National Institutes of Health, Balboa Heights,

Thirty-two horses were infected with enzootic and epizootic strains of Venezuelan equine encephalomyelitis (VEE) virus to determine pathogenicity and strain-specific patterns of antibody. Enzootic strains produced fever and mild leuko-penia in 16 horses that subsequently developed moderate amounts of neutralizing (N), CF, and HAl antibodies. In contrast, 16 horses given epizootic strains of VEE virus frequently exhibited high fever, severe leukopenia, depressed hema-tocrit, and signs of encephalitis; most developed severe viremia, and survivors produced large amounts of N antibodies. Measurements of homologous and heterologous strain-specific N antibody detected antibodies by the fifth or sixth day after infection, when viremia ended and clinical encephalitis began. The assays revealed a pattern of variant-specific antibodies similar but not identical to that previously reported for these viruses in rodent sera tested in a modified HAl system. Strain-specific patterns of response in these horses are relevant in understanding the recent, novel VEE epizootics in Central America and the United States. In 1969, before the epizootics of Venezuelan equine encephalomyelitis (VEE) in Central Amer-ica [1-5] and the United States [6], it was known that strains of VEE virus existed in enzootic cy-cles in Panama [7, 8], every Central American country except El Salvador [9, 10, and P. T. Franck, personal communication], and in Florida Received for publication December 19, 1972, and in revised form April 18, 1973