West Nile Fever in Horses

West Nile Fever is a disease caused by a virus (WNV) carried by mosquitoes.  It causes encephalitis (inflammation of the brain and spinal cord) that can affect humans and other animals as well as horses.  Prior to 1999, the virus was found only in Africa, Eastern Europe and Western Asia, but in August 1999 it was identified in the northeastern states of U.S.A. (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 U.S.A. and Canada.  In 2002, West Nile Fever was diagnosed in a stallion in official quarantine in Australia after importation from North America.  The virus has been found in Mediterranean Europe, including southern France, causing disease in horses.   

What are the effects of West Nile Fever?

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 that 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, causing encephalitis (inflammation of the brain), sometimes with fatal results.

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

• Elevated body temperature.
• Weakness or paralysis of the hindlimbs.
• Incoordination.
• 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 West Nile Fever diagnosed?

These symptoms are not specific for WNV.  Blood and sometimes cerebrospinal fluid (CSF) tests are usually needed to differentiate between different causes of encephalitis and in-coordination.  Diseases to be differentiated include other encephalitis producing viruses (Eastern, Western and Venezuelan encephalitis), protozoal encephalomyelitis (EPM), wobbler syndrome, traumatic injury, poisoning, bacterial infection and 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 (disease fighting proteins) that the horse has produced in response to the infection.

Can West Nile Fever be treated?

There is no specific treatment for West Nile Fever.  Treatment is aimed at symptomatic and supportive care for the encephalitis and the neurological abnormalities that it produces. 

Killed virus vaccines are available for use in horses and widespread use shows that it is safe and that it produces useful immunity that either prevents or reduces the severity of the disease. Annual vaccination is advised in high risk areas. Consult your veterinarian with regards to this vaccine and your horse.

How do horses pick up West Nile Fever?

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 West Nile Fever?

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 and the vaccine can be used under special license.

Control of the spread of infection in WNV areas depends upon limiting the breeding and spread of infected mosquitoes.  In U.S.A., government laboratories perform postmortem examinations on wild birds, using them as 'sentinels' for spread and intensity of infection.  Vaccination for horses is now used in high-risk areas .In such areas it is possible that the judicious use of insect repellent may reduce the risk of infection for individual horses and it is sensible to house horses in insect screened and controlled stables during times of maximum mosquito activity.  It is recommended that humans in high risk areas use insect repellent and wear long sleeves and long trousers.