Tom,
The example of SIV changing it's route of transmission is wrong. As far
as we know, SIV has always been transmitted naturally by sex. For HIV
the same holds true. Nothing about the basic biology has changed. The
addition of needles is not natural, so can't be used as an example either.
Many factors are involved in determining whether a virus can replicate in
a given type of cell. Since mosquito cells don't express the receptors
required for HIV entry, the virus would need not merely a variation in
it's spike proteins, but a completely different spike. I guess such a
thing is possible, and maybe I'll run a sub-4 minute mile before I'm 50,
but I doubt it.
When SIV jumped ship from monkeys to humans (if indeed you believe that
SIV is the ancestor of HIV), it didn't really need to change too much.
The cells of both species are similar in many respects, including
receptors, and the virus probably needed relatively few changes to
initially establish infection in humans.
Think about it...no retroviruses that we know of are transmitted by
arthropod vectors. Neither are influenza, herpes, or hepatitis. Why not?
Jay Mone'
