Why are some species associacted with different diseases?
Not all species of mosquitoes are equally good at transmitting a particular disease. For example, with West Nile fever, we’re finding that some mosquitoes, after picking up the virus in a blood meal, are quite good at replicating the virus and disseminating it while others are not. The studies (like the ones by Mike Turrell et al at Fort Detrick) are interesting: simply put, they look for virus in the legs of the mosquito at some time period after the mosquito has taken a bloodmeal. If the virus is found in the legs, then it has made it’s way from the stomach and is likely in the salivary glands, ready for the next recipient (bird, human, horse, maybe other mammal). Mosquitoes like the Japanese Rock Pool Mosquito (Ochlerotatus japonicus) have demonstrated a great ability to spread WN despite the lack of historical involvement. (Positive pools of Oc. japonicus have recently popped up in the New World where WN is an emerging disease, but not in the Old World where WN is established. Why that is has been a cause for some ongoing arguments.)
But, even if a mosquito should show a high tendency to transmit virus, it still may not become involved in the disease cycle simply because the mosquito may not encounter those other hosts that carry the virus. For example, if the virus was carried by treetop dwelling Blackburnian Warblers (Dendroica fusca) and the mosquito in question never host-seeks above the 1 meter mark, then it is unlikely to pick up the virus in the first place. If the virus is carried by winter resident birds and the mosquito overwinters as larvae (adults die after laying eggs) before the birds arrive, again a chance for interaction in thwarted.
So, I think part of the answer might be that mosquitoes differ not only in their ability to transmit disease, but also in their opportunity to do so. Why do mosquitoes differ in their ability to transmit virus? I dunno. Interesting question.
I don’t know the answer, but I’d WAG that the biochemistry and anatomy of a specific mosquito species. Some insects may have biochemical or immunological features that reduce or eliminate the development or spread of some parasites and diseases (after the exposure and transmission tendencies that brachyrhynchos mentioned have come into play), but not others.
There may also be anatomical features of the feeding appendages that tend to promote transmission in some species of mosquito and inhibit it in others.
Anyone know this subject in detail?