Blister Beetle Poisoning (Cantharidin Toxicosis) in Horses

Cantharidin toxicosis is a potentially life-threatening condition caused by ingestion of cantharidin-secreting blister beetles. In horses, this most often happens when alfalfa hay is infested with this type of beetle. There are over 7500 known species of blister beetles. Species from the Epicauta genus are most commonly associated with cases of poisoning in horses.

As little as four grams of dried beetles can cause death in an adult horse. Early treatment is crucial in minimizing the extent of gastrointestinal damage.

Symptoms depend on the amount of toxin ingested. High doses of toxin result in sudden death within hours. Lower doses cause symptoms that persist as long as the toxin is present and causes irritation.

Blister beetle poisoning is uncommon in horses. Suspected cases are emergencies that, left untreated, can lead to death.

Cases in the USA are most prevalent in semiarid regions where hay conditioning practices have been shown to increase the concentration of dead beetles in the final feed. While blister beetles are most commonly found in the Western United States, cantharidin toxicosis can occur worldwide as the toxins can survive shipping.

Cantharidin toxicosis is caused by the ingestion of cantharidin, a blistering agent secreted by insects of the Meloidae family, also known as blister beetles. The beetles feed on the alfalfa legume commonly used in hay.

Horses that consume infested hay ingest the toxin, which is rapidly absorbed and causes irritation to the gastrointestinal tract, skin, and urinary tract lining.

Diagnosis of cantharidin toxicosis is based on dietary history and symptoms.

Horses presenting symptoms of cantharidin toxicosis generally undergo the following diagnostics:

• Physical examination
• Bloodwork
• Urinalysis
• Feed testing for the toxin
• Intestinal content or urine testing for the toxin

Once diagnosed, the first step in treatment is removing access to the infested hay and stabilization of the animal's condition.

There is no antidote for cantharidin toxicosis, and treatment focuses on reducing absorption and supportive care. Treatment options include:

• Activated charcoal
• IV fluid therapy
• Medications to protect the gastrointestinal lining
• Antibiotics
• Magnesium and calcium supplementation
• Pain medication

Note: always consult a veterinarian before administering medication to horses, including activated charcoal.

Horses often require intensive care for 3-10 days.

Prognosis of cantharidin toxicosis is guarded and depends on the amount of toxin ingested, the severity of the gastrointestinal damage, and the timing of treatment. Cases that are caught quickly and treated aggressively have a good prognosis. In most cases, animals that survive the first week of treatment are able to recover completely.

If treatment is delayed or a high level of toxin is consumed, the prognosis is very poor.

Cantharidin toxicosis is not contagious, however outbreaks can occur if multiple horses consume the same contaminated feed. Eliminating alfalfa hay from the animal's diet is the most effective method for preventing cantharidin toxicosis.

Strategies to reduce risk include:

• Feeding only with uncompressed alfalfa hay (not hay cubes) to allow for inspection of hay contents
• Feeding only first-cut alfalfa hay, cut before the bloom stage
• Checking for beetles before feeding
• Destruction of contaminated hay

Cantharidin toxicosis is uncommon in horses. Horses not fed with alfalfa hay are unlikely to suffer from cantharidin toxicosis.

• Activated charcoal
• IV fluid therapy
• Gastroprotectants
• Magnesium and calcium supplementation
• NSAIDs
• Removal of alfalfa from the animal's diet