Overview of Clostridial Diseases in Animals

Clostridia are bacteria of the phylum Bacillota (formerly Firmicutes), which are large, anaerobic, spore-forming, rod-shaped, gram-positive organisms. They occur naturally in soils and in the intestinal tracts of animals, including humans.

Although many clostridial species can be found in animals, only a small number are capable of causing disease. The various pathogenic and related species can be differentiated on the basis of morphological characteristics in culture, including spore shape and position, biochemical factors, and the antigenic specificity of toxins or surface antigens. The genomes of many clostridial species have been sequenced and published.

Pathogenic clostridial strains or their toxins can be acquired by susceptible animals via either wound contamination or ingestion and result in neurotoxic, histotoxic, or enteric disease. These diseases are not transmitted from animal to animal or from animal to human.

Clinical clostridial disease occurs only when certain predisposing conditions arise, such as damage to the skin or intestinal barriers or alterations in the GI microbiota associated with certain feeds or antimicrobial treatments. A key feature of clostridial infections is the cellular and tissue damage that results from the potent effects of protein toxins that are either secreted from the cell or released upon lysis.

Another important feature of pathogenic clostridial infections is the organism's ability to sporulate, resulting in latent spores that are capable of surviving harsh environmental conditions, such as heat and desiccation. Once they are in a more suitable environment, such as the ischemic conditions of a deep wound, the spores can germinate into vegetative cells capable of producing toxins and of reproducing.

Clostridial diseases can be divided into two categories:

• diseases in which the organisms actively invade a host or locally dormant spores are activated and reproduce in tissues of the host (clostridial soft tissue infections include cellulitis, myositis, and myonecrosis), producing toxins that enhance the spread of infection (eg, blackleg, bacillary hemoglobinuria, infectious necrotic hepatitis, gas gangrene)

• diseases characterized by toxemia resulting from the absorption of toxins produced by organisms within the digestive system (enterotoxemias, clostridial abomasitis, Clostridioides difficile infection), in devitalized tissue (tetanus), or in food or carrion outside the body (botulism)

For discussion of a few additional important clostridial diseases, see the separate topics covering Tyzzer disease, gangrenous dermatitis in poultry, quail disease (Clostridium colinum infection), and enterotoxemia in rabbits (Thomasclavelia spiroformis infection).