The malaria parasite Plasmodium knowlesi is transmitted between monkeys by forest-dwelling mosquitoes. This limits transmission to humans and, at present, there are only around 300 human cases per year.
But as human populations grow and the forest shrinks, people are likely to venture into the forest more often and in greater numbers. This, said the researchers, could lead the parasite to evolve, enabling it to pass more easily between humans and monkeys.
Balbir Singh, director of the Malaria Research Centre at the University of Malaysia, Sarawak (UNIMAS), and colleagues first showed in 2004 that P. knowlesi causes deadly disease in humans. They did not know whether the disease is maintained in the human population or whether the monkeys act as wild reservoirs from which humans get infected.
Singh, who led the current study, published last week (7 April) in PLoS Pathogens, said he suspected macaque populations in Sarawak, Borneo, were a reservoir for P. knowlesi — and were the source of human malaria cases seen in local hospitals — as monkey populations in other areas are also known to act as malaria reservoirs.
Singh's team looked closely at the DNA of P. knowlesi in 108 wild macaques from Sarawak and 31 human malaria patients.
They found that the disease is maintained in the monkeys but that, as humans become increasingly exposed, it could switch host and adapt to infecting people. Macaques had multiple P. knowlesi infections, while most human patients had just one, and the parasite diversity was much higher in the monkeys, which could make it more difficult to eliminate the disease.
"These macaques are the most common non-human primate in the jungles of South-East Asia," Singh said. "It's a huge reservoir of parasites, so trying to eliminate malaria might be virtually impossible."
As deforestation and human populations increase, Singh fears the parasite could evolve, switching its preferred host to humans and leading to large-scale malaria outbreaks caused by the strain.
"Anything that brings humans into increasing contact with the forest could increase the risk," said Catherine Walton, an evolutionary biologist at the University of Manchester, United Kingdom.
Richard Coker, head of the communicable diseases policy research group at the London School of Hygiene and Tropical Medicine, United Kingdom, said population growth and deforestation would "continue to contribute to the emergence and spread of novel pathogens with increasing frequency".
PLoS Pathogens doi: 10.1371/journal.ppat.1002015 (2011)