Blister Beetles: Deadly in Horse Hay

Purchasing hay is a ritual that many horse owners have down to a science: Select a type (i.e., grass, alfalfa, or timothy), grab a handful for a smell test, examine for dust and mold, and feed to the hungry horses waiting at home. As many hay producers will be harvesting hay in the coming weeks and months and passing the fruits of their labor on to consumers, it's important for horse owners to be aware of a tiny, toxic (and potentially fatal) tagalong in some perfectly healthy-looking alfalfa hay bales: the blister beetle.

Blister beetles naturally contain and secrete a chemical substance called cantharidin, which is extremely toxic to horses. The insects--which can be found in most parts of the United States, are ½-1 inch long, often cylindrical in shape, and can be a variety of colors--feed on alfalfa flowers and can easily, although inadvertently, be included in the hay during the baling process. Once baled in the hay, the beetles will generally appear dried up and might be crushed or broken into parts (due to the bailing equipment).

"Blister beetles tend to swarm to feed on alfalfa flowers," explained Sam L. Jones, DVM, PhD, professor of equine medicine at North Carolina State University's College of Veterinary Medicine. "If large enough numbers of the beetles are incorporated into baled hay, horses can ingest the beetles."

Simply touching a blister beetle--either dead or alive--is enough to cause inflammation and blistering of a horse's skin within hours of contact.

If a horse ingests even a few beetles, the insects' cantharidin can cause ulceration and inflammation of the mouth, stomach, and intestines. Clinical signs including decreased appetite, frequent drinking and urination, colic, and depression can be apparent within hours.

In the worst case scenarios (e.g., ingesting many of beetles), cantharidin can cause endotoxemia, shock, and death within hours of ingestion.

If an owner suspects a horse has ingested blister beetles, he or she will likely need to transport the horse to the nearest equine hospital for treatment.

"There is no specific antidote for blister beetle poisoning," relayed S. Peder Cuneo, DVM, MS, extension veterinarian at Arizona Veterinary Diagnostic Laboratory at the University of Arizona. "Treatment is aimed at reducing absorption of the toxin by administering activated charcoal and mineral oil. Intravenous fluids, gastrointestinal protectants, and broad-spectrum antibiotics are also recommended."

So before feeding your horse that bale of alfalfa hay, examine the hay carefully for the presence of blister beetles. In the event that one horse in a barn suffers from blister beetle toxicity, veterinarians recommend that owners stop feeding the hay to other horses in the barn and keep a close eye for signs of toxicity in other horses.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More

Free Newsletters

Sign up for the latest in:

From our partners