After warning us in 2011 about a huge reservoir of Plasmodium knowlesi  in the monkeys and how the increasing human populations and deforestation could shift the disease to humans, Director of the Malaria Research Centre at University of Malaysia, Sarawak, Balbir Singh now reports 3 years later that sixty-eight percent of malaria hospitalizations in Malaysia last year were caused by the malaria species found in macaques.

“This is a form of malaria that was once rarely seen in people, but today, in some remote areas of the country, all of the indigenous malaria cases we are seeing are caused by the P. knowlesi parasite,” said Singh. “If the number of cases continue to increase, human-to-human transmission by mosquitoes becomes possible. In fact, this may already have happened, which would allow P. knowlesi malaria to spread more easily throughout southeast Asia,” he cautioned.

Singh’s research was presented last week at the American Society of Tropical Medicine and Hygiene (ASTMH) Annual Meeting.

Plasmodium knowlesi, “the fifth human malaria”, is mostly found is mostly found in South East Asian countries particularly in Borneo, Malaysia, Myanmar, the Philippines, Singapore, and Thailand.

Plasmodium knowlesi is absent in Africa. This may be because there are no long-tailed and pig-tailed macaques (the reservoir hosts of P. knowlesi) in Africa and many West Africans lack the Duffy antigen – a protein on the surface of the red blood cell that the parasite to uses to invade.

Mosquitoes belonging to the Anopheles leucosphyrus group are known to transmit P. knowlesi from monkeys to humans as Singh notes: “But the P. knowlesi strain of malaria should stay within Southeast Asia as there are no mosquitoes outside the region capable of carrying these parasites.”

Related: Dengue in Malaysia: Outbreak grows by another 10,000, 20 deaths

“These intriguing results are yet another example of the complexity and diversity of the interaction between man, his activities, parasites, and mosquitoes. P. knowlesi is now a significant cause of human malaria in Malaysian Borneo that must be addressed across multiple levels: research, development, implementation, funding and evidence-based policies,” said Alan J. Magill, MD, FASTMH, president of the American Society of Tropical Medicine and Hygiene.

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