Blister Beetle Toxicity in Horses

Nov 15, 2011
Preventative/Preparedness blister beetle

Blister beetle toxicosis can cause serious disease in horses that can lead to death. The causative agent, cantharidin, is a potent irritant found in the blister beetle. Alfalfa flowers attract the beetles and at harvest they may be incorporated into hay.

Clinical signs vary depending on the amount of toxin ingested but may include; fever, anorexia, colic, dipping muzzle in water, and blood in urine. Unfortunately there is no treatment that is 100% successful but supportive care is essential. Awareness and prevention is the most effective way to ensure your horse is safe from blister beetle toxicosis.

The Blister Beetle

  • 0.5-1.0 inches long, brown, black, grey, or yellow striped
  • Have eyes that follow contour of their head
  • First portion of thorax is narrower than thorax

Why are they Dangerous for Horses?

  • Each beetle contains varying amounts of a toxin called cantharidin
  • Lethal dose of cantharidin: 1mg/kg
  • 6-8 beetles can be lethal


Cantharidin is a potent and caustic irritant that can cause ulcerations throughout the GI tract and alters permeability of blood vessels.

It works by inhibiting the mitochondrial metabolism, which essentially destroys the power producer of cells as they come in contact with the toxin. Cantharidin is excreted by the kidneys and can cause renal and bladder damage during its clearance.

Clinical Signs of Blister Beetle Toxicity

Signs of toxicosis are dependent on the amount of cantharidin ingested and may become apparent within 6-8hrs. Clinical signs include:

  • Depression
  • Anorexia and colic
  • Ulceration- oral, esophageal, gastric,SI, LI
  • Excessive salivation due to oral ulcers
  • Diarrhea due to severe inflammation of GI tract
  • Increased respiratory, heart rate, and fever of >101.2F
  • Submerging muzzle in water/ playing with water
  • Frequent attempts at urination +/- blood in urine
  • Sweating
  • Severe Pain
  • Endotoxemia and Shock
  • Death


There is no specific antidote to cantharidin toxicity. Early on, mineral oil and charcoal may be given with a nasogastric tube to prevent further absorption of the toxin. Supportive therapy with fluids, electrolytes, and gastric support are the main line of treatment.

The effectiveness of such treatment depends on the time since ingestion of the beetles and the amount ingested. Fatalities with blister beetle toxicosis can be as high as 65%.


Knowledge and prevention are the best way to avoid blister beetle toxicosis

  • First-cutting alfalfa hay is usually safe if harvested before mid- May as blister beetles will not emerge from winter until late May to early June.
  • Non- crimped hay is safer as it allows the beetles to escape from bales.
  • Check your hay!!
  • Blister beetles tend to swarm so multiple beetles may be found in one flake of hay and none in the rest of the bales.


Blister Beetle image curtesy of joedecruyenaere

Written by Jen McDonald, BVetMed, MRCVS