Introduction to Parasitology Flashcards
1
Q
Protozoans
A
Single cell microorganisms; microscopic; the size of a yeast
- Four groups: rhizopods, ciliates, flagellates and sporozoans
- Most protozoans are free living; those causing disease are obligate parasites with hosts being vertebrates and arthropods
2
Q
Helminths
A
Macroscopic; millimeter to longer than a meter; multicellular worms
- Round worms: dimorphic sexes; found in digestive tract of humans
- Cestodes: flat, ribbons of proglottids (both sexes and no digestive tract)
- Flukes: flattened parasites; some in the blood, others in the biliary tree
- Filarial worms: reside in the lymph system and circulate in the blood
3
Q
Strategies of a Successful Parasite
A
- Chronicity
- Means of transmission to a new host
- Immune evasion
- Environmental resistance (e.g. cyst)
- Human, animal or environmental reservoir
4
Q
Ectoparasites
A
- Parasites living on the outside of the host
- May be vectors of infectious diseases and include:
- Lice
- Mites
- Ticks
- Fly larvae
5
Q
Diagnosis of Parasitic Diseases
A
- Collection of fecal specimens; 3 specimens are optimal; recheck 5-6 weeks after treatment of Taenia.
- Fresh observation and preservative.
- Direct observation of stool for trophozoites.
- Urine test for Schistosoma hematobium.
- Sputum for: Paragonimus, Strongyloides
- Blood smear for Babesia, Plasmodium, Chagas
- PCR (Biofire 20): available as a combo test for multiple protozoans and helminths in Biofire 20
6
Q
Drugs for Protozoan Infection: Antimalarial Quinolines
A
- Three major classes analogs of quinine; chloroquine, primaquine and mefloquine.
- Mechanism: Destroy intracellular parasites by accumulating in parasitized host cells. Inhibit heme polymerase that allows buildup of toxic hemoglobin metabolites within the parasite.
- Chloroquine and mefloquine: active in erythrocytes
- Primaquine: Accumulates in tissue cells and destroys hepatic parasites
7
Q
Drugs for Protozoan Infection: Quinones
A
- Atovaquone works against malaria and toxoplasmosis by blocking pyrimidine biosynthesis.
- Effective in patients with chloroquine-resistant P. falciparum malaria.
- Coformulate “Malarone” is used for malaria prophylaxis. Not active against malarial liver infection.
8
Q
Drugs for Protozoan Infection: Artemisinin
A
- Natural extract of the plant Artemisia annua was used for fevers in China.
- Active against both chloroquinesensitive/resistant Plasmodium falciparum.
- These compounds concentrate in parasitized erythrocytes, decompose and release free radicals that damage parasitic membranes.
- They act more rapidly than other malarial drugs.
- They should not be used in pregnancy because of their tetratogenic properties.
- Relapses occur so they have to be combined with another malarial drug.
9
Q
Drugs for Protozoan Infection: Nitroimidazoles
A
- Metronidazole (nitroimidazole) is effective against trichomoniasis, giardiasis, amebiasis and several obligate anaerobic bacteria.
- These agents cause DNA alkylation.
- A newer agent in this group is Tindazole.
10
Q
Drugs for Helminths: Benzimidazoles
A
•for larval and adult nematodes, inhibits fumarate reductase (a mitochondrial enzyme of the helminths)
11
Q
Drugs for Helminths: Avermectins
A
•paralysis of worms
12
Q
Drugs for Helminths: Praziquantil
A
•cestodes and trematodes
13
Q
Toxoplasma gondii
A
- Toxoplasma gondii – Sporozoan (Protozoa)
- Definitive hosts are felines, sexual stage of reproduction
- Thick walled oocysts are passed in cat feces, mature and are swallowed
- Oocyst wall is digested releasing trophozoites
- Enter macrophage, trafficked through the lymphohematogenous route and infect: brain, heart, retina and skeletal muscle turning into pseudocysts
14
Q
How is Toxoplasmosis Acquired?
A
- Human is an incidental host
- Common worldwide, prefers moist climate, rare in desert regions of Arizona
- Transmitted to humans from:
- Contaminated soil (78 million cats in US in houses)
- Cat feces
- Eating infected meat (pork or lamb)
- Vertical (congenital toxoplasmosis - part of TORCHS)
- Waterborne
15
Q
Toxoplasma: Pathogenesis
A
- Invade host cells with local and hematogenous spread, all nucleated cells are susceptible
- Tissue necrosis, infiltration of mononuclear cells
- Phagocytosed by macrophages (phagosome-lysosome fusion) is blocked by trophozoites, preventing acidification and killing,
- Killing of T. gondii by macrophages is enhanced by IFN-γ
- Development of effective cell-mediated immunity, TH1 response Tissue cysts formation, numerous trophozoites with markedly decreased their metabolic activity to become bradyzoites
- They are walled off by the host into a collection called a pseudocyst (a true cyst would be walled by parasite-produced material). This becomes a latent infection, but may be reactivate in the event of immunosuppression.
16
Q
Immunity in Toxoplasmosis
A
- Humoral immunity
- Cell immunity is paramount
- Transplant patients
- AIDS Patient
17
Q
Clinical Diseases of Toxoplasmosis
A
- Congenital infection: 1/500 pregnancies; 20% of infected fetuses with severe disease resulting in microcephaly, hydrocephalus, abortion, stillborn; usually with organomegaly; calcified brain lesions; chorioretinitis
- Infection in a normal host: lymphadenopathy (cervical) or generalized and presenting as “mononucleosis” syndrome
- Infection in the immunocompromised host: declining cell immunity: pneumonitis, myocarditis, serositis, encephalitis; common in AIDS patients (brain)
18
Q
Toxoplasmosis Diagnosis
A
•Biopsy, serology, PCR
19
Q
Toxoplasmosis Treatment
A
•Sulfa plus pyrimethamine for AIDS and immunocompromised patients
20
Q
Cryptosporidium partum
Cryptosporidium hominis
A
- Two species Cryptosporidium partum and C. hominis, sporozoa (Protozoa)
- Found in cattle and humans, 2-6 micron in size
- Obligate intracellular parasite (epicellular: looks like a yeast)
- Acid-fast oocyst membrane
- In Milwaukee (1994), 400,000 people were infected
- Recent outbreak in England; water contaminated by rabbit species
21
Q
Cryptosporidium Infection: The Pathogen
A
- Oocysts are fully mature when shed from feces
- When swallowed attach to the small intestinal villi
- Form schizonts, then gametocytes and then oocyst
- ~20% of the oocysts lack the thick protective wall and immediately begin a cycle of autoinfection
22
Q
Cryptosporidium Pathogenesis and Immunity
A
- Fecal oral spread (water, probably food)
- Day care centers and parents of children with acute disease
- Common in AIDS patients with diarrhea (<50 CD4 cells/mL)
- Replicates in an intracellular, but extra-cytoplasmic location, which may be part of the reason why drug treatment works so poorly.
- Superficial patchy infection of small intestine with mild histologic changes is found.
- The mechanism by which Cryptosporidium causes diarrhea is unknown.
- Animal studies in addition to the common occurrence in AIDS patients indicate that the cell-mediated immune response is of primary importance in preventing and controlling infection
23
Q
Clinical Disease: Cryptosporidiosis
A
- Most common cause of diarrhea in AIDS patient
- Rarely it can disseminate beyond the intestine, primarily to the gall bladder
- Villous atrophy, lymphocytic infiltrate in lamina propria
- In normal hosts, the incubation period is 2-14 days and illness typically lasts about 2-7 days, and then resolves without complication
- Frequent, watery stools—enterotoxin?
- Some people develop chronic diarrheal illness with frequent , foul smelling, bulky stools associated with significant weight loss
24
Q
Foodborne Cryptosporidiosis
A
- 8% of cases (most waterborne; remain viable in ice for days); 7 day incubation period
- Minimum inoculum of 10-100 oocysts (as many as 30 billion oocysts are shed by a calf in 2 weeks)
- Remain viable on fruits and vegetables usually spread by contaminated irrigation water or water used to spray insecticides
25
Q
Cryptosporidiosis Diagnosis
A
- Biofire 20 (a PCR) test
- Acid-Fast Stain
26
Q
Cryptosporidiosis Treatment
A
- No treatment for immunocompetent host
- Restore the CD4 cell count in AIDS
- Nitazoxanide is the only therapy (not effective in immunocompromised patients)
- Paromomycin (aminoglycoside)
27
Q
Cyclospora cayetanensis
A
- Apicomplexa (Sporozoa)
- Life cycle like Cryptosporidium
- Gastroenteritis syndrome—watery diarrhea
- Occurs in US due to imported berries, usually from Guatamala
- Also cause of traveler’s diarrhea
- Trimethoprim-sulfamethoxazole treatment
28
Q
Cyclospora cayetanensis Clinical Picture
A
- Symptoms: Watery diarrhea (most common)
- Loss of appetite/weight loss
- Cramping/Bloating/Gas
- Nausea/Fatigue
- If not treated, symptoms can persist for several weeks to a month or more.
29
Q
Cyclospora cayetanensis Diagnosis
A
- Acid-fast
- Biofire 20 (PCR), fluorescent under UV light
30
Q
Cystisopora (Isospora) belli
A
- Humans are the only known reservoir of Cystisospora
- Can be serious cause of diarrhea in infants because of dehydration
- PCR of stool, Biofire 20 detect disease.
- Otherwise a short 2-3 days of diarrhea
- Found throughout Africa, Latin America and Asia
- Treated with trimethoprim-sulfamethoxazole and prevented by Pneumocystis jiroveci pneumonia prophylaxis
