Inability of passively acquired antibody to protect lambs against experimental pasteurellosis

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Abstract
An experimental model of pneumonic pasteurellosis in sheep was used to investigate the role of serum antibody in resistance to this disease. Lambs vaccinated with a sodium salicylate extract of Pasteurella Haemolytica type A1 were protected against challenge with PI3 virus followed by P. haemolytica type A1 7 days later. The majority of untreated lambs and lambs which received 200 ml of antiserum to P. haemolytica or 200 ml of control serum i.p. 18 h before infection with P. haemolytica type A1 succumbed to the challenge. Lymphocytes from vaccinated lambs showed a specific proliferative response when exposed to P. haemolytica type A1 sodium salicylate extract; this response increased after exposure of these animals to P. haemolytica type A1 in aerosol. The humoral immune response alone may be incapable of affording protection against experimental pasteurellosis. Cell-mediated immunity may play an important part in resistance to this disease.