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—like Plasmodium 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 falciparum parasite, 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.
Diseases with animal vectors are difficult to combat. Consider Ebola, the terrifying disease du jour. With no warning, it occasionally transfers between an animal (possibly a fruit bat) and a human. Once this so-called “spillover event” occurs, the virus can spread rapidly within human communities. Alarm bells ring worldwide, and doctors swarm to the site of the outbreak. All that response work, however, is the equivalent of cutting the head off of a hydra. Ebola is still lurking in the jungles of Africa, waiting to strike again. As long as we keep living in proximity to whatever animal is carrying the disease, there’s very little we can do to stop more outbreaks.
Malaria is another example. Doctors and public-health experts in the early 1980s, excited by the success of the smallpox eradication effort, began to plot the downfall of other diseases. Malaria was one of their highest priorities. Rather than disappearing, though, the disease has flourished. Global malaria deaths actually increased between 1980 and 2010, and currently number up to 755,000 annually. The main problem is that the disease’s complex life cycle spans two species. How can you eradicate malaria without killing most of the Anopheles mosquitoes on the planet?
Plasmodium knowlesi combines some of the features of Ebola and traditional malaria. It has an animal reservoir (macaques) as well as a vector (mosquitoes). The parasite has so many hiding places that doctors will have a very difficult time hunting it down.
To make matters worse, P. knowlesi is a dangerous little creature. Unlike its close relative P. malariae, which replicates every three days, knowlesi replicates every day. That means a few cells of the parasite can quickly spread throughout the human body. This also makes it more infectious, allowing it to jump between humans through the mosquitoes.
Don’t get too worried yet—P. knowlesi is still a relatively rare parasite, mostly limited to a small part of Asia. It’s not the global problem that P. falciparum malaria has been for decades, and it’s not nearly as lethal as Ebola or other hemorrhagic diseases. But the expansion of the parasite should serve as a warning to us. There are things lurking deep in the jungle that we don’t want to deal with. Let’s give monkeys their space, for our own good.