Bacterial Respiratory Pathogens of Beef Stocker Cattle.

Bovine respiratory disese (BRD) is the leading cause of morbidity and mortlaity in stocker cattle, a high-risk sector of the beef cattle industry, characterized by stress, commingling, and limited or unknown prior health management. Key bacterial pathogens, including Mannheimia haemolytica, Pateurella multocida, Histophilus somni, and Mycoplasmopsis bovis, act as opportunists when host defenses are compromised. These microbes employ diverse virulence mechanisms, including toxins, adhesins, surface proteins, and immune evasion strategies to colonize the lower respiratory tract and cause disease. Increasing antimicrobial resistance and variable vaccine efficacy complicate control efforts. This review summarizes pathogen-specific characteristics, epidemiology, and current challenges associated with the prevention and control of BRD in stocker cattle systems.