Mannheimia haemolytica serotype A1 (M. haemolytica A1)

Mannheimia haemolytica serotype A1 is a Gram-negative bacterial pathogen and the principal causative agent of bovine respiratory disease complex (BRDC), one of the most economically significant infections in cattle. The bacterium normally resides as a commensal organism in the upper respiratory tract of healthy ruminants but can transition to a pathogenic state under stress conditions such as transport, environmental changes, or viral co-infections. Serotype A1 possesses unique virulence determinants that distinguish it from other serotypes, including an invasion mechanism that allows it to penetrate differentiated bovine respiratory epithelial cells—a capability absent in serotype A2. The primary virulence factor is leukotoxin, a calcium-dependent toxin that triggers inflammatory responses leading to lung edema and necrosis. Understanding the molecular basis of A1's pathogenicity is critical for developing vaccines and antimicrobial strategies, particularly given increasing antimicrobial resistance in clinical isolates.

Other names

Formerly classified as Pasteurella haemolytica