A mystery toxin killed a fifth horse in western North Carolina in November. Veterinarians now suspect this case might be due to the same substance that caused the death of four other horses earlier that month.

The fifth horse was found dead in its pasture by the owners. Veterinarians determined via a Nov. 25 necropsy that the horse died of atypical myopathy, a condition that has been recorded in grazing horses in Europe during the wet weather months. At the beginning of November, four horses at a different farm in the area died under similar circumstances.  

Veterinarians believe a toxin present in area pastures might cause the sudden illness in otherwise healthy horses. The toxin seems to be linked to cool, wet weather.

Attending veterinarian Beverly Hargus, DVM, said the weather had been wet before the most recent horse became ill and died. The horse’s pasturemate also fell ill in late December, with clinical signs similar to the first four horses. This horse is being treated with supportive therapies.

Previously, area veterinarians had recommended owners keep their horses off pastures, but because the sudden illness seems to be linked to milder temperatures, Hargus believes the risk isn’t as great. “I think we’ll be okay now that the weather is getting colder,” she said.

David Waldrep, DVM, regional Veterinary Medical Officer for the area, believes the cases might be linked to weather patterns. Waldrep said he suspects a fungal-produced ionophore is causing the toxicity.