Most Infectious Malaria Strain Came From Chimps?

Amitabh Avasthi, National Geographic News 3 Aug 09;

The most malignant known form of malaria may have jumped from chimpanzees to humans, according to a new study of one of the most deadly diseases in the world.

Malaria, a mosquito-borne illness, can be caused in humans by one of four strains of the Plasmodium parasite. More than a million people die from malaria each year.

P. falciparum is the most virulent of these strains and accounts for nearly 85 percent of all malaria infections. (See a malaria parasite picture.)

Three of the four human strains are known to have originally come from Old World monkeys. The exact origins of P. falciparum have been a mystery.

Researchers had thought that P. falciparum and P. reichenowi—the malaria strain found in chimpanzees—evolved independently from a common ancestor about five to seven million years ago.

But the new study has found that the human strain is actually a mutated form of the chimp strain.

"Current wisdom that P. falciparum has been in humans for millions and millions of years is wrong," said study co-author Nathan Wolfe, director of the Global Viral Forecasting Initiative based in San Francisco, California, and a National Geographic emerging explorer. (The National Geographic Society owns National Geographic News.)

"We now know that there was a point in time when this was primarily a disease in chimpanzees that jumped and took hold in humans."

Just One Jump

Wolfe and his colleagues analyzed tissues samples from 94 live wild and wild-born captive chimpanzees in Cameroon and Côte d'Ivoire (Ivory Coast). Eight of the animals were found to have malaria.

The team discovered that the human and chimpanzee strains have certain genetic similarities, but that the chimp strain is more genetically diverse.

Further analysis placed all 133 variants of P. falciparum found around the world under a single branch of the P. reichenowi family tree.

"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.

"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."