EXPERIMENTAL-INFECTION OF PREGNANT CATTLE WITH THE VACCINE CANDIDATE BRUCELLA-ABORTUS STRAIN RB51 - PATHOLOGICAL, BACTERIOLOGICAL, AND SEROLOGIC FINDINGS

To determine the placental tropism and abortigenicity of the vaccine c andidate Brucella abortus strain RB51 (SRB51), a rough mutant of the v irulent strain 2308, ten Polled Hereford heifers were inoculated intra venously in the 6th month of gestation. Heifers were euthanatized and examined at postinoculation week (PIW) 8 (n = 5) or at full term (n = 5). Four of five infected heifers sampled at PIW 8 and three of four i nfected heifers at term had placentitis, whereas reproductive tissues of three normal cows used for comparison had no placentitis. Numerous macrophages, immunoreactive for SRB51 antigen, as well as neutrophils, fibrin, and cell debris filled the arcade zone between chorion and ma ternal septae. Trophoblastic epithelium of the placentomal arcade zone had intracellular bacteria that were immunoreactive for SRB51 antigen . The tips of maternal septa had a lymphoplasmacytic infiltrate with s mall multifocal erosions and ulcerations of maternal epithelium. SRB51 was cultured from all tissues in which lesions were seen. Placentae o f one cow from each group had no placentitis and contained no SRB51. I n mammae, interstitial lymphoplasmacytic infiltrates and suppurative i nfiltrates within alveoli and intralobular ductules were seen in two o f five heifers at PIW 8. SRB51 was cultured from liver, spleen, lung, and bronchial lymph nodes in four of five calves at PIW 8 and three of four full;term calves, but no lesions were seen. One near-term heifer had disseminated infection, placentitis, and lymphoplasmacytic endome tritis, and delivered a premature weak calf. These results establish t hat SRB51 is less abortifacient than previously published reports with strain 19, in that only one of four heifers delivered prematurely fol lowing intravenous inoculation with SRB51, whereas intravenous inocula tion with strain 19 leads to 100% abortion. However, it also shows tha t SRB51 can infect the bovine placenta, mammary gland, and fetus, can induce placentitis, and, in some cases, can lead to preterm expulsion of the fetus.