| Popis: |
Thirteen heifers (group A) given a booster vaccination with Campylobacter fetus 4 1/2 months before breeding to bulls infected with the homologous strain had a pregnancy rate of 54%, compared with a pregnancy rate of 92% for 12 heifers (group B) given a booster vaccination 10 days before the 63-day breeding period began. None of 7 nonvaccinated heifers became pregnant, and all remained infected for at least 136 days. All three groups of heifers were bred by the same infected bulls. Nine of the heifers in group A, but none of the heifers in group B, became infected during the first estrus. Blood serum agglutination titers peaked 2 weeks after the first vaccination but decreased to prevaccination titers 6 weeks later. Titers more than double those produced by one vaccination peaked between 2 and 3 weeks after booster vaccination but in most heifers had decreased to low titers 7 weeks later. When the breeding period began, the median antibody titer was 640 for heifers in group A and 10, 240 for heifers in group B. Indications of an anamnestic response due to natural challenge were not found in any of the 9 infected heifers in group A when serum titers were determined 27 and 34 days after breeding began. Therefore, it appears that in many heifers and cows, only the antibodies produced by booster vaccination will be available to provide protection. Inasmuch as vaccinal antibody titers decrease rapidly, it was concluded that booster vaccinations should be given approximately 10 days before breeding so that titers will be high during the breeding period. |
