Blister beetles are very small beetles that have a very toxic chemical in their shells. The beetles are not much larger than a grain of rice, and are usually solid black or gray or black with yellow or red stripes.

These beetles swarm in alfalfa fields. At hay cutting, the shells or bodies of these beetles end up in the hay and then are incorporated into alfalfa bales. Toxicity can also occur from eating alfalfa pellets or cubes that contain these carcasses too. This is a regional problem, mostly occurring in the Southern United States.

Blister beetle bodies contain cantharidin, a substance that causes severe irritation and ulceration of the equine gastrointestinal and urinary tract. When eaten, cantharidin is rapidly taken up from the intestine into the blood stream. The toxin is excreted in urine, causing damage to the urinary tract. The toxin damages the lining of the entire gastro-intestinal tract.

Horses with blister beetle toxicity usually demonstrate signs of abdominal pain (colic), severe depression, and signs of shock. They may have bloody urine. Some have mouth ulcerations.

Diagnosis requires finding the beetles in hay or isolating the toxin in body fluids or tissues. Diagnosis is presumed with the presence of the typical clinical and laboratory signs.

Treatment is generally supportive nursing care, and treatment of the damaged intestinal and urinary systems.