Blister Beetle (Cantharidin toxicosis)

Since the last two posts have been associated mostly with dogs I thought it might be nice to move to another species for this post.  The topic for this post is cantharidin, a nasty little compound found in blister beetles. A quick search on Google Scholar revealed a wide variety of species that have been affected, but the most commonly affected species are those that eat alfalfa hay.  Most reports are in horses, but there are also case reports in cattle, sheep, goats, rabbits, dogs, cats, various birds, and even people. 

            There are many different beetles that contain cantharidin.  The most common in the United States are those of the species Epicauta.  The adult beetles feed on the nectar of flowering plants.  As a result if alfalfa hay is cut when it is in bloom, there is a risk of smashing up swarms of these beetles in the process.  Therefore, if hay is cut before it flowers, or if it is cut before or after the adult beetles are active, it will reduce the risk of cantharidin exposure to the unsuspecting consumers.

Cantharidin is very irritating and can cause blisters on exposed skin.  You can imagine what it does to the mouth, throat, and stomach.  It causes erosions and ulcerations in the upper gastrointestinal tract (esophagus and stomach), and because it is excreted in the urine, it can also cause severe inflammation of the bladder. 

            Horses are likely the most susceptible and, according to several sources, as little as 4-6 grams of dried beetle can kill a horse.  Some reasonable prevention techniques are to feed first or last cutting hay.  Adult beetles emerge later in the spring, so typically first cutting is done before they are active.  Similarly the last cutting is typically after they are most active, reducing the likelihood of beetle contamination in the hay.  Feeding grass hay can also be an effective alternative.  If the number of animals you are feeding is small enough, you may find that simply inspecting each flake of hay before you feed is effective.  If you see beetle parts don’t feed the hay.  Even if the beetles are removed the cantharidin can, and very likely is still present on the stems of he hay.