Horses, mules, (equids) are seriously affected with WNV. It does NOT spread from horses to mosquitoes and back to horses or man. Vaccination of equine animals is strongly encouraged. There are now three vaccines that can be used to provide protection. All are labeled as "sold through veterinarians only" and it is best if owners consult with their veterinarian about timing, use in pregnant mares, use in foals, etc.
The Fort Dodge (Invator) product has been used for several years (millions of doses) and has provided excellent protection (94%) if BOTH doses were given initially. The vaccine is also available in combinations with Western and Eastern Encephalitis, as well as tetanus. The vaccine should be given prior to the mosquito season, if possible. The first year it takes two doses with 3-4 weeks between the two doses. It will also take another 2 weeks after the second dose before the animal has protective immunity. After the first year, it only takes one dose. If an area is experiencing a HEAVY outbreak of WNV in Aug/Sep (the peak season) then the horses could be given another booster dose to provide extra protection at that time.
In January 2004, Merial released a WNV vaccine called RecombiTEK. It also requires
two doses the first year (4-6 weeks apart) and an annual booster.
In late summer 2006 the new WNV vaccine PreveNile was released by InterVet for horses. PreveNile requires only a single dose for the initial vaccination and is good for 12 months, providing a high level of immunity.