Arizona state livestock officials are warning horse owners to make sure their horses’ vaccinations are up to date for a potentially fatal equine sleeping sickness—Western equine encephalitis. The Arizona Department of Health recently found mosquitoes carrying the virus that causes WEE in southern Maricopa county and on the Gila River Indian Reservation.

Cases of WEE are most common during the summer and early fall months when mosquito populations in the area are high.

Symptoms of WEE include neurological signs such as depression and lack of coordination. A sick horse may also go down and not be able to rise. The illness is fatal in 20-50% of horses that are stricken with the disease