Colonization of mouse placentas by Brucella abortus inoculated during pregnancy.

Pregnant mice were challenged at Day 3, 7, 11 or 15 of pregnancy with Brucella abortus Strain 544 and killed at Day 18 of pregnancy for the enumeration of brucella in the spleens and individual placentas. Whatever the route of challenge--i.p., i.v. or s.c. into the foot-pad (F-s.c.) no abortions or foetal deaths were observed. Placental colonization involved either all, none or only some of the placentas in the same uterus (partial placental colonization: 25% of the mice). In the latter case, the probability of colonization was the same for all sites of implantation. Placentas were independent units as regards colonization and bacterial proliferation. Placental colonization was expressed either by (1) the class of placental infection within the uterus, which might be total, partial or nil; (2) the ratio of infected to total placentas analysed per group; or (3) the mean degree of infection per group. Whatever means of expression was chosen, placental colonization increased with the dose of challenge in parallel with splenic infection in the mouse. The challenge doses required at Day 7 to infect 50% of the placentas differed according to the route (i.p. = 54; i.v. = 5.6 x 10(2) and F-s.c. = 3.6 x 10(4) brucella). Placentas were more frequently and more intensively colonized when the challenge was performed at Days 7 and 11 than at Days 3 and 15 at pregnancy. Mice immunized with H.38 B. melitensis killed vaccine 36 days before pregnancy were found to be protected against an i.p. challenge with 2 x 10(5) brucella at Day 7 of pregnancy.