Exam 5 - (CH 28) Diagnostic Parasitology Textbook Review Questions

Question 1

Trophozoites can be described as
A. a motile and reproductive, growing form of some amebae.
B. an asexual form of some helminths.
C. the asexual, sporelike form of most microfilaria.
D. the nonmotile, vegetative form of liver flukes.

Answer 1

A. a motile and reproductive, growing form of some amebae.

Question 2

Which of the following is a permanent stain commonly used in clinical parasitology?
A. Formalin-ethyl acetate
B. Zinc sulfate
C. Methylene blue
D. Trichrome

Answer 2

D. Trichrome

Question 3

Which specimen stains optimally and is considered the ideal specimen for a malarial smear?
A. Venous blood sample collected in an ethylenediamine tetraacetic acid (EDTA) tube
B. Expectorated sputum
C. Capillary blood from finger stick
D. Arterial blood sample collected in sodium heparin

Answer 3

C. Capillary blood from finger stick

Question 4

These are bipyramidal hexagonal crystal remnants of eosinophils in the stool of some patients with an intestinal parasite infection.
A. Ameboma
B. Charcot-Leyden crystals
C. Morulae
D. Wiel-Felix granules

Answer 4

B. Charcot-Leyden crystals

Question 5

Blastocystis hominis is a protozoan that exists in this form:
A. Ameboid
B. Granular
C. Cyst
D. All of the above

Answer 5

D. All of the above

Question 6

Acanthamoeba spp. are characterized by hematogenous spread to the central nervous system (CNS) from a primary site in the
A. skin.
B. spinocerebellar tract.
C. urinary tract.
D. gastrointestinal tract.

Answer 6

A. skin.

Question 7

As few as ______ cysts can initiate infection with Giardia duodenalis.
A. 10
B. 100
C. 1000
D. 10,000

Answer 7

A. 10

Question 8

Which species is the cause of New World cutaneous leishmaniasis?
A. Leishmaniasis tropica
B. Leishmaniasis mexicana
C. Leishmaniasis braziliensis
D. Leishmaniasis donovani

Answer 8

B. Leishmaniasis mexicana

Question 9

Which of the following describes Winterbottom’s sign?
A. Fever, headache, joint, and muscle pain
B. Chiclero ulcers characterized by lesions on the ear
C. Enlargement of the lymph glands in the posterolateral triangle of the neck
D. Edema in the legs and arms in concert with severe headaches and coordination problems

Answer 9

C. Enlargement of the lymph glands in the posterolateral triangle of the neck

Question 10

Blackwater fever is characterized by
A. rice water stools.
B. hemoglobinuria.
C. buboes.
D. all of the above.

Answer 10

B. hemoglobinuria.

Question 11

Merozoites of Plasmodium malariae may be arranged in this characteristic manner.
A. Schüffner’s stippling
B. Appliqué forms
C. Fimbriated cells
D. Loose daisy petal

Answer 11

D. Loose daisy petal

Question 12

Cryptosporidium spp. infections have been associated with which of the following?
A. Swimming pools
B. Unpasteurized drinks
C. Daycare centers
D. All of the above

Answer 12

D. All of the above

Question 13

Flukes typically have this organism as the first intermediate host.
A. Snails
B. Fish
C. Pig
D. Dogs

Answer 13

A. Snails

Question 14

Which hookworm attaches to the mucosa of the small intestine by means of cutting plates?
A. Ancylostoma duodenale
B. Ascaris lumbricoides
C. Enterobius vermicularis
D. Necator americanus

Answer 14

D. Necator americanus

Question 15

Which of the following is most commonly transmitted to humans by eating undercooked pork?
A. Strongyloides stercoralis
B. Dipylidium caninum
C. Trichinella spiralis
D. Ancylostoma braziliense

Answer 15

C. Trichinella spiralis

Question 16

When is the best time to collect a blood specimen when Loa loa infection is suspected?
A. Early morning
B. Noon
C. Just before bed
D. Middle of the night

Answer 16

B. Noon

Question 17

The causative organism of river blindness is
A. Loa loa.
B. Onchocerca volvulus.
C. Mansonella ozzardi.
D. Dracunculus medinesis.

Answer 17

B. Onchocerca volvulus.

Question 18

Onchocerca volvulus microfilariae are best described as
A. sheathed with nuclei that extend all the way to the tip of the tail.
B. sheathed with nuclei that do not extend to the tip of the tail.
C. not sheathed with nuclei that do not extend to the tip of the tail.
D. not sheathed with nuclei that extend all the way to the tip of the tail.

Answer 18

C. not sheathed with nuclei that do not extend to the tip of the tail.

Question 19

Which of the following causes the fatal disease primary amebic meningoencephalitis and is associated with contaminated water sources in Texas, Florida, Virginia, and California?
A. Naegleria fowleri
B. Entamoeba coli
C. Endolimax nana
D. Acanthamoeba

Answer 19

A. Naegleria fowleri

Question 20

Dientamoeba fragilis is commonly found in patients also infected with this organism.
A. Trichomonas vaginalis
B. Entamoeba histolytica
C. Plasmodium vivax
D. Enterobius vermicularis

Answer 20

D. Enterobius vermicularis

Question 21

A trichrome-stained smear of a patient’s fecal specimen shows the presence of cysts that are oval and approximately 11 µm in size and have four nuclei containing large karyosomes with no peripheral chromatin and a cluttered appearance in the cytoplasm. What is the most likely identification of the organism? Is the organism considered a pathogen?

Answer 21

The most likely identification is G. duodenalis; these parasites are pathogenic for humans.

Question 22

A patient with a history of travel to Africa has fever and chills. The physician suspects malaria and orders a blood smear for examination. Why should you do both a thin film and a thick film? Why would final species identification be made from the thin smear?

Answer 22

Both thick and thin smears are routinely performed on blood specimens to detect parasites. Because the thick smear is made with a larger volume of blood, it is more sensitive than a thin smear. However, species identification is made from the thin smear because the red blood cells are intact and parasites have a more characteristic morphology.

Question 23

Give the major characteristics (including size) that you would use to identify eggs of the following organisms: Taenia spp., Ascaris lumbricoides, Trichuris trichiura, and hookworm.

Answer 23

The eggs of Taenia spp. are round, approximately 35 µm in diameter, have a striated border, and contain a hexacanth oncosphere. The eggs of T. solium and T. saginata cannot be differentiated. A. lumbricoides eggs are fertilized or unfertilized. The fertilized eggs are slightly oval, with a thick mammillated coat, and they measure about 75 µm long by 50 µm wide. The unfertilized eggs are more oval and are as long as 90 µm, with a thick mammillated coat or an extremely minimal mammillated layer. Eggs of T. trichiura are oval with plugs on both poles; they are about 52 µm long and 22 µm wide. Hookworm eggs are oval with broad ends, have a clear space between the thin shell and embryo, and measure 55 to 75 µm long by 35 to 40 µm wide. The eggs of the two human hookworm species, A. duodenale and N. americanus, cannot be differentiated.

Question 24

Describe the diagnostic method you would use to detect Enterobius vermicularis eggs that would not be used with the other types of eggs of intestinal helminths. Explain why.

Answer 24

The gravid females of E. vermicularis (pinworm) migrate out of the anus to deposit eggs; therefore, stool samples are not the specimen of choice. The best method for detection of pinworm eggs is to use the scotch tape or sticky paddle technique.

Question 25

Describe the microscopic characteristics you would use to differentiate the oocysts of Cyclospora cayetanensis and Cryptosporidium parvum. Include size, appearance on routine wet mount or trichrome stain, and appearance with special stains.

Answer 25

C. cayetanensis oocysts are round and average 8 to 10 µm in diameter. On a wet mount, they appear nonrefractile, with a cluster of granules. The organisms are not readily distinguishable on trichrome-stained smears and may appear as round, wrinkled, clear objects. With modified acid-fast stain, the organisms stain variably from dark pink to almost colorless and may have a wrinkled or distorted shape. Oocysts of Cyclospora autofluoresce blue under ultraviolet light, at a wavelength of 365 nm, and green at 450 nm. C. parvum oocysts are round and approximately 4 to 6 µm in diameter. Because of their small size, they are not distinguishable on wet mounts or in most permanent-stained smears. With a modified acid-fast stain, they are dark pink and round. Direct fluorescent antibody techniques that detect the oocyst in the feces are available.

Question 26

You identify a trophozoite on a trichrome-stained smear of a stool sample that is approximately 22 µm in diameter. There is a single nucleus that shows even peripheral chromatin and a small central karyosome. The cytoplasm is relatively clean but ingested RBCs are seen. What is the most likely identification of the organism? Is the organism considered a pathogen? If yes, describe the typical patient symptoms and possible complications.

Answer 26

The most likely identification is E. histolytica trophozoite. If no ingested erythrocytes had been seen, the organism would have been reported as E. histolytica– E. dispar because the organisms are morphologically identical. The presence of ingested erythrocytes indicates that it is E. histolytica. This parasite is considered pathogenic for humans. Patients with E. histolytica infection can suffer from amebic dysentery, which can manifest as fever and intense colicky abdominal pain.

Question 27

For both Cryptosporidium spp. and Strongyloides stercoralis, explain the mechanism of autoinfection in the life cycle and why this phase contributes to increased severity of infection.

Answer 27

C. parvum produces two types of oocysts, thin-walled and thick-walled. The thick-walled oocysts are passed in the feces and are infective. The thin-walled oocysts are involved in autoinfection. These structures rupture in the intestine and release sporozoites that reinfect intestinal cells. With S. stercoralis infection, the eggs rupture in the intestine and release rhabditiform larva that mature to filariform larva in the intestine. The filariform larvae burrow into the intestinal wall, gain access to the circulation, and then complete the life cycle. Autoinfections result in persistent infections with large numbers of parasites.

Question 28

You are examining a blood smear and find an extracellular structure that is approximately 18 µm long. It is tapered at both ends and has an anterior flagellum. An undulating membrane extends the length of the body. What is the genus of this organism? What is the morphologic stage of this organism? With what two diseases do you see this stage in blood?

Answer 28

The organism belongs to the genus Trypanosoma. The morphologic form is the trypomastigote. Members of the genus Trypanosoma cause African sleeping sickness, transmitted by tsetse flies, and Chagas disease, transmitted by kissing or reduviid bugs.

Question 29

Compare primary amebic meningoencephalitis and granulomatous amebic encephalitis. Include the following in your discussion: causative organism, population usually infected, route of infection, clinical symptoms, and method of diagnosis.

Answer 29

Primary amebic meningoencephalitis is caused by Naegleria fowleri. Young, healthy individuals are usually infected and present with a history of swimming in warm, stagnant water. The trophozoite gains direct access to the brain via the nasal olfactory nerve when water is forced into the nose. The infection has a rapid onset (within 2 to 7 days) after exposure, and symptoms resemble those of bacterial meningitis— photophobia, headache, vomiting, and stiff neck. Death usually occurs within 1 week of onset of symptoms. Diagnosis can be made by observing the ameboid trophozoite in a wet mount of the CSF sediment. Trophozoites may be found in biopsy tissue of the brain. Granulomatous amebic encephalitis is a chronic infection caused by Acanthamoeba spp. and is usually found in individuals with underlying conditions such as lymphoma or diabetes. The organism may enter through the skin or be inhaled, and it spreads hematogenously to the brain. It may take years for symptoms to develop. Symptoms include headache, dizziness, mental confusion, and seizures. Often, the infection is detected at autopsy. Trophozoite and cyst forms can be identified on biopsy specimens.

Question 30

For Toxoplasma gondii, Cryptosporidium spp., and S. stercoralis, compare the clinical presentation in the immunocompetent host and in the immunocompromised host.

Answer 30

Toxoplasmosis in the immunocompetent host is usually asymptomatic or presents as a transient, flulike illness. The immunocompromised patient often develops encephalitis or pneumonia. Cryptosporidium infection in the immunocompetent host causes self-limiting diarrhea. In the immunocompromised host, the diarrhea is prolonged and severe (partially because of the autoinfective part of the life cycle). The patient may lose as much as 10 L/ day of fluid and develop electrolyte imbalance. Long-term infection may lead to malabsorption syndrome. Strongyloides infection in the immunocompetent host also may be asymptomatic or present with vague abdominal symptoms that can mimic those of an ulcer. In the immunocompromised host, hyperinfection (dissemination) may result, with filariform larvae migrating to multiple body organs.

Question 31

Points to Remember

Answer 31

■ Protozoan cysts, helminth eggs, and helminth larvae can be identified on a wet mount preparation of a fecal concentrate. The identification of protozoan trophozoites and cysts is confirmed on a permanently stained smear.
■ Rapid tests (immunochromatographic) are routinely used for detection of Giardia duodenalis, Cryptosporidium spp., and Entamoeba histolytica.
■ Multiplex molecular assays for intestinal pathogens include several intestinal organisms— usually G. intestinalis, Cryptosporidium spp., and E. histolytica.
■ Fecal-oral transmission is the route of infection for enteric protozoa. The cyst is the infective stage, and the trophozoite is the stage that causes tissue damage.
■ The major intestinal protozoan pathogens include E. histolytica and G. duodenalis. D. fragilis and B. hominis cause symptomatic infections in some patients.
■ E. histolytica, E. dispar, and E. moshkovkii are morphologically identical, and specific immunoassay techniques must be performed to differentiate the pathogenic E. histolytica from the nonpathogenic E. dispar and E. moshkovkii. If the organism is ingested, RBCs are present in the trophozoite, the organism can be reported as E. histolytica.
■ N. fowleri, Acanthamoeba spp., and Balamuthia mandrillaris are free-living amebae that can infect humans. N. fowleri causes an acute condition called primary amebic meningoencephalitis, which is rapidly fatal. Acanthamoeba spp. cause keratitis, skin infections, or granulomatous amebic encephalitis, whereas Balamuthia is associated with GAE and skin infections.
■ The genera Leishmania and Trypanosoma are blood flagellates of humans transmitted by insects.
■ Human malaria can be caused by four different Plasmodium spp.— P. vivax, P. ovale, P. malariae, and P. falciparum.
■ The life cycle of malaria is complex, with asexual reproduction taking place in human RBCs and sexual reproduction occurring in the gut of the mosquito.
■ Identification of Plasmodium spp. is made by observing characteristics of the infected RBCs and the malarial organism on a Wright- or Giemsa-stained peripheral blood smear.
■ One rapid detection test (P. vivax and P. falciparum) is approved for use in the United States for diagnosing malaria. Microscopic confirmation is still performed.
■ B. microti, an intraerythrocytic parasite, morphologically resembles P. falciparum on blood smears.
■ The intestinal Apicomplexa species include C. parvum, C. belli, and C. cayetanensis; the infective stage for humans is the acid-fast– positive oocyst.
■ T. gondii causes a tissue infection that is usually asymptomatic in immunocompetent hosts. In patients with AIDS, a latent infection can reactivate and cause encephalitis or pneumonia. Congenital transmission can result in serious complications.
■ For most flukes, diagnosis is made by finding the egg in a fecal specimen. In the case of the lung fluke (P. westermani), the egg may be found in sputum; in the case of S. haematobium, the egg is found in urine.
■ Humans are the definitive hosts, and animals or insects serve as intermediate hosts for most tapeworms infecting humans.
■ Eggs of the beef tapeworm, T. saginata, and eggs of the pork tapeworm, T. solium, are identical and must be reported as Taenia sp.
■ The diagnostic stage for a roundworm may be an egg or a larval form, depending on the species.
■ Pinworm, E. vermicularis, infection is common in children and is diagnosed by finding eggs on a cellophane tape preparation or pinworm paddle. Eggs are laid on the perianal area when the female migrates out through the anus at night.
■ Human hookworm infection may be caused by N. americanus and A. duodenale. The eggs are identical and are reported as “hookworm eggs.”
■ Eggs of S. stercoralis are not usually present in the stool; the typical diagnostic stage is the rhabditiform larva.
■ T. spiralis is acquired when humans ingest raw or undercooked pork containing the larval form.
■ Diagnosis of trichinosis is made through biopsy of tissue to identify the coiled larval stage.
■ Diagnosis of filarial worm infection is made by observing the microfilariae in blood or tissue specimens. Larval characteristics include the presence or absence of a sheath and the location and arrangement of nuclei in the tail of the microfilariae.