Malaria appears to have originated in chimpanzees and jumped over to humans at some point in the last two million years, bucking the leading theory that the disease had evolved along with humans.
After gathering blood samples from nearly 100 chimpanzees in central Africa, researchers uncovered eight new strains of the parasite that causes chimp malaria. By comparing genes from the new chimp strains to genes from human malaria, scientists discovered that like HIV, our malaria bug is a gift from chimpanzees.
“The conventional wisdom on malaria is that this is a disease that has been in humans since the dawn of humanity,” said infectious disease expert Nathan Wolfe of Stanford University, who co-authored the paper published Monday in the Proceedings of the National Academy of Sciences. “In fact, what we found was really quite surprising to us: There is a tremendous diversity of these parasites in chimpanzees, and it’s a diversity that completely encompasses a much more limited diversity in human malaria.”
“There’s only one way to interpret that finding,” Wolfe said. “Namely, that this is a chimpanzee parasite that had jumped over to human populations.”
Malaria kills more than one million people each year and infects at least 500 million around the world. But until now, scientists had only a cloudy understanding of the origin of the deadliest human malaria parasite, called Plasmodium falciparum. Researchers had identified a single strain of a similar parasite in chimpanzees, but most scientists assumed that the two bugs had evolved from a common ancestor before humans split off from chimpanzees five to seven million years ago.
With just one strain of chimp malaria, however, it was impossible to know for sure. Scientists can estimate how long a bacteria or parasite has been around based on the genetic diversity between its various strains: The longer a bug has existed, the more time it has had to mutate and accumulate small differences in its genes.
After uncovering eight new types of malaria parasites in chimpanzees, Wolfe and colleagues could compare genes across different strains of chimp malaria for the first time. What they discovered came as a surprise: Unlike P. falciparum, which is very similar across all strains, there’s a huge diversity among the chimpanzee parasites. That means chimp malaria has likely been around a lot longer, Wolfe said.
“The human parasites are sitting on a very, very narrow branch in the midst of a very complex and diverse tree of chimpanzee parasites,” he said. The pattern of diversity suggests that human malaria split off from a particular strain of chimpanzee malaria at some point in our relatively recent past.
“There’s no other possible explanation for this finding,” said molecular biologist Ajit Varki of the University of California, San Diego, who specializes in the origins of infectious disease but was not involved in the research. “Looking at it from the evolutionary perspective, there’s really no way that it could have happened in any other way.”
The researchers think chimpanzee malaria was probably carried to humans by mosquitoes. And although the main transmission event happened only once, Wolfe thinks that in some remote areas, there could be an ongoing exchange of parasites between humans and chimps.
“One of the problems with malaria diagnosis is that it’s very much based on symptoms,” said Wolfe, who has himself been infected with malaria three times. “Only a few cases are diagnosed by blood studies under the microscope, and very few people in the world could identify the difference between chimp and human parasites.”
In other words, if chimpanzee malaria is circulating in some remote parts of Africa, we might never know it. In future studies, Wolfe and his colleagues plan to search for this kind of ongoing transmission, as well as try to better understand how malaria affects the chimpanzees themselves.
“We simply don’t know much about the additional health consequences of malaria in chimpanzees,” Wolfe said. “Scientists are biased toward thinking that these things don’t cause disease in species that they’ve been in for a long time, but if we find that it causes disease, it won’t be too surprising.”
Knowing the origin of malaria may have practical significance for people, too. Scientists have been searching for a malaria vaccine for a long time, Varki said, but so far they’ve failed. If chimp malaria doesn’t cause a serious infection in humans, but generates an immune response, he thinks it might be a good target for a vaccine.
Wolfe says there’s a historical lesson to be learned from malaria as well. Media coverage of bird flu, SARS, and swine flu may come and go, but animal diseases that jump over into humans don’t always die out quickly. “Just because these things are spilling over now does not mean they will disappear in five to 10 years,” he said. “They may have the potential to be around thousands of years from now.”
“It just emphasizes the importance of what we do, catching pandemics early and stopping them from spreading,” said Wolfe, who founded the Global Viral Forecasting Initiative in 2008 to try to halt pandemics before they start. “Once something spreads around the world and takes hold, it becomes a different story.”
Images: Chimpanzees in the Mfou National Park in Cameroon. Image 1: Nathan Wolfe, GVFI. Image 2: Matthew LeBreton, GVFI.