On Aug. 10, 2019, the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) was notified of a confirmed case of eastern equine encephalitis (EEE) in a horse located in the United Counties of Leeds and Grenville, ON. The unvaccinated mare was euthanized following the sudden onset and progression of neurological signs. Laboratory diagnostic testing confirmed infection with the EEE virus (EEEv).

EEE is not transmissible from horses to people.

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.

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 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 the months of August and October and end with the onset of frost.

EEEv affects mainly equine species in eastern North America but can rarely cause severe disease in humans. EEEv has also caused fatal infections in pheasants, quail, emus, alpacas, llamas and dogs.

EEEv has been reported in horses in Ontario since 1938. Equine neurological cases are posted on the OMAFRA website at http://www.omafra.gov.on.ca/english/livestock/horses/westnile.htm.

Ontario’s local public health units are conducting mosquito surveillance for both WNV and EEEv. Birds are the natural hosts for both viruses, which are transmitted to horses and humans by mosquitoes which have bitten an infected bird. As of Aug. 7, 2019, no mosquito pools have tested positive for EEEv (https://www.publichealthontario.ca/en/DataAndAnalytics/Pages/WNV.aspx).

Questions about EEE in horses can be directed to:

Dr. Alison Moore
Lead Veterinarian – Animal Health and Welfare
Ontario Ministry of Agriculture and Food
Tel: (226) 821-0364
E-mail: alison.moore@ontario.ca