Cantharidin poisoning

Attention! This is a potentially life-threatening condition for your horse. Time is of the essence, contact your veterinarian immediately.Find a Vet

Cantharidin Poisoning

Blister Beetle Toxicosis, Cantharidiasis

Cantharidin is a toxin that is found in blister beetles (Meloidae family) and false blister beetles (Oedemeridae family). The toxin is released when the beetle is crushed or handled roughly. Both insect species are found often in alfalfa fields and on occasion can contaminate alfalfa hay. Beetles are also found frequently on carelessweed (pigweed/amaranth), puncturevine (goathead), peanuts, soybeans, and many other species of plants, both wild and cultivated. All livestock are at risk of poisoning, however horses are more sensitive.

Estimated Number of Beetles for a Lethal (1 mg/kg) Dose of Cantharidin
Beetle Cantharidin Content (mg)Horse weight (lb)

Source: Adapted from Campinera et al.(1985)

There are more than 300 blister beetle species in the continental United States.
Different species and sexes of beetles have differing levels of the amount of cantharidin in their bodies, varying from 1 to 11.3% of their dry weight. This variability in cantharidin content has resulted in a wide range in number of beetles reported to cause death in horses--fatal poisonings have occurred from a few beetles to as many as 200.

Beetles are most likely to be found in the second cutting of alfalfa, which include the flowers, as blister beetles are most attracted to the flowering vegetation. Unfortunately, blister beetles have a tendency to congregate in large clusters along field margins rather than spread out, which increases the likelihood that large numbers of beetles can become harvested within the hay. The contamination of blister beetles within the hay is most frequently the result of alfalfa being crimped when cut, which crushes the beetles and traps them within the hay.

Horses can be poisoned by ingesting the bodies of living or dead blister beetles in alfalfa hay. The severity of clinical signs vary depending on the amount of cantharidin ingested. Duration of clinical signs range from 3 to 18 hours. Onset of clinical signs is usually rapid, and requires quick, aggressive treatment for a positive outcome.


Loss of appetite
Increased body temperature
Increased heart and respiratory rates
Oral erosions
Bloody diarrhea
Blood-tinged urine or urine with blood clots
Stiff, short-stride gait
Making frequent attempts to drink water, submerging muzzle
Dark, congested mucus membranes


  • History
  • Clinical signs
  • Physical exam
  • Laboratory testing - toxicology


Supportive therapy


  • Grow your own hay or purchase from quality local hay suppliers. Give preference to alfalfa hay from fields that are scouted regularly as part of an integrated pest management (IPM) program
  • Cuttings of alfalfa made before mid May and after late August are the least likely to contain blister beetles.
  • Inspect alfalfa hay for blister beetles as it is removed from the bale. Do not feed infested material to livestock, regardless of how long the hay has been stored.
  • Cutting aflafa hay without the use of crimpers and avoiding driving over freshly cut alfalfa may reduce the chances of heavy contamination of blister beetles within hay bales.


Depends on the amount consumed, and how fast treatment is sought.

Scientific Research References

Good Overviews

  • Colic Signs icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon
  •  icon

Risk Factors

  • Feeding second cutting alfalfa hay to horses