The bacteria is a normal inhabitant of the upper respiratory tract of cattle and in normal unstressed animals, it does not usually cause disease. However, animals which are exposed to various stress factors (see Table 1) are more susceptible to growth of the organism in the lower respiratory tract, resulting in severe pneumonia (1, 2). While a number of infectious agents are involved with the bovine respiratory disease complex (BRD), the bacterial species are usually the cause of mortality and therefore contribute significantly to the economic losses estimated between $500 million and $1 billion annually in North America. However, respiratory viruses such as BHV-1, PI-3, BRSV and BVD are important in the disease process as they often predispose animals to infection with P. haemolytica. The incidence of Haemophilus somnus pneumonia has also increased over the past decade and it is an important cause of bovine respiratory disease.