Written by Dr. Kaitlyn Coleman
2021-2022 Peterson Smith Equine Hospital + Complete Care Hospital Intern
Where is cantharidin found?
Cantharidin is a toxic substance found in the Epicauta genus of beetles, which are frequently associated with toxicosis in horses. These beetles are commonly known as blister beetles. The lethal dose of cantharidin is <1mg/kg, but concentrations of the toxin vary between the different species of blister beetle. Ingestion of 100, or as little as 4 to 8 dried beetles, may be lethal. Usually more than one horse is affected, and the fatality rate may be 50% or greater. The striped blister beetle (Epicauta vittata), ashgray blister beetle (Epicauta fabricii) and the “Florida” blister beetle (Epicauta floridensis) have all been known to inhabit Florida and many northern and southeastern states.
What are the clinical features of cantharidin toxicity?
Cantharidin is a highly potent irritant, which causes cell damage, vesicle formation, and necrosis on contact with skin or mucous membranes. The gastrointestinal tract is most affected in horses because of ingestion. The urinary tract can be affected from high concentrations of cantharidin in the urine causing inflammation of the bladder and kidney damage. Severity of clinical signs vary according to the dose of cantharidin ingested. Signs range from depression or discomfort to severe colic, shock and even death. Petechiation and/or reddish gums or purple discoloration on the gum line “toxic line” can be noticed as well as sweating, irritability, elevated heart and respiratory rates, fever, painful urination/red urine and profuse diarrhea. Low levels of calcium and magnesium on a serum chemistry are characteristic features of cantharidin toxicity and may be profound. Horses that ingest large amounts of the toxin show signs of severe shock and die within hours of consumption.
How is cantharidin toxicity diagnosed?
A presumptive diagnosis is made by the finding of blister beetles in hay along with the clinical signs displayed. Identifying the species of beetle inciting damage may be necessary to determine the amount of cantharidin ingested. A definitive diagnosis can be made by an analysis of the gastric contents and urine by using both high performance liquid chromatography and gas chromatography or mass spectrometry, which are all sensitive and reliable methods. A urine sample should be collected immediately if cantharidin toxicosis is suspected due to its concentration in urine becoming undetectable in 3-4 days. Laboratory findings such as prolonged decrease in serum calcium and magnesium concentrations, as well as a marked increase in serum CK concentration within 24 hours of onset are helpful findings in differentiating cantharidin toxicosis from other gastrointestinal toxicoses. A urinalysis may reveal a decrease in specific gravity (<1.010) and horses acutely affected are almost always hyperglycemic.
How is cantharidin toxicity treated?
There is no specific antidote or treatment for cantharidin toxicity. Aggressive treatment with intravenous fluid therapy and maintenance of electrolyte balance are important. Close attention to renal function and serum calcium levels are important when developing a fluid therapy plan. Administration of activated charcoal (if given early) and Biosponge via nasogastric tube can be helpful in decreasing the amount of toxin absorbed and aiding in evacuation of the GI tract. Interestingly, a study in rats found that mineral oil increased cantharidin absorption and resulted in increased fatality. Supplementation of magnesium and calcium for prolonged periods is almost always necessary as well as the administration of pain medication and diuretics. NSAIDS should be used cautiously to prevent further GI and renal damage. Alternative analgesics that can be used are lidocaine by constant rate infusion or an opioid such as butorphanol. The prognosis for horses affected improves if no complications occur.
What is the best way to prevent cantharidin toxicity?
Prevention involves feeding beetle-free hay. Scouting the field and avoiding areas that contain swarms of beetles before it is cut and during baling can also be a helpful measure of prevention. Once there is no sign of the beetles the area can be cut and harvested. Asking your hay distributor if these measures are taken during their alfalfa baling process or looking through your alfalfa bales before feeding may be necessary. First cutting hay is usually always free of blister beetles because the insects usually lie dormant and do not emerge until late May or June. The last cutting hay is also usually safe because it is harvested after the adult’s inactive period.
- Aiello, Susan E., et al. The Merck Veterinary Manual: Cantharidin (Blister Beetle) Toxicity. 11th ed., Merck & Co., Inc., 2016.
- Selander, Richard B, and Thomas R Fasulo. “EENY166/IN323: Blister Beetles (Insecta: Coleoptera: Meloidae).” AskIFAS Powered by EDIS, IFAS Extension: University of Florida, 22 Oct. 2020, https://edis.ifas.ufl.edu/publication/IN323.
- Smith, Bradford P., et al. (2020) Large Animal Internal Medicine: Cantharidin Poisoning (Blister Beetle Poisoning). 6th Elsevier.
- Qualls, HJ et al. (2013) Journal of Veterinary Internal Medicine. Sept-Oct 27(5): 1179-84.