Search for persistent epizootic Venezuelan encephalitis virus in Guatemala, El Salvador and Nicaragua during 1970-1975

Evidence was sought during 1970-1975 of persistence of equine-virulent Venezuelan encephalitis (VE) virus in regions of Central America that were heavily involved in the epidemic-equine epizootic of 1969. (a) Four sentinel horses were exposed in an arid, upland region of the Atlantic drainage of Guatemala during August-October 1970, but no horse became infected. (b) The epicenter region of the 1969 outbreak, in southwestern Guatemala and southwestern El Salvador, was studied during July 1970-February 1974; no antibody developed in sentinel horses, sentinel hamsters did not die, mosquitoes yielded no virus, wild rats had no detectable VE virus HI antibody. Unexplained decreases in populations of wild terrestrial mammals possibly limited maintenance of VE virus. However, mosquitoes were plentiful and present in the same species composition found at a focus of enzootic VE virus about 35 km northwest of the epicenter region. (c) In studies at two Guatemalan ranches near the epicenter, where horses died in 1969, VE viruse infected sentinel horses along one of three lakes on one ranch during the wet season of 1972 but not during the dry or wet seasons of 1973; the titers of neutralizing antibodies in these four horses were higher against an enzootic strain of VE virus than against an epizootic strain. During 1970 and 1971, VE virus was isolated from sentinel hamsters exposed at a marsh on the other ranch, but Vero plaque characteristics were those of enzootic VE virus. (d) The only epizootic activity of VE virus discovered in Central America in 1970-1975 occurred in Nicaragua between April and June 1972. Several hundred horses died, and N antibody, like that engendered by epizootic virus, was found in two young, unvaccinated horses. Whether this represented persistence of epizootic VE virus or reintroduction of virus is unknown.