Western equine encephalomyelitis in horses in the Northern Red River Valley, 1975

In mid-July, 1975, western equine encephalomyelitis (WEE) virus was isolated from mosquitoes collected in flooded areas of eastern North Dakota and western Minnesota. Inasmuch as clinical manifestations of WEE are usually observed in horses before human cases of encephalitis are recognized, surveillance of equine disease was initiated. Sixty-one practicing veterinarians from the are under surveillance reported 281 cases of WEE in horses from June through September, with peak incidence in late July. The high percentage of sero-positive, clinically normal, unvaccinated horses in one region suggested that many horses had developed non-clinical infections. The efficacy of vaccines used by the practitioners appears to have been execllent, as none of the horses vaccinated before the epizootic became ill during the period of surveillance. It was concluded that data collected from routine surveillance of encephalomyelitis in horses could be used to predict epidemics of WEE.