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West Nile Virus

West Nile Virus is not currently circulating in the UK, but with horses being transported globally and the ongoing impact of climate change, there’s a risk that outbreaks could occur in the future.

  • Last reviewed: 14th April 2023
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West Nile Virus (WNV) is a notifiable disease to the Department for Environment, Food and Rural Agency (DEFRA) in England and Wales1, and in Scotland and Northern Ireland to the equivalent government authority.


Birds are known as ‘natural carriers’ of the virus and are infected while in a warmer country and then migrate to other countries. A mosquito then bites an infected bird and becomes a carrier of the virus and can pass it on to other mammals when biting them. Once a horse or human has been infected, they cannot pass the virus directly to another – this makes them known as “end stage hosts”2.


Most infected horses will not show any signs3 – making it more important to be vigilant of transporting and isolation periods. The incubation period for the virus in horses is approximately 3-15 days after being bitten1. WNV causes encephalomyelitis which is a swelling of the brain and spinal cord.

Horses that do display signs may show:

  • Dullness/depression
  • Tiredness
  • Paralysis of the face (droopy eyelids or lips)
  • Head pressing
  • Listlessness
  • Unable to get up from lying down
  • Lack of coordination
  • Muscle tremors
  • Sensitivity to touch or sound
  • Seizures


Blood would need to be taken from the horse to test for the presence of the virus or an increase in antibodies which would be present if trying to fight off the disease. In some cases, cerebrospinal fluid (colorless liquid taken from around the brain or spine) may also be taken to define the exact cause of possible signs3.


There are no specific treatments for WNV. The signs presented may be managed by supportive care but this would have to be assessed on a case-by-case basis, depending on the severity of the signs. Anti-inflammatories and IV fluids may be offered to support the horse through recovery.

Many horses recover and return to full health, although there are a small minority of cases which report long-term effects such as gait abnormalities4. It’s reported that the mortality rate for horses which show clinical signs is between 35%-45%5.


Vaccination against the virus is available and is highly recommended for horses being transported to areas where the virus is present4. Humans, as well as horses, are encouraged to seek shelter from mosquitos where possible at peak times of the day  this could be providing mosquito nets in stables and humans wearing long sleeved tops and trousers2. Avoiding travel to parts of the world which are at a higher risk of transmission, is also advisable3.


  1. APHA (DEFRA). (2020) West Nile Virus.
  2. Young, A. (2020) West Nile Virus.
  3. VCA Animal Hospitals. (2022) West Nile Fever in Horses.
  4. MSD Veterinary Manual. (2019) West Nile Encephalomyelitis in Horses.
  5. Gagea, A. Scarsdale Vets. (2019) West Nile Fever.

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