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An experimental model of pneumonic pasteurellosis in sheep was used to investigate the role of serum antibody in resistance to this disease. Lambs which had been 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 had received either 200 ml of antiserum to P. haemolytica or 200 ml of control serum intraperitoneally 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, and this response increased after exposure of these animals to P. haemolytica type A1 in aerosol. The results indicate that the humoral immune response alone is incapable of affording protection against experimental pasteurellosis and that cell-mediated immunity may play an important part in resistance to this disease.

Methods of immunising lambs against pneumonic pasteurellosis, caused by several serotypes of Pasteurella haemolytica, were assessed in specific pathogen free lambs. Lambs were vaccinated intramuscularly with sodium salicylate extract (SSE) of P haemolytica, either alone or in combination with heat-killed organisms (HKO). SSE of P haemolytica type A1 protected vaccinated lambs against pneumonia resulting from challenge with the homologous serotype. SSE of type A2 also provided some protection but this was improved by vaccination with a combination of SSE and HKO.

The efficacy of a multicomponent clostridial vaccine containing Pasteurella haemolytica antigens was tested in specific pathogen free or conventionally reared lambs exposed to experimental infection with P haemolytica serotypes A1, A2 or A6. In four experiments assessment was based upon the findings of clinical, pathological and bacteriological examinations. Three experiments carried out in conventionally reared lambs demonstrated protection against challenge infection with P haemolytica serotypes A1, A2 and A6 in vaccinated lambs. However, the inconsistency of the disease induced in these experiments emphasised the need to perform definitive studies in specific pathogen free conditions. The final experiment was carried out with specific pathogen free lambs and confirmed the efficacy of the multicomponent clostridial vaccine containing P haemolytica antigen in protecting against the effects of infection with P haemolytica serotype A6. In addition, this experiment indicated that the inclusion of several components in a vaccine did not affect the efficacy of an individual antigenic component.

Two groups of 10, specific pathogen free lambs were injected with a long acting oxytetracycline preparation at a dose rate of 20 mg/kg either 24 hours before or 24 hours after exposure to an aerosol of Pasteurella haemolytica. When compared with the response of similarly infected but untreated lambs, the effect of pretreatment was to postpone the appearance of clinical signs of pneumonia for four days and the deaths of five lambs for five to six days post infection, by which time seven untreated lambs had died. Treatment 24 hours after infection caused a rapid clinical recovery which persisted until six days after infection but two treated lambs died seven days after infection. Lung lesions in the group treated after infection were significantly less extensive than those in the untreated lambs.

Vaccination of pregnant ewes with a Pasteurella haemolytica adjuvanted vaccine which contained serotypes A1, A2 and A6 antigens caused significant increases in serum antibody titres to A1 and A6 measured by the indirect haemagglutination test (IHA). The response to vaccination with the serotype A2 antigen contained in this vaccine cannot be measured by the IHA test. There were also increased antibody titres in the colostrum from the vaccinated ewes and in the serum of their lambs after sucking compared with the corresponding titres in unvaccinated ewes and their lambs. The inoculation of either 10 +/- 2- or 18 +/- 2-day-old lambs from both vaccinated or unvaccinated ewes with the same vaccine induced the active production of antibodies to serotypes A1 and A6 despite the presence of passively acquired antibodies. By four weeks after vaccination the group geometric mean serum antibody titres of lambs from both vaccinated and unvaccinated were similar whether the lambs were vaccinated at 10 or 18 days of age. Successful vaccination of young lambs with this type of vaccine is therefore possible. Optimum protection would be obtained by vaccinating ewes in late pregnancy and their lambs within the first two weeks of life.