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Flashcards in Intro to Fungi & Parasites Deck (17):

Define parasite.

Definition: a parasite is an organism which lives upon or within another living organism at whose expense it obtains some advantage.


Explain the importance of parasitic diseases for human health from a global perspective.

Parasitic diseases are very prevalent on a worldwide basis, and they have an important impact on morbidity and mortality, especially in developing countries.Many of the major parasitic diseases are uncommon in the United States.Development of effective vaccines against the major parasitic diseases of humans has been difficult and remains an important goal for world health.Because parasitic diseases are caused by eukaryotic pathogens, the biological bases for selective toxicity of anti-parasite drugs are quite different from those described previously for anti-bacterial and anti-viral drugs.


Explain why the life cycles of parasites are important for the geographic distribution, diagnosis, and control of parasitic diseases.

Many parasites which cause diseases in humans have complex life cycles, some of which involve development in one or more additional host species. The species in which the parasite undergoes sexual replication is called the definitive host, and other species in which asexual replication occurs are called intermediate hosts. The ability of parasites to infect specific tissues (tropism) is often essential for their life cycles.The geographic occurrence of parasitic diseases is often restricted by the availability of host species that are essential for the life cycles of the parasites. An understanding of life cycles is essential for understanding how parasites are transmitted to man and how such transmission can be minimized or prevented. Most protozoan parasites can replicate and increase their numbers in humans. In contrast, many worms undergo development but do not replicate in humans, and the worm burden of such parasites in humans reflects the intensity of their exposure to infection.Patients from the United States may be at risk for parasitic diseases that do not normally occur here if they are exposed as the result of travel to other parts of the world for business, pleasure, or as a consequence of military service. A detailed medical history, including details of travel and activities that could place the patient at risk for exposure to parasitic diseases, is essential for evaluation of patients with diseases caused by parasites.


Describe the taxonomic groups in which the major protozoan and metazoan parasites of humans are found.

For parasitic diseases, diagnosis is often made by direct examination of the parasites in specimens or biopsy materials collected directly from patients. Although immunological tests (detection of specific antigens and antibodies) and molecular diagnostic tests (for specific nucleic acid sequences) are sometimes helpful, morphological criteria are usually much more important for diagnosis of parasitic diseases than they are for bacterial and viral diseases.Helminths: Roundworms/nematodes, Flatworms/Trematodes/Flukes, Tapeworms/CestodesExample: SchistosomiasisProtozoans: Amoebas, Flagellates, Ciliates, Sporozoa, MicrosporidiaExample: Malaria


Describe some of the major current challenges to treatment and prevention of parasitic infections.

Major strategies for controlling schistosomiasis include: public health education (avoiding contact with infected water, building wells and latrines to provide potable water and avoid contamination of environmental water sources), use of molluscicides and environmental modification to control snail intermediate hosts for schistosomes, mass treatment of populations with anti-schistosome drugs (such as praziquantel, etc.), and research on diagnostic tests, improved therapeutics, and vaccines for schistosomiasis. Even in regions where control of schistosomiasis has been effective, the parasite has not been eliminated and transmission of infection continues to occur, albeit at a lower frequency.Prevention by minimizing mosquito contact is used in endemic areas. The efforts at mosquito control and malaria eradication during the 1950’s and 1960’s by using DDT and drugs were eventually frustrated by the concurrent appearance of DDT-resistant mosquitoes, cessation of DDT use due to its environmental effects, and development of drug resistant plasmodia. An array of drugs is used for prophylaxis and treatment of malaria. Extensive efforts are underway to develop effective vaccines against the various forms of malaria.


Describe the characteristics of fungi with respect to morphology, cell composition, and biochemical functioning. Be able to explain why fungi differ from bacteria, plants, and animals.

Fungi are eukaryotic, aerobic, unicellular or filamentous, heterotrophic organisms encased in a rigid cell wall.Fungi may reproduce by sexual and/or asexual means, and the nature reproduction is used in classification.Fungi are unrelated to bacteria, and thus, are largely insensitive to antibacterial antibiotics. To treat fungal infections we instead use antifungal antibiotics.As eukaryotes, fungal cells contain membrane bound organelles including nuclei, mitochondria, Golgi apparatus, endoplasmic reticulum, and lysosomes.As heterotrophic organisms, fungi lack chlorophyll and are not photosynthetic (autotrophic) like plants and algae, but instead obtain necessary organic substrates from their surroundings.Like plants, fungi have rigid cell walls, a feature which separates them from animals. These cell walls contain chitin (a material also found in the exoskeleton of insects) and also cellulose (a material found in plant matter). Fungi also have a cell membrane inside of the cell wall which contains ergosterol.Only a very small number of specialized fungi (Chytridiomycota) are motile, and none of the medically-relevant species are motile.


Describe three means (one Linnean, two functional) for categorizing fungi. Understand that the functional categorization schema are used cheifly to discriminate between: a) superficial and deep/systemic mycoses, or b) superficial fungi acquired from soil, animals or other humans.

Fungi are subclassified based upon their method of sexual reproduction.Linneus: fungi are one (Mycota) of the five kingdoms at the top of the phylogenetic classification scheme, as opposed to bacteria (Monera) or protozoa (Protista).Classification between where acquired: From soil (geophilic), From animals (zoophilic), From humans (anthrophilic) Notice that human-native fungi (anthrophilic) tend to trigger less immune responses than the others-- they can evade the human immune system better.A clinical way of classifying fungi is to look at whether they stay on epithelia or get into the bloodstream. Generally the former are more benign, as you would expect.


Explain the differences between yeast and hyphal forms of fungi (molds), and understand that some organisms may be dimorphic.

Yeast: a unicellular growth form where the fungus reproduces via budding to form blastoconidia, or by dividing in half through fission. Colonies of yeast are usually moist or mucoid in appearance. Medically relevant yeast includes Cryptococcus neoformans and Candida albicans.MOLDS - a filamentous growth form where the fungus reproduces via formation of spores or conidia. These filamentous elements are called hyphae. A mass of hyphae is referred to collectively as mycelium. Hyphae are often branched, and grow by apical extension. Medically relevant molds include common dermatophytes and Aspergillosis.DIMORPHIC - fungi that do not have a fixed morphology but may exist in a yeast or hyphal form are referred to as dimorphic. Typically, an environmental change, such as a change in atmosphere, temperature, or food supply, triggers a transition from one form to another. Thermal dimorphism refers to dimorphism that is dictated by temperature.


What are conidia?

CONIDIA - asexual spores usually borne off of specialized aerial hyphae (upward-projecting hyphae) called conidophores. A conidium may be large and multinucleated (macroconidia) or small and unicellular (microconidia). Some fungal species may produce both macroconidia and microconidia.


What are sporangia?

SPORANGIA - sporangia are similar to macroconidia, except that the asexual spores (endospores) are enclosed in a membranous sac that breaks and the entire structure is borne by a sporangiphore . The shapes and or colors of a sporangium may be useful in speciation.


What are chlamydospores?

CHLAMYDOSPORES - chlamydospores are thick-walled, round spores that are highly resistant to adverse environmental conditions. Chlamydospores are further classified based upon where they form along hyphae. Terminal chlamydospores form at the ends of hyphae, while intercalary chlamydospores form along and within hyphae


What are arthrospores?

ARTHROSPORES - like chlamydospores, arthrospores develop along the hyphae, but in general they are more numerous and elongated, often with a shape likened to a “barrel.” The mycelial phase of Coccidioides immitis classically demonstrates barrel-shaped arthospores. Incidentally, it is inhalation of these arthospores from the environment that yields the disease.


What are spherules?

SPHERULES - large, asexual spores that develop during the yeast phase of some organisms growth. The yeast form of the dimorphic fungus Coccidioides immitis forms spherules in tissue that are filled with endospores


What are blastoconidia?

BLASTOCONIDIA - yeasts that bud asymmetrically are said to form blastoconidia


Describe four means to diagnose fungal infections and have a general understanding that other advanced techiques (serological or PCR) may be used for certain infections or in specific circumstances.

CLINICAL DIAGNOSIS - Think Athlete’s Foot. Given the prevalence of tinea pedis (the #1 fungal infection in mankind) and the cost of advanced testing, it is unlikely that many non-dermatologists would do any testing at all, and would likely treat empirically with topical antifungal medicaments.DIRECT MICROSCOPIC EXAMINATION - In many situations, it is most appropriate to take a scraping of epithelium from the affected skin or mucosa. A drop of KOH (5-20%) or the surfactant DMSO may be added, which denatures human material and leaves the chitinous walls of fungi more visible, and this allows for direct observation for the presence of yeast and hyphae indicative of a superficial fungal infectionCULTURE - Just as for bacterial cultures, clinical material (skin, hair, nails, soft tissue, etc.) may be cultured for growth of fungal organisms. The chief advantage of culture is that it allows for direct speciation of the infecting organism, although in many clinical situations the sensitivity of culture may be less than direct examination or histological examination (via biopsy).HISTOLOGY - A biopsy of skin, hair, and nails is a sensitive and specific diagnostic technique for fungal infection. Advantages include the rapidity of diagnosis (2-3 days, which is more rapid than culture but less than direct examination), and a high degree of sensitivity, particularly when special stains are performed. Disadvantages of biopsy include a higher cost and the invasive nature of the procedure. As with all histologic techniques, procurement of a representative sample is essential to the sensitivity of the test.Can also use skin testing, serology and PCR can be used to identify fungal type and presence


Explain the pathogenesis of schistosomiasis  as paradigms of important diseases caused by metazoan and protozoan and parasites.

Cercarial invasion into the skin can cause dermatitis (swimmers itch) within 2-3 days. The acute phase of schistosomiasis (also called Katayama fever) is a serum-sickness like illness that occurs 4-8 weeks after skin invasion, coincident with worm maturation and onset of oviposition, and often accompanied by lymphadenopathy and hepatosplenomegaly. The chronic stage of schistosomiasis involved granulamatous and fibrotic changes in the liver (from S. mansoni, S. japonicum, S. mekongi or S. intercalatum) or in the bladder (from S. hematobium). These pathological changes in chronic schistosomiasis are the consequence of host reactions to deposited eggs, leading to formation of infiltrates containing large numbers of eosinophils and eventually to scarring of the liver or urinary bladder. Patients with light infestations may be asymptomatic, while those with heavier infestations often present with symptoms such as diarrhea, abdominal pain, or ascites for intestinal schistosomiasis, and symptoms ranging from bloody urine (hematuria) to bladder cancer (for urinary schistosomiasis). The diagnosis is often made by microscopic examination of stool or urine for schistosome eggs with characteristic sizes and shapes or by detection of similar schistosome eggs in tissue biopsies.Schistosomiasis is transmitted to humans by exposure to contaminated fresh water. It causes acute manifestations and progresses to chronic disease affecting the intestinal or urinary system. Schistosomiasis is estimated to affect 200-300 million people and to cause up to 200,000 deaths per year on a global basis, second only to malaria among parasitic diseases. Among the infected people, 120 million are symptomatic and 20 million have severe disease. 85% of affected people live in sub-Saharan Africa. 


Explain the pathogenesis of malaria as paradigms of important diseases caused by metazoan and protozoan and parasites.

The symptoms of malaria are primarily associated with the rupture of infected erythrocytes and release of merozoites. P. falciparum invades erythrocytes of all ages, and it can therefore achieve the highest parasitemia and mortality. P. vivax and P. ovale invade only young erythrocytes, while P. malariae invades only old erythrocytes. This process often becomes synchronized, and fever paroxysms may then have a regular periodicity (48 hr for benign tertian malaria caused by P. vivax or P. ovale; 72 hr. for quartan malaria caused by P. malariae; and 36-48 hr. for malignant tertian malaria caused by P. falciparum). Anemia, which may be disproportionate to the parasitemia, results not only from the lysis of RBCs, but also from their phagocytosis by the stimulated reticuloendothelial system, their sequestration in the enlarged spleen, and depressed bone marrow function. Hemolysis can be extreme, resulting in hemoglobinuria (blackwater fever). Physical examination reveals jaundice, hypotension and tachycardia in addition to fever and hepatosplenomegaly. Especially with P. falciparum infections, vasodilation causes hypotension and inadequate blood supply to vital organs. P. falciparum-infected red blood cells bind to the microvascular endothelium, which is especially significant in cerebral malaria (up to 50% mortality). Multi-organ failure is the major cause of death in adults. With P. malariae infections, immune complex deposition leading to glomerulonephritis is common. The patient generally mounts an immune response that makes subseqent episodes of symptomatic disease less severe. Both B and T cell responses are involved. Within a few weeks of infection, stage specific anti-plasmodium antibodies are produced. Natural immunity is short-lived, and continual re-infection is required to maintain it. People returning to endemic areas following a long absence may therefore be quite susceptible to re-infection.