Genetic and antigenic diversity among eastern equine encephalitis viruses from North, Central, and South America

Eastern equine encephalitis virus (EEEV), the sole species in the EEE antig enic complex, is divided into North and South American antigenic varieties based on hemagglutination inhibition tests. Here we describe serologic and phylogenetic analyses of representatives of these varieties, spanning the e ntire temporal and geographic range available. Nucleotide sequencing and ph ylogenetic analyses revealed additional genetic diversity within the South American variety; 3 major South/Central American lineages were identified i ncluding one represented by a single isolate from eastern Brazil, and 2 lin eages with more widespread distributions in Central and South America. All North American isolates comprised a single, highly conserved lineage with s trains grouped by the time of isolation and to some extent by location. An EEEV strain isolated during a 1996 equine outbreak in Tamaulipas State, Mex ico was closely related to recent Texas isolates, suggesting southward EEEV transportation beyond the presumed enzootic range. Plaque reduction neutra lization tests with representatives from the 4 major lineages indicated tha t each represents a distinct antigenic subtype. A taxonomic revision pf the EEE complex is proposed.