Equine Viruses

There are seven known mosquito-borne viruses which affect horses worldwide:

  • West Nile Virus (including Kunjin - an Australian strain)
  • Japanese Encephalitis Virus
  • Venezuelan Equine Encephalitis Virus
  • Western Equine Encephalitis Virus
  • Eastern Equine Encephalitis Virus
  • Murray Valley Encephalitis Virus
  • Ross River Virus

These viruses are called encephalitis viruses because the clinical signs are associated with swelling of the brain. In many horses these infections will cause only a short fever which the owner may not even notice, but in a proportion of cases encephalitis, and therefore signs of illness occur.

Signs include:

  • Reduced co-ordination of the legs, ranging from mild to paralysis
  • Difficulty eating due to lack of co-ordination
  • Dramatic change in attitude / behaviour
  • Commonly drowsiness, but narcolepsy is also possible (collapse due to suddenly falling asleep)
  • Muscle twitches (often seen in other conditions such as grass sickness, which are seen in the UK)
  • Impaired vision
  • Profuse sweating

West Nile Virus: A virus you may have heard about

There have been several outbreaks of West Nile Virus (WNV) in Europe including in France, Italy and Spain. It is not known how many horses in these outbreaks are infected without any signs, but between 40% and 60% of horses with signs of brain swelling did not survive in these outbreaks.

WNV is a notifiable disease in the UK, meaning that any case or suspected case must be notified (by a vet) to the authorities, so that steps can be taken to help protect other horses in the UK. There is a vaccine against WNV which is licensed for use in the UK and is therefore available immediately, should there be a known risk to UK horses.

However, it should be remembered that these viruses cannot be spread from one horse to another and that an infected mosquito is required to cause infection in a horse. Mosquitoes do not become infected with WNV by biting horses because horses do not produce enough virus in the blood to infect mosquitoes. Mosquitoes become infected by biting certain types of infected birds.

More information about how WNV is transmitted is found here.

You may have read about the WNV outbreak in North America and the large number of horses affected. It is thought that this size of outbreak is less likely in Europe because many birds in Europe are exposed to WNV through migration to Africa, and therefore the number of birds highly infected with WNV is likely to be lower. This should have some protective effect, reducing infections in mosquitoes and therefore horses. More information from research is required to assess the risk of mosquitoes becoming infectious in the UK, as this depends on the mosquito species, along with other factors such as temperature.


Part of the aim of this project is to increase the knowledge-base in the UK, for actions that should be taken in the event of any of these viruses entering the UK

There are vaccinations against Venezuelan, Western and Eastern Equine Encephalitis Viruses available in the USA where these viruses are currently found, therefore it is likely that if there were European outbreaks of these diseases these vaccines could be put through the licensing process in Europe.

Ross River Virus and Murray Valley Encephalitis have caused outbreaks in Australia, and Japanese Encephalitis Virus causes outbreaks mainly in Asia.