Parasitology
Core concepts
Divide parasites into protozoa (single-celled: Plasmodium, Giardia, Entamoeba, Toxoplasma, Cryptosporidium, trypanosomes, Leishmania) and helminths (worms: nematodes, cestodes, trematodes). For each, know the vector or route, the host cycle, and the diagnostic stage seen in stool, blood, or tissue.
High-yield pairs
- Plasmodium: Anopheles mosquito; thick/thin smears; cyclical fever; P. falciparum causes severe disease (high parasitemia, cerebral malaria).
- Giardia: contaminated water; malabsorption, steatorrhea; stool antigen or PCR.
- Entamoeba histolytica: liver abscess with anchovy-paste aspirate; serology.
- Toxoplasma: cat feces or undercooked meat; ring-enhancing CNS lesions in HIV.
- Helminths: eosinophilia is a clue; Strongyloides can cause hyperinfection with steroids.
Traps
- Confusing cyst (environmental/transmission) with trophozoite (disease) stages.
- Missing that eosinophilia is common in helminths but not protozoa.
- Forgetting that Strongyloides hyperinfection is triggered by corticosteroids.
- Assuming a single negative stool exam excludes parasites; multiple samples may be needed.
Worked rule of thumb
A traveler returning from sub-Saharan Africa with cyclical fevers and thrombocytopenia should have malaria smears repeated every 8–12 hours for three sets; a single negative smear does not exclude malaria. Treat P. falciparum with artemisinin-based combination therapy.