Venezuelan equine encephalitis vaccination survey in Arizona and New Mexico, 1972.

Field studies were conducted in 1972 to determine the immunization status of equines along the Mexico, Arizona, and New Mexico borders. Interviews with horse owners were conducted along roads selected at random in the counties of Santa Cruz and Yuma, Ariz., and in Dona Ana County, N. Mex. At least 450 horse owners in each county were asked about the vaccination status of their animals, and information was taken on 1,260 animals. Blood specimens were obtained from every third equine, regardless of stated vaccination status, and tested for the presence of Venezuelan equine encephalitis (VEE), western equine encephalomyelitis (WEE), and eastern equine encephalomyelitis (EEE) neutralization antibodies. Serum samples were collected from 446 equines in the 3-county area; only 227 (50.7 percent) had both a history of VEE vaccination in 1971 (including 20 vaccinated in 1972) and serum neutralization antibody against VEE. Of the remaining 220 with no detectable neutralization antibody to VEE, 197 (89.5 percent) had a history of VEE vaccination in 1971 (including 5 revaccinated in 1972), 14 (6.4 percent) had no history of vaccination, and 9 (4.1 percent) had an unknown vaccination status. Eighty-two percent (160 of 1971) of the equines with a history of VEE vaccination and presence of dectectable WEE or EEE antibodies, or both, had no detectable levels of VEE antibody. Therefore, the results of this study suggest that the presence of WEE or EEE antibodies, or both, may suppress the development of dectable vaccine-induced VEE antibody response in the equine. As a result of this investigation, the U.S. Department of Agriculture, as an added precaution, recommended the revaccination of equines in areas of the United States bordering Mexico.