A variety of infectious diseases caused by the coccobacilli Pasteurella multocida and P. haemolytica; the term also applies to diseases caused by any Pasteurella species. 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. Virulence

Pasteurella multocida is the most prevalent species of the genus causing a wide variety of infections in many domestic and wild animals, and humans. It is a primary or, more frequently, a secondary pathogen of cattle, swine, sheep, goats, and other animals. As a secondary invader, it is often involved in pneumonic pasteurellosis of cattle (shipping fever) and in enzootic or mycoplasmal pneumonia of swine. It is responsible for a variety of sporadic infections in many animals, including abortion, encephalitis, and meningitis. It produces severe mastitis in cattle and sheep, and toxin-producing strains are involved in atrophic rhinitis, an economically important disease of swine. Hemorrhagic septicemia, caused by capsular type B strains, has been reported in elk and deer in the United States.

All strains of P. haemolytica produce a soluble cytotoxin (leukotoxin) that kills various leukocytes of ruminants, thus lowering the primary pulmonary defense. It is the principal cause of the widespread pneumonic pasteurellosis of cattle. Other important diseases caused by certain serotypes of P. haemolytica are mastitis of ewes and septicemia of lambs.

All of the Pasteurella species can be isolated by culturing appropriate clinical specimens on blood agar. Multiple drug resistance is frequently encountered. Treatment is effective if initiated early. Among the drugs used are penicillin and streptomycin, tetracyclines, chloramphenicol, sulphonamides, and some cephalosporins. Sound sanitary practices and segregation of affected animals may help limit the spread of the major pasteurelloses. Live vaccines and bacterins (killed bacteria) are used for the prevention of some.