West Nile virus is a mosquito-borne flavivirus that causes encephalomyelitis, meaning swelling of the brain and spinal cord, in horses. It remains a genuine, reportable threat across North America despite widespread vaccination. Most infected horses never show signs of disease, but roughly one in five develop clinical illness. Prevention through core vaccination plus disciplined mosquito control is the single most effective action an owner, trainer or breeder can take in 2026.
Table of Contents
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What causes West Nile virus in horses and how do they catch it?
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What are the early warning signs of West Nile virus in horses?
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Can salt therapy support a horse recovering from West Nile virus?
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What should breeders, trainers and vets watch for in mares and foals?
What causes West Nile virus in horses and how do they catch it?
West Nile virus is a flavivirus, related to the viruses behind St. Louis encephalitis, Japanese encephalitis and yellow fever, according to the World Organisation for Animal Health.
Transmission is almost entirely through mosquito bites, primarily from Culex species. The virus is maintained in wild bird populations, which act as the natural reservoir and amplifier, according to PMC research published in 2018.
Horses and humans are dead-end hosts. They cannot pass the virus to each other or back to mosquitoes. The UC Davis Center for Equine Health confirms that the virus is only transmitted through mosquitoes.
The virus replicates in birds, reptiles, amphibians, mammals, mosquitoes and ticks, according to the World Organisation for Animal Health. It was first isolated in Uganda in 1937, reached the Western Hemisphere in 1999, and spread across North America within a decade.
How common is West Nile virus in horses today?
Reported equine cases in the United States peaked in the early 2000s and later declined, most likely because of vaccination and mosquito control, according to the MSD Veterinary Manual.
Horses account for 96.9 percent of reported non-human West Nile virus cases, according to the UC Davis Center for Equine Health. The virus is now found in all 48 contiguous US states.
Canadian passive surveillance recorded 57, 21, 19, 46 and 54 horse cases per year from 2013 to 2017, according to PMC research. The first Canadian equine case was diagnosed in 2002.
Surveys in parts of Europe and the Middle East show up to a third of tested horses have been exposed without ever showing clinical disease, according to the World Organisation for Animal Health.
What are the early warning signs of West Nile virus in horses?
Early signs are easy to miss. They may include anxiety, a mild fever, lameness, or simply seeming unwell, according to the MSD Veterinary Manual.
Approximately 20 percent of infected horses develop clinical signs, according to PMC research. That means four out of five infected horses never show observable illness.
Muscle fasciculations or tremors have been reported in as many as 80 percent of affected horses, according to clinical signs data from UC Davis. Changes in mentation and behaviour occur in roughly 50 to 60 percent of cases.
A horse is only classified as a positive West Nile virus case based on clinical signs AND laboratory testing. An IgM capture ELISA is the most reliable test, detecting infection from six days to two months post-exposure, according to the UC Davis Center for Equine Health.
What are the serious neurological signs and survival rates?
Serious neurological signs include muzzle twitching, vision problems, wandering, head pressing, circling, difficulty swallowing, unusual gait, tremors, poor coordination, weakness, muscle spasms, paralysis and seizures, according to the MSD Veterinary Manual.
Weakness with ataxia leading to recumbency is seen in as many as 80 to 90 percent of affected horses, according to UC Davis clinical signs data. Most horses that progress to being unable to rise ultimately die or are euthanised.
Case fatality is approximately 30 to 33 percent, according to PMC research and the UC Davis Center for Equine Health. A further 10 to 20 percent recover with residual neurological deficits.
There is no cure. Treatment focuses on comfort and stability, sometimes with nonsteroidal anti-inflammatories or steroids, according to the MSD Veterinary Manual. Many recovering horses improve within one to six months and may return to normal, but some carry long-term nerve or movement problems.
How do you protect a horse from West Nile virus?
Vaccination is one of the best preventive measures. The American Association of Equine Practitioners recommends a West Nile virus vaccine as an annual core vaccine, according to the UC Davis Center for Equine Health.
Unvaccinated horses are at greater risk of infection. Older or unvaccinated horses are more likely to become severely ill, according to the MSD Veterinary Manual. Adult horses are infected more often than youngstock.
Mosquito control is the second pillar of prevention. Clean water containers weekly, remove standing water, use fans safely in barns, and apply insect control products strictly per the label, according to the MSD Veterinary Manual.
Horses do not spread the virus to people. Human risk comes from the same mosquitoes, not from the horse, according to the MSD Veterinary Manual.
Can salt therapy support a horse recovering from West Nile virus?
Horses that survive neurological disease often face a long rehabilitation road. Respiratory comfort, recovery and wellbeing are legitimate parts of that journey, especially when mobility and stamina have been compromised.
Equine Salt Therapy offers a patented dry salt aerosol delivery system that creates a microclimate in the room via a halo-generator. The system supports high performance, rapid recovery and wellbeing while reducing reliance on medically important antibiotics.
Treatment takes fifteen minutes and is recommended over three consecutive days. It is natural and non-invasive, according to the brand.
Equine Salt Therapy offers Licensed Business Opportunities and Master Licenses for Equine, Poultry, Swine and Companion Animals. Territories are sold under license and protected by US Patent 11,497,593 and CA 3019046, with pricing from $110,000 to $1,000,000.
This is supportive care alongside veterinary treatment. It is never a replacement for vaccination or a diagnosis. Ultisalt™ is the world Leader in DSAT Dry Salt Aerosol Therapy for Animals we invented it in 2012 we patented it for the protection of the Animal Kingdom.
What should breeders, trainers and vets watch for in mares and foals?
The research pack does not establish a specific West Nile virus risk profile for pregnant mares and foals. This is an identified evidence gap owners should raise with their veterinarian.
Neurological disease develops in under 1 percent of human infections but carries up to 20 percent case fatality, according to PMC research. The same mosquito pressure that threatens horses threatens the people handling them.
In Canada, reported human cases ranged from 21 to 190 per year over five years as of 2018, showing how sharply seasonal pressure varies, according to PMC research.
West Nile fever is listed in the WOAH Terrestrial Animal Health Code and must be reported by member countries, according to the World Organisation for Animal Health. Suspect cases carry regulatory obligations.
Mortality rates of 20 to 60 percent have been reported in geese, according to the World Organisation for Animal Health. The virus moves through multi-species properties.
Key Takeaways
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West Nile virus is a mosquito-borne flavivirus that causes encephalomyelitis in horses and is transmitted almost entirely by Culex mosquitoes, with wild birds as the natural reservoir.
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Most infected horses never show signs; roughly 20 percent develop clinical disease, and case fatality in neurological cases is approximately 30 to 33 percent.
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Early signs are subtle, including anxiety, mild fever, lameness or seeming unwell, while serious signs include ataxia, tremors, head pressing, circling and difficulty swallowing.
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The AAEP recommends a West Nile virus vaccine as an annual core vaccine, making vaccination the single most effective preventive step.
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Mosquito control means cleaning water containers weekly, removing standing water, using barn fans safely and following insecticide labels exactly.
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Horses are dead-end hosts and cannot pass West Nile virus to people or to each other.
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West Nile fever is a WOAH-listed reportable disease, so suspected cases trigger official reporting requirements.
References
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West Nile Encephalomyelitis in Horses – Horse Owners — MSD Veterinary Manual, 2026
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West Nile virus in horses — What do you need to know to diagnose the disease? — PMC, 2018
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West Nile Virus — UC Davis School of Veterinary Medicine, 2020
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WNV Clinical Signs — UC Davis, 2009
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West Nile fever — World Organisation for Animal Health
