Protecting your horse from West Nile Virus
The Utah Department of Agriculture and Food (UDAF) strongly advises horse owners throughout the state to prepare to protect their animals by vaccinating them for the West Nile Virus as soon as possible each year, usually in May.
If you have already vaccinated your horse with the recommended number of shots, then a single "booster" shot will be sufficient each year. You can obtain the vaccine from most local veterinarians.
Horse owners can protect their animals by applying approved repellants to the animals and by controlling mosquitoes and mosquito breeding areas. Horse owners may also protect the animals by putting them in the barn or other enclosed structures.
Additional information from CDC
See a video about protecting your horse