Fortunately, heart disease is not common in horses but when it does occur, it can be serious. Some heart disease in horses can be caused by toxins. Dr. Megan Romano is a toxicology resident at the University of Kentucky Diagnostic Lab and she indicates that a common poisoning occurs when horses are exposed to feed containing a compound called an ionophore. Ionophores are commonly used in cattle to increase feed efficiency and promote weight gain but are toxic to horses, and toxicosis occur when this ingredient is mistakenly added to horse feed or when a horse is allowed to eat cattle feed. Ionophore exposure causes heart failure in horses with poor performance and sometimes sudden death.
Another heart toxicity is exposure to the blister beetles commonly found in alfalfa hay. If a significant amount of blister beetles are ingested, damage to the heart muscle can occur, which can be deadly. Exposure to an ornamental yew or taxus can also affect the horse's heart and all parts of the plant or toxic. Ingestion of just a small amount of the yew shrub can cause heart failure and death so you want to make sure a horse property does not have any yew or taxus shrubs that horses could consume.
Some other plants that can affect the horse's heart include white snakeroot, rhododendrons, azaleas, laurels, and fetterbush. Also milkweed are wildflowers cultivated as ornamentals and these can also be toxic to a horse’s heart. Many of these plants are not palatable to horses when growing but may be consumed if baled in hay, so always inspect your hay and if you see unusual plants, get them checked out before feeding the hay.