Beware of Blister Beetles
Martin W. Adams, PhD, PAS – Equine Nutritionist for Southern States

Blister beetles get their name from the irritating reaction they cause on the skin and mucous membranes of people and animals. Blister beetle poisoning can be a serious problem for horses. Cantharidin is the poisonous substance present in the bloodstream of blister beetles that will cause a welt or blister on your skin but can be much more harmful to horses if consumed. Horses that ingest blister beetle-contaminated hay absorb cantharidin from the intestines and can develop symptoms ranging from a mild colic for a few days to going into shock and dying within a few hours.

Adult blister beetles feed on flowering plants such as alfalfa and clover. When there are large numbers of blister beetles feeding on alfalfa and it is harvested for hay, many beetles may become trapped and killed in the hay during processing. It is not unusual for a horse to consume a large number of blister beetles, because they tend to feed together in large numbers on the flowers and might all be killed and preserved in the same bale of hay. There are specific management guidelines for prevention of blister beetles in alfalfa hay for hay growers.

There are over 200 species of blister beetles found in the central to southern United States. The only alfalfa hay that is free of blister beetles is from the extreme northern United States and Canada. Alfalfa hay from the southern and western U.S. is of the most concern, where particular species of blister beetles are present that contain high levels of cantharidin. Alfalfa hay from Kansas, New Mexico and Oklahoma, in particular, is more often associated with incidence of blister beetle poisoning. This is due to the toxicity of blister beetles from these states and the large amount of alfalfa hay produced and shipped from these states. In addition, midsummer alfalfa is more likely to contain dead blister beetles than first-cutting hay or late cuttings as the adult beetles are only active during the summer months. Early May and late September are the best harvest dates for buying alfalfa hay for horses. The process of mowing and crimping at the same time during harvest crushes and traps the beetles in the hay. Farms that use separate mowing and crimping procedures, or that use no crimping process are not likely to have a problem with blister beetle contamination.

If you suspect blister beetle poisoning, call your veterinarian immediately. Check the alfalfa hay that the horse has been eating for the presence of blister beetles. Diagnosis of blister beetle poisoning is by detection of cantharidin in urine, blood serum, or gastric contents, or through post-mortem examination for the presence of beetles in the stomach or cecal contents.

There is no antidote for cantharidin poisoning, the focus is on reduction of cantharidin absorption by administration of activated charcoal followed by mineral oil, control of pain, administration of fluids and electrolytes to correct dehydration and promote kidney function, and removal of cantharidin. The prognosis of the horse depends on how much cantharidin was absorbed in to the bloodstream and how aggressively the horse is treated. If the horse survives 2 to 3 days past ingestion of the toxin, the chances for survival are more favorable. The estimated minimum lethal dose is one milligram of cantharidin per kilogram of body weight. Blister beetles vary by species in the amount of canthardin they contain, with variation of 1 to 5 mg per beetle. A toxic dose for a 500 kilogram or 1,100 pound horse could vary from 500 beetles (1 mg of canthardin per beetle) to only 100 beetles (5 mg of canthardin per beetle). However, lower levels of beetle or cantharidin ingestion in horses have been fatal in particular cases, and even ingestion of a few beetles can cause colic.

To safely feed alfalfa hay to horses, use only square bales and divide the bale into individual "flakes" and check for blister beetles before feeding. Discard any hay containing blister beetles, removing dead beetles from the hay will not make it safe for use as the toxin can still be present. Cantharidin is odorless and colorless, so the only way to determine if the toxin is present in alfalfa hay is the presence of blister beetles. Cantharidin is also a very stable compound, and blister-beetle contaminated hay will remain toxic for a long time.

Blister beetles range from ¾ to 1¼ inches in length and are recognized by their long, narrow body, broad head, and "neck-like" appearance. Colors of blister beetles vary from black to gray to brown, and some species are spotted or striped. Two of the most toxic species are Epicauta vittata (three-striped blister beetle) and Epicauta occidentalis (striped blister beetle).

Symptoms of Blister Beetle Poisoning

• Depression, dehydration, irritability and anorexia.
• Frequent urination and diarrhea.
• Ulceration of the mouth, tongue, stomach, esophagus, and intestines.
• Increased heart rate and rapid breathing.
• Colic, straining to urinate, fever and sweating.
• Bloody urine, blood and mucous in manure.
• Gait changes and kidney dysfunction.
• Frequent play in water with the lips and tongue.
• Low blood levels of calcium and magnesium.

Selecting and Feeding Alfalfa Hay

• Use first-cutting or fall-harvested alfalfa to be certain of it being free of blister beetles.
• Find out where your alfalfa is from, so you will know when to be especially careful.
• Feed only small square bales for individual feeding.
• Divide hay into "flakes" and inspect for blister beetles.
• Discard any hay with blister beetles present.