Blackleg is an infectious disease of cattle and sheep caused by the bacterium Clostridium chauvoei. The name Blackleg is descriptive of the lesions in muscle which develop as almost black coloured and crepitant (making a cracking sound) areas of infected tissue usually occurring in the area of the hind quarters. The organism is world-wide in its distribution in soil and pastures where it can remain viable for years.
Although it has been suggested that infection in cattle arises from contaminated wounds, it is more probable that infection occurs from the ingestion of the organism. These ingested bacteria would remain as dormant spores in the tissues until predisposing factors trigger its transformation in the vegetative form which very rapidly multiplies in a particular area, and develops the gas gangrenous condition (death and decay of body tissues) known as Blackleg. In sheep it is more common to relate the disease with wounds associated with lambing, docking, castration, and shearing, but ingestion maybe an important source of infection, specially for animals grouped together in paddocks. In cattle the disease is more frequent in younger animals of 6 months to about 2-3 years of age, which are in a healthy condition.
· Sometimes animals are found dead without any warning symptoms.
· The most obvious sign is a crepitant swelling particularly in the hind or fore-quarter, which crackles when rubbed with the fingers as a result of gas produced by the organism.
· The fevered animal will be lame, and the muscles in the affected region will show trembling and sometimes violent twitching.
· Death usually occurs suddenly and often within 24 hours of the symptoms first being observed.
· In sheep an acute febrile condition develops within 1-2 days following an injury, and a typical lesion (a break or wound in the skin) can be observed near the site.
· Death occurs suddenly.
· In the central part of a Blackleg lesion, there is usually a well defined area of muscle which is dark red, dry, necrotic (death of cells or tissues) and filled with small gas bubbles, and gives a characteristic rancid odour.
· Carcasses of infected animals usually decompose rapidly.
Control and prevention
· While the use of hyper immune serum and/or antibiotics is normally effective if they are given during the early stages,
· It is normally very difficult to get in time with diagnosis and treatment for a disease which evolves so rapidly to death. In addition, treatment is expensive. Active immunization using reliable vaccines to prevent Blackquarter infection has proved to be the most effective and economical way to control the disease.