Blackleg is the common name for an infection of Clostridium chauvoei. The bacteria invade muscles, commonly in the leg. Following a reduction in oxygenation of the muscle cells (perhaps by a minor trauma) the bacteria rapidly multiply, producing gas which leads to a characteristic subcutaneous emphysema. Toxins are released into the blood stream which lead to the depression and death of the animal.
The bacteria survive for long periods in the soil and are ingested by cattle while grazing, or in infected feed. The bacteria penetrate the GI tract and are distributed into the soft tissues via the bloodstream. The bacteria enter muscle cells and can lie dormant at this stage until stimulated to reproduce by a reduction in oxygenation of the muscle cell.
We have seen an increase in the number of clinical cases of blackleg over the last year, vaccination is a cheap and effective way of preventing the disease. In cattle an initial primary course of 2 x 2ml doses are given sub cut 2-3 weeks apart and then an annual booster is given before the peak risk period. Sheep have the same dosing interval but each dose is 1ml.