Parasitology
Core concepts
Stool examination remains the foundation: direct wet mount, formalin-ethyl acetate concentration for cysts and eggs, trichrome stain for permanent morphology, and modified acid-fast stain for coccidian oocysts. Blood parasites require both thick and thin Giemsa films; thick films screen, thin films identify species and quantify parasitemia. Pinworm is diagnosed by cellulose tape preparation, not stool.
Know the key morphologic markers. Giardia lamblia shows pear-shaped trophozoites with two nuclei and falling-leaf motility. Entamoeba histolytica trophozoites ingest red cells; E. coli trophozoites do not. Cryptosporidium oocysts are acid-fast positive and small, while Cyclospora is larger and requires modified acid-fast or autofluorescence. Strongyloides is diagnosed by rhabditiform larvae in stool or by agar plate and Baermann techniques, and it can cause hyperinfection. Among blood parasites, Plasmodium falciparum shows multiple ring forms, applique forms, and banana-shaped gametocytes; Babesia shows tetrad Maltese cross forms and no pigment.
Traps
- Reporting Entamoeba histolytica without ingested red cells; use antigen or nucleic acid testing when in doubt.
- Mistaking pollen, hair, or air bubbles for ova and parasites.
- Forgetting that blood for malaria should be drawn repeatedly over hours when the first film is negative.
- Treating eosinophilia as diagnostic; it is supportive, not specific.
Rule of thumb
The presence of ingested red blood cells inside a trophozoite in a fresh stool sample supports Entamoeba histolytica over the harmless E. coli. If a patient has eosinophilia and larvae in stool, consider Strongyloides and avoid steroids that could precipitate hyperinfection.