Anyone who thinks that HIV originated by a recent recombination between
visna virus and bovine immunodeficiency virus (that's BIV, not BLV)
hasn't looked at the sequences. Sure, these are all lentiviruses, and as
such share some gene structure and function, but visna is very far from
HIV in terms of sequence similarity. BIV is somewhat closer and antibodies
to BIV CA react with HIV CA antigens, but there can't be more that 35 %
homology on the nucleic acid level.
I'll bet that HIV has been around for centuries (at least) and shared
roots most recently with the simian lentiviruses. It seems quite
possible that HIV existed in small pockets of tribal Africa and, due to
the breakdown of tribal structures and migration of people into large cities,
only recently has been recognized/associated with disease. It is also likely that the virus has become more pathogenic in recent decades, as the virus
that could kill off a whole isolated population would have little chance of
surviving over a long period. However, when most hosts are available, it
is advantagious to the propagation of the virus to become more virulent.
These theories will be very difficult to prove, but most people that I have
talked to about HIV origins think that the virus has been around humans for
a long time.
Check out some of the retrospective and current data generated about two years
during investigations into the correlation between SIV-infected (potentially)
simian cells used to raise the Sabin polio vaccine and the rise of HIV. It is
my recollection that examination of low-temperature storage stocks uncovered
no SIV sequences. Any others know about this?
Greg Tobin, Ph.D. tobin at lcms-1.ncifcrf.gov
PO Box B
Frederick, MD 21702