LOS ANGELES, Sept. 22 (Xinhua) -- U.S. researchers have found that most malaria infections in humans might be traced to one infected gorilla.
Researchers at the University of Alabama, Birmingham based their finding on a genetic analysis of the parasites that cause malaria found in primate feces to create an evolutionary family tree for the disease.
The parasite most closely related to human malaria came from gorillas, and may have made the leap from animal to human host in a single mosquito bite, according to the study published in the September issue of the journal Nature.
Five types of malaria parasites infect humans, but the most common (and most deadly worldwide) is Plasmodium falciparum. Like all Plasmodium parasites, P. falciparum, as it is called, is carried from host to host by mosquitoes. When the mosquito bites someone, the parasite infects the person's red blood cells, causing fatigue, fever and vomiting. Untreated, the infection is fatal.
For many years, researchers thought the closest relative of P. falciparum was a similar Plasmodium infection found in chimpanzees. This led to the theory that the malaria parasite originated in the common ancestor of chimpanzees and humans, and evolved into two species at the same time as chimps and humans did, about five million to seven million years ago.
But this theory became controversial after more strains of Plasmodium were found in gorillas, chimps and bonobos (part of the same genus as chimpanzees), creating confusion over the parasite's evolutionary history.
To investigate the connections between parasite species, Beatrice Hahn of the University of Alabama, Birmingham and colleagues isolated malaria parasites from primate feces specimens the researchers had collected for their work on the origins of HIV. More than 1,000 chimpanzee samples, 805 gorilla samples and 107 bonobo samples were tested, making this the largest study of its kind.
Based on the testing, the researchers estimate that between 32 percent and 48 percent of wild chimpanzees and western gorillas are infected with malaria parasites. Samples from eastern gorillas and bonobos did not show any infection.
A genetic analysis showed that none of the chimpanzee malaria parasites were closely related to human P. falciparum. However, one subtype of gorilla parasite was nearly identical to the human strain. The genetic lineage of the parasite suggests that it evolved after making a single jump from gorilla to human.
"When you take all the sequences that have been published for all human Plasmodium falciparum worldwide, and when you put it in this family tree analysis, you see that they all have one single common ancestor," Hahn said in remarks published by LiveScience on Wednesday. "That tells you that this was the result of a single cross-species transmission event."
Researchers can't say exactly when the parasite first infected humans, because little is known about the rate of evolution for P. falciparum. However, the leap probably occurred between 5,000 and 300,000 years ago, Hahn said.
An estimated 250 million people become infected with malaria each year and nearly a million die from it, according to the World Health Organization.