Malaria’s deadly leap from chimps to humans

One mosquito; one hot-blooded human target; one quick puncture of skin. Most likely, our distant ancestor reacted with no more than a scratch and a shrug.

Thus did malaria leap across the “species divide’’ between chimpanzees and humans, according to new research led by a University of Massachusetts at Amherst scientist.

Additionally, the research suggests the transmission occurred much more recently than many epidemiologists had believed - perhaps as primitive farmers started encroaching on chimp territory by felling trees and digging drainage ditches.

That fateful mosquito bite set the stage for millennia of death and suffering by unloosing an affliction that still defies modern medicine. Plasmodium falciparum, the most lethal of the malaria parasites, kills more than 1 million people a year and infects hundreds of millions more.

“In essence, we’ve pinpointed [human] malaria’s Eve,’’ said Stephen M. Rich, the UMass evolutionary geneticist. “Mother of so much misery.’’

Using genetic tools, Rich and an international team of scientists backtracked human malaria to discover its origin, finding that the most virulent strain of human malaria is genetically similar to chimp malaria, known as P. reichenowi.

The research contradicts the long-prevailing view that P. falciparum - the parasite responsible for 85 percent of human malaria infections and nearly all of the deaths - co-evolved with humans as our species split off from apelike predecessors some 5 million years ago.

“Instead, we’ve shown P. falciparum jumped from chimpanzees to humans in relatively recent times,’’ Rich said. “A disease that was benign in chimps crossed over to humans, and became deadly.’’

The discovery, described in the journal Proceedings of the National Academy of Sciences, capped more than a decade of genetic sleuthing in which Rich and colleagues from the United States, Germany, and the African nations of Cameroon and Ivory Coast isolated eight new DNA sequences of chimp malaria. Blood extracted from 94 chimps in West and Central Africa was scrutinized to locate the parasites.

Painstaking analysis of the chimp malaria showed that it possessed a wide degree of genetic variability - meaning that it was an ancient parasite, since mutations accumulate with age. Human malaria shows less genetic variability, suggesting more recent genesis as a parasite adapted to attack human blood cells.

The leap may have occurred only 10,000 years ago, the researchers theorize, as Africa’s early farmers started clearing land and otherwise impinging on chimp forests. That put humans in closer proximity to infected apes, creating the ideal circumstance for a mosquito that had first dined on the blood of a chimp to then bite and infect a human with the chimp malaria parasite.

The rise of agriculture also created pools of water perfect for spawning disease-bearing insects, while formation of the first villages - all those warm-blooded beings clustered so conveniently in huts - provided a banquet for Anopheles mosquitoes, the noisome biter that carries the human parasite.

Scientists involved in the study stressed that the precise chronology is not clear - the transfer might have occurred hundreds of thousands of years earlier. But that’s still just a twitch in time compared with previous suppositions about how long the disease has stalked humans.

Meanwhile, the study warned: “Today, human encroachment into the last forest habitats has further extended, leading to a higher risk of new pathogens, including new malaria parasites.’’

The research may help scientists better understand how deadly infectious diseases switch from animal hosts to humans. That’s a hot topic these days, as swine flu burns through schools, hospitals, and workplaces around the world. The notorious human immunodeficiency virus, known as HIV, originated in African apes. And Lyme disease, lately plaguing New England, comes from deer-borne ticks.

“The origins of P. falciparum has been one of humanity’s outstanding medical mysteries,’’ said Nathan D. Wolfe, director of the San Francisco-based Global Viral Forecasting Initiative and one of the lead scientists involved in the DNA analysis of 94 chimpanzees from Cameroon and Ivory Coast.

“We’ve solved it,’’ he said. “The research shows that the transfer is the result of a single, cross-species transmission event from a chimp.’’

The startling claim that malaria’s deadly ravages through the ages stem from a single insect bite is based on the team’s discovery that every P. falciparum variant whose DNA has been isolated can be genetically linked to just one form of the chimp parasite.

The research has stirred considerable interest among malaria researchers, although not all are convinced that the riddle has been fully unraveled.

“There’s still room for skepticism,’’ said Daniel E. Neafsey, who studies the malaria genome at Cambridge’s Broad Institute, a genetics research center. He argued that the results may be complicated if it turns out - as unrelated research suggests - that chimps are infected by two malaria parasites, not a single species.

Nonetheless, Neafsey said of the new research, “this is the biggest sample ever compiled of chimpanzee parasites. It’s very important data. . . . Understanding the ways in which human diseases originate can help us better conduct surveillance for other [animal-borne] diseases in the early stages of making the leap.’’

The finding that malaria was directly transmitted from animals is in some ways grim news. It’s been killing humans for thousands of years, scary evidence that animal-to-human infections do not necessarily burn out in a few decades or even centuries.

However, identifying the chimp parasite as the origin of human malaria provides scientists with a new opportunity to devise cures for the disease. The chimp form of malaria is quite mild, and figuring out the genetics of that relative gentleness could point epidemiologists toward an “Achilles heel’’ in the human strain.

More tantalizingly, since chimp malaria is such close kin to the human parasite, researchers may figure how to forge it into preventative medicine that could wipe out malaria the way smallpox and polio have been eradicated.

“That’s the Holy Grail,’’ said Rich. “A malaria vaccine.’’

Colin Nickerson can be reached at Nickerson.colin@gmail.com.