Vet’s View: West Nile Virus in British Columbia

West Nile was introduced to America in 1999 and first identified in Canada in 2001. Although it’s been around for quite a while, British Columbia did not have reported cases in horses for the longest time. West Nile Virus has recently been found in 7 horses, all from different premises in the Kootenay Area of British Columbia, and another case was confirmed in the Fraser Valley. The later was originally from the United States and therefore it is unknown if he contracted the disease here or at his original location. These recent cases are a reminder that although West Nile Virus is still rare, it is a serious threat to the health of our horses.

West Nile is spread by mosquito bites. All horses are therefore at risk, even the ones that don’t travel, go to shows or are in contact with other horses.  Mosquitos get the virus from feeding on infected birds. There were 2 dead crows in the Kootenays that also tested strongly positive for the West Nile Virus around the same time as the horses.

Infected horses usually show neurologic deficits such as ataxia; which is a lack of muscle coordination where horses usually can’t control where they put their feet and can stumble or even fall over when the symptoms are severe. West Nile also causes fever, depression, lethargy, head pressing against walls, head tilt, difficulty swallowing or chewing, impaired vision and problems blinking. It is a very serious disease that can be fatal. The mortality rate was originally recorded at almost 50%, but new data puts it in the 30% range. A lot of horses end up having severe symptoms needing a lengthy and costly hospitalization.

There is no treatment for the infection, only supportive care. However, West Nile Virus infection can be prevented with a safe and proven vaccine. For the vaccine to be effective, there needs to be an initial dose given followed by a booster around a month later. There needs to be an annual vaccination afterwards to keep the protection against the virus. Pregnant mares previously vaccinated should receive a vaccine 4 to 6 weeks before foaling. The recommendations for foals are similar to other vaccines, which starts at 4 to 6 months of age with the first booster 4 weeks later and a second booster  around 10-12 months of age if the dam was correctly vaccinated. If you have a pregnant mare with no history of vaccination or a young foal with an unknown history, it is best to consult your veterinarian for the appropriate steps to take to assure that your horse is protected.

Prevention also includes mosquito control, such as fly spray, keeping horses inside at dawn and dusk and drying all stagnant water areas.

In conclusion, the West Nile Virus threat is still real, the affected horses can have very severe symptoms that could require extensive medical care and it is recommended that horses get vaccinated to help protect them against this

this disease.

Marielle St-Laurent DVM, graduated from the University of Montreal in June 2010. She completed Paton & Martin’s internship program and is now a full time associate veterinarian. Marielle was a competitive Dressage rider prior to entering vet school. She recently got back in the saddle and started training and competing again.