Horse facts as told by a Shetland pony.
Gypsy, the Equine Encyclopedia, tells about blister beetles and alfalfa hay.
If you have ever taken a flake of alfalfa and shaken it out, only to find crushed striped or spotted bugs with large, spiny antennae hidden inside, you have probably encountered Blister Beetles. These toxic pests are large, approximately 1 inch long. The reason why they are harmful to horses upon ingestion is the presence of the toxin Cantharidin, which remains inside the beetles for as many as three years. Belonging to the family Meloidae, Blister Beetles come in over 200 different species and may be black with yellow or orange stripes or grayish tan paired sometimes with black spots and sometimes without.
If your horse appears to be “off his feed”, Blister Beetles can be a prime suspect. These insects are sometimes eaten by horses, usually concealed in baled alfalfa (a legume hay that is very high in protein); colic can occur. The minimum lethal quantity of blister beetles for a 1,100 lb horse is thought to be as few, or even less than, 125 beetles.
Treatment of Blister Beetle poisoning can be difficult. It has no specific treatment; in severe cases, mineral oil can be administered. Horses have either recovered or succumbed within 1-3 days. After treatment, laminitis can result.
Prevention is always the easiest and surest way. One way to do this is not to feed alfalfa or to obtain alfalfa from a field that is cut before it blooms for this reduces the possibility of hidden beetles.
Not all places are home to Blister Beetles, as they are usually found in warm, dry locations. So, if the beetles have not been found in fields nearby or you do not feed alfalfa or other flowering hays, your horses are probably safe.