Clara Mason, DVM

44 Cloverleaf Street
Winfield, WV 25213



(Excerpts from the Equine Quarterly Newsletter - Dr. Lee Townsend Department of Entomology Univ of KY - Lexington, KY)

Blister Beetles (Meloidae) are in the family of beetles that produce cantharidin. This substance is a toxic defensive chemical located in the blood of both dead and live blister beetles. Exposure to cantharidin can cause blistering of the skin and mucus membranes in mammals, especially horses that accidentally consume the beetle. Cantharidin is stable and will remain for a long time in the dead beetles that may be crushed and baled with alfalfa hay. The severity of the reaction to the cantharidin is based on the amount of chemical consumed and the size of the animal consuming it. The average Striped Blister Beetle contains approximately 4mg of cantharidin per beetle.  

The LETHAL dose for cantharidin in livestock is 0.45-1.0 mg of cantharidin per Kg of body weight of the animal.


Clinical signs of Cantharidin poisoning usually appear within hours and include irritation and inflammation of the digestive and urinary tracts, colic, urinary straining, and frequent urination.  In addition, the animal may experience bleeding and a secondary infection.  Calcium levels in horse may be drastically lowered and the heart can be damaged.  Death may occur within 72 hours especially without veterinary intervention.


The most common blister beetle associated with toxicity in horses are Meloids in the genus Epicauta, especially the striped blister beetle. Blister beetles are attracted to flowering alfalfa and may be trapped and crushed during the harvest of the hay. The blister beetle is commonly found in the Central and Eastern USA. Striped blister beetles tend to congregate in large clusters along field margins.  This can result in high concentrations of beetle in baled hay. Addition blister beetle species have been identified in poisonings in other areas of the USA.


Unfortunately, there is not an effective way to inspect hay thoroughly enough to sort out the blister beetle. 

-The beetle is not active when the first cutting of alfalfa hay is made; harvest at the late bud stage or when the first flowers open for high quality horse hay. Since the beetle is attracted to the bloom, harvest at intervals to minimize flowering of alfalfa or weeds in hay fields. 

-Practice good broadleaf-weed management.  

-Check fields for blister beetles before cutting hay from July through early September. The beetles prefer blooming plants and tend to cluster in masses near field edges.  Avoid harvesting areas where beetles are present. 

- Avoid crimping hay during harvest. Straddle cut swaths to avoid crushing beetles with tractor tires.