Blister beetles produce cantharidin, a toxic defensive chemical that horses are especially sensitive to. | Photo: Courtesy North Carolina Department of Agriculture and Consumer Services

Blister beetles belong to a family of plant-feeding insects (Meloidae) that produce cantharidin, a toxic defensive chemical. Contact with it in the blood of live or dead beetles causes blistering of the skin or mucous membranes of sensitive mammals, especially horses. Cantharidin is stable and remains toxic in dead beetles for a long time, so animals can be poisoned by eating crushed beetles in cured hay.

The severity of the reaction depends upon the amount of cantharidin ingested and the size and health of the animal. The lethal dose for livestock is estimated to be 0.45 to 1.0 mg of the chemical per kilogram of body weight.

Clinical signs associated with poisoning usually appear within hours and include irritation and inflammation of the digestive and urinary tracts, colic, urinary straining, and frequent urination. This irritation can also result in secondary infection and bleeding. In addition, calcium levels in horses might be drastically lowered and the heart can be damaged. Since animals can die within 72 hours, it is imperative to contact a veterinarian as soon as blister beetle poisoning is suspected