Characterization of Fort Morgan virus, an alphavirus of the western equine encephalitis virus complex in an unusual ecosystem

An alphavirus isolated from nestling Cliff Swallows (Petrochelidon pyrrhonota) and House Sparrows (Passer domesticus) and from cimicid bugs (Oeciacus vicarius) in eastern Colorado, for which we propose the name Fort Morgan (FM) virus, is sensitive to the action of sodium deoxycholate, unstable at pH 2.0-4.0, and demonstrates no characteristics of temperature-sensitive mutants. Unpassaged field strains are nonpathogenic, or of low pathogenicity, for suckling mice; however, plaque-purified FM virus is pathogenic for a variety of laboratory hosts. By hemagglutination-inhibition (HI), complement-fixation, and neutralization tests, cross-reactions were observed between FM virus and members of the western equine encephalitis (WEE) virus antigenic complex. Short-incubation HI tests indicated that the new isolate shared closer antigenic relationships with WEE complex virus strains from the eastern United States (Highlands J virus) than with other WEE complex viruses. On the basis of these serological findings, as well as characterization of the structural polypeptides and oligonucleotides, we suggest that FM is a distinct virus belonging to the WEE antigenic complex. A reconsideration of the taxonomy of the WEE complex and discussion of the epizoologic significance of FM virus are presented.