How poisonous are azaleas, and how much must an average-sized horse consume to be affected?

Azaleas are generally only consumed by horses when other forage is lacking, such as in the winter months. The presence of toxins varies among species, so some azaleas may be of little risk. The principal toxin is a grayanotoxin, which acts by binding to cell membranes affecting sodium channels and causing prolonged depolarization of cells. The primary effects are on the heart, nervous system, and gastrointestinal tract and include anorexia, excessive salivation, colic, and frequent defecation. Severe cases may result in muscle weakness, bradycardia (slow heart rate), cardiac arrhythmia (abnormal heart rhythm), weakness, paralysis, and coma, which may precede death. Horses eating approximately 0.2% of their body weight, or if using 1,000 pounds as an average-sized horse, two pounds of leaves, are likely to develop signs of poisoning, according to one source, while another source states that 1.75% of body weight (or 17.5 pounds of leaves for a 1,000-pound horse) causes symptoms in horses. It is interesting to note that some of the most toxic plants, including oleander and yew, are ornamentals and that horse owners should consider this when landscaping. Two excellent, though somewhat technical, books on the subject include A Guide to Plant Poisoning of Animals in North America by Anthony P. Knight and Richard Walter and Toxic Plants of North America by George E. Burrows and Ronald J. Tyrl.