blackleg or black quarter, acute infectious disease of cattle, less often of sheep, caused by an organism of the genus Clostridium. It is characterized by inflammation of muscles with swelling and pain in the affected areas. Toxins formed by the organism produce severe muscle damage, and mortality is high. Animals between the ages of six months and two years are most commonly affected. Treatment with large doses of antibiotics is only partially successful; in endemic areas, young animals can be vaccinated for prevention.