Zoonotic Malaria: Plasmodium knowlesi

Abstract

Human malaria parasite species [genus Plasmodium] originally were derived from nonhuman primate hosts and subsequently crossed the species barrier to become adapted to humans. Malaria is an ancient disease which has caused the greatest human health affliction more than any other disease known to mankind. Each year malaria cause hundreds of thousands of deaths globally [mainly children] and many million cases of clinical disease. Before the advent of the twenty-first century, human malaria was considered secondary to four Plasmodium species [fully adapted to humans as the only intermediate host]: Plasmodium falciparum, P. vivax, P. ovale, and P. malariae. In 2004 a large concentration of known nonhuman primate malaria, Plasmodium knowlesi, was reported in the human population of Sarawak, Malaysian Borneo. P. knowlesi, a known parasite of Asian macaques, had adapted to the human population for some unknown time before, but was likely misidentified as P. malariae infection. P. knowlesi is transmitted by the Leucosphyrus group of anopheline mosquitoes primarily as a zoonotic infection in forested areas. It is now the most common cause of malaria in humans in Malaysia and has spread throughout Southeast Asia. The clinical spectrum of disease is variable, from mild malaria resembling infection with P. malariae to severe disease with high parasitic load with mortality of 10% or more, mimicking infection with P. falciparum. Uncomplicated malaria by P. knowlesi can be treated with chloroquine, but severe disease is best treated with artesunate combination. Diagnosis by blood smear microscopy can be misleading and molecular methods [PCR] are more reliable. Investigation for an effective vaccine is in progress.