BROOMFIELD, Colo. — Although West Nile virus is a seasonal occurrence, the Colorado Department of Agriculture reported Thursday four of the 10 horses confirmed to have the virus have been euthanized due to its severity.

The cases were confirmed in eight Colorado counties: one in Adams, Garfield, Larimer, Mesa, Montrose, Pueblo and Rio Blanco, and three in Weld.

According to a news release from the Colorado Department of Agriculture, vaccines are an effective prevention tool but horses need an annual booster if they’ve previously been vaccinated.

The department also recommends include removing stagnant water sources, keeping animals inside during the bugs’ feeding times, which are typically early in the morning and evening, and using mosquito repellents.

The Colorado Department of Public Health and Environment has reported 72 human cases to date in 2019, and one human fatality, according to the release.