Cantharidin is found in more than 200 species of beetles throughout the continental USA. Beetles of the genus Epicauta are most often associated with toxicosis in horses. The striped blister beetles are particularly troublesome in the southwestern US.
Blister beetles usually feed on various weeds and occasionally move into alfalfa fields in large swarms. These insects live in groups and may be found in large numbers in hay when it is baled. One flake of alfalfa may contain several hundred beetles, but a flake from the other end of the same bale may have none. Animals are usually exposed by eating alfalfa hay or alfalfa products that contain blister beetles.
Cantharidin is odorless, colorless, and highly irritating. It causes blisters when in contact with skin or mucous membranes. As little as 0.1 to 0.2 ounces (4 to 6 grams) of dried beetles may be deadly to a horse. The toxicity of cantharidin does not decrease in stored hay, and cantharidin is also toxic to people, dogs, cats, rabbits, rats, cattle, sheep, and goats.
The severity of signs associated with cantharidin poisoning varies according to dose. Signs can range from mild depression or discomfort to severe pain, shock, and death. Common signs include abdominal pain, depression, loss of appetite, frequent attempts to drink small amounts of water or submerge the muzzle in water, and frequent attempts to urinate. Signs can last from hours to days. Affected horses always have dark, congested mucous membranes, even if other signs are barely noticeable. Horses that ingest a large amount of toxin may show signs of severe shock and die within hours.
Laboratory tests can detect cantharidin in stomach contents or urine. The amount of cantharidin in urine becomes too small to be detected in 3 to 4 days, so urine should be collected early in the course of disease for analysis. Microscopic evaluation of stomach contents of fatally poisoned horses may reveal fragments of blister beetles.
There is no specific antidote for cantharidin, and supportive treatment must be prompt to be successful. Administration of mineral oil helps flush the gastrointestinal tract, and repeated treatments may be recommended. Activated charcoal may be helpful if given early. Calcium and magnesium supplementation for prolonged periods is almost always recommended. Other supportive treatment includes administration of fluids, pain relievers, and medication that increases urine output. The outlook for affected horses improves daily if no complications occur.
Prevention is aimed at feeding beetle-free hay. The hay field must be scouted before it is cut and during baling, because the insects can be crushed during cutting, crimping, or baling of hay. Areas of the field that contain swarms of beetles must be avoided for a few days because most of the insects will leave. Once the beetles have left, these areas can be harvested.
First-cutting hay is almost always free of blister beetles, because the insects overwinter as subadults and usually do not emerge until late May or June in the southwestern US. Likewise, the last cutting of hay is often safe, because it is usually harvested after the adult insects are no longer active.