Eastern equine encephalitis in Massachusetts, 1957-1976. A prospective study centered upon analyses of mosquitoes.

Reappearance of eastern equine encephalitis (EEE) in Massachusetts residents in the 1970's provided an opportunity to assess the predictive value of data on rainfall, EEE in horses, and carriage of EEE virus (EEEV) by mosquitoes, factors which had been studied annually since the last EEE outbreak in 1955-1956. The cycle of multiple cases during 1973-1975 started in a second consecutive year of rainfall that exceeded the annual mean by more than 20 cm, conditions recapitulating the 1955-1956 experience. In 1973, widespread EEE fatalities in horses presaged human cases, another recapitulation of the 1955-1956 experience. However, in 1974, when horses were immunized extensively, no equine cases were seen even though three human fatalities occurred. An unseasonably early appearance of EEEV in mosquitoes was the only basis upon which the threat to humans could have been recognized. These changes in the recognition and distribution of EEEV activity from season to season illustrate the difficulty in making rational decisions regarding widespread aerial insecticide applications for mosquito control.