Vaccinated Thoroughbred Gelding Tests Positive for West Nile in California

Mosquito in process of biting and extracting blood.
Mosquito in process of biting and extracting blood. PD-USGOV-HHS-CDC

Newsdate: Wednesday, September 21, 2022, 10:00 am
Location: SACRAMENTO, California

The California Department of Food and Agriculture has confirmed that a vaccinated 8-year-old Thoroughbred gelding in Tulare County tested positive for West Nile virus (WNV) and was euthanized on September 19,2022.

Horse watching flooding river on a rainy day.

Horse watching flooding river on a rainy day

Gelding displayed neurological signs, including incoordination in all four limbs, exaggerated gait in the front limbs, and recumbency, making this the ninth confirmed case of equine WNV in California in 2022.
© 2017 by M Francis McCarthy New window.

The gelding displayed neurologic signs, including ataxia (incoordination) in all four limbs, hypermetria (an exaggerated gait) in the front limbs, and recumbency (down and unable to rise). This is the ninth confirmed case of equine WNV in California in 2022.

West Nile virus is transmitted from birds, often referred to as avian reservoir hosts, by mosquitoes, and infrequently by other bloodsucking insects, to horses, humans and a number of other mammals.

Horses and humans are considered to be dead-end hosts for West Nile Virus because the virus is not directly contagious from horse to horse or horse to human, and indirect transmission via mosquitoes from infected horses is highly unlikely as these horses do not circulate a significant amount of virus in their blood.

West Nile Virus 101

West Nile virus is transmitted to horses via bites from infected mosquitoes. Not all infected horses show clinical signs, but those that do can exhibit:

  • Flulike signs, where the horse seems mildly anorexic and depressed;
  • Fine and coarse muscle and skin fasciculation (involuntary twitching);
  • Hyperesthesia (hypersensitivity to touch and sound);
  • Changes in mentation (mental activity), when horses look like they’re daydreaming or “just not with it”;
  • Occasional drowsiness;
  • Propulsive walking (driving or pushing forward, often without control); and
  • Spinal signs, including asymmetrical weakness; and
  • Asymmetrical or symmetrical ataxia.

West Nile virus has no cure, however some horses can recover with supportive care. Equine mortality rates can reach 30-40%.

Studies have shown that vaccines can be effective WNV prevention tools. Horses vaccinated in past years need an annual booster shot, but veterinarians might recommend two boosters annually—one in the spring and another in the fall—in areas with prolonged mosquito seasons.

In contrast, previously unvaccinated horses require a two-shot vaccination series in a three- to six-week period. It takes several weeks for horses to develop protection against the disease following complete vaccination or booster administration.

In addition to vaccinations, owners should work to reduce mosquito population and breeding areas and limit horses’ mosquito exposure by:

  • Removing stagnant water sources;
  • Dumping, cleaning, and refilling water buckets and troughs regularly;
  • Keeping animals inside during insect feeding times (typically early in the morning and evening); and
  • Applying mosquito repellents approved for equine use

EDCC Health Watch is an Equine Network marketing program that utilizes information from the Equine Disease Communication Center (EDCC) to create and disseminate verified equine disease reports. The EDCC is an independent nonprofit organization that is supported by industry donations in order to provide open access to infectious disease information.


Press release by EDCC Health Watch

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