Diagnostic exercise: Myocarditis due to Histophilus somni in feedlot and backgrounded cattle.

A backgrounding operation for calves in Wyoming identified a disease syndrome presenting as lethargy, fever, and death between November and January each year. An unfixed heart was submitted for examination, along with samples of lung. There was focal red discoloration in papillary muscle of the left ventricular myocardium. Histologically, the lesion corresponded to acute necrotizing myocarditis with myriad intravascular and intralesional gram-negative coccobacilli. Histophilus somni was detected by bacterial culture and immunohistochemistry. Focal myocarditis due to H. somni occurs in fall-placed cattle in western provinces and states of North America, and it can be an appreciable source of death loss. Gross lesions are readily detected in affected hearts. The presence of such changes in papillary muscles of left ventricular myocardium in feedlot or backgrounded cattle should prompt a differential diagnosis of H. somni myocarditis.