About Blister Beetle Poisoning

Creative Commons image by Vincent P. Lucas

Blister beetles are sometimes found in hay, particularly alfalfa, where they may be inadvertently swallowed by livestock. Although sheep and cattle are often exposed to blister beetles in hay, they do not generally become ill as a result.

Horses, on the other hand, are quite susceptible to blister beetle poisoning, which can cause endotoxemia, kidney damage and rapid death. Impact on humans is generally limited to blistering of the skin from contact with the beetles and the toxin they produce; these blisters heal without treatment in a few days.


Blister beetles produce cantharidin, a highly toxic substance that causes severe blistering to skin and organs of the digestive and urinary tract. Cantharidin is a very stable chemical; heat processing, curing, and crushing hay does not reduce the risk of blister beetle poisoning to livestock. In fact, these processes make it considerably more difficult to determine whether the hay is infested with blister beetles, thereby increasing the risk of poisoning to animals fed the hay.


Blister beetle poisoning causes many symptoms in affected horses and other livestock, including colic, diarrhoea, sweating, fever, frequent drinking and urination, excess salivation, shock and sudden death. A necropsy usually reveals that the inner surfaces of the entire digestive and urinary tract are covered in numerous blisters; severe kidney damage, laminitis, and reduced blood calcium levels may also be present.


Hay is most likely to be infested when the grower does not closely monitor for blister beetles. In general, hay that is intended for consumption by cattle and sheep is more likely to be unsafe for horses, since other livestock can tolerate higher levels of cantharidin without serious effects. The first cutting of alfalfa is least likely to contain blister beetles, since the insects are not usually active until later in the season. Third and fourth cuttings frequently contain blister beetles, as does hay that was cut when the alfalfa was in bloom.


The most common species of blister beetles found in the United States are the black blister beetle, the striped blister beetle, and the margined blister beetle. The three species are about the same size--approximately 1 inch long. They have long, slim bodies, wide head, and softer bodies than most beetles. Their antennae are quite lengthy, about one-third of the body's overall length, and extend at about a 45-degree angle from the head. Unfortunately, a lack of visible blister beetles in hay does not necessarily mean that it is not contaminated; if the beetles are crushed during hay processing, they may no longer be visible but their toxin remains in the hay.


Blister beetles are found in almost all agricultural areas in the continental United States and Canada. Blister beetles in hay fields occur most frequently in arid climates, such as the Great Plains region and the Southwest. Black, striped and margined blister beetles are native to North America.