Blackleg is an infectious, non-contagious disease caused by Clostridium chauvoei. Infection occurs when animals ingest bacterial spores while grazing. The bacterial spores penetrate the intestine and are disseminated via the bloodstream to the skeletal muscle, where the spores remain dormant. Following an event that causes low oxygen conditions (i.e. bruising or damage to the muscle) in infected tissue, the spores germinate, multiply and produce toxin that results in muscle necrosis and hemorrhage. The animals affected by blackleg are usually well fed animals between 6 months and 2 years of age. The cause of death in affected cattle is usually acute toxemia. The course of the disease is often between 12-48 hours and clinical signs are often absent; however, animals may exhibit signs of lameness, tachycardia, fever, anorexia, rumen stasis and lethargy. Blackleg is primarily a disease of pastured cattle with the majority of the cases occurring during the summer months.
Throughout 2017, the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) diagnosed a significantly increased number of blackleg cases. Although a cause for the increased incidence of cases has not been conclusively determined, a possible explanation are the environmental changes that resulted from the unusual weather conditions experienced during 2017. Specifically, it is speculated that the extreme amounts of rain and flooding associated with Hurricane Harvey and the subsequent dry conditions worked to stir up the soil leading to exposure of deeply hidden bacterial spores.
The affected animals presented to TVMDL for necropsy ranged in age from 4 months to up to 1 year of age. The gross lesions included variably sized areas of skeletal muscle that was dark red and contained gas bubbles (necrotizing myositis). Due to the intramuscular gas bubbles, affected sections of skeletal muscle would often float in formalin. Most of the affected animals had a concurrent fibrinous pericarditis and necrotizing myocarditis. The skeletal muscle and myocardial lesions typically had the faint to pervasive odor of rancid butter. The diagnosis of blackleg was based on the characteristic gross lesions and a positive fluorescent antibody test results on affected tissues. The cases of blackleg at TVMDL appeared to peak after Hurricane Harvey and continued through December 2017.
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