Experimental Infection of Horses with Enzootic and Epizootic Strains of Venezuelan Equine Encephalomyelitis Virus

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Abstract
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 leukopenia in 16 horses that subsequently developed moderate amounts of neutralizing (N), CF, and HAI antibodies. In contrast, 16 horses given epizootic strains of VEE virus frequently exhibited high fever, severe leukopenia, depressed hematocrit, 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 HAI 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.

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