HEPATITIS B, the virus carried by 5 per cent of the world's population, probably spread to people from apes or monkeys, British scientists suggest.
The researchers tested three wild chimpanzees found orphaned in Cameroon for the presence of hepatitis B (HBV), and found all three were positive. DNA tests on the virus matched a strain present in a chimp living at London Zoo. The researchers say that this, along with evidence of natural infections of a hepatitis-like virus in orang-utans published last year, provides powerful evidence of naturally occurring animal epidemics.
The finding also appears to kill off other theories about the origins of the HBV pandemic, which causes a million liver-related deaths a year. One idea was that HBV spread into the Old World from the Americas as recently as 400 years ago, following contact between native Americans and Europeans. But this suggestion has been dispelled by the discovery that wild chimpanzees, orang-utans and possibly even gibbons in Asia and Africa are infected with the virus. For the same reason, the notion that modern humans carried HBV out of Africa over 100 000 years ago now seems implausible.
The best bet is that various HBV strains co-evolved with their primate hosts 10 to 35 million years ago, say the British researchers. The different strains of the virus seen in humans would have occurred through various animal-to-human crossover infections. And they point to the AIDS pandemic as a precedent.
However, team member Peter Simmonds of the University of Edinburgh admits "genuine mysteries" remain. None of the six major human strains of HBV differs genetically from any of the others by more than 11 per cent. But, given that HBV should be able to mutate quickly, like HIV, the genetic variation should be much greater if the virus did pass to humans tens of thousands of years ago. "It may be that initially the virus mutated rapidly, but beyond a certain point further mutation was not viable," he suggests.
As yet, the remaining HBVs that infect people have not been identified in apes or other animals. It could be that the sources of these infections have yet to be discovered. According to Simmonds, the prevalence of particular HBV strains in South-East Asia suggests there might be animals in this part of the world-possibly monkeys-which passed these strains to people. "It might be that we haven't looked hard enough," he says.
Michael Day
Source: Journal of Virology (vol 74, p 4253 and vol 73, p 7860)
New Scientist issue: 6th MAY 2000
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