Diatomaceous earth (silica) is a naturally occurring mineral made from fossilized remains of marine organisms. These fossilized particles are extremely small in size and have sharp edges. In theory, when insects come in contact with diatomaceous earth, these sharp edges cut and remove the waxy cuticle on their exoskeletons, which leads to their dehydration and death. The use of diatomaceous earth as a dewormer for internal parasites in horses is very questionable. Diatomaceous earth fed to the horse probably would be overwhelmed by and incorporated into the remainder of the horse’s feed so that very little of it actually would come in contact with internal parasites. And the equine intestinal tract is not an environment that would easily result in dehydration if the parasite was to become injured by the diatomaceous earth. Diatomaceous earth would affect only parasites in the horse’s intestinal tract which would be mainly adult parasites. Larval forms of parasites which usually reside in tissues outside the intestinal tract are often more damaging to the horse than adult stages. There are many deworming products (both daily use and purge type) available to horse owners that kill both larval and adult stages of internal parasites. These products have undergone rigorous scientific testing to validate their safety and effectiveness. Until the effectiveness of diatomaceous earth as an internal parasite control substance has been validated by scientific testing, using it should be avoided because it might leave the horse largely unprotected from internal parasitism. Companies that market diatomaceous earth as an internal parasite control substance usually recommend feeding the horse approximately one cup daily. Because of the extremely small particle size of diatomaceous earth, horses that continually breathe in these particles might be prone to respiratory problems. It is true that parasites initially would have little resistance to the mechanical injuries caused by contact with diatomaceous earth. However, we would hesitate to say that diatomaceous earth would result in “no chance” for parasite resistance over time.