Mosquitoes, etc. as infection vectors?

[Giovanni Maga]

In article <3p38dl$2dl at portal.gmu.edu>, tvalesky at site.gmu.edu (Tom Valesky
(CS 555)) wrote:

> Question from a layman:
> 
> Malaria spreads through mosquitos. Sleeping sickness spreads through
> tsetse fly bites. Why haven't mosquitoes, biting flies, etc., become
> infection vectors for viral diseases like Ebola and AIDS?
> 

The ability of a virus to survive in an intermediate host depends on many
factors. Basically, if the virus/host relationship is satisfactory (i.e.
allows infection, replication and spreading of the virus) without the
intervention of an intermediate host (the so-called vector), thus there
will be no selection for viruses able to survive in the vectors.
Enviromental changes as well as fluctuation of the host population could in
principle create those conditions which favour the selection of a virus
able to propagate itself through a vector. The fact that HIV does not have
this route of transmission means that in the natural hystory of the virus
it was never an advantage to spread through vectors. Possibly, all the
putative vectors with whom HIV was in contact (e.g. blood-sucking
mosquitos) were not permissive for HIV survival. BTW, being HIV infected
people long-survivors to the infection, they are the best vectors for the
virus, since they have more than enough time to spread it to other people.
There are anyway many examples of viruses passed to men by animals (not
necessarly bugs). Ebola itself could be considered a zoonotic infection,
since the virus seems only occasionally infecting humans, which are not its
natural hosts. The existence of a vector (kind of unkown) for Ebola
transmission to humans in my hopinion is very likely.
Regards, G.Maga
maga at vetbio.unizh.ch