Plasmodium knowlesi: An Important Yet Overlooked Human Malaria Parasite

To the Editor: In the May 2009 issue of Mayo Clinic Proceedings, Reyburn and Virk¹ reported a case of a young woman who had visited West Africa and presented with fever and jaundice. The authors did not mention the fifth human malaria parasite Plasmodium knowlesi, which like P falciparum, causes life-threatening illness.² Although currently P knowlesi has been reported only from Southeast Asia³ and like P vivax needs the Duffy antigen,⁴ which is not found in people in West Africa,⁵ clinicians must keep in mind a broad differential diagnosis. P knowlesi is transmitted from human to human or from the macaque monkeys (Old World monkey) via the Anopheles leucosphyrus species of mosquitoes. To date, P knowlesi has not been reported in West Africa. Ciucă et al⁶ found that P knowlesi does not sequester substantially in the microcirculation like P falciparam, and once high parasitemia is achieved, the monkey's death is secondary to rapidly developing anemia, jaundice, and renal failure, all of which are features of severe P falciparum malaria in adults. P knowlesi is currently identified only by polymerase chain reaction-based techniques; by light microscopy, it is falsely reported as P malariae, P vivax, or P falciparum.³ A detailed history of a patient's recent travel to Southeast Asia should be obtained to rule out P knowlesi.