Cathy Barnette, DVM, a paid consultant for Covetrus®

Although it is no longer a popular topic in the media like it was 20 years ago, West Nile virus (WNV) remains a significant risk to horses. In an interview with Kevin Hankins, DVM, MBA, senior technical services veterinarian at Zoetis Animal Health,² he told Covetrus, “West Nile Virus is now considered an endemic disease in the United States, so the horse owner needs to remain concerned about the disease even though it has been around for over 20 years.”

WNV vaccine is a core vaccine in horses.¹ This means that all horses, regardless of lifestyle or individual risk factors, should be vaccinated against WNV.

Here are some of the reasons why vaccination of horses against WNV is important.

1. West Nile virus is impossible to avoid.

   While most of us are used to thinking of Eastern equine encephalitis (EEE) and Western equine encephalitis (WEE) as significant equine threats, WNV is actually the most common cause of encephalitis in horses. It has infected over 25,000 horses since arriving in the United States in 1999.¹

   Twenty-four years later, this virus continues to pose a risk to horses. The virus is spread by a wide variety of mosquitoes, allowing it to infect horses in nearly any area. “Since it is a mosquito borne disease, all unvaccinated horses have the possibility of being infected,” says Dr. Hankins. In 2018 alone, the United States Department of Agriculture identified 220 equine WNV cases in 34 different states.³

2. West Nile virus has a 33% mortality rate.

   Approximately one in three horses with symptomatic WNV die of their infection.¹ While this makes WNV less fatal than EEE, a 33% mortality rate is similar to that seen with WEE.⁴

   Clinical signs can help determine an individual horse’s prognosis. Kansas State University’s Veterinary Health Center estimates that the mortality rate in horses that remain standing is only 10% to 20%, while horses that are unable to stand have a much higher mortality rate.⁵ Veterinarians have observed a “70% mortality rate if the horse is recumbent when treatment is started,” says Dr. Hankins.

3. West Nile virus can cause long-lasting neurologic effects.

   Approximately 40% of horses with symptomatic WNV still have neurologic signs six months later.¹ In a 2001 study, nearly 10% of clients who responded to long-term follow-up reported that their horse still had remaining neurologic issues 12 months after infection.⁶

   Long-term neurological changes after WNV can vary from behavioral changes to gait abnormalities. These impacts may be concerning in the case of a backyard pasture pet, but they can have dramatic impacts on performance horses who rely on careful, precise movements to compete.  

4. West Nile virus can be a costly condition to treat.

   The cost to treat symptomatic WNV infection can vary significantly. In mild cases, one veterinarian visit (to rule out other conditions) and a course of anti-inflammatory medication may be all that is needed. In more severe cases, however, horses may require hospitalization at a veterinary teaching hospital or equine specialty hospital, where they can receive fluid therapy, nutrition, and sling support. Ultimately, says Dr. Hankins, “the cost can reach into the thousands of dollars and there is still no guarantee that the horse will survive.”
   
   

5. West Nile vaccination is affordable and effective.

   WNV vaccines are very effective, when administered appropriately by a veterinarian. Leading WNV vaccines are estimated to be more than 95% effective, and unvaccinated horses are 30 times as likely to develop symptoms of WNV as a vaccinated horse.⁷

   Horses must receive an initial series of two to three vaccines, followed by a booster once annually. Once your horse has received their initial vaccine series, annual revaccination will likely cost less than $50/year. This is a small price to pay for the ability to protect against such a serious condition.

Summary

WNV is the most common form of equine encephalitis in the United States, causing signs that range from mild neurologic signs to death. Even horses that survive infection may have lasting neurologic deficits and may undergo significant treatment costs. Fortunately, safe, effective, and affordable vaccines are available to offer protection against this equine health risk.

Read more about vaccinations for horses in the vector-borne diseases resource center.

About the author: Cathy Barnette, DVM is a veterinarian and freelance writer, based in Southwest Florida. After graduating from the University of Florida in 2006, Dr. Barnette spent 14 years working in small animal general practice. Her current professional focus is veterinary writing, creating educational content for veterinary teams and their clients. Dr. Barnette is a paid Consultant for Covetrus.

References

1. American Association of Equine Practitioners. West Nile Virus. Accessed April 25, 2023. https://aaep.org/guidelines/vaccination-guidelines/core-vaccination-guidelines/west-nile-virus
2. Kevin Hankins, DVM, MBA, email communication [interview], April 24, 2023.
3. USDA APHIS. (2022). 2021 Summary of West Nile Virus Equine Cases in the United States. Accessed April 25, 2023. https://www.aphis.usda.gov/animal_health/downloads/animal_diseases/2021-wnv-report-summary.pdf
4. American Association of Equine Practitioners. Western Equine Encephalitis Fact Sheet. Accessed April 25, 2023. https://aaep.org/sites/default/files/Documents/Outside%20Linked%20Documents/DiseaseFactsheet_WEE_FINAL.pdf
5. Kansas State University Veterinary Health Center. Equine Internal Medicine: West Nile Virus in Horses. Accessed April 25, 2023. https://www.ksvhc.org/services/equine/internal-medicine/west-nile.html
6. Porter, M. B., Long, M. T., Getman, L. M., et al. (2003). West Nile virus encephalomyelitis in horses: 46 cases (2001). Journal of the American Veterinary Medical Association, 222(9):1241–1247.
7. Epp, T., Waldner, C., West, K. (2005). Efficacy of vaccination for West Nile virus in Saskatchewan horses, in Proceedings: 51st Annual Convention of the American Association of Equine Practitioners, 180-182. Accessed April 25, 2023. https://www.ivis.org/library/aaep/aaep-annual-convention-seattle-2005/efficacy-of-vaccination-for-west-nile-virus-saskatchewan-horses