A variety of infectious diseases caused by the coccobacilli Pasteurella multocida and P. haemolytica. However, the term pasteurellosis also applies to diseases caused by any Pasteurella species. For example, P. caballi is considered to have a causal role in upper respiratory infections, pneumonia, peritonitis, and mesenteric abscesses of horses; and P. granulomatis has a causal role in a chronic disease of cattle in Brazil characterized by a progressive fibrogranulomatous process. All Pasteurella species occur as commensals in the upper respiratory and alimentary tracts of their various hosts. Although varieties of some species cause primary disease, many of the infections are secondary to other infections or result from various environmental stresses. Pasteurella species are generally extracellular parasites that elicit mainly a humoral immune response. Several virulence factors have been identified.