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.
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