SUMMARY

It may be that alphaviruses, including Eastern Equine Encephalitis (EEE) and Highlands J viruses, amplify around congregations of avian reservoir hosts in upland sites rather than in swamp sites where the vector mosquitoes, Culiseta melanura, breed. To test this hypothesis, we estimated the densities of birds and Cs. melanura in swamp and upland habitats, and the alphavirus infection rates in these mosquitoes, in a 2000 hectare (ha) region in Kingston, Plymouth County, MA.

We found that the densities of nocturnally-resident birds, determined for a period from July through September (1996) varied from about 5 to 22 individuals per ha in all swamp and upland habitats studied. The mean density of communally roosting robins, however, was about 440 per ha, and communally roosting blackbirds (grackles and starlings) about 4,500 per ha. Communal roosting behavior was greatest in August, coinciding with the season of maximum density of Cs. melanura. Fiber pot collections of diurnally-resting female Cs. melanura were more numerous in upland sites near avian communal roost sites than at the perimeter of the swamp where these mosquitoes breed. Engorge mosquitoes dominated collections at the upland roostsites, whereas teneral and gravid mosquitoes dominated the swamp collections. The minimum infection rate of HJ virus was four times greater in the upland-collected mosquitoes than in those collected at the swamp perimeter (P=0.001, Fisher's Exact Test). No EEE virus was detected in 6,417 female Cs. melanura tested by chick embryo tissue culture plaque assay. Testing was conducted by the Encephalitis Surveillance Program of the Massachusetts Department of Public Health.

Cs. melanura collected by CDC light trap at the same swamp and the same upland communal bird roost site during September (1993 and 1994) were similarly tested for alphavirus infection. The minimum infection rate for HJ virus in 41 upland mosquitoes was eight times greater than that for 476 swamp mosquitoes (P=0.05, Fisher's Exact Test).

We conclude that Cs. melanura commonly disperse from swamp breeding sites and feed at sites where communally roosting passerine birds, such as starlings, grackles and robins, congregate. These regions of great nocturnal density of birds may be the venue for enzootic amplification of alphaviruses.

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