**4.3** *Mannheimia haemolytica*

Serotype 1 of *Mannheimia haemolytica* is the common isolated bacteria of the cow's respiratory tract with BRD. *Pasteurella multocida* is also an important reason of pneumonia. *Histophilus somni* is constantly recognized as an important pathogen in bacterial respiratory disease (BRD). These bacteria are normal inhabitants of the nasal pharynx of cattle. When pulmonary abscess occurs, it is generally associated with chronic pneumonia.
