Loa loa and ivermectin risk — DTM&H MCQ

Correct answer: E — Daytime blood film for Loa loa microfilarial load

The correct answer is E, daytime blood film for Loa loa microfilarial load. Loa loa microfilariae exhibit diurnal periodicity and are found in peak numbers in peripheral blood during the day, so a daytime thick blood film is the correct sampling technique to quantify microfilarial density. This matters clinically because patients with heavy Loa loa infection (suggested here by a history of Calabar swellings and eye-worm migration, both classic loiasis features) are at risk of severe, sometimes fatal, post-ivermectin encephalopathy if their pretreatment microfilarial density is very high. Before mass drug administration or individual ivermectin treatment for onchocerciasis in Loa loa co-endemic areas of Central Africa (including Cameroon), quantifying the Loa loa load allows risk stratification and, if needed, use of alternative strategies (slow dose escalation, pre-treatment with albendazole, or closer monitoring) rather than standard-dose ivermectin. Why the other options are wrong: B. Skin snip for Leishmania amastigotes: Skin snips are the correct test for onchocerciasis (detecting Onchocerca volvulus microfilariae), not for Leishmania, which is diagnosed by tissue smear or biopsy for amastigotes in cutaneous or visceral leishmaniasis, an unrelated clinical picture here. C. Midnight blood film for Wuchereria bancrofti: Nocturnal periodicity applies to Wuchereria bancrofti, not Loa loa; using a midnight film here would miss the diurnal Loa loa microfilariae entirely and is irrelevant to this presentation. D. Urine filtration for schistosome eggs: This detects Schistosoma haematobium ova and has no bearing on filarial disease or ivermectin safety screening. A. Sputum microscopy for Paragonimus eggs: This diagnoses paragonimiasis (lung fluke), presenting with haemoptysis, and is unrelated to the eye-worm migration and Calabar swellings described. Key point: Loa loa microfilariae peak in blood during the day, so pretreatment daytime microfilarial density testing is essential before ivermectin in Loa loa co-endemic regions to prevent encephalopathy.

Reference: The Lancet Microbe, 'Evaluating post-treatment Loa loa microfilarial densities to classify serious adverse events caused by ivermectin: a retrospective analysis' (2022), describing pretreatment microfilarial density thresholds (>10,000 mf/mL) associated with post-ivermectin encephalopathy risk. https://www.thelancet.com/journals/lanmic/article/PIIS2666-5247(22)00331-7/fulltext