September 23, 2010

Study: Human Malarial Parasite Came From Gorillas

The parasite that causes the most deadly strain of malaria in humans appears to have originally crossed the species barrier from gorillas, researchers reported on Wednesday.

The scientists analyzed DNA from the droppings of some 3,000 gorillas, chimpanzees and bonobos, and found that the strain of malaria parasite most common in humans is virtually identical to one of many strains that infect gorillas.

Beatrice Hahn of the University of Alabama at Birmingham and colleagues used the droppings to study the origins of the human immunodeficiency virus, and then used a similar approach to search for DNA from malaria parasites such as Plasmodium falciparum.

"We had them all nicely organized, genetically characterized in our freezer," Hahn said during an interview with Reuters.

Plasmodium falciparum, the most lethal of the five known strains of malaria parasites, causes several hundred million human infections each year, about one million of which are fatal.

"Wild apes, in particular the common chimps and the western gorillas, are naturally infected with at least eight or nine different Plasmodium species," Hahn said.

Although chimps had long been the primary suspects, Hahn's data found that only gorillas are infected by a Plasmodium species virtually identical on the genetic level to the type that infects humans.

"Now, how many mosquitoes were biting however many humans or gorillas I do not know," she said.

"But the end result is, based on sequence analysis of 105 human Plasmodium parasites, it looks like there was a single transmission."

This means the parasite only had to infect a single person or a small group of people before rapidly taking hold and spreading throughout the rest of the world.

According to the World Health Organization, malaria kills an estimated 800,000 people a year.  The parasite is spread when a mosquito feeds on an infected person and carries it to another.

Although there is no vaccine or cure for malaria, drugs can control the infection and help prevent its spread.

Hahn's study could ultimately have implications for efforts to contain malaria, said Dr. Larry Slutsker, head of the U.S. Centers for Disease Control and Prevention's malaria program.

"If we were trying to eradicate, meaning we were trying to rid the planet of every last parasite and there was a reservoir in western gorillas, that would have implications for eradication. I don't think we are there, obviously," he told Reuters.

Gorillas would likely have to be included in any such initiative so the parasite could not move into humans again.

It is possible that the parasite may not have spread from ape to human via a mosquito, Slutsker said.  The parasite can also be spread by direct blood transfer, the way many experts believe HIV first spread to humans.

The researchers have not yet identified when this may have happened.  HIV is a rapidly mutating virus, and scientists can use these changes as a "molecular clock" to date such alterations.  However, malaria parasites change much more slowly, wrote Edward Holmes of Pennsylvania State University in a commentary accompanying the study, which was published in the journal Nature.


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