Despite a horse's apparently sturdy exterior, his various internal body systems are delicate–particularly his gastrointestinal (GI) system. The normal GI tract is populated with various "good" bacteria and protozoa–referred to as a horse's normal flora–that serve one main purpose: to prevent the growth of other microbes, particularly pathogenic (disease-causing) bacteria such as Salmonella spp and Clostridium spp. This is achieved by two mechanisms. One is the physical presence of the "good" microbes that physically blocks the growth of pathogens, and the second is related to the natural flora's production of short-chain fatty acids and other metabolites that inhibit the growth of potential pathogens.

"Any factor that alters the normal population of bacteria that inhabit the GI tract can potentially result in a fatal diarrhea," explains Rodney Belgrave, DVM, MS, Dipl. ACVIM, an internal medicine clinician at Mid-Atlantic Equine Medical Center, located in Ringoes, N.J.

While horse owners are likely aware of some of the more common or well-known causes of diarrhea, other factors contributing to the development of diarrhea in horses continue to be overlooked. This article will look at some of the less commonly considered causes of diarrhea