Livestock owners buying alfalfa should know their supplier well and find out what precautions were taken to avoid blister beetles in hay. If presence of beetles is suspected, hay should be inspected thoroughly before feeding. Purchase of alfalfa harvested before May or after September reduces blister beetle risks significantly. If toxicity symptoms are seen in horses, a veterinarian should be called immediately. If a large amount of cantharidin is consumed, a horse may die within six hours.
Small amounts consumed cause mild colic.
A frequent symptom of cantharidin illness of a horse is placing the muzzle in water and playing in the water with the lips and tongue. Severe poisoning may result in low blood calcium and magnesium, which can cause stiffness and an exaggerated goose-stepping gait. Horses that survive 24 hours may strain frequently, and small amounts of blood may be seen in the urine. In addition to having a veterinarian examine a horse showing symptoms of toxicity, remaining hay also should be inspected for presence of blister beetles.