Clostridia are relatively large, anaerobic, sporeforming, rod-shaped, gram-positive organisms. They are found either as living cells (vegetative forms) or as dormant spores. Their natural habitats are soils and intestinal tracts of animals, including people. Dormant spores of several clostridial species have been found in healthy muscular tissue of horses and cows. The endospores are oval, sometimes spherical, and are located centrally, subterminally, or terminally. The vegetative forms of clostridia in tissue fluids of infected animals occur singly, in pairs, or rarely in chains. Differentiation of the various pathogenic and related species is based on cultural characteristics, spore shape and position, biochemical reactions, and the antigenic specificity of toxins or surface antigens. The genomes of many clostridia have been sequenced and are available online. Pathogenic strains or their toxins may be acquired by susceptible animals by either wound contamination or ingestion. Diseases thus produced are a constant threat to successful livestock production in many parts of the world.
Clostridial diseases can be divided into two categories: 1) those in which the organisms actively invade or when locally dormant spores are activated and reproduce in the tissues of the host, with the production of toxins that enhance the spread of infection (the gas-gangrene group, the clostridial cellulitides group); and 2) those characterized by toxemia resulting from the absorption of toxins produced by organisms within the digestive system (the enterotoxemias), in devitalized tissue (tetanus), or in food or carrion outside the body (botulism). Clostridial diseases are not spread from animal to animal.