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  • West Nile Virus (WNV)

What is West Nile Virus?

West Nile Fever is a disease caused by a flavivirus (WNV) that is carried by mosquitoes. It causes encephalitis (inflammation of the brain and spinal cord) that can affect humans and other animals. Prior to 1999, the virus was found only in Africa, eastern Europe and western Asia, but in August 1999 it was identified in the north eastern states of USA (New York and the New England states), causing fatal illness in a variety of animals, including horses and people. Since then it has spread dramatically across USA and Canada. The virus has been found in Mediterranean Europe, including southern France, causing disease in horses, however the disease has not yet been confirmed in the UK. Although disease has not been found in the UK, there has been serological evidence of viral challenge found in birds in Britain.

What are the effects of WNV?

Infection with WNV does not always cause signs of clinical abnormality. Many infected animals show no sign of illness although they produce detectable blood antibody levels. In areas where the virus occurs, wild birds, which are commonly infected by mosquitoes, then act as a source of replicating virus for mosquitoes to bite and then infect other animals. Horses and humans can be infected, however both horses and people are 'end-stage' hosts and are not infectious to other horses or people.

Clinical signs in horses range from a loss of appetite and depression to any one, or combination of the following:-

  • Elevated body temperature.
  • Weakness or paralysis of the hind limbs.
  • An apparent loss of orientation, resulting in walking in circles or aimless wandering and bumping into walls.
  • Blindness or excessive excitability.
  • Coma, collapse and death.

How is WNV diagnosed?

Blood and sometimes cerebrospinal fluid (CSF) tests are usually needed to differentiate between different causes of encephalitis and inco-ordination. Diseases to be differentiated include other encephalitis producing viruses (Eastern, Western and Venezuelan encephalitis, not usually seen in UK), protozoal encephalomyelitis (EPM, only seen in horses imported from USA), wobbler syndrome, traumatic injury, poisoning, bacterial infection and chronic/advanced liver disease.

Where a horse is suspected of having West Nile Fever, blood tests can be performed to test for the presence of the virus or antibodies that the horse has produced in response to the infection.

Can WNV be treated?

There is no specific treatment for West Nile virus - treatment is aimed at symptomatic and supportive care until natural resolution or death occurs, or euthanasia is required on humane grounds.

Vaccines using killed virus are available for use in horses in the USA and widespread use there shows that it is safe and that it produces useful immunity that either prevents or reduces the severity of the disease. An inactivated virus vaccine is now available for use in Europe, although it is not yet in widespread use outside areas where the disease is known to occur. This vaccine should be administered by deep intramuscular injection and the initial course consists of two injections three to five weeks apart followed by annual boosters.

How do horses pick up WNV?

Horses and people are 'end-stage' hosts and the virus is not transmitted from one to another. Wild birds are the most important environmental source of infection and mosquitoes pick up the virus when they feed on an infected bird. The virus is then transmitted to the next animal or animals that the mosquito bites. This means that the virus can be spread over vast distances in relatively short time by mosquito transmission where there is a wild bird reservoir of infection.

How can I prevent my horse from getting WNV?

One approach is to avoid geographical areas where disease occurs, however for horses that need to compete internationally, this may not be appropriate. Vaccination appears to be a sensible precaution for such horses. Control of the spread of infection in WNV areas depends upon limiting the breeding and spread of infected mosquitoes.


Fortunately, West Nile Fever has not yet occurred in recognisable disease form in the UK and most of mainland Europe. It is now considered endemic in Italy and Romania. It is probable that the virus will continue to spread internationally via infected mosquitoes and wild birds.

It is not clear whether an imported infected horse might be able to infect a local mosquito and thereby transmit the virus to a previously uninfected wild bird population. This is considered an unlikely scenario. Many veterinary surgeons expect the virus to arrive at some time in the future and all cases of neurological disease must now be considered a risk for WNV and examined and tested accordingly.

If you import horses into UK, particularly from WNV endemic countries, you should discuss sensible quarantine and insect control measures with your veterinary surgeon.