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Equine Health Resource

Blister Beetle Toxicity

Synonyms: Cantharidin Toxicosis or Toxicity

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.

Prognosis & Relevant Factors

The prognosis is fair with aggressive and appropriate veterinary treatment. The prognosis depends to a great extent on the quantity of toxin swallowed.

Even a small amount of cantharidin can be fatal if left untreated. Horses treated promptly by a vet have an improved prognosis.


  • How did you reach this diagnosis?
  • What can I do to prevent this in the future?

    Always visually inspect alfalfa hay as you are feeding it to horses. Look for the tiny beetle bodies in your hay. Hay farmers can reduce the likelihood of Blister beetle infestation by cutting alfalfa before the bloom.

    Certain methods of cutting hay may reduce the likelihood of incorporating beetles into the hay.

    Helpful Outside ResourcesCredible Equine Health Information on the Internet

    Author: Doug Thal DVM Dipl. ABVP


    Lavoie JP, Hinchcliff KW eds. Blackwell's 5 Minute Vet Consult: Equine. 2nd Ed. Ames: Wiley Blackwell 2008.


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