Most Infectious Malaria Strain Came From Chimps?

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"This suggests that there has been only one transfer. P. falciparum is the result of a single cross-species transmission event," said Wolfe, whose work appears this week in the Proceedings of the National Academy of Sciences.

The scientists aren't sure how the chimp strain mutated to become infectious to humans or when it might have made the jump, although the oldest known human cases of malaria date back thousands of years.

The team believes it's possible that, as early humans settled into an agrarian lifestyle in Africa, their likely encroachment into chimpanzee territory provided the parasite with new opportunities.

Parasite Evolution

Biologist Dan Hartl of Harvard University noted that, until now, only one sample of the chimpanzee parasite had ever been studied.

"Researchers had believed that P. falciparum probably originated from parasites in birds," said Hartl, who was not involved in the study.

"[Wolfe and colleagues'] paper proves that is not true, and that data from those early studies were misleading."

Late last year, a team led by Arnab Pain, a malaria researcher at the Wellcome Trust Sanger Institute in Cambridge, U.K., announced they had sequenced the genome of P. knowlesi, a monkey malaria parasite that can also infect humans.

Pain agrees that Wolfe and his colleagues have conclusively proven that the chimpanzee malaria parasite was transferred only once to humans.

But researchers would need to know the entire genetic makeup of the chimp parasite to find out what changes it underwent, he added.

"How much the parasite changed, we don't know the full story yet."

Ongoing Leaps

According to study co-author Wolfe, the new work hints that even today similar disease-causing parasites may be ready to make the leap from monkeys and apes to humans.

(Related: "Chimps Do Get 'AIDS,' Study Finds.")

"Our study suggests that there is a [version of P. reichenowi] out there that is very similar to P. falciparum—it has a tremendous amount of genetic diversity, and is present in animals that are close to humans in a very geographically distributed area," Wolfe said.

That means there's a chance that a new malaria parasite might make its way into people, he said.

The work also highlights how long a disease can last once a parasite has made that leap, Wolfe added.

"What this finding demonstrates is that the kinds of jumps we're having right now—HIV, SARS, etc.—could very well be the beginning of something that lasts for thousands of years."

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