Blackleg is an acute, febrile, highly fatal disease caused by Clostridium chauvoei bacterium, affecting cattle and sheep worldwide. Blackleg is characterized by sudden onset of emphysematous swelling, commonly affecting heavy muscles (clostridial myositis) of the hip, shoulder, chest, neck, back, or sometimes other areas.

Blackleg usually occurs in outbreaks of a few cases per day in cattle from 6 to 24 months of age, however it has been seen in calves as young as 6 weeks old and in cattle as old as 12 years of age. It often occurs in farms which have had recent excavations or flooding, as the spores remain active in the soil for many years and can become sources of infection to cattle when disturbed to the surface.