Blackleg is an acute, febrile disease of cattle and sheep caused by Clostridium chauvoei (feseri) characterized by emphysematous swelling, usually in the heavy muscles. It is found worldwide.

Clostridium chauvoei (feseri) is found naturally in the intestinal tract of animals. It probably can remain viable in the soil for many years, although it does not actively grow there. Contaminated pasture appears to be a source of organisms. Outbreaks of blackleg have occurred in cattle on farms in which recent excavations have occurred, which suggests that disturbance of soil may activate latent spores. 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.
In cattle, blackleg infection is endogenous, in contrast to malignant edema ( Malignant Edema). Lesions develop without any history of wounds, although bruising or excessive exercise may precipitate some cases. Commonly, the animals that contract blackleg are of the beef breeds, in excellent health, gaining weight, and usually the best animals of their group. Outbreaks occur in which a few new cases are found each day 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. In sheep, the disease is not restricted to the young, and most cases follow some form of injury such as shearing cuts, docking, crutching, or castration. Endogenous blackleg in sheep is uncommon in the USA; it is much more common in New Zealand where blackleg is seen more frequently in sheep than in cattle.

However, these signs are usually of such short duration that they may be missed. More frequently, a thrifty calf or yearling is simply found dead. Gas is detectable under the skin and this produces a crackling sensation when the skin is rubbed with the hand.

If the skin over the affected area is removed, excess bubbly bloodstained fluid can be seen, and the muscle immediately below will be dark in colour. However, when the affected muscle is inside the carcass, such as when the heart muscle is affected, no external evidence of the disease is found.

Figure 2 shows diseased heart muscle from a 12-month-old heifer that died suddenly from blackleg. Notice the haemorrhage and the ‘half cooked’ appearance of the diseased muscle tissue.

Because the disease is accompanied by rapid decomposition, the post-mortem changes may be masked by gas formation, especially if examination of the carcass is delayed for more than a few hours.

The only effective means of controlling blackleg is by vaccination. Several makes of multivalent vaccine (‘5 in 1’ or ‘7 in 1’) are available commercially and care should be taken to follow the manufacturer’s instructions.

A rapidly fatal, febrile disease in well-nourished young cattle, particularly of the beef breeds, with crepitant swellings of the heavy muscles suggests blackleg. The affected muscle is dark red to black and dry and spongy; it has a sweetish odor and is infiltrated with small bubbles but with little edema. The lesions may be in any muscle, even in the tongue or diaphragm. In sheep, because the lesions of the spontaneously occurring type are often small and deep, they may be overlooked. Occasionally, the tissue changes caused by C septicum , C novyi , C sordellii , and C perfringens may resemble those of blackleg. At times, both C septicum and C chauvoei may be isolated from blackleg lesions, particularly when the carcass is examined ≥24 hr after death, which allows time for postmortem invasion of the tissues by C septicum . Field diagnoses are confirmed by laboratory demonstration of C chauvoei in affected muscle. The samples of muscle should be taken as soon after death as possible. The fluorescent antibody test for C chauvoei is rapid and reliable

Control:
A bacterin containing C chauvoei and C septicum is safe and reliable for both cattle and sheep. Calves should be vaccinated twice, 2 wk apart, at 2-6 mo of age; in high-risk areas, revaccination may be necessary at 1 yr and every 5 yr thereafter. When outbreaks are encountered, all susceptible cattle should be vaccinated and treated prophylactically with penicillin to prevent new cases, which may develop for up to 10 days, at which the bacterin provides protection. In some areas, multicomponent clostridial vaccines are warranted. Treatment of clinical cases with parenteral and multiple local injections of penicillin may be attempted but is frequently unsuccessful.

 

Ref:  dairy , merck