Cantharidin Toxicity in Horses

Blister beetles are a type of insect found throughout the United States. The beetles harbor a very powerful and toxic poison called cantharidin, but, unlike other types of insects, it does not spread its most toxic poison through biting. Adult blister beetles feed on alfalfa flowers and crops, the same crops used for horse and cattle feed, and when the crops are harvested the beetles are often killed in the process, contaminating the crops with their body parts and fluids and causing illness in the horses that eat the contaminated feed.

Male blister beetles share the cantharidin toxin with the female during the mating process, when they use it to cover their eggs to protect them from predators. But, this helpful toxin becomes deadly when a sufficient quantity is ingested by animals. Bites from the blister beetle are not as worrisome. Bite reactions are usually limited to raised, blister-like lesions, but these will heal over time, with the same amount of care that would be given for any other type of irritating insect bite.

Symptoms and Types

• Fever
• Colic
• Anorexia
• Diarrhea or soft stool
• Intestinal lining irritation (displayed as stomach pain)
• Kidney damage
• Blisters on the mouth and dipping face in water in attempt to provide relief from blisters
• Damage to urinary tract (displayed as abnormally frequent urination and urinary tract infection)
• Increased heart rate, possible arrhythmia and hypocalcemia

Causes

• Eating contaminated fresh or dry alfalfa; contamination occurs when blister beetles are crushed into the alfalfa during the crimping process
• Cured hay does not lose toxicity, nor does the age of hay affect its levels of cantharidin

Diagnosis

A veterinarian should be summoned if your horse is exhibiting any of the symptoms mentioned above or if you suspect that your feed was contaminated by blister beetles. You will need to give your veterinarian a thorough history of your horse's health, including a background history of symptoms, and possible incidents that might have precipitated this condition, including its recent dietary history. The history you provide may give your veterinarian clues as to which organs are being affected secondarily.

For those who live in an area where blister beetles have been found, and for horses that are known to consume alfalfa -- especially the first crop of the year -- it may be a bit easier for your doctor to diagnose this type of poisoning.

Your doctor will need to perform a urinalysis in order to correctly diagnose the presence of cantharidin. A blood profile to measure the level of toxicity in the body will also be conducted, as well as an analysis of samples from the stomach contents.

Laboratory tests to determine kidney damage and damage to the urinary tract are also very useful in making a final determination as well, and if heart dysfunction appears to be present, an electrocardiogram (EKG) recording can be used to examine cardiac electrical conduction (which underlies the heart’s ability to contract/beat).

Prompt and effective treatment, within the first three days, is essential for avoiding lasting heart or kidney damage. If you suspect that your horse is ill as the result of blister beetle poisoning, it would be better to have its urine tested immediately rather than wait until the toxin has progressed throughout the body.