West Nile Virus is a Flavivirus and the virus causes temperature reactions and encephalitis in horses and humans. Birds are the natural reservoir hosts and mosquitoes transmit the virus to horses and humans during blood feeding.

Not all horses and humans infected with West Nile Virus will develop clinical symptoms and those infected individuals are not contagious to one another. Horses and humans are therefore considered “dead-end” hosts.

However, if a horse does develop clinical symptoms of West Nile Virus, 90% of those horses will show neurological symptoms. The fatality rates for horses with the neurological form of West Nile Virus is 30 – 40 %.

There is no cure for West Nile Virus, with treatment merely being supportive of the symptoms shown.

Prevention is thus the best way to ensure that your horse does not contract West Nile Virus. Vaccination and mosquito control are the methods utilised to prevent spread of the disease.

Vaccination consists of a primary course of two vaccinations, four weeks apart and thereafter an annual vaccination. The optimal time to vaccinate your horse is in December. Horses require light work for two days after vaccination and occasionally may develop some swelling and mild pain at the injection site. The insurance companies do not stipulate that your horse has to be vaccinated against West Nile Virus, even in the case of a mortality claim. This is the opposite with African Horsesickness, where the insurance companies require that your horse be vaccinated by a veterinarian against African Horsesickness in the case of a treatment or mortality claim.

Mosquito control involves the use of repellents, eradication of mosquito breeding sites in areas of stagnant water and stabling of horses between dusk and dawn. Fans in the stable also help to prevent mosquitos from feeding on the horses.