Malaria parasite in mosquito midgut
Malaria parasite in mosquito midgut

We often think of mosquitoes as willing co-conspirators in spreading human1 pathogens. But of course, in most cases the mosquito would be just as happy to get rid of the pathogen themselves; even if it doesn’t cause as severe as disease in the mosquito as in humans, it’s not doing them any good.2

So why don’t the mosquitos get rid of these pathogens, rather than carrying them around to infect yet more vertebrates? We know that insects have a fairly elaborate immune system, albeit one that’s quite different from ours.

The answer seems to be (at least partially) that — just as with pathogens of vertebrates — the mosquito pathogens have evolved ways of evading the immune response, so that the mosquitos can’t eliminate them.

Our findings provide support for the hypothesis that mosquito-borne pathogens have evolved to evade innate immune responses in three vector mosquito species of major medical importance.

Bartholomay, L., Waterhouse, R., Mayhew, G., Campbell, C., Michel, K., Zou, Z., Ramirez, J., Das, S., Alvarez, K., Arensburger, P., Bryant, B., Chapman, S., Dong, Y., Erickson, S., Karunaratne, S., Kokoza, V., Kodira, C., Pignatelli, P., Shin, S., Vanlandingham, D., Atkinson, P., Birren, B., Christophides, G., Clem, R., Hemingway, J., Higgs, S., Megy, K., Ranson, H., Zdobnov, E., Raikhel, A., Christensen, B., Dimopoulos, G., & Muskavitch, M. (2010). Pathogenomics of Culex quinquefasciatus and Meta-Analysis of Infection Responses to Diverse Pathogens Science, 330 (6000), 88-90 DOI: 10.1126/science.1193162

  1. And other animal[]
  2. Are there any agents that are pathogenic to vertebrates, and mutualistic in their arthropod host? Not that I know of – there are lots of amazing mutualistic agents of arthropods [example here in spiders: “More Symbionts and Flight“], but as far as I know they tend to be highly specialized to one host.[]