Bovine respiratory disease (BRD) associated with either Mannheimia haemolytica or Pasteurella multocida is often due to secondary bacterial invasion by these organisms. Secondary bacterial invasions are infections caused by bacteria that invade tissue after an initiating event, such as a previous viral infection, which has established conditions that allow these secondary bacteria to invade tissue and cause disease. These secondary bacterial invaders are commonly found in the environment and are associated with healthy animals. Under normal conditions, they cause no problems.
Two bacteria, Mannheimia haemolytica (f Pasteurella haemolytica) and Pasteurella multocida, are often associated with bovine respiratory disease (BRD) or shipping fever in cattle and are often referred to as secondary bacterial invaders. Mannheimia haemolytica, the bacteria most frequently isolated from pneumonic lungs in cattle, and Pasteurella multocida often compound respiratory disease initiated by other pathogens (viruses, bacteria, mycoplasma). These two bacteria are considered as part of the normal bacterial flora found in the upper respiratory tract of most cattle but are not considered as normal flora of the lungs. As long as these two organisms only inhabit the pharynx or upper respiratory tract, clinical respiratory disease, or BRD associated with them is uncommon. The animal's normal bodily defenses keep these bacteria in check: in a healthy animal, they replicate slowly, are destroyed by antibodies and removed by macrophages. Respiratory tract infections (pneumonia) due to these two bacteria occur when the organism is inhaled. Under conditions of impaired pulmonary defenses, a severe necrotizing fibrinous pleuropneumonia develops. Spread of these organisms is by direct contact, or by ingestion of feed and water contaminated by nasal and oral discharges from infected cattle. Therefore these two bacteria are easily spread between cattle, especially when calves are crowded (as in shipment) or closely confined (as in a dairy calf nursery).
Pneumonia associated with either Mannheimia haemolytica or Pasteurella multocida often occurs when the animal's normal defenses are compromised. Examples of compromised defense mechanisms include, damage to the cells lining the upper respiratory tract by viruses such as infectious bovine rhinotracheitis virus (IBR), parainfluenza virus (PI-3), or bovine respiratory syncytial virus (BRSV). Damage to the tracheal lining could also occur due to inhaled irritants such as exhaust fumes or dust. The respiratory defense mechanism could also be depressed due to immunosuppression associated with bovine viral diarrhea (BVD) virus. The production of mucosal antibodies against these bacteria could be suppressed due to environmental or nutritional stress. When the defenses are compromised, the bacteria become attached to the lining of the respiratory tract (colonize), reproduce rapidly, and spread throughout the lungs. The severity of the disease depends upon the pathogenicity of the bacterial organism(s) and the associated infections (IBR, PI-3, BVD, and BRSV, other viruses or bacteria). M. haemolytica is often associated with the more acute cases of BRD, while P. multocida is often associated with the longer-lasting cases of BRD.