Following on the heels of the beginning of the start eastern equine encephalitis (EEE) season, warnings are going out about another mosquito-borne disease in parts of North America, West Nile virus (WNV) encephalitis. While mosquitoes have been swarming around here for months, WNV only becomes a real concern in southwestern Ontario starting in mid-to-late August. This time of year, the virus starts increasing in the mosquito population, and the types of mosquitoes that bite both birds and mammals (and therefore act as a bridge between the bird reservoirs and susceptible mammals like horses and people) become more common and more active.

Reminders about WNV have been issued in Ontario over the past few days because of increasing numbers of mosquito pools testing positive for the virus. Mosquito pools are groups of mosquitoes that are caught and tested together to see if the virus is present. The more pools that are positive, the more mosquitoes that are positive and the greater the WNV activity in the area.

People are being reminded to take measures to reduce mosquito breeding grounds on their property, which mainly means getting rid of standing water. Avoiding mosquito bites through basic measures, such as staying away from mosquito-infested areas (particularly at dawn and dusk), wearing light coloured clothing with long sleeves and long pants, using insect repellent containing DEET and making sure windows have intact screens, is also emphasized.

Recommendations are similar for horse owners; reduce mosquito breeding sites and reduce mosquito exposure of your horses (although this can be easier said than done). Vaccinating horses against WNV should also be considered. The dramatic decline of WNV infection in both people and horses after it first emerged back in 2001/2002 was quite impressive, and WNV hasn’t ended up being the major problem it could have been. In 2011, there was only 1 confirmed case in a horse in Ontario. Presumably, there were more undiagnosed cases but this shows how the disease has leveled off to be an uncommon but still present issue.

Despite disease due to WNV being rare in Ontario (remember that the virus itself is not rare because the mosquito pools are still testing positive), WNV shouldn’t be ignored because it still has the potential to cause severe illness in horses and people. When deciding whether or not to vaccinate your horse, consider carefully the risk of mosquito exposure, WNV activity in your local area, WNV history in horses and people in the area and your own level of risk aversion.