Blister Beetle Toxicity

Blister Beetle toxicity is not common in the eastern United States. Blister beetles are typically found in the arid regions of the western U.S. This year because of the drought in the eastern U.S., alfalfa hay may be shipped in from areas where blister beetles are common and cases of blister beetle poisoning could be seen in the East. For this reason, horse owners in the East that are forced to buy alfalfa hay from other sources will benefit from a basic knowledge of blister beetles.

Cantharidin is the chemical in blister beetles that causes toxicity when consumed by horses. It is produced only by the male beetle and is passed to the female during mating. Adult blister beetles mate during the summer. The females then lay their eggs in the soil and the eggs hatch in the fall. The larvae live on grasshopper eggs and then over winter in the soil. They emerge from the soil in late spring or early summer as adults. The adult beetles prefer to feed on alfalfa blossoms but will also feed on the leaves. They also feed on goldenrod, pigweed, and many other plants. Some species such as the striped blister beetle are gregarious and large numbers will feed in a small area of a field. This characteristic allows large numbers of beetles to end up in a small number of hay bales or even in just a few flakes of hay within one bale. Horses consume dead beetles that were killed during the hay making process and remained in the flakes of hay. The most common sign of cantharidin toxicity in horses is colic. This is induced by the painful and irritating effects of cantharidin on mucosal surfaces such as the lining of the stomach and intestines. Sudden death is also possible with ingestion of large doses. Other less common signs of cantharidin toxicity are neurologic disease, difficult urination and /or blood in the urine (since the toxin is excreted in the urine), heart problems, and oral lesions. Diagnosis requires at least five hundred milliliters (one pint) of urine, gastric, or intestinal content to be submitted for testing. Detection of cantharidin at any concentration is considered significant. The concentration of cantharidin in blister beetles varies between individuals, between species, and between males and females.

Because of the variation in cantharidin concentration, the exact number of beetles needed to be consumed to produce signs of poisoning is variable.

The table below , adapted from Capinera et al.1985. J. Econ.Entomol. 78:1052-55, is an estimate of the number of blister beetles required to cause mortality in a horse of various weights. Weight of Horse

Species 275 lbs 550 lbs 825 lbs

Black blister beetle 550 1,100 1,700

Spotted blister beetle 175 345 120

Three-striped blister beetle 40 80 120

There is no antidote so symptomatic therapy is the only option. Contaminated hay should be destroyed since cantharidin is very stable even with long term storage and can also poison other species of livestock.

Producers of alfalfa hay can do a number of things to try to prevent contamination of their hay. Insecticides can be used. Use hay mowers that don’t crimp the hay and avoid driving on freshly mowed windrows. Crimping hay or driving on the windrows is a major cause of death for blister beetles. If the beetles are not killed in the hay making process, they will crawl out as the hay dries and move to other feeding grounds. Blister beetles do not migrate very far and are often found within fifty yards of the field perimeter. By not harvesting the field perimeter, the producer can also help prevent contamination of the hay. Finally, harvest the hay before the alfalfa and/or the weeds bloom since it is the blooms that tend to attract and keep the beetles in the field.

As a purchaser of alfalfa hay, what can be done to reduce the risk of cantharidin poisoning? Find out where the hay has come from. If it is an area where blister beetles are common, ask what was done to avoid contamination. Buying hay harvested before May or after September significantly reduces the risk. Examine the hay carefully before feeding. Good pictures of some of the species of blister beetles can be found at www.uky.edu/Agriculture/Entomology/ entfacts/fldcrops/ef102.htm or www.agweb.okstate.edu/pearl/plantsoil/crops/f-2072.pdf . These pictures show the most common species found in alfalfa hay. If you find insects in your alfalfa hay resembling any of these, or if your horse shows any signs of cantharidin toxicity after eating alfalfa hay, call your veterinarian immediately.

References:

Gulick M.,MacAllister,C,DVM.Equine Cantharidiasis.Compend Contin Educ Pract Vet 18:77-83,M,DV,1996.

Helman,R,DVM.,Edwards,W,DVM.Clinical Features of Blister Beetle Poisoning in Equids: 70 cases (1983-1996).J Am Vet Med Assoc 211:1018-1021,1997.

Townsend,L.,Blister Beetles In Alfalfa, http://www.uky.edu/Agriculture/Entomology/entfacts/fldcrops/ef102.htm accessed 10/8/02.

Mulder,P.,Shawley,R.,Blister Beetles and Alfalfa,http://www.agweb.okstate.edu/pearl/plantsoil/crops/f-2072.pdf accessed 10/8/02.

The American Board of Veterinary Toxicology,Blister Beetle Intoxication or Cantharidin Poisoning, http://www.abvt.org/canth.html accessed 10/13/02.

Kinney K, Peairs F, Swinker A., Blister Beetles in Forage Crops, http:www.ext.colostste.edu/pubs/insect/05524.html accessed 12/11/02.