Blister beetle poisoning in horses is a potentially fatal condition caused by the ingestion of blister beetles, which are found in alfalfa and other forage crops.

Blister beetles, also known as oil beetles, secrete a blistering agent called cantharidin, which is highly toxic to horses. Ingestion of as little as five grams of beetles is enough to cause death in an average-sized horse.

Symptoms of cantharidin ingestion in horses include a stiff gait, appetite loss, frequent urination, kidney failure, depression, diarrhea and fever. Currently, there is no antidote for cantharidin toxicosis in horses, and treatment is supportive and symptomatic in nature.

Prevention of blister beetle poisoning involves careful management and inspection of forage, especially alfalfa. Always purchase hay from reputable sources and inquire about the measures taken to reduce the risk of beetle contamination.

The risk of blister beetle exposure can vary depending on geographical location. Consult with a local veterinarian to understand the risk in your region and for more information if you have concerns.

Causes of Blister Beetle Poisoning

Blister beetles are a group of insects belonging to the Meloidae family, known for their ability to produce a potent toxin called cantharidin. This substance causes blistering on contact with skin and is used by the beetles as a defense mechanism. [2]

When consumed by horses, cantharidin is highly irritating and can cause severe inflammation and blistering of the gastrointestinal and urinary tracts.

Blister beetles feed on flower blossoms, particularly in alfalfa plants, which makes them a common contaminant in alfalfa hay. [1] Alfalfa hay can become contaminated with cantharidin when it is baled while a blister beetle swarm is feeding on the plants.

Alfalfa is widely used in horse diets in a variety of formats including hay, cubes, treats and pelleted feeds. [1] To reduce the risk of blister beetle poisoning, closely inspect alfalfa before feeding it to horses and source hay from trusted suppliers.

Risk Factors

There are approximately 7,500 different species of blister beetles, found all over the North American continent. The risk of cantharidin toxicosis is greatest in the western United States, where there is the highest population of blister beetles. [5]

Most species of blister beetles spend the winter in their larval stage, maturing into adults with the onset of spring when flowers are blossoming. [3] The beetles reach peak activity during the summer months, but cantharidin toxicosis can affect horses at any time and in any location. The toxin does not degrade when contaminated hay is shipped or stored for long periods.

Male beetles secrete more cantharidin than female beetles, so the quantity of toxin in a sample of beetles is difficult to predict. [1] Veterinarians consider 5 grams of beetles as a potentially lethal dose in horses, but ingestion of any amount warrants immediate medical attention and treatment. [5]

Horses of any sex, age, or breed can suffer from blister beetle poisoning. As with all forms of poisoning, the risk of toxic exposure is directly correlated to your horse’s body weight and toxin dosage. Young foals and miniature horses are at higher risk of severe symptoms and are more sensitive to lower dosages. [4]


The severity of blister beetle poisoning in horses depends on the number of beetles ingested, the concentration of cantharidin, and the sensitivity of the individual horse.

Cantharidin is highly toxic, and ingestion of as little as 0.5 – 1 mg/kg body weight can be lethal. This means that just five grams of dried beetle material can be lethal to an average 1,000 lb (450 kg) horse. [5]

When a horse consumes alfalfa infested with blister beetles, the cantharidin secreted by the beetles is rapidly absorbed by the gastrointestinal tract. [5] Affected horses often develop blisters in the mouth and upper GI tract before more severe symptoms set in. [5]

Once absorbed, cantharidin disrupts the function of protein phosphatases, which are enzymes found throughout the body that are crucial for cellular processes involved in metabolism, reproduction, and building new proteins. [4]

Cantharidin inhibits these enzymes by removing phosphate groups from proteins, resulting in significant disruption of normal cell functions and signaling pathways. This can lead to widespread cellular damage, particularly affecting tissues and organs with high metabolic rates or rapid cell turnover, such as the gastrointestinal and urinary tracts. [4][5]

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Symptoms in Horses

The severity and range of symptoms associated with blister beetle poisoning vary depending on the amount of toxin consumed by the horse. Cantharidin can affect multiple organ systems, primarily targeting the gastrointestinal, urinary, and cardiovascular systems. [5]

Gastrointestinal Symptoms

Gastrointestinal symptoms and clinical findings associated with cantharidin toxicity in horses include: [5][11]

Cardiovascular Symptoms

Horses with cantharidin poisoning may develop a number of cardiovascular symptoms, including: [4]

  • Tachycardia: a rapid heartbeat
  • Hypovolemic shock: severely low blood pressure
  • Endotoxemic shock: toxins released by dead bacteria in the GI tract enter the blood stream, resulting in sepsis
  • Myocardial necrosis: death of heart muscle tissue

Urinary Symptoms

The renal system consists of the kidneys, ureters, bladder, and urethra, and is primarily responsible for filtering waste products from the blood and the production and excretion of urine.

Once cantharidin enters a horse’s system, it can cause damage to the glomeruli in the kidneys, which are crucial for filtering blood and producing urine. [5]

Clinical findings and symptoms of renal dysfunction associated with blister beetle poisoning include: [1][4][5][10]

  • Hyposthenuria: production of abnormally dilute urine
  • Cystitis: inflammation of the lining of the bladder
  • Urethritis: inflammation of the urethra
  • Hematuria: presence of blood in urine
  • Pollakiuria: abnormally frequent urination in small volumes
  • Polyuria: excessive urination volume
  • Dysuria: difficulty and discomfort during urination

In severe cases, renal symptoms can escalate to acute renal failure. [5]

If a horse is unable to pass urine at all, it is a life-threatening emergency. Seek immediate veterinary attention if your horse is showing signs of pain while urinating or an inability to urinate.


Other Symptoms

Other clinical signs associated with cantharidin poisoning are more generalized and may include: [4][5]

  • Muscle tremors and stiff gait
  • Fever
  • Lethargy
  • Aggression
  • Bruxism (teeth grinding)
  • Seizures
  • Dehydration

In rare cases, the ingestion of cantharidin-infested alfalfa can cause laminitis. [6]


Diagnosis of cantharidin toxicosis in horses is suggested based on clinical signs and a history of exposure. Additional diagnostic tests may include: [5]

  • Examination of alfalfa-containing feed to identify the presence of blister beetles
  • Urinalysis to detect cantharidin in the urine
  • Liver biopsy
  • Serum analysis
  • Other blood tests

Differential Diagnosis

In cases of suspected cantharidin toxicosis without a known history of exposure, other conditions and toxicosis that cause similar symptoms are taken into consideration, including: [5]


Blister beetle poisoning is a medical emergency that requires immediate veterinary attention. Horses showing symptoms require hospitalization for appropriate care.

There is currently no antidote for cantharidin toxicosis available, and treatment typically involves supportive care to relieve symptoms and prevent complications. [5]

The main goals of initial treatment are halting further ingestion of the toxin, supporting detoxification, relieving abdominal pain, and maintaining blood pressure and hydration. [5] Supportive care may include administration of intravenous fluids and stall rest. [5]


Consult with your veterinarian to determine the most appropriate treatment protocol for your horse. Depending on the severity of the symptoms, your veterinarian may recommend pain medication to reduced abdominal discomfort, including: [5]


Cantharidin is rapidly absorbed into the horse’s system. As a result decontamination, or the removal of ingested feed from the gastrointestinal tract, is rarely practical once symptoms arise.

However, detoxification to block the toxic effects of ingested feed may be attempted using: [9]

  • Activated charcoal
  • Epsom salts (magnesium sulfate)
  • Di-tri-octahedral (DTO) smectite
Administration of detoxifying agents should only be performed by a veterinarian. It is not safe to administer activated charcoal without veterinary guidance. Do not administer mineral oil, as this may lead to further poisoning. [5]


The prognosis for horses affected by blister beetle toxicosis varies from guarded to extremely poor. As with all forms of poisoning, the severity of symptoms and expected outcome depend on the quantity of toxin ingested and the timing and efficacy of medical intervention.

The fatality rate of untreated blister beetle poisoning is as high as 65%. Prompt and comprehensive medical attention can reduce this rate to 20%. [11]

Horses that ingest a lethal dose of cantharidin generally die within three to eighteen hours from the onset of symptoms, notwithstanding treatment. Horses that survive two days from the onset of clinical signs have a higher recovery rate. [4][5]

Long-Term Complications

Horses that survive cantharidin toxicosis can develop long-term complications. Up to 10% of horses that recover from blister beetle poisoning develop acute laminitis, a painful inflammatory hoof condition. [6]

Symptoms of acute laminitis include: [7]

  • Hot hooves
  • High digital pulse
  • Recumbency
  • Muscular tension

Preventing Blister Beetle Toxicosis

Since there is no antidote for cantharidin poisoning, preventing exposure to blister beetles is the best way to ensure the safety and well-being of horses.

Alfalfa crops are favored by blister beetles, so if you are feeding alfalfa hay make sure it is sourced from areas with low beetle populations or harvested in a way that minimizes beetle contamination.

While removing access to alfalfa hay entirely is the safest option to avoid cantharidin poisoning, doing so is impractical for many horse operations. Alfalfa is a nutrient-dense forage and an excellent source of protein. This makes it well-suited to horses with higher energy requirements such as performance horses and pregnant and lactating mares.

Given the popularity of alfalfa products, the key to preventing cantharidin toxicosis in horses lies in sourcing uncontaminated alfalfa through careful coordination between growers, suppliers, and horse owners.

Strategies for Horse Owners

Practices that horse owners can implement to reduce the risk of blister beetle toxicosis include: [5]

  • Choosing high-quality forage suppliers from low-risk areas with protocols to prevent beetle contamination
  • Inspecting hay and pastures for the presence of blister beetles
  • Purchasing hay that hasn’t been crimped (mechanically crushed) before baling
  • Monitoring horses for symptoms, especially after introducing new batches of hay to the diet
  • Training all farm personnel on the risks of blister beetle poisoning

Consult with your veterinarian for additional prevention strategies and to understand more about the signs of cantharidin toxicosis. You can also keep up to date with your local agricultural extension service for reports that might indicate increased blister beetle activity in your area.

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Strategies for Hay Producers

Alfalfa producers can take several steps to prevent blister beetle poisoning in horses, reducing the likelihood of beetle contamination during the growing and harvesting processes: [5]

  • Timely Harvesting: Harvest alfalfa and other forages before they flower, as blister beetles are attracted to the blossoms. Avoid harvest from July to September, the peak activity period for blister beetles.
  • Avoid Crimping: Crimping, which involves crushing the hay to speed up drying, can trap or kill blister beetles in the hay, leading to contamination. Avoiding crimping or crimping hay post-harvest allows beetles to escape before baling.
  • Regular Monitoring: Regularly inspect fields for blister beetle activity, especially during peak seasons (typically mid-summer). Understanding local beetle populations and their behavior can help in planning the safest times for harvesting.
  • Insect Control: Many species of blister beetles do not swarm, so hay producers typically don’t take special measures if these beetles are spotted in crops meant for hay or silage. However, the striped blister beetle is an exception because it swarms, leading producers to actively manage these particular beetles when they’re present in their fields. [1]
  • Quality Control: After harvesting and before baling, inspect the hay for any signs of blister beetles. This may involve manual checks and could be aided by machinery designed to remove foreign objects.
  • Coordination: Work closely with veterinarians, agronomists, local equine extension services and horse owners to understand their concerns and requirements for beetle-free hay. Communication is key to maintaining a safe supply chain.

By implementing these strategies, hay producers can significantly reduce the risk of blister beetle contamination in their products, helping to protect the health of horses.


Cantharidin toxicosis is a life-threatening form of poisoning in horses, typically caused by ingestion of blister beetles in contaminated alfalfa hay.

  • Cantharidin, secreted by blister beetles, is highly toxic to horses and can be fatal in doses as small as five grams of beetles
  • Blister beetles commonly feed on alfalfa, posing a significant risk to horses consuming alfalfa hay
  • Symptoms of cantharidin toxicosis include rapid heart rate, lack of appetite, frequent urination, fever, stiff gait, seizure, and sudden death
  • There is no cure for blister beetle poisoning in horses; treatment focuses on detoxification and supportive care
  • The prognosis for affected horses varies but is generally guarded to extremely poor
  • Prevention involves effective coordination between horse owners, veterinarians, and hay producers to minimize the risk of blister beetle exposure

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  1. Kinney, K. et al. Blister Beetles in Forage Crops. Colorado State University Extension. 2010.
  2. Romano, M.C. Cantharidin Toxicosis in Animals – Toxicology. Merck Veterinary Manual. 2022.
  3. Allen, C. Blister Beetles. Texas A&M Extension Entomology. 2018.
  4. Plumlee, K.H., Ed. Clinical veterinary toxicology. St. Louis, Mo: Mosby, 2004.
  5. Hovda, L.R. et al. Blackwell’s Five-Minute Veterinary Consult Clinical Companion: Equine Toxicology. Wiley Blackwell, 2022.
  6. Holbrook, T.C. Treating Cantharidin Toxicosis. Compendium Equine. 2009.
  7. Laminitis: Prevention & Treatment. American Association of Equine Practitioners.
  8. Heuzé, V. et al. Alfalfa (Medicago sativa). Feedipedia, 2016.
  9. Wilson, D.A., Ed. Clinical veterinary advisor: The horse. St. Louis, Mo: Elsevier Saunders, 2012.
  10. Cotovio, P. et al. Acute Kidney Injury by Cantharidin Poisoning Following a Silly Bet on an Ugly Beetle. Clin Kidney J. 2013. doi: 10.1093/ckj/sft001.
  11. Moore, J.N. Colic in Horses. MSD Veterinary Manual. 2019.