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Blackleg, black quarter, quarter evil, quarter ill (Latin: Gangraena emphysematosa) is an infectious bacterial disease most commonly caused by Clostridium chauvoei, a Gram-positive bacterial species. It is seen in livestock all over the world, usually affecting cattle, sheep and goats. It has been seen occasionally in farmed bison and deer.  The acute nature of the disease makes successful treatment difficult, but there is an effective vaccine available to provide animals with protective immunity.
Most losses due to blackleg occur when the cattle are between the ages of six months and two years of age, although it can occur when they are as young as two months. Typically, cattle that have a high food-intake level and are well-conditioned tend to be the most susceptible to blackleg. Furthermore, many blackleg cases occur during the hot and humid summer months or after a sudden cold period. However, cases can occur at any time during the year. 
Blackleg is most commonly caused by Clostridium chauvoei, but Clostridium feseri and other Clostridial species can be isolated from some lesions. C. chauvoei are Gram-positive, rod-shaped, anaerobic, and motile bacteria that can produce environmentally-persistent spores when conditions are not ideal for growth. These spores can remain in the soil for years in an inactive state, and return to their infectious form when consumed by grazing livestock. Contaminated pasture is a predominant source of these organisms, which are also found naturally in the intestines of animals.  Cases can occur over many years in areas where there is soil or manure contamination with the bacteria, and it is exceedingly difficult to remove the spores from the environment.
C. chauvoei produce a large amount of gas as a metabolic by-product when growing and reproducing, hence the alternate infection name gas gangrene, present in humans.  This gas builds up in infected tissue, usually large muscles, and causes the tissue to make a crackling or popping sound when pressed. Large gas-filled blisters can also form, which can be extremely painful as they build up in the tissues.
When infection begins, the animal may develop a fever, and the affected limb can feel hot to touch. The limb usually swells significantly, and the animal can develop lameness on the affected leg. Crepitation (the sensation of air under the skin) can be noticed in many infections, as the area seems to crackle under pressure.  Once clinical signs develop, the animal may only live a short while, sometimes as little as 12 hours. Occasionally, cattle will succumb to the disease without showing any symptoms, and only a post-mortem reveals the cause. During a necropsy, a diagnosis is usually made very quickly, as the affected muscle is usually mottled with black patches, which are dead tissue, killed by the toxins that the bacteria release when they infect live tissue. If viewed under a microscope, small rod-like bacteria can be seen to confirm the diagnosis.
The use of a 7-way clostridial vaccination is the most common, cheapest, and efficacious preventative measure taken against Blackleg. Burning the upper layer of soil to eradicate left-over spores is the best way to stop the spread of blackleg from diseased cattle. Diseased cattle should be isolated. Treatment is generally unrewarding due to the rapid progression of the disease, however penicillin is the drug of choice for treatment. Treatment is only effective in the early stages and as a control measure. Dr. Oliver Morris (O.M.) Franklin made a significant contribution to the welfare of cattle and the livestock industry with his development of the blackleg vaccine. Franklin developed the original method of giving the vaccine while at Kansas State Agriculture College using live cattle. Franklin and another graduate veterinarian founded the original Kansas Blackleg Serum Co. in Wichita in 1916.
When an animal has died as a result of the blackleg disease, obey the following instructions:
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