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United Kingdom
Horse looking over fence

West Nile Virus Disease

West Nile virus (WNV) is a virus that is transmitted from migrating carrier birds to mammals through the bite of various mosquito species, particularly Culex species. Horses and humans have been known to be particularly affected by West Nile virus, which travels through the bloodstream to the brain and the spinal cord, resulting in an inflammation which can cause severe and potentially fatal neurological symptoms. Originating in Africa many years ago, the virus has spread throughout the world, and is now known to also occur in Australia, Asia, Europe, and North America. After being initially diagnosed in the United States in 1999, WNV rapidly became a nationwide epidemic which debilitated tens of thousands of horses, with a substantial mortality rate. Although this virus can only be transmitted directly by mosquitoes (not from horse to horse, nor from horses to humans) every unprotected horse is at risk, particularly during the mosquito season. WNV most recently began to re-appear in Europe in a new outbreak which started in Italy in 2008, where the disease continues to affect both horses and humans. There is no cure for West Nile virus disease, but individual horses can be protected through vaccination.

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  • Symptoms may appear within 3-15 days of being infected

    • Fever and flu-like symptoms
    • Loss of appetite
    • Depression or lethargy
    • Inability to swallow
    • Impaired vision
    • Head pressing or tilt
    • Stumbling or tripping
    • Muscular weakness or twitching
    • Aimless wandering or circling
    • Partial paralysis
    • Inability to stand up unsupported
    • Convulsions
    • Coma
  • West Nile virus infection should be considered if a horse exhibits any, or a combination, of the neurological symptoms described above. These symptoms are particularly relevant if a horse has not been vaccinated against West Nile virus, and if there is possible exposure to the mosquito vector. Potential cases must be quickly diagnosed in order to begin the necessary supportive treatment; therefore it is very important to seek veterinary attention should a horse display such abnormal behaviour. Definitive diagnosis to rule out other neurological diseases (for example, rabies) is only possible through laboratory tests, and it is important to maintain good vaccination records to assist in the diagnosis of this disease. Laboratory tests usually involve blood sampling in order to diagnosis specific WNV antigens or antibodies present in the blood serum.

  • Horses will naturally produce antibodies specific to West Nile virus, and may overcome a mild case of WNV disease. However, all diagnosed cases require veterinary attention. There is no specific treatment to cure WNV disease: treatment is “supportive” – managing the disease itself, whilst giving appropriate nursing care. If a horse exhibits serious neurological symptoms, it will be necessary to provide care in a clinical setting. Such horses need to be in an environment that is safe, with padded walls and thick bedding, and may need to wear protective head gear to avoid self-harm. Those which are unable to stand unassisted may have to be put into a special supportive sling.

    Adequate hydration and nutrition must be ensured, and if a horse is unable to achieve voluntary intake, it may be necessary to administer fluids and nutrition intravenously. Other supportive treatment may include nonsteroidal anti-inflammatory drugs, sedatives, and anti-viral drugs, although the success of the latter is still indeterminate. Recovery of these clinical cases can take many months. If a horse survives a severe encephalitis, there may nevertheless be permanent damage to the central nervous system of the horse.

  • Vaccination of the horse and exposure to the mosquito vector are both important to prevent West Nile virus disease. 


    Vaccination can greatly reduce the risk of horses suffering the consequences of West Nile virus. This effectiveness has been successfully shown in the United States where the number of reported equine cases has declined year-on-year since the outbreak peak in 2002.  To protect horses against West Nile virus, the vaccination programme should be completed well before the start of the mosquito season (which can vary greatly from year to year), so that the horse is well protected before it is actually at risk of being bitten.

    Vector (mosquito) control

    Insecticides are a simple and effective method of reducing adult mosquito populations.  To control the main source of mosquitoes, activities should be focused at eliminating the mosquitoes’ breeding ground in order to reduce the number of mosquito larvae.

    • Change the water from troughs or other drinking receptacles at least every 4 days – but preferably daily.
    • Remove any containers (tyres, old feed buckets) where water could collect and keep your horse’s environment tidy.
    • Keep lids on any bins and containers.
    • Clean and remove debris on a regular basis from gutters to prevent water collecting in them.
    • If available and not banned for use in your country consider purchasing mosquito-eating fish for use in ponds, etc.

    Reduce horse exposure to mosquitoes

    • Keep horses stabled during dawn and dusk, when mosquitoes are most active
    • Turn off lights so as not to attract mosquitoes at night, or alternatively use fluorescent lights which do not attract mosquitoes.
    • Affix mosquito-proof screens to stable windows. 
  • How many bites does a horse need to receive from infected mosquitoes for it to become infected?

    It is not known how many bites a horse requires for it to become infected, and may depend on the level of viremia in the infected mosquitoes.

    How long from a horse being infected will it take for symptoms to appear?

    Symptoms may appear in 3 to 15 days. However, not all horses may show signs of the disease.

    Can horses infected with West Nile virus infect others?

    No, West Nile virus is transmitted by the bite of infected mosquitoes. The mosquito is only infected by taking a blood meal from birds. WNV transmitted to horses by the bite of infected mosquitoes does not replicate into high enough levels within the horse to infect another mosquito subsequently biting that same infected horse. Horses (and humans) are therefore considered ‘dead end’ hosts.

    Must a horse is found to be infected with West Nile virus be reported to any authority?

    Yes, as a notifiable disease any horse infected with West Nile virus must be notified to the country’s appropriate authorities.

    Can a horse that has been exposed naturally to West Nile Virus still be vaccinated?

    It is important to achieve maximum protection against WNV by vaccinating all horses, regardless of whether it is suspected that they may have been naturally exposed to the virus.  However, any horse that is obviously sick should not be vaccination whilst exhibiting symptoms of illness.

    Will an equine vaccination programme interfere with WNV surveillance studies (ie testing undertaken in a given region to determine whether West Nile virus is present in populations of mosquitoes, birds and/or horses in that area)?

    WNV vaccination should have no significant impact upon WNV surveillance studies.  Horses which are naturally infected will have two types of antibodies in their blood serum (IgG and IgM).  Horses which are vaccinated (and not naturally infected) are unlikely to have IgM antibodies in their blood serum.  Therefore, surveillance tests can usually distinguish between vaccinated and naturally infected horses.

    Why should a horse be vaccinated against West Nile virus if the disease has not been isolated in a specific European country or region?

    West Nile virus is already circulating in the bird population in Europe and the Culex species of mosquitoes that transmit this disease to horses are distributed widely over  Europe. As such an outbreak could potentially strike at any time, the only way to reduce the risk of a horse succumbing to the disease is through vaccination. Furthermore,  vaccinating in the face of an outbreak is potentially too late to protect horses, as full vaccine protection is only achieved 3 weeks following the completion of the primary vaccinations.  Additionally, any horses travelling to areas where West Nile virus is endemic (e.g. America and northern Italy) would need to be vaccinated at least 6 weeks before travelling to ensure that they are protected when they enter that country.

    From what age may a foal be vaccinated?

    Foals may be vaccinated from 6 months of age.