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.