Equine mosquito-borne illnesses

Photo: OleksandrZastrozhnov/Adobe Stock

Along with mosquito season comes a rise in potentially fatal mosquito-borne illnesses in horses, including West Nile Virus (WNV) and Eastern Equine Encephalitis (EEE). Amanda Trimble, BVMS, PGCertVetEd, MS, a clinical assistant professor of equine internal medicine at the Texas A&M College of Veterinary Medicine and Biomedical Sciences, explained how WNV and EEE affect horses, and steps owners can take to prevent infections.¹

“About 1/3 of affected horse — typically those that are unvaccinated — may die from [WNV], while more than 90% of unvaccinated horses die from an EEE infection,” Trimble said in a Texas A&M College of Veterinary Medicine and Biomedical Sciences News story.¹ “With both, disease and clinical signs may still occur in horses that are vaccinated but are usually much less severe. However, horses that survive may have permanent neurological deficits.”

West Nile Virus (WNV)

According to the UC Davis Veterinary Medicine Center for Equine Health, horses represent 96.9% of non-human WNV cases. Since first being introduced to the United States in 1999, the virus is now found in all of the contiguous 48 states. The virus enters the bloodstream through a bite from an infected mosquito, where it multiplies.²

In order to diagnose WNV in horses, veterinarians first rule out other possible neurological diseases. An lgM Enzyme-Linked Immunosorbent Assay (ELISA) test measures the amount of lgM antibodies in a given sample, and is considered the most reliable method to detect WNV infection, capable of identifying the virus within 6 days, and after 2 months after infection.²

Although many horses do not show signs of illness, Trimble recommends that owners contact a veterinarian if they suspect their horse has the virus, based on any of the following symptoms:¹

• Lethargy
• Fever
• Anorexia (low appetite)
• Neurological signs (sudden onset tremors, weakness, poor coordination, stumbling, or falling)
• Recumbency

Eastern Equine Encephalitis (EEE)

Commonly referred to as “sleeping sickness,” EEE is a viral disease that causes inflammation of both the brain and the spinal cord. EEE is the most common in the southeastern United States, with the primary vector being the mosquito Culiseta melanura, although it can be transmitted by other species of mosquito, as well. Once infected, horses are not required to be quarantined, as they do not carry enough of the virus to be contagious.³

The virus is diagnosed through bloodwork. Possible tests include ELISA tests or polymerase chain reaction (PCR) on cerebrospinal fluid (CSF) samples. Trimble recommends contacting a veterinarian based on an owners suspicion and recognition of the following symptoms:¹

• Lethargy
• Fever
• Anorexia
• Neurological Signs (inability to swallow, tremors, head pressing, seizures, circling, blindness, rapid changes in behavior, poor coordination, stumbling, or falling)
• Recumbency

Prevention

Trimble recommends creating a prevention plan with a veterinarian, which includes vaccination. The vaccines for both WNV and EEE are considered ‘Equine Core Vaccines’ by the American Association of Equine Practitioners.⁴ She recommends bi-annual vaccinations for horses in locations where mosquitoes are present year-round.¹

“These diseases do not need other horses to spread, and they can cause severe and potentially fatal neurological disease if a horse is not properly protected from them,” Trimble said.¹ “The vaccines are safe and effective and can help decrease the spread and severity of the disease.”

Additionally, Trimble recommends the implementation of environmental management strategies, meant to reduce the mosquito population in the horse’s proximity. Steps can include:¹

• Reduction of standing water
• Installing mosquito lights/zappers
• Using fly sheets/boots
• Frequent application of bug repellent
• Adding a garlic supplement to horses’ feed
• Keeping horses inside at dusk, dawn, and overnight
• Daily cleaning of water buckets and tanks
• Running fans
• Introducing mosquito-eating fish to ponds/lakes

Regarding both viruses, Trimble said the care a veterinary team can provide is mostly supportive. “Care is largely supportive. For non-severe cases, this may include anti-inflammatories and keeping the horse hydrated and eating,” Trimble said.¹ “More severe cases, which includes horses with severe neurological signs or recumbency, may need to be hospitalized or referred to a facility that can provide intravenous fluids and more intensive medical care.”

References

1. Say ‘Neigh’ to West Nile Virus, Eastern Equine Encephalitis. Texas A&M Veterinary Medicine and Biomedical Sciences News. September 12, 2024. Accessed September 16, 2024. https://vetmed.tamu.edu/news/pet-talk/west-nile-virus-eastern-equine-encephalitis/
2. Young A. West Nile Virus. UC Davis Veterinary Medicine Center for Equine Health. August 19, 2020. Accessed September 16, 2024. https://ceh.vetmed.ucdavis.edu/health-topics/west-nile-virus
3. Young A. Eastern Equine Encephalitis (EEE). UC Davis Veterinary Medicine Cetner for Equine Health. August 28, 2020. Accessed September 16, 2024. https://ceh.vetmed.ucdavis.edu/health-topics/eastern-equine-encephalitis-eee
4. Vaccination Guidelines. American Association of Equine Practitioners. Accessed September 16, 2024. https://aaep.org/guidelines-resources/vaccination-guidelines/