Blister Beetles Toxic to Horses

Blister beetles, which are toxic to horses, could be a problem in North Dakota this year.

“While blister beetles are common in many areas of the U.S., horse owners in North Dakota usually have not been concerned with this toxic insect,” says Carrie Hammer, a professor in North Dakota State University’s Animal Sciences Department. “However, horse owners need to be cautious because horses that consume alfalfa hay contaminated with these beetles have a high risk for serious illness and death.”

Several species of blister beetles can be found in the U.S., and all produce cantharidin, a toxin that causes inflammation and blistering of internal body tissues. Although all species produce the toxin, the cantharidin content varies among species. The striped blister beetle is known for consistently having higher toxin concentrations.

“A common question from horse owners is: ‘How many beetles can my horse consume before I need to worry?’” Hammer says. “Due to the variation in toxin concentration among beetles, this is a difficult question to answer. However, most studies report ingestion of 25 to 300 beetles is enough to kill an average-size adult horse.”

Owners should consult their veterinarian immediately if they suspect their horse has consumed blister beetles.

“Clinical signs of blister beetle poisoning usually occur six to eight hours after ingestion,” says Gerald Stokka, NDSU Extension veterinarian. “Affected horses often show signs of colic and depression, although urinary, cardiovascular and nervous system problems also can occur. Horses consuming a toxic dose can die quickly (within three to 18 hours).”

Blister beetles tend to congregate in certain areas of a field. Thus, hay bales from those areas often contain high beetle numbers, whereas bales from other areas of the field may be beetle-free.

Hammer and Stokka encourage horse owners to check alfalfa bales carefully prior to feeding the alfalfa to their horses and discard any contaminated bales.

“Even the juice from crushed beetles can cause illness; therefore, owners should not simply remove dead beetles in hopes of feeding the hay,” Hammer adds.