Pasteurellosis is most commonly seen in pigs as a complication of mycoplasmal pneumonia (see Mycoplasmal Pneumonia), although swine influenza, Aujeszky disease, Bordetella bronchiseptica, or Haemophilus parahaemolyticus may also cause changes in the lungs that lead to disease caused by Pasteurella spp. The causative organism usually is P multocida. A normal inhabitant of the porcine upper respiratory tract, it produces an exudative bronchopneumonia, sometimes with pericarditis and pleuritis. Primary, sporadic, fibrinous pneumonia due to pasteurellae, with no epidemiologic connection with mycoplasmal or other pneumonia, may also be seen in pigs. In both primary and secondary forms, chronic thoracic lesions and polyarthritis tend to develop. Diagnosis is based on necropsy findings and recovery of pasteurellae from the lesions. Nontoxigenic strains of capsular type A are the predominant isolates from cases of pneumonia. Toxigenic strains of P multocida, in the presence of B bronchiseptica, are now associated with atrophic rhinitis (see Atrophic Rhinitis in Pigs).
Septicemic pasteurellosis and meningitis occasionally occur in piglets. Mannheimia haemolytica has been recovered from aborted fetuses, and septicemia may also occur in adult pigs. There are no distinctive lesions, and the pathogenesis is obscure. Porcine strains of M haemolytica are often untypeable and do not belong to the common ovine and bovine serotypes. However, some outbreaks in the UK have been associated with close contact with sheep.
Control of the secondary, pneumonic form of the disease is generally based on prevention or control of mycoplasmal pneumonia. Early and vigorous therapy with antibiotics, or in combination with sulfonamides, is indicated to prevent chronic sequelae of all forms of the disease. An increasing resistance to some antibiotics has been noted among the pasteurellae.