Wisconsin Farm: Blister-Beetle-Infested Hay Kills 15 Horses

In September 2019, Cindy Kanarowski-Peterson of Red Ridge Ranch, in Mauston, Wisconsin, owned 110 horses that carried tourists from surrounding states on trail rides, hayrides, sleigh rides, camping trips, and at summer camps. People even stopped by just to say “hi” and give their favorite horse a pat.

By January 13, Cindy had lost 15 of her horses to a mysterious disease. “It started with just a couple of horses with light colic symptoms,” she says. “They’d eat their grain, stop eating, stand there, look at me like they had a belly ache, hang their heads, and maybe lie down,” she says sadly. She responded as she usually does to colic, “and they popped out of it.”

But as one horse after another repeated the cycle and then started losing weight and even dying, she racked her brain trying to determine a cause. Was it grain? Was it worms?

“Finally, one week, it was all of a sudden much, much worse,” she says. “I was exhausted and thought, I can’t keep doing this!

Our weather had been so wet the past two years that we couldn’t get into our hayfields; we’d had to buy winter hay in online auctions. I’d just begun feeding some Wyoming hay. The first vet I called sent grain samples for analysis. Test results took over three weeks and indicated no problem with the grain. Meanwhile, my horses were still dying.”

She called another veterinarian, who performed a necropsy on the latest victim, finding a huge hole in his stomach. “The horse was blistered down his esophagus, all the way through his insides, down his intestinal wall, like someone had poured acid down him,” Cindy says. “The vet said it was blister beetles. We stopped feeding that hay Dec. 13, but after that more horses died. I’m sure they had so much damage they just couldn’t recover. It was sad and sickening and frustrating and a feeling of helplessness every day.”

What You Can Do

The severity of a horse’s clinical signs will vary according to how many beetles or how much of the (cantharidin) toxin the horse has consumed, says Kate Christie, DVM, Dipl. ACVIM, an internal medicine specialist at Rood & Riddle Equine Hospital, in Lexington, Kentucky. “Even when people touch the beetle or the toxin gets on our skin, we get blisters that usually rupture within three to eight hours.

“The first signs are ulcerations in the mouth, not wanting to eat, drooling, or drinking lots of water because it makes them feel better,” she adds. “The first signs owners might see are depression and colicky signs and diarrhea. In time the toxin can affect the kidneys and heart, as well. In severe cases where the horse has eaten a large number of beetles or a large amount of the toxin, death can occur rapidly.”

Christie says that if your horse gets sick from ingesting blister beetles or their toxin, your veterinarian might treat him with activated charcoal or Bio-Sponge, which basically limit toxin absorption. Gastroprotectants help heal the ulceration, and if your horse has diarrhea, supportive care with intravenous (IV) fluids can replace electrolytes. “There’s no specific antidote, unfortunately,” she adds.

Hay Disposal

After your horse is diagnosed and being treated, what do you do with all your blister-beetle-infested hay? “I’d like to get rid of it,” Cindy says. “At this point it’s evidence so it’s sitting in my building.”

“Sheep and cattle can also be affected by blister beetles,” Christie notes. Other options include burning, burying, and composting the hay.

Blister Beetle Basics

Blister beetles (about 2,500 species worldwide, with more than 400 known in North America) exist in many shapes, sizes, colors, and locations, with some in habitats where horses are highly unlikely to encounter them, says PJ Liesch, MS, director of the University of Wisconsin-Madison Insect Diagnostic Lab. Populations peak at various times, as well.

Liesch describes blister beetles as a diverse family of typically oblong beetles that can range anywhere from ¼-inch to more than 1 inch long. The most common species seen in the Midwest and many other areas of the U.S. tend to be either black or yellowish striped, but might also be black, grayish, or dark metallic turquoise.

Although blister beetle problems in horses seem to arise mostly in alfalfa hay that’s harvested while it’s blooming heavily, blister beetles aren’t alfalfa-specific. “Usually they show up and nibble on (alfalfa or other) plant leaves and flowers, then within a few days they just move on,” Liesch says. “It’s rare that they’re present right at harvest time. And, they may just be on the edge of a field rather than throughout.”

The beetles’ tendency to “eat and run” means there’s no need for you to worry about the potential of your pastures being infested with blister beetle eggs from hay you’ve bought, he says.

However, not all blister beetles feed on foliage; the larvae of one common genus in the U.S, Epicauda, feed on grasshopper eggs.

Minimizing Risk to Your Horses

First, know your hay producer and communicate that you’re buying hay to feed to horses.

Hay harvesters can reduce the chances of blister beetle infestation in their hay by:

  • Harvesting alfalfa before its peak bloom, and
  • Avoiding crimping hay when harvesting, as that process might crush and incorporate the beetles and/or their toxin into the hay. Simply cutting and allowing the hay to air dry allows any beetles that might be present to wander away before the hay is baled.

Once baled, identifying blister-beetle-infested hay becomes difficult. “They might be crushed to the point where identification would be very difficult without a microscope,” Liesch says. “However, they differ from most beetles in that they have a softer body, similar to a firefly. If you step on one, you won’t hear a hard crunch like you would with most beetles.”

Picking the beetles out of hay won’t necessarily keep your horse from getting sick. “The toxin can persist for quite a long time in the hay, and even if the beetles themselves had tumbled out of the hay during harvesting, you could still have cantharidin toxin present that could harm your horse,” Liesch says.