NextBillion - An Initiative of the William Davidson Institute at the University of Michigan

Thursday, June 11, 2015

If you’re not a doctor, you probably think of malaria as a single disease. That’s not quite right. There are several parasites that cause what we call malaria—likePlasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae—and each brings a slightly different form of the disease. Now, in a northeastern corner of Malaysia, doctors are concerned about the rapid expansion of yet another malaria-inducing parasite named Plasmodium knowlesi. Incidence of the parasite increased tenfold between 2004 and 2011, and many public-health researchers are worried things are going to get worse.

An important difference between P. knowlesi and most other human malaria parasites is that it also infects other primates. Although the widespread falciparumparasite, for example, is thought to have derived from a gorilla disease and many related strains are still found in apes, the human parasite no longer infects other species. Knowlesi, in contrast, can infect both humans and monkeys, and it remains a common infection in macaques.

The rapid expansion of knowlesi infections in recent years is most likely a result of humans pushing deeper and deeper into the jungle that used to buffer us from the disease.

In past decades, the occasional knowlesi infection has almost always occurred in people working deep in the jungle. The first confirmed human case came in 1965, when an American soldier surveying the Malaysian jungle came home with the cyclical fevers characteristic of malaria. The doctors then inoculated the blood of some rhesus macaques, which were already known to carry the disease, with the patient’s infection. When the monkeys died, the physicians knew they were dealing with knowlesi. (Medical research was, and in many ways remains, exceedingly cruel to other creatures.)

The ability of knowlesi to pass between monkeys, mosquitoes, and humans makes it a major public-health issue as well as an environmental one. The rapid expansion of knowlesi infections in recent years is most likely a result of humans pushing deeper and deeper into the jungle that used to buffer us from the disease. In short, habitat destruction isn’t just killing wildlife—it’s killing us.