Blister beetle-infested hay can be toxic to horses

Alfalfa High numbers of the insect reported in fields in western North Dakota this summer

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The University of North Dakota Extension Service is warning that blister beetles, which are toxic to horses, could be a problem in the state this year.

According to, the insect is also found in southern Alberta.

NDSU equine specialist Carrie Hammer says there have been several reports of alfalfa fields infested with blister beetles in western North Dakota this year.

“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,” she says in an NDSU release.

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.

Hammer says that due to the variation in toxin concentration among beetles, it’s difficult to say how many beetles are toxic, but most studies report ingestion of 25 to 300 beetles is enough to kill an average-size adult horse.

Clinical signs of blister beetle poisoning usually occur six to eight hours after ingestion. 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, the release says.

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

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