Blackleg is an acute, highly fatal, infectious, non-contagious disease of cattle and sheep caused by Clostridium chauvoei, an anaerobic, soilborne bacterium. The disease is characterized by emphysematous swelling and necrotizing myositis that commonly affects large muscles. Infection occurs when animals ingest bacterial spores while grazing, and the spores penetrate the intestine and are disseminated via the bloodstream to the skeletal muscle, where the spores remain dormant. Following an event that causes low oxygen conditions (i.e., bruising or damage to the muscle) in infected tissue, the spores germinate, multiply, and produce toxin that results in muscle necrosis and hemorrhage. The animals affected by blackleg are usually well-fed animals between 6 months and 2 years of age, and the cause of death in affected cattle is usually acute toxemia. The course of the disease is often between 12-48 hours, and clinical signs are often absent; however, animals may exhibit signs of lameness, tachycardia, fever, anorexia, rumen stasis, and lethargy. Blackleg is primarily a disease of pastured cattle with the majority of the cases occurring during the summer months.

Producers and feedlot operators can take steps to increase the immunity of their herds and reduce the chances that their animals will contract blackleg. Annual vaccination of adult cows will boost their immune response, plus they will produce antibodies in colostrum milk to provide stronger protection for calves. It is important to note that although blackleg is infectious, it is not contagious from individual to individual. So an infected animal cannot spread the disease directly to other members of the herd. It is also not necessary for cattle to have open wounds to contract blackleg — consumption and subsequent bruising and injury are sufficient to activate the disease.