By hunting and handling fresh primate meat, thousands of rural Africans might be infected with a virus in the same general category as HIV, according to a new study by researchers at Johns Hopkins University and the U.S. Centers for Disease Control and Prevention.
The study is the first to document virus transmission from primates to humans in a natural setting.
"This is the first real-world evidence that these viruses cross species boundaries. And this appears to be something that is happening regularly," said the study's lead author, Nathan Wolfe, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.
The virus appears harmless to humans. But scientists say the finding highlights the continuing danger from cross-species transmission, particularly for those involved in the African practice of hunting monkeys, chimpanzees and other primates for food and trade.
Other researchers praised the study, which was published in today's issue of The Lancet, Britain's leading medical journal.
"The data they present are certainly convincing," said microbiologist Beatrice Hahn, a professor at the University of Alabama, Birmingham and a leading expert on simian viruses.
The scientists studied 1,099 rural Cameroonians who reported direct exposure to fresh primate blood and body fluids, mainly through hunting and butchering. Of that group, 10 people, or 1 percent, had antibodies to simian foamy virus (SFV).
First identified three decades ago, SFV exists naturally in every primate species but humans. It is distantly related to simian immunodeficiency virus (SIV), the original source of human HIV.
Both SFV and SIV are retroviruses, which replicate by inserting their genes into host cells. Although it doesn't seem to harm its natural primate hosts, SFV has a lethal effect on cells in the lab. In an artificial environment, it causes cells to foam up and die - hence the name.
The 1 percent infection rate found in the study could mean that in parts of Africa, humans might contract SFV relatively often. Wolfe estimates that bushmeat hunting and handling might expose as many as 1 million Central Africans to primate blood and bodily fluids. If the study's results are extrapolated to this larger group, 10,000 people could be infected with the virus.
Wolfe and others emphasized that these figures are vague guesses, and said more research is needed to pinpoint the actual numbers of human SFV infections.
Scientists have known since the mid-1990s that SFV could jump from primates to humans. The virus was found in about a dozen North American zoo workers and primate researchers, who had likely contracted it through close contact with primates.
These humans, some of whom have been infected for more than 20 years, show no signs of any illness related to the virus, said CDC virus researcher Walid Heneine. And unlike HIV, the primate virus does not seem to spread through human contact.
But Heinine, a co-author of the Lancet report, said the American sample wasn't large enough to definitively rule out a threat to humans. "We need more research to find out if SFV is harmful or transmissible," he said. Heinine and Wolfe said they hope to examine health among African populations at high risk for SFV infection.
Even if SFV doesn't harm humans, researchers said, the study raises concern about the overall level of primate-human viral transmissions in sub-Saharan Africa.
"SFVs are a very good indicator of how much virus traffic there is across species," Heinine said.
In recent years, scientists have become increasingly worried about viruses that jump species. Once in the new species, these bugs can sometimes mutate into more lethal and infectious forms.
As humans move into formerly uninhabited areas, they come into contact with creatures that might harbor such bugs; with its booming bushmeat trade, Africa is a key area of concern.
"There are tens of millions of tons of wildlife being taken out of the forest in Central and West Africa every year," said Michael Hutchings, co-chairman of the Bushmeat Crisis Task Force, a nonprofit group dedicated to stopping the trade.
The more cross-species transmission occurs, the greater the chance that one of these viruses will mutate into a human killer such as AIDS. "It's just a probability game," said Wolfe.