One California horse is dead and another recovering after being poisoned by cookies laced with oleander.

Oleander (Nerium oleander) is a leafy, flowering, drought-resistant plant used in ornamental landscaping in several parts of the United States, including California, Utah, Arizona, and Texas. It contains cardiac glycosides that can cause various clinical signs of toxicity in horses, including colic, lack of coordination, profuse sweating, difficulty breathing, abnormal heart function, muscle tremors, and potentially death from cardiac failure. As little as one ounce can be fatal to some horses.

“Oleander causes weakness (and) depression,” said Anthony P. Knight, BVSc, MS, Dipl. ACVIM, professor in the College of Veterinary Medicine at Colorado State University. “The toxin is like digitalis; it works on the heart muscle.”

Sergeant Cecile Focha, public information officer for the Sonoma County, California, Sheriff’s Office said that an owner reported that two of her horses appeared to have been poisoned on Feb. 12. One horse survived, but another subsequently died; a dog also died in connection with the incident, she said. Necropsies revealed that the horse and the dog had died due to oleander toxicity, Focha said.

Leslie Webb, owner of the horse and dog that died, said the home-made cookies contained oatmeal, shredded apple, shredded carrot, and some had been soaked in molasses. The cookies had been placed on the ground along the fence line near a roadside. Others were placed in a paddock containing the hor