The incubation period is 3 - 14 days. Mortality is low.
Signs are usually most severe in very young, very old or debilitated horses.
Epidemics can occur where horses are congregated. EVA is transmitted by aerosolized nasal secretions and by venereal spread. A carrier state can develop in stallions with the virus present in semen. There is no specific treatment. Supportive treatment is useful where indicated.