It’s that time of year again… at least in some areas.

West Nile virus (WNV) infection has been identified in a horse in Colorado. It’s not really a surprise. West Nile virus is one of those pathogens that we know is coming back every year, we just don’t know exactly when. The date of return varies a bit from year to year, but tends to be fairly consistent within a region (e.g. if West Nile cases started to crop up in a certain state in mid-August last year, they are likely to start again at roughly the same time this year).

The timing of onset of WNV cases depends on a few things, including WNV circulation in birds, climate and mosquito populations. The latter is quite important since only certain mosquitoes like to bite both birds and mammals. These particular mosquitoes species (called bridging vectors) are the concern, since they are more likely to bite an infected bird, and then possibly transmit the virus to a horse (or human) if they bite them next. Mosquito populations aren’t the same all year and in all regions, which explains in part why WNV cases don’t start earlier in the year, and why there are some major regional variations in disease despite the widespread presence of mosquitoes.

In Ontario, I suspect we have a few more weeks before we get the first reports of cases for 2014, but the WNV season is approaching here as well.

Photo credit: Rennett Stowe (click image for source)