Transfusion-transmitted malaria — DTM&H MCQ

Correct answer: B — Direct inoculation of erythrocytic stages through transfusion

The correct answer is B, direct inoculation of erythrocytic stages through transfusion. Transfusion-transmitted malaria occurs when a donor unit containing asexual blood-stage (erythrocytic) Plasmodium parasites is infused directly into the recipient's bloodstream, bypassing the mosquito vector entirely. This explains the absence of hepatosplenomegaly (no chronic exposure or established hepatic cycle) and the three-week interval, which corresponds to the typical incubation period for transfusion-acquired falciparum malaria rather than a mosquito-borne infection. Because the sporozoite and hepatic (exoerythrocytic) stages are skipped, the parasite enters the blood already as merozoites or trophozoites, producing rapid parasitaemia once the transfused red cells are haemolysed and release parasites. UK blood services guidance recognises this transfusion risk and screens or defers donors from malaria-endemic areas accordingly. Why the other options are wrong: A. Activation of dormant liver hypnozoites: Hypnozoites are unique to Plasmodium vivax and Plasmodium ovale, not P. falciparum, and this mechanism requires a natural mosquito-inoculated sporozoite infection with a hepatic phase, which did not occur here. E. Ingestion of oocysts in contaminated water: Plasmodium has no faecal-oral or waterborne transmission route; oocysts develop only in the mosquito midgut and are not infective to humans by ingestion. C. Skin penetration by cercariae: Cercarial skin penetration is the transmission mechanism for schistosomiasis, an unrelated trematode infection, not malaria. D. Inoculation by a sandfly bite: Sandflies transmit Leishmania and some arboviruses, not Plasmodium, which requires an Anopheles mosquito vector for natural transmission. Key point: Transfusion-acquired falciparum malaria bypasses the sporozoite and hepatic stages entirely, presenting with erythrocytic-stage parasitaemia and no hepatosplenomegaly within weeks of transfusion, unlike vector-borne infection.

Reference: JPAC (Joint United Kingdom Blood Transfusion and Tissue Transplantation Services Professional Advisory Committee), Donor Selection Guidelines, Malaria entry: https://www.transfusionguidelines.org/dsg/wb/guidelines/ma001-malaria