Veterinarians Urge Vaccination against West Nile Virus

As of Aug. 21, more than 110 cases of equine West Nile virus (WNV) have been reported this year in the United States, according to the United States Geological Survey. The majority of these affected horses were either not vaccinated against the disease, or not up-to-date on their WNV vaccination.

Since the mosquito season is far from over in some areas of the country, some horse owners might be wondering if a vaccine booster might benefit their animals, or if it's too late to inoculate a currently unvaccinated horse. To get the answers to these questions, TheHorse.com caught up with Rocky Mason, DVM, an ambulatory equine practitioner from Hagyard Equine Medical Institute in Lexington, Ky.

"Given the recent rise of clinical cases associated with the West Nile virus, I would recommend to owners of unvaccinated horses to consider vaccinating their horses now with a current West Nile vaccine product," Mason said. "That initial shot needs to be boostered in three to six weeks depending on which commercial product was used. Following that initial two shot series, horses should be vaccinated annually.

"It takes roughly 10 to 14 days for a horse to mount and generate protective antibodies," he continued. "If the horse is previously unvaccinated it may take two weeks past the second shot to mount protection so the recommendation is to get started now."

Like with previously unvaccinated horses, Mason recommends starting a new vaccination series in horses with an out-of-date or incomplete vaccination status.

"Horse owners of previously vaccinated horses that have lapsed on their annual vaccine booster will need to go through the initial two-shot series similar to that of unvaccinated horses in order to be in compliance with the commercial product claims," he explained. "Despite the lapse these horses typically mount an immune response to the first vaccination, but it may not be at a high enough level to prevent viremia, disease and encephalitis. To be safe, the two shot series is recommended in this group as well."

Mason explained that all commercially available WNV vaccines "have claims for protection of up to a year as determined by challenge models." That said, even some well vaccinated horses might benefit from a booster this fall, he said.

"More susceptible individuals (young, old, or the immunosuppressed) may require more frequent boostering especially during times of increased exposure like now," he explained. "It is important to consult with your veterinarian to best determine the needs of your individual horse and its risk assessment. It is very likely given the increased exposure and challenge that the recommendation to booster your horse will be made."

Finally, Mason stressed that it's not too late in the year to vaccinate against WNV, even if horses are unvaccinated at present.

"West Nile virus is dependent on mosquitoes for transmission, there are many areas in the country that have insects well into the fall and even winter, necessitating year around immunity. It is important to consult with your veterinarian as only he or she will know when best to time annual boosters in your area."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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