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Deforestation Is Clearing the Way for a Rare Monkey-Borne Malaria

A formerly-rare form of malaria, which is capable of jumping from macaques to humans, is now the leading cause of malaria hospitalizations in the country.

by Derek Mead
Nov 3 2014, 8:20pm

Deforestation in Sarawak, Malaysia. Image: Wakx/Flickr

Malaysia's forests are disappearing at a faster rate than just about anywhere else in the world. And with the rise of deforestation has come another troubling trend: A formerly-rare form of malaria, which is capable of jumping from macaques to humans, is now the leading cause of malaria hospitalizations in the country.

It's not fully clear how tightly linked the two phenomena are, but Dr. Balbir Singh, director of the Malaria Research Center at the University of Malaysia in Sarawak, is working to find an answer.

In research presented today at the American Society of Tropical Medicine and Hygiene annual meeting, Singh explained how the parasite Plasmodium knowlesi has grown from being a minor cause of malaria in Malaysian Borneo to causing some 68 percent of malaria hospitalizations in the region last year.

This finding comes after decades of heavy deforestation, largely driven by the area's palm oil and timber industries. The disease is carried by macaques and then can be transmitted to humans by mosquitoes, and it's thought that deforestation is pushing infected macaques closer to human populations.

"Right now we can only speculate, but deforestation for me seems to be a major, major reason for the increase," Singh told me.

The range of confirmed knowlesi malaria cases. The dashed line is the range of the mosquito species that carry the disease, while the inset is the range of long-tailed macaques, which are also carriers. Image: Dr. Balbir Singh

A variety of Plasmodium species of protozoa cause malaria in humans with a variety degree of seriousness, with P. falciparum being by far the most deadly worldwide. But Singh said the P. knowlesi, the species focused on in his work, is of concern because it replicates in just 24 hours, as compared to 48 hours for P. falciparum or 72 hours for P. malariae, a more benign species. That means that the disease can spread quickly, and before doctors necessarily realize what's going on.

"We have seen that due to [the rapid reproduction] of P. knowlesi, some rural patients have come into clinics ill, but doctors didn't detect any parasite, so they were sent home thinking it was the flu," Singh said. "They returned a few days later and the [blood] count [of the parasite] was very high, and they died."

Singh found that knowlesi malaria is now three times more prevalent than the the falciparum variety in Borneo, making it a major emerging health concern. It also comes at a time that the region faces massive deforestation: A 2013 study found that Malaysia lost 14.4 percent of its forest cover from 2000 to 2012, the highest rate of deforestation in the world over that period. And the loss of forests have opened up new regions to industry, pushing macaques and humans closer.

Knowlesi malaria in monkey blood sample. Image: CDC/ Dr. W.A. Rogers, Jr.

"When people go into the forest where they have their farms, or go hunting, or work in the timber camps, these are the ones that are being exposed to the mosquitoes that are feeding on macaques," Singh said.

"This is anecdotal evidence: When talking to people [whose communities have been affected by the disease], they say 'We never had malaria here until they started clearing land for plantations, and then there were more macaques,'" he continued.

That's where Singh's findings get a bit troubling: Currently, a few species of macaque appears to be the largest host population of P. knowlesi, which causes only mild malaria in the monkeys. But the disease can be transmitted human-to-human (via mosquitoes, like all malaria-inducing parasites), and if the number of macaque-human infections continues to grow, it could eventually lead to P. knowlesi finding a self-sustaining mass in the human population.

A long-tailed macaque. Image: Drriss & Marrionn/Flickr

Singh said that identifying the disease has proven troublesome in the past because it looks similar to other species under a microscope, and part of the increase may be due to better diagnoses using more modern molecular ID techniques. He said that part of this problem is because there is a there "a serious shortage" of classical research, adding that "these days students prefer to sit at their computer and analyse DNA data" than actually go into the field. That's left a bit of a gap in understanding the emerging disease that his team hopes to rectify.

"Human-to-human infections may be happening now, but there are thousands of macaques, how do you prove it?" he said. "It could be that the macaques came around foraging for food, and then were bit by mosquitoes and transmitted to humans. You can transmit human-to-human, it's not a dead-end, and it may well be occurring now."

Additional reporting by Jason Koebler.