91av

HIV may have jumped from chimps to people as early as 1910, says a team of
researchers who have modelled the virus’s history on a supercomputer. The result
has sparked new debate about whether or not HIV was transmitted from chimps to
people during polio vaccination programmes.

The virus seems to have originated when the primate version, SIV, was
transmitted to humans in Africa. People may have become infected when they
butchered chimpanzees and monkeys for food.

But in The River, a book published last year, journalist Edward
Hooper revived a theory that doctors had inadvertently sparked off the AIDS
epidemic with contaminated polio vaccines
(91av, 13 November 1999, p 54).
He gave evidence that the kidneys of chimps that lived near
Kisangani in Zaire were used to produce a vaccine called CHAT. Between 1957 and
1960, the vaccine was given to more than a million Africans in the Democratic
Republic of Congo, Rwanda and Burundi, the countries in which AIDS first
appeared.

But this now seems unlikely, according to Bette Korber of Los Alamos National
Laboratory in New Mexico. Korber’s work suggests HIV may have infected people
decades earlier. “Our work certainly doesn’t disprove his hypothesis,” she says.
“But in my opinion it makes it less likely.”

Korber’s team studied HIV-1 group M, the group responsible for the vast
majority of AIDS cases. Previous work showed 10 of the 11 subtypes of group M
diverged from an original virus all at once. Such “starburst” radiation is
thought to happen when a viral population suddenly increases, for example when
the virus enters a new host.

To find out when HIV diverged like this, Korber’s team looked at DNA
sequences of more than 160 HIV-1 group M viruses and came up with 100 possible
evolutionary trees describing how the viruses are related. They then winnowed
the trees down to a handful that were statistically most probable.

These suggested the viruses diverged from a common ancestor between 1910 and
1930, with a margin of error no later than 1950, Korber told a conference on
retroviruses in San Francisco last week. The starburst could have happened in
chimps before the transfer to humans. But Korber’s colleague Beatrice Hahn of
the University of Alabama in Birmingham says this is implausible because it
would imply that viruses that had diverged from the original virus to exactly
the same degree had independently jumped from primates to people at least 10
times. “You would have to have at least 10 transmissions of viruses that were
all evolutionarily equidistant to each other.”

“That is exactly what I believe may have happened,” counters Hooper. He says
the virus may have already diversified in the descendants of a single infected
chimp. When they were killed for vaccine testing, some of their kidney cells
were used for producing CHAT that was sent to central African distribution
sites. These strains, all equally evolved, would have jumped to humans all at
once and continued to evolve.

James Moore, an expert on AIDS at the University of California, San Diego,
says that while he is sceptical of the vaccine theory, the new result doesn’t
rule it out. He thinks that looking for SIV in local primates could even prove
it’s true. “If it turns out the chimps near Kisangani are harbouring something
like group M, you can almost put the polio hypothesis in the bank.”

Scientists are collecting chimp faeces from the region to look at the strains
of SIV they contain. Meanwhile, six phials of the original polio vaccine have
been recovered. They have been sent to labs that will screen them for SIV and
HIV.