Blister Beetle Poisoning in Donkeys

Blister beetle poisoning is a life-threatening toxicosis caused by cantharidin, a chemical found in blister beetles. Donkeys are exposed when they eat hay, most often alfalfa, that contains crushed beetles or beetle parts. Cantharidin stays toxic even after the hay is dried and stored, so older hay is not automatically safer.

This toxin is highly irritating to body tissues. After ingestion, it can injure the mouth and digestive tract, then affect the kidneys, urinary tract, and heart. Clinical signs can range from mild discomfort to severe shock and death within hours, depending on how much toxin was eaten.

Most published veterinary guidance focuses on horses and other equids, but the same toxin risk applies to donkeys. Because donkeys may show pain more subtly than horses, early signs can be easy to miss. If your donkey seems quiet, uncomfortable, or suddenly off feed after eating hay, treat it as an emergency and contact your vet right away.

The cause is ingestion of cantharidin-contaminated hay. Adult blister beetles are attracted to flowering plants, especially alfalfa blooms. During harvesting, beetles may be crushed into the forage, releasing toxin into the hay. Even a small amount of contaminated material can be dangerous for an equid.

Risk tends to be higher in parts of the Midwest and Southwest United States, where blister beetles are more common. Hay harvested during periods of active beetle swarming is more concerning. Crimping equipment can increase risk because it crushes insects into the forage rather than allowing them to escape.

Stored hay does not lose cantharidin toxicity over time. That means a bale can remain dangerous months later. Donkeys are usually poisoned by eating contaminated alfalfa hay, but mixed forage containing alfalfa can also be a concern.

This is not caused by a bite or sting. The problem is swallowing the toxin in feed. If one donkey on the property is affected, other equids that ate from the same lot of hay may also be at risk and should be discussed with your vet.

Your vet usually starts with the history and physical exam. A recent change in hay, especially alfalfa, plus signs like colic, mouth irritation, urinary discomfort, diarrhea, dark gums, or shock can raise strong suspicion. Because donkeys may show quieter signs than horses, your vet may rely heavily on the feeding history and subtle exam findings.

Diagnostic testing often includes bloodwork and electrolyte testing to look for dehydration, kidney injury, and abnormalities such as low calcium or magnesium. Urinalysis may help assess urinary tract irritation and kidney involvement. Heart monitoring may be recommended if there is concern for arrhythmias.

A more specific diagnosis can be made by detecting cantharidin in urine or stomach contents. Urine is most useful early, because detectable levels can fall within about 3 to 4 days. In fatal cases, microscopic examination of stomach contents may reveal blister beetle fragments.

There is no single bedside test available in every field setting, so diagnosis is often a combination of exposure history, clinical signs, and supportive lab findings. If you suspect contaminated hay, keep the bale and lot information available for your vet and remove that hay from all equids immediately.

Prevention centers on hay sourcing and harvest practices. The biggest risk is alfalfa hay from areas where blister beetles are common. Ask hay suppliers where and when the hay was harvested, whether the field was scouted for beetles, and whether the crop was crimped during harvest. Hay cut before heavy beetle activity or after activity has dropped is generally lower risk.

Inspect each new load of hay before feeding, but remember that visual checks are not perfect. Beetles may be crushed into flakes and hard to see. If you find suspicious insects or if one bale seems contaminated, stop feeding that lot to all equids and contact your vet.

Many veterinary references note that first cutting is often lower risk because adult blister beetles usually emerge later in the season, and later cuttings may also be safer once adult activity ends. Still, no cutting is guaranteed safe. Buying from growers who avoid harvesting heavily infested patches and who use methods that reduce beetle crushing can lower risk.

Store hay by lot when possible so you can trace problems quickly. If your donkey develops sudden colic or mouth irritation after a new batch of hay, remove the feed immediately and save samples for your vet. Fast action can protect the rest of the herd and may improve the outcome for the affected donkey.