Epizootic/epidemic cycle: equids are amplifying host (high, prolonged viraemias) and infect a wide range of mosquitoes which are not restricted to equid hosts
o non-vector spread of disease via direct contact or aerosols has been proposed; horse-to-human and human-to-human transmission has not been recorded
o role of non-equid vertebrates in transmission is not clear but likely minor
Swiftness of spread depends on: VEE virus subtype, density of competent vectors and population of susceptible hosts; large, geographically disperse epizootics of VEE have depended on ability of virus to produce high viraemias in equids.