Blister beetle toxicity

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Blister Beetle Toxicity

Blister Beetle Toxicosis, Cantharidiasis

Blister beetle toxicity occurs when horses eat hay contaminated with crushed blister beetles. These beetles secrete an irritating and toxic substance called canthanridin. There have been several instances in the United States where horses have died from eating hay contaminated with blister beetles.

There are thousands of different species of blister beetles, however the most commonly found are the striped blister beetle.
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.

Blister beetles are attracted to alfalfa, especially the flowers, and are most abundant during the mid-summer months, and thus more likely to be found in second cutting of alfalfa hay bales. In general, early and later cuttings of hay are less likely to have beetles present.

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. When the hay is cut and baled at the same time, generally with a crimper, this makes it harder for the beetles to escape the harvesting process and traps them into the hay. Therefore, it is better to purchase alfalfa where a crimper was not used. This means the hay was cut and left to dry then baled at a later time, which allows the beetles time to escape.

Clinical signs of blister beetle toxicity

If horses eat blister beetle contaminated hay, they usually develop blisters in their mouths and in their gastrointestinal tract, resulting from the irritation caused by the toxic substance (hence why the beetles are referred to as "blister beetles"). The severity of clinical signs varies depending on the number of beetles ingested. Most horses develop a reduced appetite, lethargy, colic signs, and seen dunking their mouths into the closest water source. Horses may also be seen stretching out to urinate and making frequent attempts to urinate.


Treatment is aimed at reducing further absorption of the toxin, preventing dehydration, correcting electrolyte abnormalities and reduction in pain. Most horses will require hospitalization for a few days to possibly a week or more.


Colic signs
Loss of appetite
Increased body temperature
Increased heart and respiratory rates
Mouth blisters
Making frequent attempts to drink water, submerging muzzle
Dark, congested mucus membranes
Frequently attempting to urinate
Stretching out as if to urinate


  • 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

General Overviews

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Risk Factors

  • Feeding second cutting alfalfa hay to horses
  • Buying hay that was baled using a crimper.