Antibody response of wild birds to natural infection with alphaviruses

From 1986 to 1990, we conducted our second longitudinal study in the central (upstate) New York (CNY) area on the wild avian hosts of eastern equine encephalomyelitis (EEE) virus. Field-collecting methods mirrored a study conducted from 1978 to 1980 at the same endemic focus. Over the 5-yr study period, we captured 6,296 birds representing 99 species and took 4,174 blood samples from representatives of 83 species. Gray catbirds, song sparrows, and veerys were the three dominant species captured and bled, accounting for 40 and 55% of birds captured and bled. Blood clots were assayed for virus and sera tested for hemagglutination-inhibition (HI) antibodies to EEE and Highlands J virus. Virus isolations from birds defined two epiornitics of EEE virus in 1988 and 1990, and an epiornitic of HJ virus in 1986. Infected birds responded with the production of HI antibodies with titers indicative of recent infection (HI > or = 1:160), and titers of sera positive during the epiornitics were significantly higher than positive sera during nonepiornitics. The 1990 EEE epiornitic extended from mid-July to the end of September, providing data to compare infection rates among species, habitats, and combinations of species with habitats. Few significant differences were found. The HJ epiornitic was only the second time this virus has occurred in CNY. Song sparrows were identified as the primary amplifying avian host of both viruses, although our capture and serological data would suggest a role for gray catbirds as the species most likely involved in yearly virus reintroduction. However, the cryptic nature of enzootic virus maintenance remains unresolved for the CNY virus foci. The appearances of HJ and EEE viruses were not epidemiologically linked, and there were no virus isolations from adults returning on site or virus isolations without concurrent isolations from mosquito vectors. Whether EEE and/or HJ virus are consistently present in or sporadically introduced into the inland foci of CNY area still has not been determined.