Experimental Studies of Rhesus Monkeys Infected with Epizootic and Enzootic Subtypes of Venezuelan

Adult rhesus monkeys were infected with five subtypes of Venezuelan equine encephalitis (VEE) virus (lA, IB, Ie, ID, and IE). The animals were studied clinically, and samples of serum were taken for determinations of viremia and antibody, leukocyte counts, and biochemical studies. Only monkeys inoculated with VEE subtype lA, IB, or Ie developed fever and signs of illness, although high titers of circulating virus were detected in the sera of all animals. Viremia appeared on day 1 after inoculation and disappeared by days 4-6. Most animals, including those infected with subtype ID or IE, were leukopenic. The concentra-tion of glutamic-oxalacetic transaminase in serum was elevated only in monkeys infected with subtype IA or Ie. Sera obtained 12 and 47 days after inoculation were tested by serum dilution neutralization and by conventional and "kinetic" HAl tests. The ability of these sera to distinguish among VEE subtypes was some-what limited; the antigenic differences observed were nevertheless in general agreement with those described by previous workers. This study supports the hypothesis that enzootic VEE subtypes are less pathogenic than subtypes responsible for major epizootics. Current interpretations of the epizootiology of Venezuelan equine encephalitis (VEE) rely heav-ily on antigenic distinctions demonstrated between strains isolated in different regions of tropical and subtropical America [1]. The strains of VEE virus that cause epidemics and equine epizootics have been found to belong to type I of the present clas-sification system when tested by the "kinetic " or short-incubation HAl test. These viruses are read-Received for pUblication July 12, 1973, and in revise