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Eastern Equine Encephalitis Warning

Published: August 14, 2018 2:14 pm ET

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On August 8 and 9, 2018, the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) was notified of two confirmed cases of Eastern Equine Encephalitis (EEE) in horses in Haldimand County.

Both horses, from separate farms, were unvaccinated for EEE and had no history of travel. They were euthanized following the sudden onset and progression of neurological signs. Laboratory diagnostic testing confirmed infection with EEE virus. Private veterinarians have confirmed that three more horses have demonstrated acute onset of neurological disease within a five kilometre radius of these two horses and two have been euthanized.

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. Equine neurological cases are posted on the OMAFRA website.

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 August 8, 2018 no mosquito pools have tested positive for EEEv.

Questions with respect to equine health issues should be directed to:

Dr. Alison Moore
Lead Veterinarian – Animal Health and Welfare
Ontario Ministry of Agriculture and Food
Tel: (519) 826 – 4514
E-mail: [email protected]
Agricultural Information Contact Centre:
E-mail: [email protected]


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