Equine Cantharadin Intoxication (Blister Beetle Poisoning)

Greg Skelton

Jill M. Blackmer, DVM, MS, DACVIM, DABVP

Introduction:

Equine cantharadin intoxication, or Blister Beetle Poisoning, needs to be considered as a possibility anytime a horse being fed alfalfa is thought to have colic. There are over 200 species of blister beetles in the United States. The beetles belong to the species, Epicauta. Two species that inhabit arid areas such as the south-southwestern United States are the striped blister beetle and the 3-striped blister beetle. Since most of the alfalfa grown in these areas is sold to horsemen throughout Louisiana, these are usually the species identified in cantharadin toxicosis in horses. When blister beetles feed, they swarm over small areas of the vegetation. This is the reason a large number of beetles can be found in a single bale of alfalfa from one location in the pasture, yet not be present in the majority of other bales from other areas of the pasture.

Life Cycle:

Understanding the life cycle of blister beetles can play an important role in prevention of cantharadin toxicity to horses fed alfalfa. Blister beetles have one generation per year meaning they breed, lay eggs, and die within the calendar year. An adult female will lay up to 100 eggs in holes dug into the soil. The eggs hatch into larva in the fall and immediately start feeding on grasshopper eggs also laid in the ground. The larvae become pupa after two weeks and they remain in the soil over the winter. In late spring, adults emerge and begin feeding on alfalfa. The life cycle of the blister beetle along with appropriate harvesting techniques should allow the first and last cuts of alfalfa to be blister beetle free.

Pathogenesis:

Cantharadin is the poisonous substance present in the blister beetle. It is extremely toxic and ingestion of as few as six to eight beetles can kill a horse. This compound itself is odorless and colorless so we must rely on visual detection of blister beetles or part of beetles in alfalfa hay as the method of detection. Horses that ingest the toxin in beetle-contaminated alfalfa can develop ulcers in the mouth, esophagus, stomach, small intestine and large intestine. The toxin is absorbed from the intestinal tract and excreted through the kidney, causing damage to the kidney and urinary tract. Cantharadin can be absorbed through the skin; however this is rare in horses. Cantharadin is a very strong irritant so much so that when applied to skin lesions resembling blisters can appear. When the toxin is exposed to the more vulnerable tissues of the gastro-intestinal tract, serious damage can occur. Cantharadin doesn’t lose its toxicity when present in stored hay. This is why alfalfa that is shipped from locations with blister beetle infestation is toxic to contaminated hay.

Cantharadin, through the alteration of mitochondrial metabolism, ultimately causes loss of cell membrane’s selective permeability. Cell membranes are responsible for protection of the cell along with the transport of specific materials in and out of the cell. When cells lose the ability to regulate passage of materials, imbalances of materials occur resulting in the death of the cell. When the integrity of the gastrointestinal mucosa is compromised, the result is typically diarrhea along with the further loss of proteins, electrolytes, and water.

Clinical Signs:

Clinical signs that develop in horses are very dependent on the amount of toxin that was ingested. Signs can range from mild depression with abdominal discomfort to severe pain, and even death. Signs of abdominal pain include restlessness with sweating, pawing at the ground, and irritability. Horses are often seen playing with their water bucket (sham drinking) along with diarrhea. Because cantharadin is rapidly excreted from the body via the kidneys, signs consistent with a urinary tract inflammatory process such as straining to urinate (stranuria) are also frequently seen due to disruption of the lining in the bladder. Horses can have endotoxemia associated with cantharadin toxicity and die before passing any diarrhea. Low levels of calcium in the blood can lead to tremors, thumping, and lowered cardiac output.

Treatment:

As mentioned earlier the dose a horse ingests of cantharadin is directly related to its toxicity, treatment is focused at dilution of the toxin from the body and ultimately its removal from the patient. Fluid therapy will help to adequately rehydrate the horse along with lowering blood concentrations of cantharadin. Non-steroidal anti-inflammatory drugs (NSAIDS) are generally given for alleviation of pain along with their protective effects in case the horse becomes endotoxemia. NSAIDS are potentially toxic to kidneys if a horse is dehydrated, so ensuring the horse is sufficiently hydrated is indicated before administration of NSAIDS. Any electrolyte abnormalities caused by the cantharadin toxicity can also be placed in the supportive intravenous fluids or given via feed supplements if the horse is still eating. Since horses often exhibit low levels of blood calcium during cantharadin toxicity, replacement of calcium is necessary but doesn’t go without risks. Monitoring calcium levels while administering calcium therapy is indicated because if calcium levels become to high, the horse is prone to deleterious cardiovascular effects such as depression in heart rate and disruption of electrical signals in the heart which are responsible for coordinating contraction and relaxation at appropriate times between the chambers of the heart.

Treatment in more acute presentations can also be directed towards lowering the absorption of cantharadin. Horses are given mineral oil to coat the stomach and intestines lowering the absorption of cantharadin. Activated charcoal may also be used, but it must be followed by mineral oil as well to lower absorption of activated charcoal by the gastrointestinal tract. The prognosis of the horse is dependent on how much toxin was absorbed into the bloodstream and how aggressively the horse is treated upon presentation to the veterinarian. If the horse survives 2-3 days past ingestion, a more favorable prognosis is given.

Prevention:

Inspection of individual flakes before feeding to the horse is very important in preventing toxicity. Breaking the alfalfa flakes apart before feeding will allow people to better visualize blister beetles.

If possible, not using bales from the periphery of the alfalfa should be done as this is generally where the highest number of blister beetles can be found.

References:

Guglick R , et al. 1996.Equine Cantharadiasis. Compendium of Continuing Education for Practicing Veterinarians 18:77-83.

Helman RG and Edwards WC. Clinical features of blister beetle poisoning

in equids 70 cases. Journal of the American Veterinary Association. 1997. October 211 (8), 1018-1021.

Merck Veterinay Manual, 8th Edition. Ed: Susan Eillo. Pub: Merck and Whitehouse Station, Newstation, NJ, pp 2301.

Reed SM and Bayly WM. 1998, Equine Internal Medicine. Ed: WM Bayly, Pub: W. B. Saunders, Philadelphia 1042 pp.

Schoub TR, Panciera RF. 1989. Acute Cantharadiasis. Journal of the American Veterinary Medicine Association: 1978, 75-77.