Efficacy of calfhood vaccination with Brucella abortus strain RB51 in protecting bison against brucellosis.

In the studies reported here, protection induced by calfhood vaccination of bison with 1.2-6.1 x 10(10)CFU of Brucella abortus strain RB51 (SRB51) against a virulent strain of B. abortus was evaluated. Non-vaccinated and SRB51-vaccinated bison were intraconjunctivally challenged during midgestation with 3 x 10(7)CFU of virulent B. abortus strain 2308 (S2308). Maternal and fetal tissues were obtained within 24hour after abortion or parturition. Incidence of abortion was greater (P<0.05) in non-vaccinated as compared to SRB51-vaccinated bison (62% and 15%, respectively), with abortions occurring between 5 and 8 weeks after experimental challenge. Calves from bison vaccinated with SRB51 had a reduced (P<0.05) prevalence of fetal infection with S2308 as compared to calves from non-vaccinated bison (19% and 62%, respectively). Although the ability to recover the 2308 challenge strain from maternal tissues did not differ (P>0.05) between nonvaccinates and vaccinates (100% and 78%, respectively), calfhood vaccination with SRB51 reduced (P<0.05) recovery of S2308 from uterine or mammary gland tissues. In bison which did not abort, S2308 was routinely recovered in low numbers from maternal lymphatic tissues; particularly the parotid, bronchial, supramammary, and mandibular lymph nodes. The RB51 vaccine strain was not recovered at any time from maternal or fetal samples obtained at necropsy. Histological lesions associated with Brucella-induced abortions were suppurative placentitis, fetal broncho-interstitial pneumonia, and fetal histiocytic splenitis. The results of our studies suggest that calfhood vaccination of bison with SRB51 is efficacious in protecting against intramammary, intrauterine, and fetal infection following exposure to a virulent strain of B. abortus during pregnancy. As brucellosis is transmitted horizontally through fluids associated with the birth or abortion of an infected fetus, or vertically to the calf through the ingestion of milk containing B. abortus, our data suggest that calfhood vaccination with SRB51 will be beneficial in preventing transmission of brucellosis in bison.