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Transmission of malaria from monkeys now dominant cause of human malaria hospitalizations in Malaysia

New study released at ASTMH annual meeting targets deforestation as a possible problem for increasing human encounters with macaques

The majority of hospitalizations in are now caused by a dangerous and potentially deadly monkey-borne once rarely seen in humans, and deforestation is the potential culprit in a growing number of infections that could allow this virulent malaria strain to jump from macaque monkeys to human hosts, according to research presented at the American Society of Tropical Medicine and Hygiene (ASTMH) Annual Meeting.

An analysis of malaria patients hospitalized in in 2013 showed that 68 percent had been sickened by Plasmodium knowlesi, said , PhD, director of the Malaria Research Center at the University of Malaysia in Sarawak. The parasite is increasingly associated with malaria deaths and is three times more frequent as a cause of severe malaria in Borneo than the more common P. falciparum parasite that is currently considered the world’s most deadly form of the disease.

The main host of knowlesi malaria has been the long-tailed and pig-tailed macaques found in the tropical forests of Malaysia and elsewhere in Southeast Asia. The infections are concentrated in areas of Malaysia where over the last decade massive loss of native forest to timber and palm oil production has led to substantially increased human interactions with macaques. That puts knowlesi malaria in the company of a growing list of dangerous emerging and re-emerging diseases – including Ebola and AIDS – that are being passed from animals to humans as development peels back more and more layers of tropical forest previously uninhabited by humans.

“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,” Singh said. “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.”

Evidence to date has strongly suggested that victims of P. knowlesi malaria have been bitten by mosquitoes that had first bitten an infected macaque, making humans a dead-end host for the parasite. Of concern, however, is recent research that the parasite could change so that it can jump from person to person via mosquito bites, without requiring a monkey as part of its life cycle. Laboratory tests in the 1960s indicated that a mosquito variety in Malaysian Borneo that carries the two most common human malaria parasites – P. falciparum and P. vivax – also can spread the knowlesi parasite. Moreover, P. knowlesi was recently found in Vietnam in mosquitoes that transmit falciparum and vivax malaria, raising the possibility that human-to-human transmission is already occurring.