WNV Confirmed in Two Ontario Horses

WNV Confirmed in Two Ontario Horses

West Nile is transmitted to horses via bites from infected mosquitoes.

Photo: James Gathany/Centers for Disease Control

The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) received notice on Aug. 21 that two horses in that province tested positive for West Nile virus (WNV).

A 10-year-old gelding from Simcoe County was euthanized after developing severe neurologic signs and seizures. Meanwhile, a 25-year-old mare from Bruce County developed severe neurologic deficits in all four limbs as well as facial hyperesthesia (hypersensitivity to touch), but is recovering under veterinary medical supervision.

The OMAFRA said Ontario veterinarians should consider WNV as a differential diagnosis in horses with neurologic signs, and can identify positive cases through appropriate testing. West Nile is an immediately notifiable disease under both the federal Health of Animals Act and the provincial Animal Health Act.

West Nile Virus is endemic in Ontario and cases occur in the horse population at varying levels each year. Most equine cases occur between August and September, although cases can occur into October if environmental conditions permit the survival of the mosquito vector species.

Health Alert: West Nile Virus

Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation; hyperesthesia; changes in mentation (mentality), when horses look like they are daydreaming or "just not with it"; occasional somnolence (drowsiness); propulsive walking (driving or pushing forward, often without control); and "spinal" signs, including asymmetrical weakness. Some horses show asymmetrical or symmetrical ataxia. Equine mortality rate can be as high as 30-40%.

Horse owners should also consult their private practicing veterinarian to determine an appropriate disease prevention plan for their horses. Vaccines have proven to be a very effective 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.

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