ECOLOGY AND EXPERIMENTAL EPIDEMIOLOGY OF WESTERN ENCEPHALITIS

Garter snakes Thamnophis spp are capable of overwintering the Western encephalitis WE virus and possesses enough virus in the blood to infect a Culex tarsalis mosquito in the spring when these snakes come out of hibernation. These mosquitoes C. tarsalis are readily infected by blood meals from infected snakes and infected C. tarsalis mosquitoes readily infect snakes when blood meals are taken. Snakes may exhibit a cyclic viremia, that is positive, then negative, then positive virus in the blood stream. In studies of about 1800 lip tagged snakes, foci of virus may be present in one area, but absent in other areas. Garter snakes wander less than 100 yards from their native area. Snakes have been found naturally infected in nature with WE virus. Climatological conditions determine when mosquitoes will bite snakes in nature, and temperatures between 50 and 65F appear to be optimum. These temperatures are part of the spring and late summer or early fall weather conditions. Mosquitoes do not bite snakes during the hot months of the year. Snakes infected 2 to 11 days before hibernation will allow a viremia to be carried over the winter months. Snakes infected 19 or more days before they hibernate fail to produce an overwintering viremia, but develop antibody. Thus a sharp drop in temperature in the late summer or early fall will determine the overwintering of virus in snakes that are infected that go into hibernation soon after being infected. Thus virus in these snakes is available the next spring for mosquitoes.