Most horse owners have heard of blister beetle poisoning in horses. Blister beetle poisoning results from ingestion of cantharidin, a toxic chemical carried by numerous species of blister beetles. Over 200 species of blister beetles occur throughout the U.S. (see Figure 1).

The most common species involved in equine cantharidiasis (cantharidin poisoning in the horse) in the Southwest are the longitudinally black and yellow striped blister beetles (Epicauta occidentalis and Epicauta temexa). Many horse owners mistakenly think blister beetle poisoning is confined to the Southwest. Blister beetle poisonings have been reported in most of the U.S., ranging from the eastern seaboard to as far west as Arizona and Colorado, and as far north as Illinois. Other species of blister beetles (striped, spotted, and non-striped) have also induced toxicosis.

Cause
Cantharidin, a stable, non-volatile anhydride of cantharidic acid, has been detected mainly in the beetles’ hemolymph (circulatory fluids consisting of blood and lymph), although it is also found in genitalia and other tissues. Cantharidin is actually produced by the male beetle; however, it is transferred to the female during mating.

Male beetles contain higher concentrations of cantharidin. Although cantharidin concentrations in beetles have been reported to vary widely, typical concentrations range from near 0 to 5%. Levels as high as 12% have been reported. poisoning in horses.

Because species, gender, and mating status affect the amount of cantharidin in blister beetles, estimates on levels of beetle ingestion which cause poisoning in horses vary. Some research estimates 100 to 150 beetles would be lethal; however, lower levels of ingestion can cause a variety
of illnesses, especially those  related to digestive disorders. Because the beetles are known to
swarm in large numbers, small amounts of forage can contain enough beetles provide large
doses of cantharidin.

Blister Beetle Life Cycle
Blister beetles complete one generation per year. Mating occurs in the summer, and females lay eggs in shallow cavities in the soil. Eggs hatch in the fall and larvae immediately begin to search and feed on grasshopper eggs. Grasshopper eggs are located within several inches of the soil, and are laid in clusters. Blister beetle larvae burrow in the soil over winter and emerge in late spring or summer as adult beetles.

Blister beetles are pollen eaters, but will also consume blossoms and leaves of flowering plants. Alfalfa hay is the usual forage associated with blister beetle poisoning in horses. Beetles are not thought to consume non-flowering plants such as grasses; however, beetles have been found in grass hay containing flowering plants, such as nightshade and goldenrod. Although not documented in controlled studies, ingestion of processed alfalfa products, such as alfalfa pellets and silage, may also contain cantharidin, as the compound is heat stable. The lack of documented cantharidin poisoning with processed products, such as dehydrated alfalfa pellets, may be due to the dilution effect of mixing beetle contaminated hay with large amounts of other alfalfa during the feed manufacturing process.

Symptoms of Cantharidiasis
Clinically, horses display a number of signs, such as colic, that are associated with the irritant effect of the toxin on the mucosal membranes of the digestive and urinary tracts. Additionally, changes in serum electrolytes, most notably reduced calcium concentrations, are potential poisoning signs. Reduced serum calcium concentration may cause a number of clinical signs, such as generalized muscle tremors and diaphragmatic flutter.

As stated in a recent review on equine cantharidiasis, the potential clinical signs are varied and numerous. In addition to clinical signs suggesting abdominal pain, behavioral signs include repeated splashing of the muzzle in water, decreased sensory awareness, gait stiffness, hind-limb ataxia, abnormal facial expression, and depression or anxiety. Additionally, mucous membrane congestion, fever, muscle tremors, cardiac arrhythmia, fever, prolonged capillary refill time, spasms of the larynx, frequent urination, difficulty in swallowing, synchronous diaphragmatic flutter (thumps), dehydration, profuse sweating, and elevated heart and respiratory rates are listed as potential signs. Shock and sudden death often result.

Treatment and Prognosis
As with all maladies of a critical nature, horse owners must consult with their practicing veterinarian to determine the specific course of therapy. Although, it has long been reported that prognosis is poor and mortality is high, prognosis can be improved if the toxicosis is recognized early and appropriate therapy is applied.

Prognosis is good when horses survive the first several days, although long term studies on problems following poisoning are lacking. Because there is no known antidote for cantharidin, treatment is aimed at removal of the source of toxin, reduction of toxin absorption, protection of the digestive tract mucosa, control of pain, and supportive measures to maintain the horse’s fluid and electrolyte balance. Diagnostic tests include sampling urine for cantharidin. Stomach and cecal contents are examined post-mortem.

Prevention
Because there is no known antidote, avoiding ingestion of blister beetles is the only answer for horse owners wanting to avoid cantharidin poisoning. Alfalfa producers harvesting hay in areas of known blister beetle populations should employ harvesting techniques which reduce the incidence of beetles in alfalfa hay. These techniques include scouting alfalfa fields for the presence of beetles immediately prior to harvesting, and using insecticide and harvesting methods that decrease the chance of dead beetles remaining in hay during baling.

PREVENTATIVE Measures for Reducing the Incidence of Blister Beetles in Alfalfa Hay Concentrate scouting for blister beetles along field borders, immediately cut and spray infested areas if found. Avoid harvesting field borders. Spray border areas when spraying fields for control of other insects to reduce beetle migration. Cut hay without using crimpers and avoid wheel traffic on freshly cut alfalfa.  Spray entire fields and borders with a short residual insecticide just before harvesting. Purchase alfalfa hay harvested before May or after September. Or, purchase alfalfa hay grown in areas free of blister beetles. Inspect hay before feeding if presence of blister beetles are suspected.  Source: P. Mulder, R. Shawley, and J. Caddel. Blister Beetles and Alfalfa, OSU Extension Facts F-2072 Blister Beetles and Alfalfa. 1996

Because beetles tend to congregate in large numbers in small areas, scouting programs prior to cutting can be helpful. Usually, beetles do not migrate long distances; consequently, beetles are usually found in the outer areas of fields. Entomologists at Oklahoma State University have not collected adult striped beetles before mid-May. Usually, the adult blister beetle doesn’t become active until mid-June in Oklahoma. Following similar patterns in other states suggests the first cutting of alfalfa can be relatively beetle-free if it is harvested before beetles become active. Similarly, hay harvested late in the season (October) should also be free of beetles, as activity ceases in early fall.

Spraying fields with a short residual insecticide just prior to harvesting is also an effective tool to reduce the presence of beetles. Regardless of scouting or spraying measures, most blister beetle contaminated hay is the result of the beetle being crushed during the harvesting process. This usually happens when the harvesting process kills beetles. The crimper roller of swathers kills beetles, and the remains are left in the hay.

Cutting without using crimpers decreases the chance of ingesting beetles, as it is common for many of the live beetles to leave the area shortly after cutting.

 

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