Pathogenesis and the role of the Pasteurellae

The combination of stress and infectious agents are responsible for the pathogenesis of pneumonia in cattle.

Mannheimia haemolytica and Pasteurella multocida are normal commensal bacteria of the upper respiratory tract of healthy cattle. The congregation of large groups of cattle may result in an increased Pasteurellae load in the upper respiratory tract.

Stress factors and possible viral infections interact to suppress host defense mechanisms. Subsequently bacterial commensals in the upper respiratory tract proliferate and eventually colonize the lower respiratory tract causing a bronchopneumonia.

Areas of pneumonia in a bovine lung