The scientists who make this assertion say the so-called simian foamy virus (SFV) appears to be harmless to animals and humans and cannot be transmitted from person to person.

But, they say, very little is known about SFVs.

It cannot be ruled out, they warn, that the viruses might be pathogenic in humans or become that way, and could already moved out of central Africa -- a path notoriously taken by the human immunodeficiency virus (HIV).

"Contrary to conventional wisdom, retroviral zoonosis [animal-borne disease] is widespread, arising in various locations where people are naturally exposed to non-human primates," their study, published on Saturday in The Lancet, suggests.

"Although we cannot estimate the total number of such infections, widespread contact with such primates throughout rural forested regions of central Africa suggests that many such infections probably exist."

In their study, published on Saturday's issue of the Lancet, the team carried out the widest survey on the ground on bushmeat viruses.

They interviewed and took blood samples from 1,800 people living in nine villages in a lowland forest in southern Cameroon.

Sixty-one percent of these reported they had had direct exposure to fresh meat from chimpanzees, gorillas or monkeys.

Ten, or one percent, tested positive for SFV antibodies, a sign that they had been exposed to the virus.

Virus isolated from three of the 10 and put through a genetic ID scan showed that the strains had distantly originated in three distinct species -- the gorilla, the mandrill and De Brazza's guenon.

Until now SFVs were only believed to have been transmitted to humans via animals in zoos and laboratories, where it is considered relatively infectious compared with other primate viruses.

A French-based expert on bushmeat viruses, Martine Peeters, said the risk was that so-called foamy viruses might behave like their cousin, HIV, and another retrovirus called the human T-lymphotropic virus, another agent that came from simians.

If different viral strains or retroviruses are together in the same host, there is the risk that they will swap genes, eventually emerging into a harmful form.

"Human infection with simian retroviruses, which have a long incubation period, might spread unrecognised for several years and lead to another disease epidemic," Peeters, of the Institute for Development Research (IRD) in Montpellier, said.

The cross-species jump from primates to humans is the widely accepted cause of the global AIDS pandemic, although no-one is making any suggestion that SFVs -- if in fact they ever do mutate into something harmful -- would be a pernicious as HIV.

The HIV is believed to have leapt the species barrier to humans in central Africa in the first part of the 20th century.

The source was the simian immunodeficiency virus (SIV) which leapt from animal primates to humans. There, it not only survived in its new host but found a way of evolving by hijacking and replicating in human immune cells.

After years of being geographically holed up among villagers in tropical forests, HIV leapt into the wider world, developing into a global pandemic in the 1980s thanks to jet travel and sexual contact.

The Lancet study is led by Nathan Wolfe of Bloomberg School of Public Health at Johns Hopkins University in Baltimore, Maryland.

Bushmeat has been a traditional source of food for people living in forests in central and western Africa. The risk of infection does not come from eating the meat but mainly from handling fresh meat, for the virus can be transmitted through tiny cuts in the skin.

TERRA.WIRE