On June 17, 2021, the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) was notified of a confirmed case of eastern equine encephalitis (EEE) in a 26-year-old horse located in the United Counties of Stormont, Dundas and Glengarry.

Veterinarians in Ontario should consider EEE as a differential diagnosis in horses exhibiting neurological signs and can identify positive cases through appropriate testing. IgM antibodies to the EEE virus (EEEv) can be detected in serum from horses with neurological signs. Clinical signs of EEE including circling, head-pressing, ataxia and depression, can mimic a variety of encephalitides including rabies, West Nile virus (WNV), botulism, hepatic encephalopathy, equine protozoal myeloencephalitis, and equine herpes myeloencephalopathy. Most equine cases of EEE in Ontario occur between August and October and end with the onset of frost.

Effective equine vaccines for EEE are available and veterinarians should encourage clients to keep their horse’s vaccinations current. Once clinical infection develops, treatment options are limited to supportive care. The mortality rate in unvaccinated horses is high.

EEEv affects mainly equine species in eastern North America, but can occasionally cause severe disease in humans, including permanent brain damage or death. EEEv has also caused fatal infections in pheasants, quail, captive whooping cranes, emus and dogs.

EEEv has been present in the Ontario horse population since 1938.

For more information, please view the AAEP factsheet on EEEv here.

(EDCC)