The efficacy of a live Listeria monocytogenes combined serotype 1/2a and serotype 4b vaccine.

In each of two experiments, sheep and lambs were vaccinated by a subcutaneous injection of a test vaccine (consisting of a combined serotype 1/2a and serotype 4b live Listeria monocytogenes culture) and challenged 16 days later with a mixture of the homologous wild strains. After challenge, the mortality rate of vaccinated sheep was 28.1% and that of controls 71.9%; that of lambs was 25.0%, although these had been inoculated with the LD70 dose. Furthermore, in each of two field trials in Listeria-infected flocks, primiparous pregnant ewes were vaccinated. In the first field trial, 3 or 110 lambs died of listeriosis of those born of vaccinated (n = 564) or unvaccinated (n = 3345) ewes, respectively. In the second, the perinatal mortality rate of lambs born of vaccinated or unvaccinated ewes was 7.6 or 30.3%, respectively, and the mean birth weight of lambs born of vaccinated or control ewes was 2.2 or 1.8 kg, respectively; the mean milk production of vaccinated ewes was 106 and that of controls 83 l; no Listeria was isolated from milk samples of vaccinated ewes. It is concluded that the vaccine is efficacious for the protection of sheep from listeriosis.