Myositis due to Mannheimia haemolytica infection in a beef heifer.

In this study, a 10-month-old, 447 lb (203 kg), mixed-breed beef heifer with elevated rectal temperature, weight loss and clinical signs suggestive of respiratory disease was presented. This heifer was from a group of 360 mixed-breed, high-risk heifers purchased and transported to the research feedlot at the Oklahoma State University Willard Sparks Beef Research Center, USA. Despite extensive consecutive treatment with antimicrobials, the heifer continued to have elevated rectal temperature and weight loss. The heifer died 22 days after arrival at the feedlot. Necropsy examination revealed an unusual localized necrotizing myositis that concentrated along the musculature of the ventral cervical area and into the adjacent right axilla. Severe bronchopneumonia, confirmed grossly and histologically, was the presumed the cause of death. A pure culture of Mannheimia haemolytica was cultured from the skeletal muscle and the lung. The strain of M. haemolytica isolated from the lung and muscle was resistant to oxytetracycline and tilmicosin, and susceptible to ceftiofur and enrofloxacin by antimicrobial sensitivity testing. This infection was likely due to septicemia following severe fibrinous bronchopneumonia and pleuritis caused by M. haemolytica.