Younger horses in training, repeat at 2 to 3 month intervals. All brood mares at least during 5th, 7th and 9th months of gestation. There are many types of encephalitis, and a large number of the cases are caused by viruses. The predominant toga viral encephalitides in the western hemisphere are associated with the Eastern, Western and Venezuelan Equine Encephalitis virus. The structure of the viruses and the associated clinical presentations are fairly similar, but the character of epidemics and the immunology are distinct.
Clinical signs are more profound in unvaccinated animals.
Acute clinical signs of EEE and WEE are nonspecific and include mild to severe fever, poor appetite, and stiffness. An incubation period of 1-3 weeks then occurs. Many cases of WEE do not progress beyond this point. With EEE, progression is more common. Once nervous signs develop, the viremia is past.