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 subcu taneous injection of a test vaccine (consisting of a combined serotype 1/2a and serotype 4b live Listeria monocytogenes culture) and challen ged 16 clays later with a mixture of the homologous wild strains. Afte r 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 i noculated with the LD, 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 vaccinat ed or unvaccinated ewes was 7.6 or 30.3%, respectively, and the mean b irth 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 10 6 and that of controls 83 1, no Listeria was isolated from milk sample s of vaccinated ewes. It is concluded that the vaccine is efficacious for the protection of sheep from listeriosis.