Blister beetle poisoning in horses is a potentially fatal condition caused by the ingestion of blister beetles, which are found in alfalfa and other forage crops.

Blister beetles, also known as oil beetles, secrete a blistering agent called cantharidin, which is highly toxic to horses. Ingestion of as little as five grams of beetles is enough to cause death in an average-sized horse.

Symptoms of cantharidin ingestion in horses include a stiff gait, appetite loss, frequent urination, kidney failure, depression, diarrhea and fever. Currently, there is no antidote for cantharidin toxicosis in horses, and treatment is supportive and symptomatic in nature.

Prevention of blister beetle poisoning involves careful management and inspection of forage, especially alfalfa. Always purchase hay from reputable sources and inquire about the measures taken to reduce the risk of beetle contamination.

The risk of blister beetle exposure can vary depending on geographical location. Consult with a local veterinarian to understand the risk in your region and for more information if you have concerns.

Causes of Blister Beetle Poisoning

Blister beetles are a group of insects belonging to the Meloidae family, known for their ability to produce a potent toxin called cantharidin. This substance causes blistering on contact with skin and is used by the beetles as a defense mechanism. [2]

When consumed by horses, cantharidin is highly irritating and can cause severe inflammation and blistering of the gastrointestinal and urinary tracts.

Blister beetles feed on flower blossoms, particularly in alfalfa plants, which makes them a common contaminant in alfalfa hay. [1] Alfalfa hay can become contaminated with cantharidin when it is baled while a blister beetle swarm is feeding on the plants.

Alfalfa is widely used in horse diets in a variety of formats including hay, cubes, treats and pelleted feeds. [1] To reduce the risk of blister beetle poisoning, closely inspect alfalfa before feeding it to horses and source hay from trusted suppliers.

Risk Factors

There are approximately 7,500 different species of blister beetles, found all over the North American continent. The risk of cantharidin toxicosis is greatest in the western United States, where there is the highest population of blister beetles. [5]

Most species of blister beetles spend the winter in their larval stage, maturing into adults