Vaccination with a Listeria strain of reduced virulence against experimental Listeria abortion in goats.

A live Listeria monocytogenes strain of reduced virulence, strain Aer, obtained by three successive mutations in regard to streptomycin and erythromycin, was used as vaccine on goats in two successive experiments. Animals were vaccinated either once or twice by subcutaneous route with doses varying from 6 x 10(8) to 1.5 x 10(10) CFU. No side effect was observed, except in one goat, vaccinated as pregnant, that aborted and from which the vaccinal strain was reisolated. The goats were challenged by subcutaneous route with 5 x 10(8) CFU L monocytogenes virulent strain ATCC 19115 at 95-100 days of pregnancy. Thirteen out of 23 controls (56.5%) and 11 out of 24 once vaccinated goats (45.8%) aborted whereas only 5 out of 22 (22.7%) twice vaccinated goats aborted. Significant protection against a severe challenge can thus be afforded by vaccination and recall with the live strain L monocytogenes Aer.