Studies of Venezuelan encephalitis in migrating birds in relation to possible transport of virus from South to Central America.

All of 13 species of northward migrating shorebirds and 7 species of songbirds captured on the Pacific coast of Guatemala during April and May of 1974--1976 were susceptible to infection with small doses of either an epizootic or an enzootic strain of Venezuelan encephalitis (VE) virus. They produce moderate to high levels of viremia for 2--4 days post-inoculation; levels high enough to infect both epizootic and enzootic vector mosquitoes. Viremias were often sufficient even on the 3rd day after inoculation, a time that might represent the end of a migratory flight, assuming that the physiological state of the birds after capture reflected that during migratory flight. Birds of many taxa react similarly to infection with strains of VE virus, and have the potential for being moderately to highly effective amplifying hosts. However, whether northward migrating birds could have been the agents for the introduction of the epizootic Ecuadorian strain that initiated the middle-American epizootic of 1969--1971 is less clear. Data are not available for the extent, rates or routes of migration between the region of Ecuador and Central America, but the best information on the real speed of migration from elsewhere indicates that even warblers that fly more slowly than shorebirds could make the flight in 72 hours or less. Still, that there are geographically segregated subtypes of VE virus suggests that avian transport has been of minimal importance over long time spans. The role of inactivated vaccines in the middle-American epizootic remains an open question.