An unidentified disease is sweeping through Waipio Valley, Hawaii, causing the island’s wild horses to lose their mobility. Several of the horses have died due to the effects of the disease.

Hawaiian agencies and veterinarians have been working to understand why Waipio Valley’s wild horses are suddenly losing their ability to walk. “Symptoms first show through an abnormal gait as the horse’s muscles begin to atrophy, usually starting with the hind limbs,” according to the Hastings Tribune.

“It most likely looks like a disease caused by mold spore toxins,” says veterinarian Kelleyerin Clabaugh. “That is not a diagnosis, but it is a high suspicion.”

Clabaugh, who began the investigation of the disease, says that the island’s domestic horses have not been affected since the outbreak started. She also believes that the disease is being caused by something the wild horses are ingesting. She has taken blood samples from two horses.

Veterinarian and Hawaii County councilman Tim Richards believes the disease is neurological. Richards, along with a team from the Hawaii Department of Agriculture, performed a necropsy on a recently deceased horse.

“These horses that have been affected have been in one area and nothing has spread to any of the domestic horses down there, so we’re kind of leaning toward something these animals are consuming,” says Jason Moniz, leader of the Hawaii Department of Agriculture’s investigative team and veterinary program manager for the Animal Disease Control Branch.

Although the cause of the disease and the disease itself have not been definitively identified, those involved in the investigation are working hard to understand why these horses are losing function in their legs. Moniz says that he expects the results from Richards’ necropsy to be returned in a few weeks.