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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Vaccines have proven to be a very effective EEE prevention tool. Horses that have been vaccinated in past years will need an annual booster shot; in areas with a prolonged mosquito season, veterinarians might recommend two boosters annually—one in the spring and another in the fall. However, if an owner did not vaccinate their animal in previous years, the horse will need the two-shot vaccination series within a three- to six-week period.

In addition to vaccinations, horse owners also need to reduce the mosquito populations and their possible breeding areas. Recommendations include removing stagnant water sources, keeping animals inside during the bugs’ feeding times, which are typically early in the morning and evening, and applying mosquito repellents approved for equine use.

The EEEv has been present in the Ontario horse population since 1938. In 2014, 24 EEE cases were reported.

Local public health units will follow up on positive equine EEE cases to determine whether the horse’s exposure was from local mosquitoes or travel-related. The public health unit will also ensure that the risk of human exposure to mosquitoes in the area, which could be carrying EEEv, is minimized.