Blister Beetle in Horses

Blister Beetle in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

Most common symptoms

Anemia / Destructive / Lethargy / Pain / Swelling


0 Veterinary Answers

Most common symptoms

Anemia / Destructive / Lethargy / Pain / Swelling

Ask a Vet
Blister Beetle in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

Jump to section

What is Blister Beetle?

The blister beetle is usually found in alfalfa which has been harvested in certain areas of the country and during certain seasons of the growing year. When consumed by your horse, he can become quite sick and can even develop life-threatening complications, especially when medical treatment is not available or provided. There are certain conditions under which alfalfa hay (a very common feed for horses) becomes infested by the beetle. It is vital that the animal be evaluated and treated if any suspicion of blister beetle poisoning is noted.

Blister beetles are basically a type of beetle which secretes a substance called cantharidin which is quite irritating to any animal that consumes or comes into contact with it. The technical designation for blister beetle toxicity is cantharidin intoxication.

Symptoms of Blister Beetle in Horses

Cantharidin intoxication (blister beetle poisoning) will likely show up as blisters in the mouth of your horse as well as throughout their gastrointestinal tract. Other signs of possible exposure and poisoning include:

  • Blisters in the mouth of the animal as previously mentioned
  • Dunking of their mouths into a source of water
  • Decreased eating
  • Abdominal pain (such as colic)
  • Stretching out to urinate
  • Frequent attempts to empty bladder
  • Lethargy and lack of enthusiasm 

Symptoms are dependent upon the amount of beetles consumed by the horse. Possible changes in electrolytes due to severe levels of toxicity can cause abdominal cramping also referred to as the thumps. This symptom indicates that the horse suffers from more severe toxicity. 


There are four types of blister beetle species that have been found to be fairly common throughout the eastern and central United States.

  • Ashgray - Epicauta fabricii

  • Black - E. pennsylvanica, found largely in Illinois
  • Margined - E. pestifera

  • Striped - E. vittata; found largely in the southwestern USA

While each species varies in its average content of cantharidin, the striped beetle has been consistently shown to possess higher concentrations than the other blister beetle species. Additionally, the striped blister beetle is a species which seems to live in larger communities or clusters and are usually found along the edges of the fields instead of being more spread out. This results in a higher risk of increased numbers of the harmful beetles being found in baled hay.

Causes of Blister Beetle in Horses

The only real cause of blister beetle poisoning in horses is the cantharidin contained in it, though the concentration of this toxin will vary depending on the species of blister beetle that has been consumed.  After the cantharidin from the beetle is eaten by the animal, it is absorbed from the digestive tract and then eliminated by the kidneys. It will cause blisters to form anywhere it makes contact with skin or mucous membranes, for example, inside the mouth, or within the digestive and urinary tract. The cantharidin is also quite toxic to humans, cattle, sheep, goats, dogs, cats, rabbits, and rats. The signs of poisoning can be seen within hours of exposure either by contact or consumption. The exposure happens primarily from the consumption of alfalfa hay which was infested with the blister beetle during hay growth and the beetle was not eliminated during the harvesting process. The beetle’s life cycle makes it most commonly found during the mid-summer months of the growing season which makes the second hay cutting the most likely to be infested by them and, accordingly, the biggest possible threat to your horse.

It is suggested that the earlier and later cuttings of the hay would most likely be safer (first, fourth and possibly fifth cuttings) as the life cycle of the beetle is spent. During the harvesting process, if the hay is cut and baled the same day, using a piece of equipment called a crimper, the likelihood of beetle infestation is greater because the blister beetle has not had an opportunity to escape captivity. Conversely, if the hay is cut and allowed to dry before being baled at a later time, the beetle is given time to escape and it will be less likely to inhabit the hay.

Diagnosis of Blister Beetle in Horses

The use of high-performance liquid chromatography, gas chromatography, or mass spectrometry analyses can be utilized to detect cantharidin in the contents of the gastric system or the urinary system though the concentration of cantharidin in urine is hardly traceable in 3 to 4 days. Since this is so, it is necessary to collect the urine sample as early as possible when symptoms are noted if a thorough analysis is desired. Various laboratory findings will also be helpful in determining the cause of the abdominal crisis in your horse.


Treatment of Blister Beetle in Horses

While there is no specific antidote for cantharidin poisoning, prompt and aggressive medical attention and therapy is vital for a good prognosis of your horse. Mineral oil can be given orally to help achieve digestive elimination of the poisonous substance and be prepared as it may take repeated doses to achieve this. Additionally, activated charcoal is also helpful if it is given orally early enough. While other oral adsorbents having di trioctahedral smectite as an ingredient may aid in elimination, their effectiveness has not been established. Calcium and magnesium supplementation for lengthy periods will very likely be recommended. Some other options include: 

  • Administration of fluids, analgesics and diuretics
  • Maintenance of normal blood pH and serum electrolyte concentrations

The prognosis can improve on a daily basis as long as there are no complications.

Recovery of Blister Beetle in Horses

The most obvious prevention recommendation is to make all attempts possible to feed your horse beetle-free hay. It is very important to pay close attention to the timing of the cutting of the hay and the season of the year. Generally, in the life cycle of the blister beetle, the subadult and adult blister beetles don’t come on until late May or June in the southwestern USA, making the last cutting of hay the safest to feed. If your situation includes a horse that has already been exposed, make every effort to reduce further toxin absorption, increase the water intake, correct any electrolyte imbalances and try to reduce any pain the is present in the animal.

Your horse may require hospitalization, lasting a few days to perhaps a week. The blister beetle poisoning can occur with only the ingestion of a small of amount of the beetle and casual observation will not necessarily see them in the hay. Exercising all possible care when feeding alfalfa hay, which we all know is favorite of many horses and owners, is vital for the health of your horses. Get medical attention at the earliest possible opportunity when any of the symptoms are noted. This is vital to the survival of your horse.

*Wag! may collect a share of sales or other compensation from the links on this page. Items are sold by the retailer, not Wag!.

Blister Beetle Average Cost

From 308 quotes ranging from $2,000 - $8,000

Average Cost


Blister Beetle Questions and Advice from Veterinary Professionals

Blister Beetle Average Cost

From 308 quotes ranging from $2,000 - $8,000

Average Cost