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Cyclospora Size Diagnosis — SCE Infectious Diseases MCQ

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EasyParasitologyCyclospora Size DiagnosisSCE Infectious Diseases

A 40-year-old HIV-positive man (CD4 30, not on ART) presents with chronic watery diarrhoea and weight loss. Stool examination reveals acid-fast oocysts of 8–10 µm on modified ZN stain. Co-trimoxazole 960 mg BD for 7 days is prescribed with good clinical response. What organism was treated?

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Correct answer: D — Cyclospora cayetanensis — 8–10 µm acid-fast oocysts responding to Co-trimoxazole is diagnostic

Size matters in parasitology: Cyclospora cayetanensis oocysts are 8–10 µm on modified ZN (versus Cryptosporidium 4–6 µm, Cystoisospora 20–30 µm). Cyclospora responds well to Co-trimoxazole at standard dose (960 mg BD for 7 days). Cryptosporidium (smaller, 4–6 µm) has no reliable antimicrobial — immune reconstitution is the treatment. Cystoisospora (larger, 20–30 µm) responds to high-dose Co-trimoxazole (960 mg QDS for 10 days). This size-based differentiation on acid-fast staining is a classic exam question.

Reference: BHIVA 2022 – OI guidelines; WHO 2023 – Cyclosporiasis