Pasteurella multocida pneumonic infection in sheep: Prevalence, clinical and pathological studies

One hundred and twenty six (9.4%) of 1335 pneumonic lung samples of sheep collected over a 4-year period were positive for Pasteurella multocida. Four, 6 months old conventionally reared lambs free of antibodies against P. multocida were intratracheally inoculated with P. multocidabiovar A, serovar 3 (A:3). Three of four infected lambs had typical pneumonic pasteurellosis characterized clinically by anorexia, coughing, nasal discharge, dullness, but none was found to be fevered, with one lamb becoming recumbent and dying on day 6 post-infection. At necropsy, three lambs had swollen and congested lungs, which were associated with profuse growth of P. multocida, and little fibrin exudation typical of subacute bronchopneumonia. Histologically, there were marked thickening of the interalveolar septa with congested capillaries and no necrosis. Most alveoli were filled with mainly macrophages, oedema fluid and little fibrin. These results imply that although P. multocida infection of the respiratory tract of lambs produces less fulminating form of pneumonic pasteurellosis, it nonetheless contributes to the incidence of the disease in the field.