Sources of virus
Equids (horses, donkeys and zebras) are a primary source of epizootic virus strains during an outbreak
o inter-epidemic maintenance of epizootic strains is not known though three hypotheses prevail:
incomplete inactivation of subtype I-AB VEE vaccines (suggested by sequencing studies)
epizootic/epidemic subtype I-AB and I-C VEE strains arise from genetic mutation of enzootic subtype I-D strains (supported by genetic studies)
recent studies of epizootic I-C VEE strains in Venezuela would indicate persistent, low-level sylvatic maintenance in and yet unknown cycle (cryptic transmission cycle)
Haematophagous vectors become infected with high titres of virus from infected horses
Enzootic/endemic strains remain in tropical ecosystems in cycle between rodents and mosquitoes; and for some subtypes, birds also.