Blister Beetle Poisoning in Horses

Anna O'Brien, DVM
By Anna O'Brien, DVM on Mar. 2, 2012

Cantharidin Toxicity in Horses

Blister beetles are a type of insect found primarily in the southwest and Midwest regions of the United States. These beetles harbor a very powerful toxin called cantharidin, but, unlike other types of insects, it does not spread this toxin through biting. Adult blister beetles feed on alfalfa flowers and crops, the same crops used for horse and cattle feed, and when the crops are harvested the beetles are often killed in the process, contaminating the crops with their body parts and fluids and causing illness in the horses that eat the contaminated feed. 

Blister beetles are extremely toxic when ingested by horses: as few as five to ten beetles may be fatal to a horse.  The cantharidin toxin affects many bodily systems.  It is extremely irritating to the digestive tract and causes blisters and erosions from the lips and tongue all the way through to the lining of the intestines, which causes abdominal pain (colic) and diarrhea.  This toxin also causes damage to the kidneys and the heart.

Symptoms and Types

  • Colic
  • Anorexia
  • Diarrhea or soft stool
  • Kidney damage
  • Blisters on the mouth
  • Playing in water in attempt to provide relief from blisters
  • Damage to urinary tract (displayed as abnormally frequent urination and discolored urine)
  • Increased heart rate, sometimes irregular heart beats (arrhythmia)
  • Diaphragmatic flutter (“thumps”) due to low blood calcium  (hypocalcemia)


  • Eating contaminated fresh or dry alfalfa; contamination occurs when blister beetles are crushed into the alfalfa during the crimping process
  • Cured hay does not lose toxicity, nor does the age of hay affect its levels of cantharidin


Call your veterinarian if your horse is exhibiting any of the symptoms mentioned above or if you suspect that your hay was contaminated by blister beetles. You will need to give your veterinarian a thorough history of your horse's health, including a background history of symptoms, and possible incidents that might have precipitated this condition, including its recent dietary history. The history you provide may give your veterinarian clues as to which organs are being affected secondarily.

To definitively diagnose cantharidin toxicity, your veterinarian will submit a sample of your horse’s urine to a diagnostic lab that specifically tests for the presence of this toxin. Stomach contents can also be submitted. Of course, direct identification of blister beetles in the hay is the easiest and fastest way to diagnose this condition. If the horse dies before a diagnosis is made, samples from the gastrointestinal tract and the kidneys can still be submitted for a post-mortem determination of cause of death.

Laboratory tests to determine kidney damage are also very useful in making a final determination of the horse’s clinical condition as well, and if heart arrhythmia appears to be present, an electrocardiogram (ECG) can be used to examine cardiac functionality.


There is no antidote for cantharidin toxin. The success of treatment relies primarily on the speed of diagnosis, and the amount of cantharidin that was ingested.

Any horse that has been affected by cantharidin will be in need of intensive supportive therapy, including the administration of IV fluids to flush out the kidneys, rehydrate the animal, and return balance to the body’s electrolytes. Activated charcoal should also be administered in attempt to neutralize any toxin that is left in the gastrointestinal tract and mineral oil may be administered via a nasogastric tube to facilitate further rapid evacuation of the intestinal contents. Ulcer medication should be given, as well as broad-spectrum antibiotics, to prevent secondary bacterial infections.  Pain medication will also likely be given.  

Living and Management

Even when caught early, cantharidin toxicity has a guarded prognosis. 


The blister beetle congregates in swarms to feed on alfalfa fields during mating, which is normally in mid- to late summer.  Knowing where your hay is coming from along with close inspection of the hay this time of year in areas that harbor this insect are two ways to help prevent against this potentially lethal condition.

Anna O'Brien, DVM


Anna O'Brien, DVM


Anna O’Brien, DVM is a large animal veterinarian. A 2008 graduate of Purdue University, she currently works in Maryland, just outside of...

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