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HEPATITIS B, the virus carried by 5 per cent of the world鈥檚 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
鈥済enuine mysteries鈥� 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. 鈥淚t may
be that initially the virus mutated rapidly, but beyond a certain point further
mutation was not viable,鈥� 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鈥攑ossibly monkeys鈥攚hich passed these strains to people. 鈥淚t
might be that we haven鈥檛 looked hard enough,鈥� he says.