Seventh California Horse Tests Positive for WNV

Seventh California Horse Tests Positive for WNV

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

Photo: iStock

California animal health officials have confirmed an additional case of West Nile virus (WNV) in a horse in that state.

“On Aug. 18, 2017, an unvaccinated yearling Warmblood filly in San Joaquin County displaying severe neurologic signs was confirmed positive for WNV,” said a statement from the California Department of Food and Agriculture (CDFA). “Due to the severity of clinical signs the filly was euthanized.

“For 2017, a total of seven horses have been confirmed positive for WNV,” the CDFA continued. “The positive horses were located in Glenn, Contra Costa, Fresno, Kern (2), Riverside, and San Joaquin counties. Five horses were unvaccinated and two horses had unknown vaccination status. Five horses died or were euthanized and two horses are recovering.”

Health Alert: West Nile Virus

West Nile is transmitted to horses via bites from infected mosquitoes. Clinical signs for WNV include flulike signs, where the horse seems mildly anorexic and depressed; fine and coarse muscle and skin fasciculation; hyperesthesia (hypersensitivity to touch and sound); 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.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More