Bovine Respiratory Disease (BRD) has been, is now, and for the near future will be the number one problem in calves, stockers, and feeder cattle. Three of the major bacterial causes of BRD are Mannheimia (Pasteurella) haemolytica, Pasteurella multocida and Mycoplasma bovis. This review of clinical signs is intended to help you better understand the disease pattern. For this discussion we will consider the M. haemolytica and P. multocida as Pasteurellas producing similar clinical signs. Of first importance is the fact that both the Pasteurellas and the M. bovis are normal inhabitants of the upper respiratory tract (nasal passage, larynx, tonsils, upper trachea, etc). They are opportunistic organisms that rely on stress to suppress the immune system and allow them to “set up house” in the lower respiratory tract (lower trachea, bronchioles, alveolar ducts) and cause clinical BRD. Because of this phenomenon, upper airway swabs for laboratory diagnosis are not reliable. Lower airway transtracheal washings or a broncho-alveolar flush should be used instead.
The clinical signs associated with BRD are temperatures of 104° F to 106° F, a clear watery to heavy yellow or green nasal discharge, a moist cough with shallow rapid breathing, depression, and an unthrifty appearance. If inflammation or infection of the pleural lining (chest cavity) occurs, there tends to be an arched back with a grunting sound on expiration.
With the Pasteurella organisms these clinical signs tend to appear within 5 to 7 days of a stressful event (branding, castrating, weaning, shipping, sale barn, viral infection, weather change, lack of water or feed, etc). With the Mycoplasma bovis organism these signs tend to appear 20 to 45 days or more following a stressful event. The M. bovis organism is a slow-growing pathogen and tends to “smolder” for some time before it causes clinical signs in the animal. Also, with an M. bovis infection, if the organism enters the blood stream and migrates to other organs, you can see joint and/or ear infections with or without the BRD signs.