Search for epizootic-like Venezuelan encephalitis virus at enzootic habitats in Guatemala during 1969-1971.

Seventy-four strains of Venezuelan encephalitis (VE) virus recovered from sentinel hamsters or mosquitoes at enzootic habitats in Guatemala in the two years following the 1969 epidemic-equine epizootic were examined for ability to produce small plaques in Vero African green monkey kidney cell cultures, like isolates obtained during the epizootic. (a) One strain recovered from a sentinel hamster in late October 1969 at an enzootic habitat near the epicenter of the hemagglutination-inhibition (HI) and equine-virulence properties like epizootic virus; this strain retained its small plaque characteristic after inoculation and recovery from bloods of three horses. (b) None of the other 73 strains produced uniformly small plaques, but 31 formed a few small plaques among large ones. Virions from small plaques of five strains were cloned twice in Vero cell cultures. Four clones produced uniformly small plaques after one more passage in Vero cells; three had hemagglutination-pH properties compatible with epizootic virus or intermediate between epizootic and enzootic virus, but HI tests with these three hemagglutinins or with antibody to the fourth cloned strain showed them to be like Central American enzootic virus. One of three cloned strains tested in horses produced encephalitis and death in one of four horses; another strain produced encephalitis with recovery in one of two horses. (c) Thus these small Vero plaque clones resembled Central American enzootic strains of VE virus in HI and equine-virulence tests, and the small Vero plaque characteristic was not a satisfactory marker for consistently isolating equine-virulent, epizootic VE virions. Nevertheless, this technic led to recognition of one epizootic strain isolated at an enzootic habitat in Guatemala at the end of 1969 outbreak. Whether this strain was there before the outbreak or subsequently penetrated the habitat is uncertain. During the next two years, this strain did not become dominant in that enzootic focus.