Effects of house sparrow age and arbovirus infection on attraction of mosquitoes.

To test the hypothesis that arbovirus infection or advancing age increases the attractiveness of avian hosts to vector mosquitoes, we used an olfactometer that measures the response of mosquitoes to the passive, vertical diffusion of host-related cues. St. Louis encephalitis or western equine encephalomyelitis virus infection in house sparrows (Passer domesticus (L.], regardless of age, had no detectable effect on Culex quinquefasciatus Say or Cx. tarsalis Coquillett attraction, respectively, at close range. Sparrow age, however, was associated positively with increased Cx. quinquefasciatus attraction; i.e., nestlings were significantly less attractive than adults, and there was a trend for increased mosquito attraction as nestling age increased. When birds were examined in groups, attraction was not equal to the sum of the individual birds in the group. Nestlings, either alone or in a group, were less attractive than a single adult. One adult plus three older nestlings were more attractive than one adult with three young nestlings, but no more attractive than one adult alone. We speculate on the epidemiological significance of these laboratory results to arbovirus epizootic transmission.