Summary

Blister beetle cantharidin toxicity is a serious and potentially fatal risk to horses fed contaminated alfalfa hay. Blister beetles, commonly found in alfalfa fields, contain cantharidin, a toxin that remains active even after the insects are crushed during harvesting. Ingesting as few as 25 beetles can cause severe irritation to the digestive tract and kidneys, leading to colic, oral ulcers, diarrhea, frequent urination, low blood calcium, heart damage, and sudden death. Diagnosis is based on clinical signs, bloodwork, and detection of beetles in hay. Survival requires rapid, aggressive veterinary treatment. Risk can be reduced by feeding first‑cutting alfalfa, purchasing certified beetle‑free hay, and carefully inspecting hay before feeding.