Western Equine Encephalitis submergence: lack of evidence for a decline in virus virulence.

The incidence of Western Equine Encephalitis (WEE) in humans and equids peaked during the mid-20th century and has declined to fewer than 1-2 human cases annually during the past 20 years. Using the mouse model, changes in WEE virus (WEEV) virulence were investigated as a potential explanation for the decline in the number of cases. Evaluation of 10 WEEV strains representing a variety of isolation locations, hosts, and all decades from the 1940's to the 1990's yielded no evidence of a decline in virulence. These results suggest that ecological factors affecting human and equine exposure should be investigated to explain the decline in WEE.