West Nile virus is a mosquito-borne infection that affects horses every year across the prairies.
The virus is carried by Culex tarsalis mosquitoes which tend to increase in numbers in the early fall months of August and September. Horses are particularly more vulnerable to this infection as they are in near constant contact with the mosquitoes and their immune system is often weakened to the virus.
West Nile virus first became a problem for horses in western Canada around 2002. It was first isolated in North America in New York City in the fall of 1999. It has now spread into almost all of Canada and parts of the Caribbean.
There are several different clinical syndromes the virus can produce but the most common signs of infection in horses are: depression, going off feed, fever (temperature >38.5C and typically 40C), weakness of the hind legs that progresses to weakness in all four legs leading to recumbency or laying down and unable to rise.
As the disease progresses, the horse will slip into a coma with high fever and often pass away shortly thereafter. From the time of infection to showing clinical signs of the disease varies from two to 10 days.
Once infected, death can occur in as little as 48 hours. The virus primarily affects the brain and spinal cord/cerebral spinal fluid. The disease is diagnosed based on the clinical signs and testing for antibodies or testing for virus particles in the blood.
Treatment for the disease is supportive only. If a horse develops weakness in the hind end and is diagnosed with West Nile virus, the vet will give medications to help decrease the inflammation in the central nervous system and will give intravenous (IV) fluids.
Unfortunately, there is no specific cure or treatment for the virus in horses. Not all horses will die from infection but many that live will have long-term side effects.
The main means of protecting your horses from West Nile Virus is through routine vaccination. Since the first outbreaks, several good vaccines have been developed for horses. Veterinarians highly recommend including West Nile virus vaccination as part of the preventative health plan for your horse.
Spring vaccination followed by a booster vaccine in August/September each year will help to prevent your horse from contracting this dangerous disease. Speak with your vet for more information and to get your horse vaccinated.