Ecology of Western Equine Encephalomyelitis in the Eastern United States

Abstract

Publisher Summary Western equine encephalomyelitis (WEE) has been recognized as a serious public health problem in western North America for more than 30 years. WEE appears to exist endemically in numerous foci in that region, with a low incidence rate among humans. Severe outbreaks, however, have occurred periodically. For example, during 1941 a severe epidemic involving more than 3000 cases in humans occurred in North Dakota, Minnesota, and in the adjacent areas of Canada. The case fatality rate ranged from 8% to 15%. Epizootics among horses are more common. More than 600 cases of WEE were diagnosed among horses in the central and western United States during 1975. More recently, the virus of WEE has been found to be widely distributed in the eastern United States in regions along the Atlantic and Gulf seaboard; however, WEE has not been recognized as a public or veterinary health problem in the east. This situation presents an interesting epidemiological problem: a virus present in the different regions is causing disease in one area but not another area even though susceptible hosts are present. The reasons for the absence of WEE in humans and equines in the east are not clearly understood but probably are a reflection of parallel variations in the enzootic transmission cycles of this virus in different regions of the United States. Several reviews are available on the ecology of WEE virus in the areas of the western United States. This chapter examines data from studies on the ecology of WEE virus conducted in the eastern United States over the past two decades since the virus was first recognized in this region.