Black Leg - PDF

BLACK LEG Etiology: -True blackleg, the clostridial myositis of skeletal muscles, is associated with Clostridiun (feseri) chauvoei, a gram positive, spore forming, rod-shaped bacterium. The spores are highly resistant to environmental changes and disinfectants and persist in soil for many years. E...

BLACK LEG Etiology: -True blackleg, the clostridial myositis of skeletal muscles, is associated with Clostridiun (feseri) chauvoei, a gram positive, spore forming, rod-shaped bacterium. The spores are highly resistant to environmental changes and disinfectants and persist in soil for many years. Epidemiology Occurrence:- The disease is enzootic in particular areas, especially when they are subjected to flooding. The case fatality rate in black leg approaches 100%. Source of infection: - Black-leg is as soil-borne infection. The portal of entry is through the alimentary mucosa after ingestion of contaminated feed or associated with erupting teeth. The bacteria may be found in the spleen, liver, and alimentary tract or normal animals, and contamination of the soil and pasture may occur from the infected feces or decomposition of carcass of animal dying of the disease. True black leg develops when spores that are not lodged in normal tissue are caused to proliferate by mechanism such as trauma or anoxia. Transmission: - In cattle the disease occurs without the history of trauma In sheep the disease is almost always associated with wound infection. Outbreaks in sheep have occurred after vaccination against entero-toxemia. It is presumed that the vaccine causes sufficient tissue damage to permit latent spores of the organism to proliferate. A special occurrence of this disease in found in fetal lambs. Ewes exposed to infection at shearing develop typical lesions but ewes treated with penicillin are unaffected. However, pregnant ewes treated with penicillin show distended abdomen, weakness and recumbency due to edema and gas formation in the fetus, from which Clostridium chauvoei can be isolated. Risk Factors Environment risk factors: - Seasonal occurrence with most cases occurring in the warm months of the year. It is also found in the areas with high rainfall. Animal Risk Factors: - True black leg is usually thought of as a disease of cattle and occasionally sheep. In case of cattle disease is usually confined to the age group between 6months and 2 years. In case of sheep there is no restriction of age group. Risk factors for cattle include rapidly growing cattle and a high plane of nutrition. Whereas, in case of sheep elevation in the nutritional status of sheep by increasing protein feeding increases their susceptibility. In pigs, black leg is not common, although a Gas Gangrene type of lesions may be associated with C. chauvoei or C. septicum infection. Pathogenesis Toxin formed by the organism produces a severe necrotizing myositis locally in skeletal muscles, and a systemic toxemia that is usually fatal in cattle and sheep occur in which the lethal lesion is clostridial cardiac myositis. Clinical findings Cattle: - Severe lameness, usually with pronounced swelling of the upper part of the affected leg. Depression, complete anorexia and ruminal stasis, and a high temperature (41°C, 106°F) and pulse rate (100-120/min). Pyrexia is not present in all cases. In the early stages the swelling is hot and painful to the touch but soon becomes cold and painless, and edema and emphysema can be felt. The skin is discolored and soon becomes dry and cracked. Lesions are also present in other locations such as base of the tongue, the heart muscle, the diaphragm and psoas muscles, the brisket, and the udder. Sheep: - When lesions occur in the limb musculature, there is stiff gait and the sheep is disinclined to move because of severe lameness in one limb or, more commonly, in several limbs. S/C EDEMA is not common and Gaseous Crepitation cannot be felt before death. Discoloration of the skin may be evident but skin necrosis and gangrene do not occur. In cases where infection occurs through wounds of the skin, vulva, or vagina there is extensive local lesion. Lesions of the head may be accompanied by severe local swelling due to edema and there may be bleeding from the nose. High fever, anorexia, depression and death occur very quickly. Sheep and cattle with cardiac myositis associated with C. chauvoei are usually found dead. Horses:- Pectoral edema, stiff gait and incoordination are recorded. Clinical Pathology There are no constant changes in hematological parameters or serum biochemistry. Necropsy findings Cattle:- Cattle found dead of black leg are in characteristic position; lying on one side with affecting hindlimb stuck out stiffly. Bloating and putrefaction occur quickly and bloodstained froth exudes from the nostrils and anus. Clotting of the blood occurs rapidly. Freshly cut surfaces are often dry and may have a metallic sheen. The heart and all skeletal muscles, including those of the tongue, diaphragm and lumbar region must be checked as the lesion may be small and escape cursory examination Sheep:- The muscle lesions are more localized and deeper and the sub-cutaneous edema is not so marked, except around the head. The isolation and identification of C. chauvoei and C. novyi is difficult because of the fastidiousness of these species in culture and rapid P/M contamination of the tissue by clostridial species from the GIT. Labs use Fluorescent antibody tests performed on tissue smears to complement (or substitute) anaerobic culture. False black leg may be associated with C. septicum and C. novyi but this disease is more accurately classified as malignant edema. Treatment: - Penicillin and Surgical debridement of lesions, including fasciotomy, is indicated if the animal is not moribund. Large doses (40000IU/kg BW) should be administered, commencing with crystalline penicillin intravenously and followed by longer acting preparations. Black Leg antiserum is unlikely to be of much value in treatment unless very large doses are given. Control Cattle:- Annual Vaccination of all cattle between 3 and 6 months with two vaccinations given 4 weeks apart followed by an annual booster vaccination is recommended. Maternal immunity persists for at least 3 months and will interfere with active immunity in calves vaccinated before this age. In an outbreak all unaffected cattle should be vaccinated immediately and injected with penicillin at a dose of 10,000IU/kg BW I/M or a combination or penicillin and benzathine penicillin. Sheep:- Maiden Ewes should be vaccinated twice, the last vaccination given about 1 month before lambing and a subsequent yearly booster given at same time before lambing. If an outbreak commences in a flock of ewes at lambing time, prophylactic injections of penicillin and antiserum to ewes requiring assistance are recommended. A single vaccination of wethers can also be carried out 2-3 weeks before shearing if infection is anticipated. Clostridial Vaccines have poor antigenicity in sheep and goat than in cattle. In both sheep and cattle it is advisable to use combined vaccine containing at least C. chauvoei, C. septicum and C. novyi where these organisms occur in the area and cause clostridial myositis. Vaccine failure has been associated with an inadequate spectrum of the antigens in the vaccine, and in these circumstances a bacterin prepared from a local strain of C. chauvoei is preferred. It is important that carcasses of animals dying of blackleg are destroyed by burning or deep burial to limit soil contamination.