Blackleg is an acute, febrile, highly fatal disease of cattle and sheep caused by Clostridium chauvoei and characterized by emphysematous swelling, commonly affecting heavy muscles (clostridial myositis). It is found worldwide.
C chauvoei is found naturally in the intestinal tract of animals. Spores remain viable in the soil for years and are purported to be a source of infection. Outbreaks of blackleg have occurred in cattle on farms in which recent excavations have occurred or after flooding. The organisms probably are ingested, pass through the wall of the GI tract, and after gaining access to the bloodstream, are deposited in muscle and other tissues (spleen, liver, and alimentary tract) and may remain dormant indefinitely.
In cattle, blackleg infection is endogenous. Lesions develop without any history of wounds, although bruising or excessive exercise may precipitate disease in some cases. Commonly, the animals that contract blackleg are of the beef breeds, in excellent health, and gaining weight. Outbreaks occur in which a few new cases are found each day, sometimes for several days. Most cases are seen in cattle from 6–24 mo old, but thrifty calves as young as 6 wk and cattle as old as 10–12 yr may be affected. The disease usually occurs in summer and fall and is uncommon during the winter. Interestingly, in sheep, the disease is almost always the result of a wound infection and often follows some form of injury such as shearing cuts, docking, crutching, or castration. The case fatality rate approaches 100%. In New Zealand, blackleg is seen more frequently in sheep.
. Initially, there is a fever but, by the time clinical signs are obvious, body temperature may be normal or subnormal. Characteristic edematous and crepitant swellings develop in the hip, shoulder, chest, back, neck, or elsewhere. At first, the swelling is small, hot, and painful. As the disease rapidly progresses, the swelling enlarges, there is crepitation on palpation, and the skin becomes cold and insensitive with decreased blood supply to affected areas. General signs include prostration and tremors. Death occurs within 12–48 hr. In some cattle, the lesions are restricted to the myocardium and the diaphragm.