Plasmodium knowlesi.

Abstract

Plasmodium knowlesi has been known as a parasite of South-East Asian monkeys since the first quarter of the last century. It was experimentally transmitted to humans in the 1930s and for a period was used in the treatment of neurosyphilis. Subsequently, naturally occurring human infection was described. In recent decades, human cases have been recognized in Indonesia, Borneo, Singapore, Malaysia, Myanmar, Vietnam, and the Philippines and occasional cases have occurred elsewhere in returning travellers. The morphological features share some characteristics with P. falciparum in that there are delicate ring forms, sometimes two ring forms per cell and sometimes double chromatin dots and accolé forms (left and center); the parasitized cells are not enlarged and have fine cytoplasmic dots, which were referred to as Sinton and Mulligan's dots in the older literature. The presence of young rings, accolé forms, and high parasitemia may lead to confusion with P. falciparum. However, in addition to delicate rings there are also more chunky rings and, later, somewhat ameboid trophozoites (left and center). Schizonts may also be seen with up to 16 merozoites (right), whereas these are quite uncommon in the peripheral blood in P. falciparum infection. A common problem is mis-identification of P. knowlesi as P. malariae, especially as both parasites produce band form trophozoites. If a diagnostic laboratory sees what appears to be P. malariae in a person who acquired malaria in the Asia-Pacific region, PCR for P. knowlesi should be undertaken, especially as P. knowlesi produces a much more severe infection and the treatment given should be appropriate for both those possibilities. These images are from in vitro cultured parasites kindly provided by Dr. Don Van Schalkwyk of the London School of Hygiene and Tropical Medicine. none