A review of the geographic distributions of all recorded human outbreaks of arthropod-borne encephalitis in the United States revealed that most western encephalitis (WE) outbreaks have occurred at or above the 70°F June isotherm, whereas St. Louis encephalitis (SLE) outbreaks generally have occurred at or below this isotherm.
Average SLE and WE antibody rates in avian sentinel flocks (mostly domestic chickens) at 11 sites in the western United States from 1954 through 1960 indicated that high WE transmission rates occurred at all latitudes, whereas maximum SLE transmission rates were limited to the warmer latitudes at or below the 70°F June isotherm.
SLE and WE virus transmission rates in avian sentinel flocks near Greeley, Colorado, during 1954 through 1960 showed marked annual variations and these variations were found to be closely related to spring temperatures. The highest SLE rates and the lowest WE rates occurred during years when the April–June period was unusually warm. the lowest SLE rates and the highest WE rates occurred during years when the April–June period was unusually cold. A similar relationship between temperature and WE virus transmission rates was apparent for shorter periods of observation at nine other sentinel sites in the western states and for a 10-year period of observation near Bakersfield, California. The relationship between SLE virus transmission rates and temperatures at these additional sites was similar to that at Greeley, except for the three southernmost sites where temperature was apparently not a limiting factor.
Indices were developed for use in predicting enzootic activity levels of SLE and WE viruses. The index for SLE is the date when 10 day degrees above 75°F are first accumulated, and the index for WE is the date when 10 or 50 day degrees above 70°F are first accumulated. The possible use of these indices for forecasting human outbreaks is discussed.