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 that is extremely toxic to horses. The Texas Veterinary Medical Diagnostic Lab indicates only about 4 grams of dried beetles is 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.  This is the reason most folks in Texas do not feed Texas alfalfa to horses but get their alfalfa from further north where this beetle is not as commonly found. 

Horses with cantharidin toxicosis develop colic, decreased appetite, depression and sweating, blood in the urine, dark mucous membranes, increased heart rate and respiratory rate and low blood calcium.  Many horses will also develop oral ulcerations and will salivate excessively due to these 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 are 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 cecal contents tested for the toxin, and even beetles in hay can be identified at the lab.