A variety of plants of different families contain toxins that directly or indirectly affect the heart. Cardiotoxic plants cause severe acute illness and sudden death in the horse. Horses are usually poisoned when they are given access to ornamental shrubs and lawn clippings.

Lily of the Valley, Oleander, Hellebores, Laurel, Azalea, Rhododendron, Foxglove, Death Camas and Hemp Dogbane all contain cardiac glycosides, which can cause signs similar to heart attack. Certain types of Milkweed contain cardenolides that cause cardiac arrest. English, Canadian and Japanese Yew contain toxic alkaloids (taxine) that inhibit proper heart function. Avocado leaves, fruit and bark is poisonous to horses, as it contains the cardiac toxin persin.

Signs of poisoning by these plants include (among many others) colic, high heart rate, irregular heart beat, sweating, trembling, diarrhea, labored breathing and sudden death.

Usually these horses are found dead, with evidence that they consumed the plant yielding the diagnosis. The glycosides may be found in blood or tissue of properly submitted samples.

There is no known treatment for these poisons.