West Nile virus (WNV) is a mosquito-borne arbovirus known to cause severe illness in humans, birds, and horses.

Affected horses can develop inflammation in the brain (encephalitis), as well as neurological symptoms such as head pressing, difficulty swallowing, and poor coordination. [1][2]

Although only 10% of horses infected with WNV show signs of the disease, the mortality rate can be as high as 57% in those with symptoms. [3]

There is no specific treatment for West Nile virus in horses. Management is primarily supportive, aiming to reduce symptoms and prevent complications. [11]

Every year, sporadic cases of West Nile virus are reported across North America. Fortunately, equine vaccines against WNV and mosquito control measures have helped reduce cases in horses. [4][5]

West Nile Virus (WNV) in Horses

West Nile virus (WNV) belongs to a family of RNA viruses known as Flaviviridae. [6] The virus was first isolated in 1937 from a Ugandan woman. Since then, it has spread worldwide and is now a leading cause of viral encephalitis. [6]

In 1999, the first equine case of WNV was reported in the United States, with thousands of cases reported in the following years. [18]

Mosquitoes are the primary vector for WNV, which means they are the main agents that transmit the disease between hosts. Mosquitoes contract the virus from infected birds, and then transmit it to humans, horses, and other animals.

All horses can contract this virus, but unvaccinated, young, and elderly horses face a heightened risk of severe illness. [4]

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify Youtube
Mad Barn - Equine Nutrition Consultants | Mad Barn USA

Transmission

The transmission of West Nile Virus to horses is facilitated by mosquitoes that contract the virus by feeding on infected birds.

Wild birds are a reservoir for WNV, which means they carry the virus without getting sick. [2][6] Birds also act as amplifier hosts, allowing the virus to replicate. House sparrows (Passer domesticus) and other Passerine birds such as crows and robins have the highest levels of circulating virus in their blood. [3]

Mosquitoes, especially the Culex species spread the virus by carrying it from infected birds to other animals. [3] The virus replicates in the salivary glands of infected mosquitoes. When the mosquito feeds again, it transfers the virus to the next animal. [3]

Mosquitoes only contract the virus from viremic animals. When an animal is viremic, it has a high enough viral load in its blood that the mosquito ingests the virus and becomes a carrier. [3]

In horses and humans with WNV, the viral load in the blood is minimal, making them incapable of infecting mosquitoes. Both species are considered “dead-end” hosts, meaning they do not contribute to the transmission cycle of the virus. This also means that the virus cannot spread from horse to horse, or from horse to human and vice-versa. [1][3]

During the winter, WNV is thought to persist in mosquitoes, and remains at low levels in host species and/or migratory birds. [3] This keeps the virus endemic in the region, leading to new infections the following year.

Disease Outbreaks

West Nile Virus infections in horses can occur as sporadic cases or as outbreaks within a region. The most significant outbreak in North America took place between 1999 and 2002.

The virus was first detected in New York and rapidly spread to Canada, Mexico and the Caribbean. This WNV outbreak resulted in nearly 15,000 confirmed encephalitis cases in horses, 4,000 cases in humans, and the death of 16,500 birds. [3]

To reduce the risk of outbreaks, it is important to remain vigilant for signs of disease in your horse and implement preventative measures.

Pathogenesis of WNV Encephalitis

West Nile Virus infection in horses can lead to encephalitis, which is characterized by inflammation of the brain. Affected horses can develop inflammation throughout the nervous system, affecting the brain, spinal cord, and protective tissues. [5]

The virus is able to cross the blood-brain barrier, to access the central nervous system (CNS). Replication of the virus within the CNS and the ensuing immune response results in damage to neurons and surrounding cells.

The neurological symptoms associated with WNV, such as ataxia (incoordination), arise from infection of the pons and medulla of the brainstem, as well as the neighbouring cranial nerves. [7]

The virus can also replicate in other tissues including the spleen, liver, heart, lymph nodes, and lungs. [3] Although rare, WNV infections can also harm these tissues, as illustrated by a case of WNV-induced hepatitis in a South African foal. [8]

Clinical Signs

Not all horses that contract West Nile virus will show overt clinical signs of infection. However, horses that are symptomatic can exhibit a range of clinical signs, primarily of a neurological nature.

The first signs of WNV in horses are usually vague, consisting of general discomfort, lameness, and fever. [4] Behavioral changes are also indicative, with some horses appearing confused, lethargic, or agitated.

In horses with severe cases of WNV, the main signs are neurological impairments resulting from damage to the CNS. Impairments can be bilateral (affecting both sides of the body) or asymmetric (only affecting one side of the horse). [7]

Neurological Symptoms

The neurological signs of WNV infection in horses include: [2][3][9]

  • Muzzle twitching
  • Abnormal mental activity (from lethargy to hyperexcitability)
  • Head and/or lip drooping
  • Facial paralysis
  • Dysphagia (difficulty swallowing)
  • Ataxia (incoordination)
  • Hind limb weakness
  • Inability to stand
  • Sensitivity to touch or sound
  • Muscle weakness
  • Trembling or muscle twitching
  • Skin twitching

Abnormal Behaviours

Horses may also display abnormal behaviours, such as:

  • Head pressing
  • Circling
  • Teeth grinding

Some symptoms involve visual impairment, including aimless wandering, proprioceptive deficits (lack of awareness of body position), absent menace response, or even blindness. [4][7]

Risk Factors

Fortunately, 90% of horses infected with WNV do not show any clinical signs. [3] However, there is a high fatality rate among symptomatic horses with West Nile encephalitis.

Some horses are more susceptible to severe West Nile encephalitis, including horses that are: [1][4]

  • Young (<5 years)
  • Unvaccinated against WNV
  • Aged (>15 years)

Given the risk of fatality with WNV infection, it’s important to contact your veterinarian immediately if you notice your horse displaying neurological symptoms.

Diagnosis

Horses displaying ataxia (uncoordinated movement) along with at least two other neurological symptoms are strongly suspected of having a WNV infection. [9]

Your veterinarian will consider your geographic region to gauge mosquito activity and track any reported WNV cases. The health history and vaccination record of your horse are also critical for accurate diagnosis.

Your veterinarian will also want to know when your horse was last vaccinated against WNV, as certain vaccines can influence the interpretation of test results. [9]

Antibody Testing

A blood test for antibodies against WNV is considered the most reliable method of detection. Cerebrospinal fluid (CSF) can also be used for this test. [3]

This test measures the concentration of immunoglobulin M (IgM) antibodies against WNV circulating in your horse’s blood. This is the preferred diagnostic method because IgM antibodies are short-lived, so their presence indicates a recent infection. [9]

A paired immunoglobulin G (IgG) test is also available. For this test, an initial blood sample is taken when clinical signs first appear with a follow-up test 14 days later. A four-fold rise in IgG antibodies against the virus indicates a significant infection in the horse. [3]

Post-Mortem Assessment

In deceased or euthanized horses, the brain and spinal cord can be examined to detect the virus either by testing for its genetic material or using staining techniques for visual identification. [6]

Microscopic (histologic) examination can also reveal inflammation in the brain and spinal cord. [3]

Differential Diagnoses

Other viral diseases that affect the nervous system may appear similar to WNV. These diseases should be ruled out by the veterinarian at the time of diagnosis. [6]

Examples of equine viruses with similar clinical signs include: [3][11]

Less likely causes of neurologic symptoms in horses include: [2][3]

In rare cases, migration of parasite larvae such as Strongylus vulgaris into the CNS can also result in neurological impairments.

Disease Notification

Accurate diagnosis of WNV is crucial because it’s an immediately notifiable disease. This means that upon detection, it must be reported to government agencies to ensure that appropriate measures are taken to prevent further spread and potential outbreaks.

When one horse in an area is diagnosed with WNV, it signals the virus’s presence and indicates a high risk of infection for other horses and humans. [6]

In the USA, confirmed cases of WNV in horses are reported to the United States Department of Agriculture (USDA) and state veterinary authorities.

In Canada, confirmed cases are reported to both federal and provincial animal and public health authorities. Additionally, the virus is notifiable to the World Organization for Animal Health (OIE). [10]

These agencies work collaboratively with local health departments to monitor and manage the spread of the disease, ensuring that there’s a coordinated response and the public is informed of any potential risks.

Treatment

Much like other viral diseases, treatment of West Nile virus is focused on providing the horse with supportive care. Antiviral medications are often ineffective in horses. [11]

Anti-inflammatory medications, such as NSAIDs or corticosteroids, are frequently administered to manage symptoms. [4][5]

The NSAID flunixin meglumine (banamine) has been reported to reduce muscle tremors and twitches. [2] If necessary, the sedative acepromazine can be given to reduce anxiety. [2]

If the horse struggles to eat or drink, intravenous fluids may be needed to prevent dehydration. Horses might need supportive care for up to six months before they return to normal functioning. [4]

While treatments, including antibodies targeting the virus, have been explored, their effectiveness remains uncertain. [4]

Recumbent Horses

In severe cases, affected horses will remain recumbent (lying down) for long periods of time. Horses may still be alert and thrash around, putting themselves and their attendants at risk of injury.

To prevent injuries, tranquilizers such as dexamethasone sodium, mannitol, and detomidine hydrochloride may be necessary. [2]

Horses that remain recumbent for long periods are prone to developing infections in wounds, cellulitis, and pneumonia. These horses may require antibiotics to treat secondary infections. [2]

Using slings to keep recumbent horses upright may prevent additional injuries and complications. [12] For horses with WNV that remain recumbent, euthanasia can be the most humane option. [2]

Prognosis

During West Nile virus outbreaks, mortality rates for symptomatic horses have ranged between 38% and 57%. [3] Horses with severe symptoms such as convulsions, paralysis and recumbency have a poor prognosis and a higher risk of death. [4][5]

Reports indicate that 10 – 40% of affected horses recover, though they may exhibit lingering neurological issues, including gait and behavioural abnormalities. [1][5]

Recuperated horses are believed to have life-long immunity from WNV, making them less susceptible to severe reinfections. [1]

Prevention

West Nile Virus presents a significant health and economic risk to equine populations worldwide. The following section provides a comprehensive overview of measures to prevent the transmission of WNV in horses.

Vaccines

Equine vaccines for West Nile Virus are widely available and considered effective for protecting horses against this disease. [13][14]

In North America, the WNV vaccine is considered a core vaccine, which means it should be given to all horses on a regular schedule. [1] Other core vaccines for horses include:

  • Eastern/Western Equine Encephalomyelitis
  • Rabies
  • Tetanus

Vaccination Schedule

Talk to your veterinarian about an appropriate vaccination schedule for your horse depending on your location, prevalence of WNV in the area, and current public health guidelines. [4]

Below is a typical vaccination schedule for WNV in adult horses, broodmares, and foals.

Adult horses

  • Previously vaccinated – normal risk: Repeat vaccination annually in the spring [15][16]
  • Previously vaccinated – high risk: Horses in high risk areas may be vaccinated every six months
  • Not previously vaccinated or unknown history: Provide an initial series of two doses, 4 – 6 weeks apart followed by yearly or semi-annual boosters depending on risk
  • Aged adult horses (<15 years): More frequent vaccinations to maintain immunity [1]

Pregnant Mares

While no research exists on WNV vaccination in pregnant mares, it is acceptable practice due to the high risks of WNV infection compared to vaccine side effects. [1]

For broodmares with unknown vaccination history, it is recommended to vaccinate them when not pregnant. Otherwise, vaccinate annually and 4-6 weeks before foaling. [16]

Vaccinating the pregnant mare also helps protect the foal from severe disease. Antibodies produced by the mare’s body following vaccination will be passed to the foal through colostrum – the mare’s first milk.

Foals and Young Horses

  • Normal-Risk Foals: Provide an initial three-dose series starting at 4 – 6 months old. [17]
  • High-Risk Foals: Foals can be vaccinated under 4 months of age if they are in a high risk area or born to unvaccinated mares at risk of WNV. [17]
  • Young Horses (1 – 5 Years): More frequent vaccination may be recommended to protect against severe disease. [1]

Mosquito Control

Reducing your horse’s exposure to mosquitoes is also important for preventing WNV. The following environmental management practices will help control mosquito populations and reduce the spread of this virus:

  1. Eliminate stagnant water to remove mosquito breeding sites. On a weekly basis, drain water containers, barrels, and other typical stagnant water spots like old tires. [4]
  2. Install fans and blowers in your barn to deter mosquitoes from landing on horses and transmitting the virus. Follow other tips to improve airflow. [4]
  3. Bring horses inside during dusk and dawn, which are peak times for mosquito activity. [6]
  4. Apply insecticides or mosquito repellents during periods of high mosquito activity. Follow label instructions for correct application. [4]

Additional Measures

Crow and other corvids are early indicators of WNV outbreaks. An increase in reported bird fatalities often precedes reports of this diease in humans and horses. [3]

Report bird carcasses to provincial or state wildlife authorities to aid in the surveillance of West Nile Virus. Reporting helps authorities inform local horse owners about increased risks, so they can implement preventative measures to protect their horses.

Summary

  • Wild birds are reservoirs of West Nile Virus, who then transmit the disease to biting mosquitoes.
  • Mosquitos are the primary vectors of WNV, and can spread the virus to horses and humans.
  • While most horses with WNV do not show signs of the disease, some infected horses exhibit neurological symptoms and are at high risk of mortality.
  • There are no treatments available for WNV, but many horses recover with appropriate supportive care.
  • Vaccinating your horse is the best way to prevent infection. Ask your veterinarian when to vaccinate your horse.
  • Reduce your horse’s exposure to mosquitoes by removing stagnant water, using bug stay and maintaining good airflow in your stable.

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.

References

  1. West Nile Virus. AAEP. Accessed Oct. 04, 2023.
  2. Long M.T. et al., Equine West Nile Encephalitis: Epidemiological and Clinical Review for Practitioners. Emerging Infectious Diseases. 2002.
  3. Castillo-Olivares J. and Wood J., West Nile Virus Infection of Horses. Vet. Res. 2004.
  4. Long M.T., West Nile Encephalomyelitis in Horses. Merck Vet Manual. Accessed Oct. 04, 2023.
  5. Paré J. and Moore A., West Nile Virus in Horses — What Do You Need to Know to Diagnose the Disease?. Can Vet J. 2018. View Summary
  6. Ciota, A.T.West Nile virus and its vectors. Curr Opin Insect Sci. 2017.
  7. Sellon D.C. and Long M.T., Equine Infectious Diseases E-Book. Elsevier Health Sciences. 2013.
  8. Venter, M. et al. Lineage 2 West Nile Virus as Cause of Fatal Neurologic Disease in Horses, South Africa. Emerg Infect Dis. 2009. View Summary
  9. Young A., West Nile Virus. UC Davis. Accessed Oct. 04, 2023.
  10. Government of Canada. Immediately notifiable disease. Accessed Oct 5. 2023.
  11. Reed S.M. et al. Disorders of the Neurologic System. In: Equine Internal Medicine (Fourth Edition). W.B. Saunders. 2018.
  12. Porter M.B. et al. West Nile Virus Encephalomyelitis in Horses: 46 Cases (2001). Journal of the American Veterinary Medical Association. 2003. View Summary
  13. Long, M.T. et al. Efficacy, duration, and onset of immunogenicity of a West Nile virus vaccine, live Flavivirus chimera, in horses with a clinical disease challenge model. Equine Vet J. 2007. View Summary
  14. Ng, T. et al.Equine vaccine for West Nile virus. Dev Biol (Basel). 2003. View Summary
  15. Desanti-Consoli, H. et al. Equids’ core vaccines guidelines in North America: considerations and prospective. Vaccines (Basel). 2022. View Summary
  16. American Association of Equine Practitioners Infectious Disease Committee Vaccinations for adult horses. AAEP. Accessed Oct 5, 2023.
  17. American Association of Equine Practitioners Infectious Disease Committee Vaccinations for foals. AAEP. Accessed Oct 5, 2023.
  18. Weese, JS. West Nile virus encephalomyelitis in horses in Ontario: 28 cases. Can Vet J. 2003. View Summary