Recent Advances in Research on the Eastern Encephalitis Virus **

Knowledge of the ecology of the Eastern Encephalitis (EE) virus has accumulated rapidly since the agent was first isolated in 1933. The sporadic incidence of infections, lack of direct contact except among pheasants in the same pen, association of outbreaks with swampland areas, and the occurrence of infections following the peak of the mosquito season led to the theory that the virus was transmitted by mosquitoes. Consequently, in early investigations attention was directed to coastal areas and the probability of salt marsh mosquitoes as vectors. However, in reports of epidemics among pheasants in New Jersey by Beaudette and co-workers,"7 the pattern of spread of the virus produced doubt concerning arthropod transmission. Where cases were confined to pheasants in one pen and failed to appear among those in adjacent pens separated only by large mesh chicken wire through which mosquitoes might readily pass, it was obvious that mosquitoes were not responsible for transmission. Similar circumstances were encountered in the course of investigating EE activity among pheasants in Connecticut' as well as the occurrence of high mortality at a time when cold weather suppressed adult mosquito activity. However, in the study of daily mortality records it was found that initial deaths occurred during a period of warm weather when mosquito activity was high. This led to consideration of primary transmission into a pheasant flock, probably by an infected mosquito or wild bird, and from the initial case, secondary contact transmission that accounted for the rapid transfer of virus from pheasant to pheasant within the flock. The secondary transmission accounted for spectacular mortality which persisted beyond the mosquito season into the cool fall weather. The postulation of the existence of an extra-arthropod secondary type of transmission and the subsequent