By: Kellie Vinal
The World Health Organization (WHO) and textbooks alike have long listed four species of parasite that cause malaria in humans: Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium falciparum, the last of which is most deadly. However, the research of Dr. Balbir Singh, director of the Malaria Research Center at the University of Malaysia, suggests that Plasmodium knowlesi, a parasite known to naturally infect pig-tailed and long-tailed macaques, is infecting humans with significantly increased incidence in Malaysia. In fact, analysis of malaria-infected hospital patients in Malaysian Borneo revealed that 68 percent of malaria cases were caused by Plasmodium Knowlesi in 2013.
In macaques, P. knowlesi infection is typically asymptomatic and produces low-grade parasitism . In humans, however, the disease is more severe, as P. knowlesi replicates most quickly of all Plasmodium species which accelerates disease development . P. knowlesi infection causes a spectrum of disease in humans that varies in severity and can potentially be fatal, but the disease is treatable with antimalarial drugs if caught early. In an acute infection of P. knowlesi, patients initially exhibit nonspecific symptoms such as chills, fever, muscular pain or tenderness, and poor appetite, which in some cases are accompanied by abdominal pain, cough, and gastrointestinal symptoms . If left untreated, patients may experience renal failure, respiratory distress, and liver dysfunction as the disease progresses .
Overall, human malaria cases have been on the decline in Malaysia due to increased measures of prevention such as insecticide-treated mosquito nets and indoor insecticide spraying. However, mosquitos carrying this form of monkey malaria live exclusively outdoors, negating the impact of current indoor-focused prevention methods. More effective methods include long-sleeved clothing and expensive insect repellant, which may not be feasible for those at risk. Although P. knowlesi is a prominent cause of malaria in Southeast Asia, the health threat of this particular parasite is unlikely to spread outside this region due to the geographic distribution of mosquitoes capable of carrying P. knowlesi.
 Lee, K., Cox-Singh, J., Brooke, G., Matusop, A., and Singh, B. (2009) Plasmodium knowlesi from archival blood films: Further evidence that human infections are widely distributed and not newly emergent in Malaysian Borneo. Int J Parasitol, 39, 1125-1128.
 Divis, P.C., Shokoples, S.E., Singh B., and Yanow, S.K. (2010) A TaqMan real-time PCR assay for the detection and quantitation of Plasmodium knowlesi.Malar J, 9, 344.
 Cox-Singh, J., Hiu, J., Lucas, S.B., Divis, P.C., Zulkarnaen, M., Chandran, P., Wong, K.T., Adem, P., Zaki, S.R., Singh, B., and Krishna, S. (2010) Severe malaria – a case of fatal Plasmodium knowlesi infection with post-mortem findings: a case report. Malar J, 9, 10.
 Daneshvar, C., Davis, T.M., Cox-Singh, J., Rafa’ee, M.Z., Zakaria, S.K., Divis, P.C., and Singh, B. (2009) Clinical and laboratory features of human Plasmodium knowlesi infection. Clin Infect Dis, 49(6), 852-860.
 Singh, B., and Daneshvar, C. (2013) Human infections and detection of Plasmodium knowlesi. Clin Microbiol Rev, 26(2), 165-184.
 Ahmen, A.M., Pinheiro, M.M., Divis, P.C., Siner, A., Zainudin, R., Wong, I.T., Lu, C.W., Singh-Khaira, S.K., Millar, S.B., Lynch, S., Willmann, M., Singh, B., Krishna, S., and Cox-Singh, J. (2014) Disease progression in Plasmodium knowlesi malaria is linked to variation in invasion gene family members. PLOS Neglected Tropical Diseases, 8(8), e3086.