Blister beetles produce a toxin that causes what looks to be a standard blister. These bugs do not deliver the toxin via a bite, nor are they capable of delivering noticeable bites to humans. Blister beetles secrete their toxin from their leg joints, meaning that exposure occurs through handling or ingestion.
Blister beetles are medium-to-large-sized insects of the family Meloidae. Their name comes from their ability to secrete a toxin, cantharadin, from their leg joints that causes painful blisters upon contact with human skin. Blister beetles possess chewing mouth parts but seldom bite, and bites do not result in visible signs or health symptoms. Careless handling of a blister beetle, however, results in the beetle secreting its toxin in defense.
As well as skin blisters, cantharadin causes swelling and conjunctivitis if eye contact occurs. Ingestion of blister beetles is even more dangerous. Ingestion is rare in humans but affects livestock, which occasionally consume the beetles in alfalfa. Several species of blister beetles feed on alfalfa flowers. While harvesting kills the beetles, body parts and fluids remain in the alfalfa. Symptoms of blister beetle poisoning include mouth blisters, diarrhea, kidney and urinary tract damage, and rapid heart rate. Less than 10 blister beetles may be fatal to an adult horse.
The toxin cantharadin, in controlled doses, is used medically in the treatment of resistant warts. A doctor puts a small amount of cantharadin on a wart, causing the skin to blister. The doctor then removes the wart along with the blistered skin.