ovale appear with enlarged pale Schuffner’s dots, conspicuous cells that are often oval, fimbriated, or crenated; they do not appear normal.

Question 66

Most of the population of West Africa is resistant to Plasmodium vivax infection due to

1 Absence of an antigen that acts as a receptor for Plasmodium vivax infection
2 Prevalence of sickle cell disease
3 Previous infection by Plasmodium vivax
4 Presence of a specific antibody against Plasmodium vivax
5 Presence of a specific antigen for Plasmodium vivax

Answer 66

Absence of an antigen that acts as a receptor for Plasmodium vivax infection

Most of the population of West Africa is resistant to Plasmodium vivax infection due to absence of an antigen that acts as a receptor for Plasmodium vivax infection and not due to prevalence of sickle cell disease, previous infection, or due to the presence of a specific antigen or antibody.

Question 67

Which of the following is true concerning amebas and amebiasis?

1 Amebas are distinguishable from rhizopods based on size and morphology
2 Amebas are smaller than true protozoa
3 Of all amebas, only Entamoeba histolytica is a routine cause of human disease
4 Cattle are the principal hosts and reservoir of Entamoeba histolytica
5 All amebas possess mitochondria

Answer 67

Of all amebas, only Entamoeba histolytica is a routine cause of human disease

Amebas are not distinguishable from rhizopods; amebas are synonymous with rhizopods! Amebas are also among the most primitive of all protozoa. Of the different amebas, it is true that only Entamoeba histolytica is a routine cause of disease in humans. The principal hosts and reservoirs of Entamoeba histolytica are actually humans. Several genera of amebas do not possess mitochondria, and exist in the anaerobic colonic environment.

Question 68

Which of the following is true concerning trichomoniasis?

1 Trichomonas is a flagellate protozoa
2 Trichomonas is the only flagellate protozoan known to cause human disease
3 Trichomonas is also referred to as a hemoflagellate
4 Trichomonas vaginalis only exists in the cyst form
5 Nonvenereal transmission of Trichomonas vaginalis is common

Answer 68

Trichomonas is a flagellate protozoa

Trichomonas is one of several different flagellate protozoa known to cause disease in humans. Trichomonas is not a hemoflagellate, unlike Leishmania and Trypanosoma, which are. Trichomonas vaginalis lacks the cyst form, existing only as a trophozoite. Nonvenereal transmission of Trichomonas vaginalis is very uncommon.

Question 69

Which of the following is true concerning malaria?

1 The incidence of malaria is highest in areas at greater than 1800 m in altitude.
2 Plasmodium ovale is the most common cause of malaria.
3 Approximately one million cases of pediatric malaria end in death each year.
4 Worldwide, it is believed that there are between 25 and 50 million individuals infected with malaria.
5 Despite popular perception, fever rarely occurs with malarial infection.

Answer 69

Approximately one million cases of pediatric malaria end in death each year.

Malaria generally occurs in areas at less than 1800 m in altitude. Plasmodium ovale is primarily found in Africa and is rare compared to the other Plasmodium species causing malaria. It is true that nearly one million children die each year as a consequence of malaria infection. Worldwide, there are between 200 and 300 million malaria-infected individuals. Fever is a hallmark of malaria.

Question 70

Which of the following is true concerning trypanosomiasis?

1 There are two distinct forms of trypanosomiasis, African and American
2 American trypanosomiasis is also known as Piaget’s Disease
3 African trypanosomiasis is caused by Trypanosoma cruzi
4 American trypanosomiasis is caused by Trypanosoma brucei
5 Nagana is the most virulent form of American trypanosomiasis

Answer 70

There are two distinct forms of trypanosomiasis, African and American

The two distinct forms of trypanosomiasis are known as African and American. American trypanosomiasis is also known as Chagas’ Disease. African trypanosomiasis is caused by Trypanosoma brucei, while American trypanosomiasis is caused by Trypanosoma cruzi. Nagana is actually a disease of cattle that has had particularly ruinous effects in the region of Central African. It is caused by a trypanosome related to Trypanosoma brucei.

Question 71

Which of the following is true concerning leishmaniasis?

1 Worldwide, it is estimated that there are approximately five million individuals with leishmaniasis
2 Leishmaniasis is transmitted by phlebotomine sandflies
3 Kala azar is caused by Leishmania mexicana
4 The only known cure for diffuse cutaneous leishmaniasis is amphotericin B
5 The incidence of kala azar is highest on the Australian continent

Answer 71

Leishmaniasis is transmitted by phlebotomine sandflies

Worldwide, it is actually estimated that there are 20 million individuals with leishmaniasis. Each year, on the order of 400,000 individuals acquire new leishmaniasis infections. Leishmaniasis is transmitted by phlebotomine sandflies. Kala azar (literally, “black disease”) is caused by Leishmania donovani, and occurs on every continent except the Australian continent. While amphotericin B is one of the drugs that is given for treatment of diffuse cutaneous leishmaniasis, actual cure of the disease occurs quite rarely. Pentamidine may also be given as treatment.

Question 72

From what disease (parasitic infestation) is he most likely suffering?

1 Schistosomiasis
2 Trypanosomiasis
3 Leishmaniasis
4 Kala-azar
5 Fascioliasis

Answer 72

Schistosomiasis

The image included here is a scanning electron microscopy image (SEM) of a schistosome parasite magnified 256X.
Blood flukes cause schistosomiasis (bilharziasis). The intermediate host for blood flukes is a fresh water snail. Cercariae (the larva) are able to get through human skin. The freshwater swimming this man did on his vacation exposed him to the infection.
Trypanosomiasis is caused by a protozoa. Chagas disease, or American trypanosomiasis usually causes a mild febrile illness but may cause cardiac or gastrointestinal symptoms in patients who are chronically infected. Sleeping sickness or African trypanosomiasis is caused by T. brucei is transmitted by the tsetse fly. It is characterized by fever, daytime somnolence, anemia, thrombocytopenia, and other central nervous system effects.
Leishmaniasis is caused by protozoa of the species Leishmania. Kala-azar is caused by Leishmania donovani. Leishmania species produce a wide variety of clinical syndromes from cutaneous ulcers to fatal visceral disease.
Fascioliasis is caused by the liver fluke, Fasciola hepatica. In its acute phase, it causes fever, malaise, abdominal pain, gastrointestinal symptoms, urticaria,
anemia, jaundice, and respiratory symptoms.

Question 73

The disease known as sleeping sickness is caused by Trypanosoma brucei and spread by the bite of the tsetse fly. After infection, the number of parasites in the blood fluctuates periodically; this cycle of parasitemia, remission, and recrudescence is due to destruction of trypanosomes by host antibody, followed by the emergence of parasites expressing different surface antigens or variant surface glycoproteins (VSGs). What is this way to evade the host immune response called?

1 Anergy
2 Antigenic variation
3 Molecular mimicry
4 Gene conversion
5 Latency

Answer 73

Antigenic variation

Antigenic variation is the most striking example of successful adaptation by microbes and is exemplified by the ability of the trypanosome causing sleeping sickness to avoid destruction by the host’s immune system. Each parasite is covered by hundreds of thousands of molecules of VSGs. Antibodies produced after each wave of parasitemia are specific for one VSG only. The parasite possesses a number of genes that code for its VSGs and can shed the old coat of VSGs and put on a new one not recognized by antibodies. The antibodies do not trigger the switch; it occurs spontaneously. Thus, by presenting an immunodominant antigen that it can vary, the parasite diverts its host’s attention away from essential, constant elements on its surface.
Anergy is a state of unstable metabolic arrest affecting lymphocytes that can lead to apoptosis. It occurs when a lymphocyte receives an antigenic signal without the normally necessary co-stimulatory second signal.
If a microbial antigen is very similar to normal host antigens, the immune response to this antigen may be weak or absent, giving a degree of tolerance. The mimicking of host antigens by microbial antigens is referred to as molecular mimicry. There is evidence that antibodies formed against microorganisms sometimes cross-react with host tissues and cause disease.
In the generation of polymorphism, several genetic mechanisms contribute to the generation of new alleles. Some new alleles are the result of point mutations but many arise by gene conversion, in which one sequence is replaced in part by another from a homologous gene.
Some viruses can enter a state known as latency in which the virus is not being replicated. In the latent state, the virus does not cause disease, but because there are no viral peptides to signal its presence, the virus is not eliminated. Latent infections can be reactivated.

Question 74

What is another complication of the disease in this patient?

1 Alopecia
2 Anemia
3 Hypercalcemia
4 Megaesophagus
5 Macrocytosis

Answer 74

Megaesophagus

Megaesophagus – dilation of the esophagus - may be seen in Chagas disease. A symptom of megaesophagus is dysphagia. The esophageal dilation is similar to that seen in patients with achalasia.
Chagas disease is also referred to as American trypanosomiasis. Chagas disease is seen in the Western Hemisphere, primarily in South America, Central America, and Mexico. Chagas’ disease is caused by the protozoa Trypanosoma cruzi. Transmission of Chagas disease is by contact with feces of an infected insect. There is an acute stage and a chronic stage.
The chronic infection can lead to cardiomyopathy. The cardiac changes seen with Chagas disease include enlarged heart and EKG findings as seen in this patient (right bundle branch block and premature ventricular contractions). The chronic infection can also lead to megaureter, megaesophagus, and megacolon.
Alopecia is baldness. Hypercalcemia is an elevated blood level of calcium. Anemia is a decreased oxygen carrying capacity of the blood. Macrocytosis refers to an increase in erythrocyte size. Alopecia, hypercalcemia, anemia, and macrocytosis are not complications of Chagas disease.

Question 75

What is the most likely diagnosis?

1 Plasmodium vivax malaria
2 Plasmodium falciparum malaria
3 Plasmodium ovale malaria
4 Plasmodium malariae malaria
5 Babesiosis

Answer 75

Plasmodium falciparum malaria

Malaria is an acute and sometimes chronic infection of the blood stream caused by a parasite of the genus plasmodium.
The 4 species of plasmodium causing human malaria include:
Plasmodium falciparum
Plasmodium vivax
Plasmodium ovale
Plasmodium malariae
P. falciparum causes most of the deaths and severe disease related to malaria. Anemia and liver dysfunction and more common in children. Crescent-shaped gametocytes on blood smear are characteristic of the P. falciparum species. Other common presenting symptoms of malaria include chills and fever, which are often associated with splenomegaly. In the early stages of the disease, the febrile episodes occur irregularly but eventually become more synchronous, assuming the usual tertian (P. vivax, P. Falciparum, P. ovale) and quartan (P. malariae) periodicity. Other manifestations include diarrhea, abdominal pain, headache, and muscle aches and pains.
P. falciparum malaria can result in high parasitemias, which can lead to severe hemolysis with hemoglobinuria and profound anemia. Erythrocytes infected with growing trophozoites, schizonts of P. falciparum, become sequestrated in small vessels of the body, and they may lead to occlusion of these vessels, causing symptoms related to capillary obstruction and tissue anoxia. Involvement of the brain is known as cerebral malaria, in which the patient becomes disoriented, progressing to delirium, coma, and often death.
Malarial parasites undergo sexual phase (sporogony) in anopheles mosquitoes and an asexual stage (schizogony) in humans that results in the production of schizonts and merozoites. In the blood stream, some merozoites eventually differentiate into gametocytes (gametogony), which when ingested by female anopheline mosquitoes, mature into male microgametes and female macrogametes. Fusion of a microgamete and a macrogamete results in the formation of the motile ookinete, which migrates to the outside of the stomach wall and forms an oocyst. Within the oocyst, numerous spindle-shaped sporozoites are formed. The mature oocyst ruptures into the body cavity, releasing the sporozoites, which then migrate through the tissues to the salivary glands, from which they are injected into the vertebral host as the mosquito feeds. The time required for the development in the mosquito ranges from 8 to 21 days.
The sporozoites injected into the vertebrate host reach the hepatic parenchymal cells within minutes and initiate the proliferative phase known as exoerythrocytic schizogony. Release of merozoites from ruptured hepatic schizonts initiates the blood stream infection or erythrocytic schizogony and eventually the clinical symptoms of malaria.
P. vivax and P. ovale differ from P. falciparum and P. malariae in that true disease relapses of the former species may occur weeks to months following subsidence of previous attacks. This occurs because of renewed exoerythrocytic and eventually erythrocytic schizogony from latent hepatic sporozoites, which are known as hypnozoites. Recrudescences of disease due to P. falciparum or P. malariae arise from an increase in the number of persisting blood stage forms to clinically detectable levels, not from persisting liver stage forms. P. vivax and P. ovale parasites primarily infect young erythrocytes, whereas P. malariae affects older erythrocytes, and P. falciparum infects erythrocytes of all stages.

Question 76

What is the most likely etiologic agent?

1 Entamoeba histolytica
2 Escherichia coli
3 Klebsiella
4 Staphylococcus aureus
5 Hepatitis B

Answer 76

Entamoeba histolytica

Amebic liver abscesses (caused by Entamoeba histolytica) produce material that looks like “anchovy paste”. Amebic liver abscesses are located on the right side of the liver more often than on the left side. They can be solitary or multiple; however, they are more often solitary. Amebic abscesses in the liver are also more common in males. Fever and an elevated white blood count are consistent with an amebic liver abscess.
Escherichia coli are considered an enteric bacterium. It is a gram-negative rod. Escherichia coli can produce many types of infections: urinary tract infections (both cystitis and pyelonephritis) are commonly caused by Escherichia coli. Infection with Escherichia coli can cause pyogenic liver abscesses. However, a pyogenic liver abscess due to Escherichia coli does not produce abscess material that looks like “anchovy paste”.
Klebsiella is considered an enteric bacterium. Klebsiella is a gram-negative rod. Klebsiella can produce many types of infections: cystitis, pneumonia, and meningitis are a few examples of infections that can be caused by Klebsiella. Infection with Klebsiella can cause pyogenic liver abscesses. However, a pyogenic liver abscess due to Klebsiella does not produce abscess material that looks like “anchovy paste”.
Staphylococcus aureus are gram-positive cocci. Staphylococcus aureus can produce many types of infections: osteomyelitis, cellulitis, food poisoning, and wound infections are a few examples of infections that can be caused by Staphylococcus aureus. Staphylococcus aureus can cause pyogenic liver abscesses. However, a pyogenic liver abscess due to Staphylococcus aureus does not produce abscess material that looks like “anchovy paste”.
The hepatitis virus is double stranded DNA virus with a lipoprotein coat. It is also referred to as the Dane particle. Exposure to blood or blood products is the means that Hepatitis B is commonly transmitted. Hepatitis B is the cause of hepatitis; it is not a cause of hepatic abscesses. Hepatocellular carcinoma can occur with long standing infection.

Question 77

What is the most likely cause of his symptoms?

1 Giardia lamblia
2 Pneumocystis jiroveci
3 Babesia microti
4 Toxoplasma gondii
5 Trypanosoma cruzi

Answer 77

Giardia lamblia

Giardia lamblia is a protozoan that can cause gastrointestinal discomfort, malabsorption, and diarrhea. It is transmitted by the fecal-oral route and by drinking infected water.
Pneumocystis jiroveci (Pneumocystis carinii) is a fungus that can cause pneumonia in an immunosuppressed host. It is transmitted by inhalation.
Babesia microti is a protozoan that can cause fever. In a vulnerable host, it occasionally causes a more severe illness. It is transmitted by ticks.
Toxoplasma gondii is a protozoan that can be congenital or acquired. It can produce both mild and severe symptoms. When the symptoms are severe, it can cause encephalitis, fever, hepatitis, pneumonitis, myocarditis, retinochoroiditis, or a brain abscess. It is transmitted by cat feces or by eating raw meat. On CT scan, there can be contrast-enhancing lesions.
Trypanosoma cruzi is a protozoan that causes Chagas disease. It is transmitted by triatomine insects.

Question 78

What is the causative agent?

1 Trypanosoma brucei gambiense
2 Leishmania donovani
3 Trypanosoma cruzi
4 Leishmania tropica
5 Trypanosoma brucei rhodesiense

Answer 78

Trypanosoma cruzi

Chagas disease is caused by the protozoan Trypanosoma cruzi. The image shows a micrograph of Trypanosoma cruzi in a blood smear (using Giemsa staining technique).
Chagas disease is also referred to as American trypanosomiasis; it is seen in the Western Hemisphere, primarily in South America, Central America, and Mexico. Transmission of Chagas disease is by contact with the feces of an infected insect. There is an acute stage and a chronic stage. The chronic infection can lead to cardiomyopathy. The cardiac changes seen with Chagas disease include an enlarged heart and EKG findings as seen in this patient (i.e., right bundle branch block and premature ventricular contractions). The chronic infection can also lead to megaureter, megaesophagus, and megacolon.
West African sleeping sickness is also called West African trypanosomiasis; it is caused by Trypanosoma brucei gambiense.
Leishmaniasis is a disease caused by the protozoan parasite of the genus Leishmania, which includes Leishamnia donovani and Leishmania tropica. Leishmania donovani produces visceral leishmaniasis, which is marked by fever, lymphadenopathy, and splenic enlargement and hepatomegaly. Leishmaniasis tropica produces cutaneous leishmaniasis, which produces a painless ulcer.
Rhodesian trypanosomiasis is caused by Trypanosoma brucei rhodesiense. It is also called East African sleeping sickness.

Question 79

What is most likely responsible for this man’s exposure to blood flukes?

1 Swimming in the freshwater pond
2 Eating contaminated food
3 Drinking drinks made with ice from the local water
4 Being bitten by a bug
5 Swimming in the ocean

Answer 79

Swimming in the freshwater pond

The correct response is swimming in the freshwater pond.
The image that appears here is a scanning electron microscopy image (SEM) of a schistosome parasite magnified 256x. Blood flukes cause schistosomiasis (bilharziasis). The intermediate host for blood flukes is a freshwater snail; cercariae (the larvae) can get through unbroken human skin.
Schistosomiasis is not transmitted by eating contaminated food, drinking drinks made with ice from the local water, being bitten by a bug, or swimming in the ocean.

Question 80

What is the most probable diagnosis?

1 Taenia saginata
2 Ancylostoma duodenale
3 Enterobius vermicularis
4 Giardia intestinalis
5 Tinea pedis

Answer 80

Ancylostoma duodenale

The most likely diagnosis is Ancylostoma duodenale, or hookworm infestation.
Hookworm is a soil-transmitted nematode, and 2 parasites are known to cause human infestations: Necator americanus and Ancylostoma duodenale. The larvae enter the skin from soil contaminated with human feces. When the larvae enter the skin, usually through the bare foot, it may produce a “ground itch.” They embolize through the lung, are brought up to the mouth on coughing, and are swallowed. They then feed on the mucosal capillaries in the jejunum. Symptoms such as abdominal pain, nausea, and vomiting can occur; however, its long-term complications such as iron deficiency anemia and protein deficiency, leading to growth retardation in children, are more serious issues. Treatment is with albendazole.
Beef tapeworm, or Taenia saginata, is ingested through raw or undercooked beef. It rarely causes symptoms. It is identified by the round, radially striated eggs with the internal oncosphere containing 6 refractile hooks.
Pinworm, or Enterobius vermicularis, presents as anal itching. It is diagnosed by the presence of the parasite or its eggs in the perianal region, and stool exam is generally not recommended due to the sparsity of worms/eggs in stool.
Giardiasis is a protozoal infection caused by Giardia intestinalis (lamblia). It is diagnosed by the identification of cysts and tropozoites in the stool.
Tinea pedis causes dermatophytosis in the foot, also called athlete’s foot. Stool examination has no role in this condition.

Question 81

What is an appropriate therapy?

1 Praziquantel
2 Ivermectin
3 Mebendazole
4 Thiabendazole
5 Piperazine

Answer 81

Praziquantel

T. solium is a zoonotic cestode. Pig is the normal intermediate host harboring the larval forms and humans are the definitive hosts harboring the adults. Intestinal infection is acquired by eating undercooked pork containing cysticercus cellulose. Cysticerci develop into adults in the human intestine. If these rupture, an intense inflammatory response may occur, marked by eosinophilia and fever. Meningoencephalitis is one of the worst complications of this infection.
Humans can become accidental intermediate hosts and develop cysticercus cellulose by ingestion of water or food contaminated with T. solium eggs. The eggs release the hexacanth embryo (oncosphere) in the small intestine. The oncospheres penetrate the intestinal wall and are carried in systemic circulation to different parts of the body where they develop into cysticercus cellulose in several weeks. In persons harboring adult worms, cysticerci can occur by autoinfection.
The treatment of choice for T. solium infection is praziquantel. In addition, it is used to treat many other helminthic infections. It appears to act by interfering with calcium permeability in the cell membrane. Albendazole may also be used.
Ivermectin is the drug of choice for strongyloidiasis and onchocerciasis. It acts by interfering with GABA transmission.
Mebendazole and thiabendazole act by inhibiting microtubule synthesis. Mebendazole is used to treat ascariasis, trichuriasis, hookworm, and pinworm infections.
Piperazine is used to treat ascariasis. It is not generally effective against other helminthic infections. The mechanism of action is blockade of acetylcholine receptors.