Blister Beetles are extremely toxic to horses when the insects are accidentally ingested. They may contaminate alfalfa hay which is a very popular feed for its excellent nutritional profile. Only six grams, or two tenths of one ounce, of blister beetle parts are potentially lethal to a fully grown horse. The insects’ toxic potency does not decrease upon death of the beetle; the whole insect is not required to induce toxicity.
Blister Beetles produce a chemical called cantharidin that causes “blisters” (thus the name) in the mucous membranes of the gastrointestinal tract. High blood levels of the toxin can also result in lesions of the heart, kidneys, and urinary bladder. None of these symptoms are obvious to the horse owner, whose only clues to the problem maybe colic, inappetence, drooling, “thumps” or “hiccups”, diarrhea, or bloody stool or urine. Unfortunately, outward signs may be subtle despite a lethal toxicity, and death may occur without much warning.
On blood-work, the veterinarian will see evidence of renal damage and reduced levels of blood calcium and magnesium. The doctor may also detect abnormal heart rhythms associated with toxic shock.
There is no antidote for the Blister Beetle toxin. Horses are treated supportively and sources of the beetles are removed to avoid further toxicity. Intravenous fluids, electrolyte supplementation, and parenteral nutrition may all be given to avoid dehydration and subsequent kidney damage. If the horse has recently ingested the blister beetle contaminated alfalfa, the veterinarian may administer activated charcoal to absorb the toxin. Sucralfate is a drug that forms a protective layer over gastric ulcers and may provide some relief of pain and avoid further tissue damage. Other medications given may include H2 blocker antacids and proton pump inhibitors to reduce acid production and absorption of the toxin, mineral oil to accelerate passage of the ingesta, and prophylactic antibiotics to prevent sepsis (systemic bacterial infection).
The only way to prevent this deadly toxicity is to secure a reputable source of alfalfa (which could still become contaminated) or eliminate alfalfa from the horse’s diet. The veterinarian can help guide the horse owner in this manner.