Blister Beetle Toxicity in Horses

By Marty New

Blister beetles are insects terrifying to horse owners everywhere. Blister beetle poisoning results from ingestion of cantharidin, a toxic chemical carried by numerous species of blister beetles. Over 200 species of blister beetles occur throughout the United States. The most common in Oklahoma is the striped blister beetle. This beetle has several black and yellow or orange stripes along the back. Many horse owners mistakenly think blister beetle poisoning is confined to the Southwest. Blister beetle poisonings have been reported in most areas of the United States. Several species with spotted, black and gray coloration have been found in the Southwest and all contain the blistering agent cantharidin.

Cantharidin is comparable to cyanide and strychnine in toxicity. It is in the hemolymph (circulatory fluids consisting of blood and lymph) of blister beetles. Cantharidin is highly toxic when ingested by horses and may cause illness or even death. It is a very stable compound that retains toxicity to horses even when dried remains of beetles that have been killed in the harvesting process are fed along with the forage. Cantharidin is produced only by male blister beetles and is stored until mating. Thus, mating status determines whether females contain the toxin.

To learn more about blister beetles, read the latest issue of OKFR!