Plasmodium knowlesi malaria — DTM&H MCQ

Correct answer: C — Plasmodium knowlesi malaria

The correct answer is C, Plasmodium knowlesi malaria. This zoonotic parasite of macaques is endemic to forest fringe areas of Malaysian Borneo and is morphologically almost indistinguishable from P. malariae on blood film, frequently causing misdiagnosis. The key discriminator is tempo: P. knowlesi has a 24 hour asexual erythrocytic cycle, so parasitaemia can rise extremely rapidly (unlike the 72 hour cycle of true P. malariae), producing daily fever, thrombocytopenia and abdominal pain, and can progress to severe or fatal disease within a short period. Exposure history (trekking near forest edges where macaque reservoirs and Anopheles vectors overlap) combined with this rapid clinical evolution should prompt species reassignment and urgent treatment as for falciparum-equivalent severe malaria, including consideration of intravenous artesunate. UK guidance therefore mandates PCR confirmation whenever a film reports P. malariae from a South East Asian, particularly Bornean, exposure. Why the other options are wrong: A. Relapsing Plasmodium vivax malaria: vivax has a 48 hour cycle, causes hypnozoite relapse rather than rapid parasitaemia rise, and does not resemble P. malariae morphologically. B. Visceral leishmaniasis: causes fever, hepatosplenomegaly and pancytopenia over weeks to months, not a positive malaria blood film with rapidly rising parasitaemia. E. Uncomplicated Plasmodium malariae infection: true P. malariae has a 72 hour cycle with characteristically low, stable parasitaemia and indolent course, incompatible with rapid deterioration over 24 hours. D. Babesiosis acquired from Ixodes ticks: Ixodes ticks and Babesia are not endemic to Malaysian Borneo, and babesiosis is not reported as resembling P. malariae on film. Key point: A Borneo travel history plus a "P. malariae-like" film with rapidly rising parasitaemia over 24 hours signals P. knowlesi, which must be treated as potentially severe malaria pending PCR confirmation.

Reference: Lalloo DG, Shingadia D, Bell DJ, et al. UK malaria treatment guidelines, 2016 (PHE Advisory Committee on Malaria Prevention in UK Travellers), Journal of Infection; PMC7132403, https://pmc.ncbi.nlm.nih.gov/articles/PMC7132403/