Blister Beetle Poisoning in Horses

Blister beetles can be deadly to horses. They are small, narrow-bodied insects that often make their homes in flowering alfalfa fields, especially in the Central and Western United States. When crushed, these beetles release cantharidin, a potent toxin that can be deadly to horses.¹

How Do Blister Beetles Affect Horses?

Horses that consume blister beetle-infested hay can experience severe health problems or even death.¹ Cantharidin is an irritating toxin. When consumed by the horse, it will cause blistering of the mouth and gastrointestinal tract. Exposure to this toxin can also cause clinical signs such as colic, excessive salivation, fever, and even heart failure. (See more about clinical signs below.)

The level of cantharidin is highly variable and released when the beetle is crushed. As little as 4 to 6 grams of blister beetles can be deadly to an 1,100-pound horse.¹ Even small parts of beetles are toxic to horses. Whole insects need not be present to be dangerous.¹ It should be noted that cantharidin remains toxic to the horse and does not dissipate or decrease over time when the hay is stored.

Early signs of blister beetle poisoning can be very nonspecific, making it difficult to diagnose. The American Association of Equine Practitioners (AAEP) identified the following clinical signs of blister beetle toxicity:

Endotoxic shock.
Colic.
Blisters in the mouth and on the tongue.
Watery feces or bloody stool.
Cardiac issues.
Bloody urine.
Diaphragmatic “thumps,” also called synchronous diaphragmatic flutter. These rhythmic contractions of the abdominal musculature mirror the horse’s heartbeat. The origin is usually traced to electrolyte imbalance. It resembles “hiccups” in the horse.
Bloodwork might show low levels of magnesium and calcium.
Renal damage.
Dehydration.

The Most Commonly Affected Hay

Blister beetle poisoning is most commonly associated with alfalfa hay; however, they can also be present in grass hay produced in arid regions. Blister beetles are attracted to the pollen, leaves, and blooms of alfalfa forage. Young, developing beetles feed on grasshopper eggs which is why blister beetles are more prevalent in arid climates such as the southern and central plains states. Blister beetles can swarm alfalfa fields when the plants are blooming (mid-summer) and grasshoppers are present, and then can get crushed in the hay during the baling process.

Certain U.S. states, such as Kansas, New Mexico, and Oklahoma, historically have higher incidences of blister beetle contamination than other states due to the more arid temperatures and increased prevalence of grasshoppers. However, blister beetles can be found in alfalfa hay grown in any region of the United States.

Horse owners need to be aware of these high-risk areas and take necessary precautions when sourcing hay for their horses. Learn as much as possible about where your hay is produced, who produced it, how it was cut, and the stage of maturity at the time of harvest. This information will help horse owners identify potential risk factors when sourcing their hay.

Protecting Your Horses

You can minimize the risk of blister beetle poisoning during harvest. The University of Minnesota Extension noted that the risk of poisoning cannot be eliminated in locations with a history of blister beetles. Below are tips for avoiding these insects:

Earlier cuttings of alfalfa tend to have minimal infestations because blister beetle populations are usually low until midsummer (August).
Beetles are more likely to be found in flowering alfalfa hay and when grasshoppers are present.
Hay that is simultaneously cut and crimped with a mower conditioner is more likely to kill beetles, keeping them in the hay.
Cutting alfalfa at less than 10% bloom reduces the number of beetles, but hay cut at this maturity is often too nutrient-dense for most horses.

“Most commonly, beetles are abundant during the midsummer months, so the second cutting of alfalfa is the stage that is most likely to be consumed by beetles and a potential threat to horses,” noted Kansas State University. “In general, early and later cuttings of hay are less likely to have beetles present. The first and fourth (if the crop permits a fifth cutting, this should be OK as well) are considered safer cuttings.”

Final Words

There is no cure for blister beetle poisoning. Supportive care for horses exposed to cantharidin while hospitalized will help to further reduce absorption of the toxin, increase the state of hydration, reduce pain, and correct potential electrolyte imbalances.

Horse owners should not be afraid to purchase alfalfa hay from endemic areas. However, they must understand the risks of blister beetle poisoning, recognize the clinical signs of cantharidin poisoning, and take precautions when purchasing their hay.