(AP) — State officials have confirmed 12 cases of a fatal horse disease in northwest Nebraska.

The Nebraska Department of Agriculture said the cases of equine infectious anemia were all found in one horse herd.

State veterinarian Dr. Dennis Hughes says the disease only affects horses, mules and donkeys, and is usually fatal to those animals. No other animals or humans can be infected.

The disease is typically transmitted through insects, such as horseflies or deerflies, but it can also be spread among horses through infected needles. Hughes says infected horses have no treatment options.

Symptoms of the disease include fever, depression, weight loss, swelling and anemia.