By Jess Hallas-Kilcoyne - Commonly found in soil, decaying animal carcasses, and, sometimes, decaying plant material, Clostridium botulinum is the bacterium responsible for producing the toxins that cause botulism. These powerful toxins prevent the release of neurotransmitters that control muscle contractions, resulting in weakness and, often, paralysis. Botulism in horses, as in humans, is frequently fatal.