Studies on the role of mammals in the natural history of western encephalitis in Hale County, Texas.

Abstract

Following an epidemic of western encephalitis (WE) in Hale County, Texas during the summer of 1963, studies were initiated to determine if small mammals were naturally infected with WE virus in that area. An antibody survey in December 1963 revealed that more than half of the rabbits and only 2% of the rodents sampled were positive for WE antibody. Collection and testing of rabbits continued through April 1964, and 52% of the 137 tested had antibody to WE. Simultaneous collecting and testing of 518 rodents and other mammals for detection of overwintering virus gave negative results. In 1965 and 1966, mammal trapping studies revealed Mus musculus and Sigmodon hispidus to be the most abundant rodent species in rural study sites. The rabbit, Lepus californicus, was commonly the most abundant Lagomorph. During May-November 1965, 572 small mammals were tested for virus and in July, WE virus was isolated from 5 of 249 Lepus californicus, 2 of 12 Gynomys ludovicianus, 1 of 44 Citellus tridecemlineatus, and 1 of 103 Sigmodon hispidus. Virus was not isolated from 197 small mammals collected during April, September, and October 1966, but HAI antibody tests of 115 specimens collected during autumn 1966 again indicated high WE antibody rates among rabbits. The results of these studies indicate that small mammals are not basically involved in the maintenance of WE virus. However, small mammals may become infected during periods of high levels of enzootic transmission, and Lepus californicus appears to be the one most frequently infected during such periods.