If you feed alfalfa to your horses, you need to know about blister beetles. Blister beetles that are sometimes found in alfalfa hay can cause a serious problem in horses. They contain a chemical called cantharidin, which is extremely toxic to horses. The Texas Veterinary Medical Diagnostic Lab indicates only about four grams of dried beetles are required to be fatal in a horse. Although there are 200 species of these beetles in the United States, the striped blister beetle in the southwestern part of the country causes the most problems as they feed on alfalfa blooms. In Texas, horse owners often do not feed Texas-grown alfalfa to their horses but get their alfalfa from areas where this beetle is not as commonly found. 

Horses with cantharidin toxicosis develop colic, decreased appetite, depression, sweating, blood in the urine, dark mucous membranes, increased heart and respiratory rates, and low blood calcium. Many horses will also develop oral ulcerations and will salivate excessively due to erosions in the mouth. 

Treatment is not very successful as many of these horses will not survive. Treating pain is important, and most horses are severely dehydrated, so intravenous fluids are required. Giving the horse mineral oil in the stomach can sometimes tie up the toxin, as can giving activated charcoal. These horses have severe ulcerations in their stomach and small intestines, so using omeprazole to decrease acid and sucralfate to coat the stomach is helpful. Depending on the amount of toxin ingested, horses require early and aggressive treatment to survive. Horses that do not survive can have their stomach contents or fecal contents tested for the toxin, and a lab can identify beetles in hay.   

Your veterinarian is your best resource for information on possible toxins that might affect your horse.