EEE Confirmed in Bruce County, Ontario, Horse

The 12-year-old unvaccinated mare was euthanized following the sudden onset and progression of neurologic signs.

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On Oct. 12, the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) received notice of a confirmed case of Eastern equine encephalitis (EEE) in a horse in Bruce County, the agency said in an equine health alert.

The 12-year-old unvaccinated mare with no travel history was euthanized following the sudden onset and progression of neurologic signs. The horse was found down in the field, became unresponsive, and was euthanized. A post-mortem examination was performed at the Animal Health Laboratory in Guelph, Ontario, and EEE was confirmed by brain-tissue testing.

A viral disease, EEE affects the central nervous system and is transmitted to horses by infected mosquitoes. Clinical signs of EEE include moderate to high fever, depression, lack of appetite, cranial nerve deficits (facial paralysis, tongue weakness, difficulty swallowing), behavioral changes (aggression, self-mutilation, or drowsiness), gait abnormalities, or severe central nervous system signs, such as head-pressing, circling, blindness, and seizures. The course of EEE can be swift, with death occurring two to three days after onset of clinical signs despite intensive care. Once clinical infection develops, treatment options are limited to supportive care; fatality rates reach 75-80% among horses. Horses that survive might have long-lasting impairments and neurologic problems.

The OMAFRA recommended that Ontario veterinarians consider EEE as a differential diagnosis in horses exhibiting neurologic signs. Positive cases can be confirmed through appropriate testing. IgM antibodies to the EEE virus (EEEv) can be detected in serum from horses with neurologic 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 September, although cases such as this one can occur later in the year if environmental conditions permit the survival of the mosquito vector species

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