Cathy Barnette, DVM, a paid consultant for Covetrus®

West Nile virus (WNV) is the leading cause of equine encephalitis, with over 25,000 cases reported since the virus was first discovered in the United States in 1999.¹

Kevin Hankins, DVM, MBA, senior technical services veterinarian at Zoetis Animal Health, told Covetrus in a recent interview² that “West Nile Virus was a threat to the equine population when it first was introduced into the United States in 1999 and remains a threat to the equine population today.”

In 2018 alone, WNV was diagnosed in 220 horses from 34 different states, and the virus has now been found in all 48 of the continental United States.³ WNV can be transmitted by a wide variety of mosquito species; this versality likely accounts for its widespread distribution.

Symptomatic WNV has a 33% mortality rate; one in three horses with symptomatic infection will die.¹ Even if a horse does survive symptomatic infection, there is a 40% likelihood that the horse will have long-lasting neurological deficits, such as gait abnormalities and behavioral changes, persisting for at least six months.¹ These residual deficits could significantly impact the career of a performance horse, while also negative affecting the quality of life of even a backyard pasture pet.

The Financial Cost of West Nile Virus

Even in horses that survive WNV infection, the financial costs associated with infection are often substantial for the horse owner. According to Dr. Hankins, “The cost can reach into the thousands of dollars and there is still no guarantee that the horse will survive.”

Some horses are fortunate to experience only mild, flu-like signs. These horses can typically be treated on an outpatient basis, with anti-inflammatory medications and slight changes in management. Costs associated with a mild case of WNV may be relatively low. An initial veterinary visit, medications, and a recheck veterinary visit may cost you several hundred dollars.

Severe cases of WNV, however, require more aggressive treatment. You may notice that your horse stops eating and drinking which is due to neurologic changes, so they may need to be hospitalized for intravenous fluid therapy and nutritional support. Horses that are unwilling or unable to remain standing may need to be supported with an overhead sling system, to prevent pressure sores and other complications associated with encephalitis. A severe case of WNV may require weeks of hospitalization, depending on the individual horse and how their immune system responds to infection. This treatment can easily cost you several thousand dollars.

Even when averaging mild cases and severe cases, treatment of WNV can be costly. A 2002 North Dakota study estimated the cost to treat one symptomatic case of WNV at approximately $1,500 per horse.⁴ Given estimated inflation rates over the last 20 years, this means that the average cost of treatment is likely $2,500 or more today. Clearly, WNV is a costly condition in addition to a dangerous one.

Benefits of West Nile Virus Vaccinations

Fortunately, effective WNV vaccines are available. Given the widespread prevalence of WNV, relatively high mortality rate, and risk of residual neurologic defects, the WNV vaccine is now regarded as a core vaccine, and it is recommended for all horses in North America.¹

Leading WNV vaccines are estimated to be over 95% effective, with unvaccinated horses being 30 times as likely to develop symptoms of WNV.⁴ This means that vaccination nearly eliminates the risk of symptomatic WNV infection.

You should ensure your horse receives their first WNV vaccine from your veterinarian at approximately four- to six-months old (depending on the mare's vaccination status), with a total of three initial vaccines administered at three- to six-week intervals.¹ After this initial vaccine series, horses should be revaccinated once yearly. In areas where WNV prevalence is very high, veterinarians may recommend your horse has a vaccine booster as often as every six months, to ensure optimal protection.¹

WNV vaccination is commonly administered with other vaccines during your horse’s annual veterinary wellness examination, avoiding the need for a separate veterinary farm call. In most cases, WNV protection is available from your veterinarian for less than $50/year. “Vaccination is a very small cost compared to possible treatment costs, or loss of the horse due to infection with West Nile Virus,” says Dr. Hankins.

Read more about WNV vaccination 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 17, 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 State. Accessed April 17, 2023. https://www.aphis.usda.gov/animal_health/downloads/animal_diseases/2021-wnv-report-summary.pdf
4. Ndiva Mongoh, M., Hearne, R., Dyer, N. W., et al. The economic impact of West Nile virus infection in horses in the North Dakota equine industry in 2002. Tropical animal health and production. 2008;40, 69–76.
5. Epp T, Waldner C, West K. Efficacy of vaccination for West Nile virus in Saskatchewan horses. Proceedings: 51st Annual Convention of the American Association of Equine Practitioners. Posted and updated Dec. 7. 2005. Accessed April 17, 2023. https://www.ivis.org/library/aaep/aaep-annual-convention-seattle-2005/efficacy-of-vaccination-for-west-nile-virus-saskatchewan-horses