Blister beetles pose a threat to horses and some livestock every year, but they’re most problematic in hot, dry weather.
Blister beetles produce cantharidin, a toxin that causes inflammation and blister formation in digestive tissues. When they swarm in alfalfa fields, many wind up dead in cut hay bales. Ingestion of 30 to 50 blister beetles can be deadly to a horse, but even fluid from crushed beetles is toxic.
Signs generally appear within six to eight hours after a horse eats contaminated alfalfa hay. These horses usually show signs of abdominal pain, but the toxin can target multiple organ systems, including the kidneys. Affected horses can die within hours.
There’s no cure for blister beetle ingestion, but a veterinarian can provide supportive care — the earlier, the better. If you live in a region with blister beetles, the best way to sidestep the problem may be to avoid feeding horses alfalfa hay altogether.