West Nile Virus and EEE Vaccines for Horses.

Learn why it is important to appropriately vaccinate your horse against West Nile virus and Eastern equine encephalitis.
Vaccinating your horse can help prevent EEE and WNV. | Getty Images

Cases of West Nile virus (WNV) and Eastern equine encephalitis (EEE) are becoming more common across horses in North America; however, preventative vaccines against both are typically effective when administered properly. “Vaccinations for WNV and EEE are highly effective in minimizing disease, if given appropriately,” said Charlie Broaddus, DVM, VDACS state veterinarian.

“The vaccines are effective for six to 12 months, so horses should be revaccinated at least annually,” he continued. “In areas where the disease occurs frequently, most veterinarians recommend vaccination every six months.”

Understanding Eastern Equine Encephalitis

A viral disease, EEE affects the central nervous system and is transmitted to equids by infected mosquitoes. Clinical signs in horses 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; and
  • Severe central nervous system signs, such as head-pressing, circling, blindness, and seizures.

The disease can progress rapidly, with death occurring two to three days after onset of clinical signs despite intensive care in some cases. Equids that survive might have long-lasting impairments and neurologic problems.

Understanding West Nile Virus

West Nile virus is also 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;
  • Hyperesthesia (hypersensitivity to touch and sound);
  • Changes in mentation (mentality), 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 has no cure however some horses can recover with supportive care.

Tips for Preventing Disease

Studies have shown that vaccines can be effective in protecting horses against EEE and West Nile when administered appropriately. 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 or where the disease is diagnosed frequently.

In contrast, previously unvaccinated horses require a two-shot vaccination series in a three- to six-week period. Full immunity takes several weeks to achieve.

In addition to vaccinations, owners should work to reduce the mosquito populations and possible breeding areas and horses’ exposure by:

  • Removing stagnant water sources;
  • Dumping, cleaning, and refilling water buckets and troughs regularly;
  • Keeping animals inside during the bugs’ feeding times (typically early in the morning and evening); and
  • Applying mosquito repellents approved for equine use and/or fly masks, sheets, and boots.






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