Placental pathology, immune responses, bacteriologic findings and clinical signs in pregnant cattle vaccinated with Brucella abortus strain RB51

To determine the placental tropism, abortigenicity, immunogenicity and effect on TNF-{dollar}alpha{dollar} levels, pregnant cattle were vaccinated IV (n = 10) or SC (n = 5) with the vaccine Brucella abortus strain RB51 (SRB51), SC with strain 19 (S19) (n = 5), or saline (n = 5) at 6 months gestation. Eight of ten IV vaccinated heifers developed placentitis and fetal infection. Strain RB51 was cultured from all tissues in which lesions were seen. No lesions were seen in SC vaccinated cattle and SRB51 was isolated from 2/5 superficial cervical lymph nodes in the area draining the site of SC vaccination. One premature calf born to an IV vaccinated heifer had mild interstitial pneumonia and disseminated SRB51 infection. No lesions were seen in other calves from IV vaccinated heifers, heifers vaccinated SC with SRB51 or S19, and neither SRB51 nor S19 could be recovered from calves of SC vaccinated cattle. Maternal PBMC from SRB51 vaccinates and S19 vaccinates showed proliferative responses to both {dollar}gamma{dollar}-irradiated SRB51 and S19 which were greater than controls. Strain RB51 vaccinates did not develop antibodies detected by the standard tube agglutination test, but did develop antibodies to SRB51 that reacted in a dot ELISA test with irradiated SRB51. Radioimmunoassay for bovine TNF-{dollar}alpha{dollar} showed no elevations in plasma from vaccinated cattle that differed from controls (P {dollar}>{dollar} 0.05). Similarly, TNF-{dollar}alpha{dollar} levels in amniotic or allantoic fluids from vaccinated cattle were not different from controls (P {dollar}>{dollar} 0.05). Immunohistochemistry for TNF-{dollar}alpha{dollar} revealed increased immunoreactivity within trophoblastic epithelial cells which was most extensive in IV vaccinated cattle with vaccine induced placentitis. These results indicate that SRB51 is less abortifacient than previously published reports with S19; however, SRB51 can infect the bovine placenta and fetus, can induce placentitis, and in some cases, lead to preterm expulsion of the fetus. Undesireable effects of SRB51 are dependent on dose and route of administration as SC vaccination with a lower dose does not result in placental or fetal infection.