The parasite Plasmodium vivax isn’t as well known as its deadly cousin P. falciparum, which dominates sub-Saharan Africa. But the “other malaria,” which is rare in Africa, sickens some 75 million people a year in Asia and the Americas. Now, new genetic evidence shows how the parasite might have gotten its start: in African ape and human populations, before hitching a ride off the continent with early human migrants.
Until recently, scientists assumed P. vivax had originated in Asian macaques and jumped to humans there, before spreading to Europe and the Americas. But in 2010, scientists started to find evidence of P. vivax in African chimpanzees, gorillas, and bonobos. That suggested an African origin for the parasite. However, there was only sparse genetic evidence to support the theory; most data from ape parasites came from just a few incomplete sequences recovered from primate feces.
Now, researchers have managed to sequence nearly the entire genomes of parasites that infected six chimpanzees and one gorilla. Blood samples for the chimpanzees came from sanctuaries in Cameroon and Gabon and from a wild chimp in the Ivory Coast. The gorilla sample came from a piece of bushmeat collected in Cameroon.
This new, closer look at the parasite’s genes shows that the ape parasites are vastly more diverse than those that infect humans, scientists report this week in the Proceedings of the National Academy of Sciences. That adds weight to the idea that P. vivax once infected both apes and humans in Africa and tagged along with migrating humans to Eurasia and the Americas, says David Conway, a malaria expert at the London School of Hygiene & Tropical Medicine who wasn’t involved in the work.
The early spread of modern humans into different parts of the world “was probably accompanied by only a few founding strains of the parasite that have given rise to most of today’s human P. vivax,” Conway says. Richard Culleton, a malaria expert at Nagasaki University in Japan, agrees. The new data strongly suggest modern human P. vivax “escaped” from Africa some time before the human population there became immune, he says. Today, P. vivax infection is rare in Africa because most people there lack the protein that the parasite uses to enter red blood cells.
Further supporting that argument, the parasites in African apes and humans elsewhere appear to be closely related, says Paul Sharp, an infectious disease geneticist at The University of Edinburgh who led the study together with Beatrice Hahn, an infectious disease expert at the University of Pennsylvania Perelman School of Medicine. “We were looking for evidence that the ape parasites were in some way clearly different,” he says. “Have they diverged to the point where they are separate species? But we couldn’t find any indications that they are separate.”
That is consistent with occasional reports of visitors to Africa coming home infected with P. vivax, probably after being bitten by a mosquito that had bitten an infected ape, Sharp says. That means even if P. vivax were eliminated from Asia and the Americas, it could hitch another ride out of Africa anytime and start a new outbreak elsewhere in the world. “This might mean that we can never eradicate P. vivax malaria—unless we manage to get rid of it in the chimp and gorilla populations too,” Culleton says.