The Veterinary Corner series, brought to you by Merck Animal Health, features insights from leading veterinarians on some of the most pressing health issues affecting horses today. 

When the temperatures rise, the mosquitoes come out to fly—and bite. And while the flying fiends are a pest to us all, it’s extra important to protect our horses from contracting serious and often fatal diseases like Eastern Equine Encephalomyelitis (EEE), Western Equine Encephalomyelitis (WEE) and West Nile virus (WNV). Keep reading to learn more about these deadly diseases, when to vaccinate your horse and other management strategies you can implement today. 

Know What You’re Up Against 

Eastern Equine Encephalomyelitis, Western Equine Encephalomyelitis and West Nile virus aren’t contagious between horses or humans, but they can be spread quickly through bites from infected mosquitoes. These diseases cause inflammation of the brain, which can lead to debilitating neurological problems in the horse. Protecting your horse begins with making sure vaccinations are up to date and administering boosters appropriately.  

EEE causes severe and often fatal neurologic disease and has a mortality rate of up to 95%.¹ Cases are primarily found in the Southeastern United States but have been reported as far north as Canada and as far west as Texas. In 2020, Michigan reported more cases of EEE than Florida,² and in 2023, New York confirmed nine equine cases, primarily in the northern part of the state, leading to euthanasia for all affected horses. These incidents underscore the importance of staying vigilant against EEE, no matter where you live.  

WEE has a lower mortality rate peaking around 40%.¹ Despite having caused minimal cases over the past decade, WEE continues to be detected in birds and mosquitoes west of the Mississippi.

WNV can result in fever and neurologic problems. It has become endemic to most of North America since it was first seen in the United States in 1999, and its mortality rates can reach over 30%.³ While recovery is possible, of the horses that do survive only 59 to 79% may experience a full recovery, with many exhibiting residual effects such as gait and behavioral abnormalities six months after diagnosis.⁴

Contact your veterinarian immediately if your horse exhibits any of the following signs:  

• Moderate to high fever (>101.5° F) 

• Severe depression or other behavior changes 

• Impaired vision  

• Lack of appetite or the inability to swallow 

• Circling or head pressing  

• Weakness, incoordination or muscle twitches 

• Paralysis of one or more limbs 

• Inability to stand  

• Convulsions 

These mosquito-borne diseases can pop up just about anywhere and they carry a high risk to the horse, which is why the AAEP recommends every horse be vaccinated against WNV and EEE/WEE every year.⁵ A six-month revaccination interval is recommended for horses less than 5 years of age and horses residing in endemic regions with extended mosquito seasons.⁵ 

Prioritize preventive strategies

Mosquito activity is ever-changing, due in part to warmer, wetter weather extending mosquito season and weather events like hurricanes shifting mosquito patterns. This leads to continual disease activity in expected areas and greater disease activity in unexpected states. That’s why it’s so important to develop a comprehensive preventive strategy.

Now is the time, if you haven’t already, to vaccinate your horses. If there is any doubt about your horse’s vaccination status, it should immediately receive an initial shot followed by a booster in three to four weeks.  

Consider how travel affects your horse’s vaccination needs. Even horses considered fully vaccinated should receive a booster before traveling to areas with year-round mosquito activity—like the Southern U.S. The horse’s lifestyle and geographic location can also impact disease risk.  

Farm management strategies can also help safeguard your horse. Control mosquito populations on your property to help mitigate mosquito-borne diseases: 

• Use insect repellents frequently 

• Stable horses at dusk and dawn when mosquitoes are most active 

• Eliminate or minimize standing water on property 

• Stock tanks or ponds with mosquito-feeding fish 

• Remove organic debris and items in which standing water can collect 

The Bottom Line 

As always, it’s important to work with your veterinarian to assess disease risk and develop a vaccination protocol that meets the needs of the individual horse. Merck Animal Health’s PRESTIGE® vaccine line includes a number of products that provide mosquito-borne disease protection. To learn more visit PrestigeVaccines.com.  

References 

¹ “Arboviruses,” Equine Disease Communication Center: Disease Fact Sheet, EDCC and AAEP, 2017, https://aaep.org/resource/arboviruses/  

² 2020 Summary of Eastern Equine Encephalitis Cases in the United States. USDA Animal and Plant Inspection Service. https://www.aphis.usda.gov/sites/default/files/2020-eee-report-summary.pdf 

³ “West Nile Virus,” Equine Disease Communication Center: Disease Fact Sheet, EDCC and AAEP, 2017, https://aaep.org/resource/west-nile-virus/ 

⁴ Wilson JH, Davis A, Bender JB, Minicucci, LA. Residual Effects of West Nile Viral Encephalomyelitis in Horses. In: 49th Annual Convention of the American Association of Equine Practitioners, 2003, New Orleans, Louisiana, (Ed.) 

⁵ AAEP Core Vaccination Guidelines (aaep.org)  

About The Author 

Dr. van Harreveld is a senior equine professional services veterinarian with Merck Animal Health. He has a strong interest in equine surgery and has completed extensive education in the area. Before joining Merck Animal Health, Dr. van Harreveld founded the Vermont Large Animal Clinic, an equine field service and referral hospital, that he operated in the Burlington, Vermont, area for over 20 years.