When to Vaccinate Against Mosquito-Borne Diseases
As spring approaches (slowly... at least here in Ontario!), people once again start thinking about vaccination programs for their horses. A question that comes up every year is when is the best time to vaccinate against mosquito-borne diseases? Depending on your region, the pathogens of concern may include West Nile virus (WNV), as well as Eastern/Western or Venezuelan equine encephalitis virus (EEE, WEE and VEE, respectively). Often people get the generic response of "30 days before mosquito season," which isn't always very helpful.
Why? It comes down to some basic mosquito biology and timing.
All mosquitoes are not alike. There are two main groups of mosquitoes when it comes to transmission of diseases like WNV that circulate primarily in the bird population. One group is called "amplifiying vectors." These mosquitoes feed on multiple birds, thereby spreading WNV and increasing the number of WNV infected birds. However, these mosquitoes rarely feed on other types of animals, so they are not very important in transmission of WNV to horses and people. The other group of mosquitoes is the "bridging vectors." These mosquitoes feed on many different types of animals, not just birds. They can pick up WNV by feeding on infected birds, and then transmit the virus when they subsequently feed on horses or people. This is the most important type of mosquito in terms of disease transmission. Areas that have larger populations of more efficient bridging vectors tend to have greater problems with WNV.
The "30-day" part of the recommendation comes from the concept that we want to vaccinate animals about 30 days prior to the risk of exposure, so that the vaccine has ample time to work. The hard part is deciding when to start counting. One of the important things to consider is when do cases typically occur? If, as in many areas, WNV cases are not seen until late August or September at the earliest, vaccinating in April is probably not optimal. Your veterinarian and/or public health department may be able to tell you when West Nile virus activity is typically highest in your area. While there's no guarantee that things will be the same every year, it's unlikely that the WNV season would start well in advance of earlier years. Here in Ontario, disease due to West Nile is rarely reported in horses any more, but the cases that do occur still tend to appear in late August, at the earliest. Therefore, I'd be hesitant to vaccinate for WNV in the spring. If a horse is vaccinated in late April, it will be four months between vaccination and the time of peak risk. The period of peak protection would occur over a few months when there is little evidence of risk of exposure. So, if I was going to vaccinate against WNV, I'd start in late July. If a horse had been vaccinated in the spring, I'd consider a booster in the fall to make sure that the periods of peak immunity and peak risk conincide.
