Why Blister Beetles may threaten your horse

To the blister beetle, alfalfa is a delicious, nutrient-rich grass. The beetle’s diet is made up of pollen, leaves of flowering plants and blossoms, and as such, an alfalfa crop is the ideal home to feast, grow and reproduce. Unfortunately, alfalfa is also an extremely popular legume for horses. Selected for its high protein and high energy content, alfalfa is perfect for horses in medium level training programs, mares in foal and convalescing horses as well as a hay replacement when other forage supplies are low. As a crop, it is in high demand and bailed seasonally for livestock feed. Unfortunately, blister beetles are sometimes swept up into the bail. Given the beetle’s potential toxicity to horses, this is no joke, as consumption of a blister beetle can have serious consequences for your horse. So what are blister beetles? And what would happen if I found one in my horse’s alfalfa?

Blister Beetles 101?

Blister Beetles
Consumption of Blister Beetles can have dire consequences for your horse*

The blister beetle, known as Epicauta spp, secretes a defensive chemical called Cantharidin, which is designed to act as a blistering agent, hence, its name. It is highly toxic and especially poisonous for horses and ponies. Blister beetles are members of the Meloidae family and can be found throughout the United States and Canada. Their average body length is about 0.3 to 1.3 inches. Most alfalfa infestation occurs during late summer and early fall, when the adult blister beetle population also peaks.

Superficial contact with Cantharidin can have punishing consequences for both humans and horses, causing skin blistering within a few hours of contact. However, as little as half a milligram of Cantharidin per kilogram of horse, can be lethal. While this works out to 125 beetles for an average size horse, you may think, “this isn't a problem as my horse doesn't eat living alfalfa crops,” but unfortunately, it’s not just the live insects that are harmful. Toxin residues can remain active long after the beetle dies. Therefore, when the beetle is found in bales of dry alfalfa, it can still be toxic to the horse.

How Can the Blister Beetle Affect Horses?

Should a horse ingest a toxic level of the cantharidin, left untreated, the horse will normally die within 72 hours. The toxin can cause damage to the horses urinary and digestive systems resulting in:

  • Colic
  • Diarrhea
  • Fever
  • Increased heart rate
  • Increased respiratory rate
  • Polyuria (frequent urination)
  • Collapse

Sadly, there is no specific treatment for Blister beetle poisoning. Should a case be suspected, the veterinarian will normally administer IV fluids to reduce toxin absorption, antibiotics to prevent infection and a suitable medication such as a gastroprotectant to protect the horse’s digestive tract. Success rates depend upon how much cantharidin has been ingested, how long ago it was eaten and the horse’s current condition.

How Can I Minimize the Risk of My Horse Encountering Blister Beetles?

Naturally, the best medicine is prevention! Keeping your horse away from the Blister beetle will ensure he cannot be poisoned, so what are some the easiest ways to avoid contact with this dangerous bug.

  • Purchase first cutting hay. This is because Blister beetles are not active early in the season and as such, are less likely to be found in this batch.
  • Purchase your alfalfa from a trusted local supplier who will allow you to closely inspect their hay products. Good suppliers will do their own due diligence to minimize the risk of their customer’s horses encountering this potentially deadly pest.
  • Inspect any alfalfa you plan to feed very closely for Blister beetles. Do not feed any alfalfa hay that you think could contain blister beetles.
  • Blister beetles are attracted to alfalfa’s flowers therefore, try to purchase hay cut prior to flowering to minimize the risk of Blister beetles being present.
  • If you have problems with Blister beetles locally and cannot find a safe supply, consider eliminating alfalfa hay from your horse’s diet and switching to a different type of forage.

Alfalfa dwelling Blister beetles contain a toxin which can be deadly to horses if eaten. You can minimize the risk of your horse ingesting the poison by sourcing cuts of hay which are less likely to contain the beetle and closely checking any alfalfa hay before it is fed to your horse.

*Image courtesy of Dollar Photo Club