A horse in Jefferson County has contracted the West Nile virus — marking the state’s first case among horses this year, officials said Wednesday.

Vaccines for horses have proven to be very effective in preventing the virus and those who have been vaccinated in the past need an annual booster, the Colorado Department of Agriculture said in a news release.

Horse owners are urged to reduce mosquito populations by removing stagnant water sources, using mosquito repellent and keeping animals inside during the bugs’ feeding times, which are typically early in the morning and evening, the release said

If a horse shows clinical signs consistent with a neurologic disease, its owners should take them to a veterinarian, according to the agency.

Infectious or contagious equine neurologic diseases can be reported to the Colorado State Veterinarian’s Office.

To date, there have been 20 human cases of the West Nile Virus this year, according to the Department of Public Health and Environment.