Subcutaneous and conjunctival vaccination with a live attenuated strain of Salmonella Abortusovis: effect of gestation on serological response of ewes.

Annual serological testing of flocks vaccinated by the subcutaneous route with a live, attenuated strain of Salmonella Abortusovis has previously demonstrated the persistence of agglutinating antibodies. It has however been impossible to determine whether the antibodies originated from the vaccination or from an enzootic infection. The serological response, as measured by a microtechnique of seroagglutination using a stained antigen, was studied in an isolated flock of 30 adult ewes. The trial period after the subcutaneous vaccination was 34 months, which included 3 lambings. Ten female offspring of these ewes were also studied. They were vaccinated by the conjunctival route and were studied for 18 months, which included 1 lambing. As is common for natural or experimental infection, high antibody titers were obtained 10-15 d after vaccination in both groups. The values then decreased and rose again to significant levels at each subsequent pregnancy. The vaccinal strain was never isolated from vaginal swabs taken at each lambing and there was no suggestion of stimulation by an external antigen. Gestation may therefore stimulate the antibody response. This suggests that the immunity conferred by this vaccination may last for at least 3 lambing periods. The consequences of these observations on vaccination protocols and serological diagnosis are discussed.