Cystoisosporiasis in HIV — DTM&H MCQ

Correct answer: C — Cystoisospora belli infection

The correct answer is C, Cystoisospora belli infection. This protozoan parasite classically causes chronic, watery, non-bloody diarrhoea with malabsorption and wasting in patients with advanced HIV and low CD4 counts, exactly as described in this wasted, dehydrated child. Its oocysts are ellipsoidal, variably acid-fast on modified Ziehl-Neelsen staining, and distinctly larger than Cryptosporidium oocysts, which is the key laboratory discriminator in the stem. This size and staining pattern is used specifically to separate Cystoisospora from Cryptosporidium and Cyclospora on stool microscopy in resource-limited settings such as rural Malawi. Cystoisospora responds to co-trimoxazole, making correct identification clinically important as well as diagnostically distinct. Why the other options are wrong: B. Giardia duodenalis infection: Giardia trophozoites and cysts are seen on wet mount or trichrome stain, not modified acid-fast stain, and Giardia is not acid-fast at all. A. Entamoeba histolytica colitis: This typically causes bloody dysentery or amoebic liver abscess rather than chronic watery wasting diarrhoea, and trophozoites/cysts are identified by iodine or trichrome staining, not acid-fast methods. D. Strongyloides hyperinfection: This presents with larvae in stool or sputum (seen on wet mount, Baermann concentration, or agar culture), abdominal pain and cutaneous larva currens, not acid-fast oocysts. E. Vibrio cholerae infection: This causes acute profuse watery diarrhoea from a bacterium, diagnosed by stool culture or dark-field microscopy for motile comma-shaped bacilli, not by acid-fast oocyst staining, and is not a chronic wasting illness. Key point: Variably acid-fast oocysts on modified Ziehl-Neelsen stain that are larger than Cryptosporidium point specifically to Cystoisospora belli in chronic HIV-associated diarrhoea.

Reference: Manson's Tropical Diseases, 24th edition, chapter on Intestinal Protozoa (Cystoisospora belli/Isospora belli): oocyst morphology and modified Ziehl-Neelsen staining in HIV-associated chronic diarrhoea (core reference text for the UK DTMH curriculum).