OBSERVATIONS OF EQUINES, HUMANS AND DOMESTIC AND WILD VERTEBRATES DURING THE 1969 EQUINE EPIZOOTIC AND EPIDEMIC OF VENEZUELAN ENCEPHALITIS IN GUATEMALA1
Thirteen strains of Venezuelan (equine) encephalitis (VE) virus were isolated from bloods and nasal or throat swabs taken from ill equines and humans in Guatemala during July-September 1969. Five of five human virus strains tested differed slightly from a prototypic endemic Mexican-Guatemalan strain in plaque-reduction neutralization tests. Virus titers ranged to 104,5 and 103.0 SMicLD50/0.01 ml of equine and human blood, respectively, and 101.5 and 105.5 of equine nasal swab and human throat swab specimens. Absence of detectable virus in acute-phase bloods correlated with presence of VE virus antibody in equine sera whereas it correlated with absence of convalescent antibody responses in human sera. Based on epidemiologic observations and antibody tests of relatively small samples at selected study sites, it was estimated that about 35% of equines died from VE virus infection during the epizootic in Guatemala, 5% were mildly ill, and at least 10% were inapparently infected. About 3 to 4% of humans had been infected by VE or an antigenically closely-related arbovirus before the epizootic, 5 to 10% had mild, non-encephalitic febrile VE virus disease and 0 to 3% had inapparent infection with VE virus. Post-epizootic sera from dogs and cattle contained VE virus hemag-glutination inhibition (HI) and neutralization (N) antibodies, and a few sera from pigs and bats, but none from domestic fowl, wild birds or rodents had HI antibodies. Since no outbreaks of disease occurred in these animals during the epizootic, the possibility that some of them might have been silent amplifiers of VE virus was considered.