From 2003-19 there were 842 cases of this disease in horses reported to the Canadian Food Inspection Agency.
There was a huge peak of cases in 2003 when the virus was first reaching the Prairies. Vaccines were just becoming available and most horses had no immunity to this disease (from vaccination or natural infection), making it a highly susceptible population.
Spread by mosquito bites, West Nile virus causes disease in birds, horses and people. First identified in New York in 1999, the disease spread across the eastern United States and Canada, reaching the Prairies in 2002.
Among domestic animals, horses are at greatest risk of this disease. The virus multiplies in tissues at the site of the mosquito bite and then spreads to the lymph nodes. From there, it enters the spinal cord and brain, causing dangerous clinical signs in the form of muscle tremors, clumsiness, weakness, lameness, colic and in severe cases, an inability to stand.
No specific treatment for West Nile virus exists, so clinical cases are given supportive care that includes fluids and anti-inflammatory medications. Unfortunately, one-third of horses that develop clinical signs will die or are euthanized. Horses that survive the infection may never return to normal.
For this study, the researchers also analyzed cases from 2015-19 in closer detail. In terms of regional differences, they found that 82 percent of cases were in Western Canada, compared to 18 percent in eastern regions (Ontario, Quebec and the Atlantic provinces).
Saskatchewan had the most cases overall with 33 cases per 100,000 horses. British Columbia has only ever had 22 cases and there were no cases in the Atlantic provinces. Western provinces experience an earlier season compared to the East. Based on this data, the researchers recommend that vaccinations should be given by mid-April in the West to ensure there is adequate time for a protective immune response to develop before the mosquito season gets started and the virus begins to spread.
Another important result was that 96 percent of cases were among horses that were not vaccinated. With only four percent of cases among vaccinated horses, these results suggest that vaccine failure (where adequate immunity doesn’t result from the vaccine) is a relatively rare occurrence and the vaccines are highly effective at preventing disease.
When analyzed by breed, most cases were among Quarter horses. This is the most common breed in the West and so the result makes sense given that most cases were in this region.
There are also management differences to consider — horses that are outdoors as opposed to those kept in barns are more likely to encounter mosquitoes and thus the virus.
It is unlikely that there is some breed-associated trait that makes Quarter horses more susceptible to infection. Cases were present in low numbers in other breeds such as Standardbreds, draft horses, Warmbloods, Paints and Thoroughbreds.
There were cases in all ages except foals. Similarly, there was no difference in the number of males versus females affected. These results support that all horses are potentially susceptible to infection.
Among cases, 32 percent of horses died or were euthanized due to the disease, which is similar to results in other studies.
This study provides important details on the Canadian experience with West Nile virus in horses, including the benefit of vaccines. In addition to the currently recommended annual vaccination, reducing exposure to mosquitoes is also useful for prevention.
West Nile virus is well-established in the West and will continue to circulate during peak mosquito months (June to September). Even though we expect fewer infections overall, be sure to discuss vaccination against this disease in early spring with your veterinarian.
Dr. Jamie Rothenburger, DVM, MVetSc,PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine.
