A controversial theory that the AIDS epidemic was accidentally started by Western health workers in Africa has been seriously undermined by a new study of the origins of HIV. The results show that viruses such as HIV can jump between closely related primates that eat each other, indicating that African markets where ape meat is sold could be a source of new diseases.
Tracking down the evolutionary history of HIV has been a thorny issue. In 1999 researchers showed that the nearest ancestor to HIV-1, the most widespread of the AIDS viruses, is a virus called SIVcpz, which appears to produce no symptoms in African chimpanzees. A controversial theory holds that polio vaccines administered in Africa in the 1950s were accidentally tainted with SIV viruses which then mutated in humans to become HIV-1. Many scientists have criticized the idea as implausible, but there was little evidence for alternative transmission routes.
Reporting in the 13 June issue of Science, a team led by Paul Sharp, an evolutionary biologist at the University of Nottingham, U.K., argues that chimps became infected by the HIV-1 precursor virus by eating monkeys. A comparison of the amino acid sequences of four proteins from SIVcpz with those of the other SIV strains reveals that SIVcpz was created by the merging of two strains that infect monkeys.
Like humans, chimpanzees are omnivores that hunt other animals when they can. The team argues that chimpanzees could have become infected by eating freshly killed monkeys, allowing the two viruses to mix their DNA. It is not clear whether SIVcpz mutated into something closer to HIV-1 before or after it was passed to humans. But what is clearer than ever, says Sharp, is that these viruses can jump the species line.
"I'm totally convinced," says Eddie Holmes, an evolutionary biologist at the University of Oxford, U.K. "It shows that butchering animals really is a viable mechanism of viral transmission." The study also obviates the theory that HIV was introduced through polio vaccination, Holmes says, as chimpanzee meat sold in Africa is commonly infected with SIVs.