In infants, approximately 1 in 50 infections leads to clinical illness, while in adults illness occurs in only 1 in 1000. The last two known human cases in the nation occurred in Wyoming in 1994. Map 1 shows the distribution of the 639 human cases from 1964 to 1999. SLE St. Louis Encephalitis is caused by a flavivirus transmitted to humans by Culex spp. Like the WEE virus, the main hosts are migratory birds.
Humans act as dead-end hosts.
The three Idaho cases noted on Map 2 occurred in 1969. West Nile virus is a flavivirus closely related to the SLE virus, and requires similar ecologic niches. It has been theorized that the West Nile virus was first introduced into the continental United States, in eastern New York, during the fall of 1999, as its presence had never been documented prior to that time. WNV caused morbidity and mortality in humans, primarily in the Borough of Queens. The outbreak was originally believed to be caused by the SLE virus due to the close antigenic relationship between SLE and WNV causing cross-reactivity in certain immunologic tests and the fact that WNV had never before been recognized as a pathogen in the western hemisphere.