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 …