Plasmodium falciparum malaria — DTM&H MCQ

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EasyMalariaPlasmodium falciparum malariaDTM&H

A 32-year-old British aid worker returns from South Sudan after a 6-month deployment. He presents with a 3-day history of cyclical fevers, rigors, and sweating occurring every 48 hours. He took no chemoprophylaxis. A thin blood film shows ring-form trophozoites with occasional banana-shaped gametocytes. His parasitaemia is 8%. What is the most likely diagnosis?

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Correct answer: D — Plasmodium falciparum malaria

The correct answer is D, Plasmodium falciparum malaria. Ring-form trophozoites with crescent (banana-shaped) gametocytes on thin film are morphologically diagnostic for P. falciparum, as no other human Plasmodium species produces this gametocyte shape. A parasitaemia of 8 percent is also characteristic, since UK guidance identifies falciparum as the species capable of producing high-density infection, with parasitaemia above 2 percent already flagging increased risk of severe disease and progression to multi-organ involvement. Sub-Saharan Africa, including South Sudan, is holoendemic for P. falciparum, and travellers without chemoprophylaxis returning from this region with fever should be presumed to have falciparum malaria until proven otherwise given its potential for rapid deterioration. Why the other options are wrong: C. Plasmodium vivax malaria: Vivax produces round, non-crescentic gametocytes and rarely sustains parasitaemia above 2 percent because it preferentially invades reticulocytes; it is also uncommon in West and Central sub-Saharan Africa due to Duffy antigen negativity. E. Plasmodium ovale malaria: Ovale also produces round gametocytes and typically causes low-grade parasitaemia, with a tertian (48-hour) but mild relapsing course, not high-density infection with crescent forms. A. Plasmodium malariae malaria: Malariae classically causes a quartan (72-hour) fever pattern, not the 48-hour cycle described, and produces low, often persistently low-level parasitaemia with round gametocytes. B. Plasmodium knowlesi malaria: Knowlesi is a zoonotic infection acquired from macaques in Southeast Asian forested regions (notably Malaysian Borneo), not South Sudan, and its blood film morphology mimics malariae, not falciparum crescents. Key point: Crescent (banana-shaped) gametocytes on blood film are pathognomonic for Plasmodium falciparum and, combined with high parasitaemia, mandate urgent treatment as potentially severe malaria per UK guidelines.

Reference: UK Malaria Treatment Guidelines Writing Group (Lalloo DG, Shingadia D, Bell DJ, Beeching NJ, Whitty CJM, Chiodini PL; PHE Advisory Committee on Malaria Prevention in UK Travellers), 'UK malaria treatment guidelines 2016', Journal of Infection 2016;72(6):635-649. https://www.journalofinfection.com/article/S0163-4453(16)00047-5/fulltext