Parasites study sheet
| Parasite | Classification | Lab and identification clues | High-yield clue | Associations |
|---|---|---|---|---|
| Acanthamoeba species | Protozoa; Free-living amoeba; Trophozoite (acanthopodia) + double-walled cyst; Ubiquitous soil/water organism | Corneal scraping wet-mount vocabulary; Double-walled cyst morphology; Spiny acanthopodia trophozoites; Non-nutrient agar with E. coli overlay concept | Contact-lens wearer with painful keratitis, plus double-walled cysts and spiny (acanthopodia) trophozoites, is the key study clue. | Contact-lens / poor lens hygiene risk vocabulary; Keratitis eye-infection study link; Granulomatous amebic encephalitis in immunocompromised; Soil and freshwater environmental source |
| Ascaris lumbricoides | Helminth; Nematode; Intestinal roundworm | Ova/parasite egg vocabulary; Mammillated egg morphology; Adult worm term recognition | Large roundworm + thick mammillated egg + lung migration vocabulary. | Intestinal obstruction vocabulary; Eosinophilia concept; Soil-transmitted helminth framing |
| Babesia microti | Protozoa; Apicomplexa; Piroplasmida; Intraerythrocytic parasite | Giemsa-stained thin/thick blood smear vocabulary; Maltese-cross tetrad of merozoites; Ring forms lacking hemozoin pigment; PCR confirmation vocabulary | Ring forms plus the rare Maltese-cross tetrad inside red cells, without malarial pigment, is the core exam clue. | Ixodes scapularis (blacklegged tick) vector; White-footed mouse (Peromyscus) reservoir; Northeastern/upper-Midwest US epidemiology; Co-infection with Lyme disease study link |
| Balantidium coli | Protozoa; Ciliate; Trophozoite + cyst stages; Largest protozoan pathogen of humans | Cilia-covered motile trophozoite; Kidney-bean macronucleus vocabulary; Cytostome (mouth-pore) feature; Stool ova-and-parasite exam concept | A large, cilia-covered trophozoite with a kidney-bean-shaped macronucleus is the unmistakable study clue. | Pigs as primary reservoir host; Fecal-oral / contaminated-water transmission; Colon-invasion / dysentery vocabulary; Zoonotic epidemiology framing |
| Clonorchis sinensis | Trematode (fluke); Adults reside in bile ducts; Snail first host, freshwater fish second host; Small operculated eggs | Small operculated eggs with shoulder/knob vocabulary; Eggs identified on stool examination; Biliary imaging concepts | Raw freshwater fish transmission, adults living in the bile ducts, and a chronic link to cholangiocarcinoma are the core clues. | Raw/undercooked freshwater fish consumption; Biliary obstruction and cholangitis vocabulary; Chronic infection linked to cholangiocarcinoma; East/Southeast Asia distribution |
| Cryptosporidium | Protozoan; Apicomplexan; Oocyst concept | Modified acid-fast stain vocabulary; Antigen/PCR concepts; Oocyst morphology | Tiny acid-fast oocysts + waterborne diarrhea vocabulary. | Waterborne outbreak vocabulary; Immunocompromised-host severity framing; Chlorine-tolerant oocysts and recreational-water outbreak vocabulary |
Assembled from the free PetriKey reference. Every row is published in full on its own reference page, with that page's own sources. Educational study reference only - not medical advice, and not for diagnosis, treatment, dosing, specimen handling, or any clinical decision-making.