Climatological Conditions Associated with Outbreaks of Eastern Encephalitis

Abstract

Summary An analysis made of the weather conditions associated with human outbreaks of eastern encephalitis in the United States revealed that the major outbreaks occurred in Massachusetts and New Jersey, and that a weather pattern of excessive rainfall during the summer of the outbreaks and the preceding autumn was consistently associated with the disease outbreaks. The only other state which reported four or more human cases in a single year was Louisiana with ten reported cases in 1947, during which no specific weather pattern was apparent. Equine outbreaks of eastern encephalitis in Massachusetts and New Jersey coincided with human outbreaks and were, therefore, associated with the same weather pattern. The relationship between the amount of precipitation and the number of equine cases was less consistent in Delaware and Maryland, and only occasional associations were noted between excessive amounts of rainfall and equine outbreaks in Florida, Louisiana, and Texas. It seems reasonable that weather conditions might be used to predict human and equine outbreaks of eastern encephalitis in the northeastern United States. In the Taunton, Massachusetts, area there would appear to be danger of an eastern encephalitis outbreak during any year when 26 inches or more of rainfall had accumulated during the previous August through October and the current June and July. In states south of New Jersey, the available data are not adequate to provide a basis for predicting epidemics or epizootics.