Pasteurellosis encompasses a number of disease syndromes in cattle, including both systemic and respiratory infections. Pasteurella multocida and P. haemolytica are the two bacterial species most commonly associated with pneumonia in feedlot cattle, although the latter is the most important in terms of prevalence and economic impact on producers. Pasteurella haemolytica is composed of over a dozen serotypes, although type A1 is most commonly associated with shipping fever pneumonia.