Effects of parenteral or intranasal respiratory vaccination and revaccination in auction-derived feedlot heifers

Further understanding of the safety and efficiency of live-attenuated bovine respiratory syncytial virus (BRSV) vaccines is important due to the discovery of BRSV infection enhancing Histophilus somni (H. somni) prevalence and because the utility of revaccination in the feedlot is controversial. The study objectives were to determine the effect of route of modified-live virus (MLV) vaccine administration (intranasal vs. parenteral) and intranasal MLV revaccination on d 14 upon growth and health outcomes and to determine if vaccination strategy influenced frequency of carriage of BRSV and H. somni in the nares. Auction-derived beef heifers (n = 3,517; initial BW = 275 ± 0.9 kg) were received in 12 arrival blocks and gate sorted into 4 treatment groups. Treatments included: 1) parenteral MLV vaccination on d 0 (INJ), 2) intranasal MLV vaccination on d 0 (INT), 3) parenteral MLV vaccination on d 0 and revaccination with intranasal on d 14 (INJ-R), 4) intranasal MLV vaccination on d 0 and revaccination with intranasal on d 14 (INT-R). Pen was the experimental unit, with a total of 12 pens per treatment and 65 to 76 heifers per pen in a randomized complete block design. Performance, morbidity, mortality, carcass traits, and BRSV and H. somni frequency of carriage in revaccinated groups on d 0 and 60 was determined. All data was analyzed in SAS 9.4 (SAS Inst., Cary, NC) and differences were determined significant when P ≤ 0.05 and a tendency was observed for a P-value of 0.05 < P ≤ 0.10. Morbidity (P = 0.95), mortality (P = 0.80), and other health variables (P > 0.74) did not differ. However, an improvement in G:F (P = 0.04), increased REA (P ≤ 0.01), and percentage of edible livers (P ≤ 0.01) was noted for INJ and INJ-R. Likewise, severely abscessed livers (P ≤ 0.01) were decreased for INJ and INJ-R. There was a day effect for both BRSV and H. somni frequency of carriage in the naris (P = 0.09 and P < 0.01, respectively) such that these pathogens increased in frequency from d 0 to 60, but no treatment effect (P = 0.23 and P = 0.71, respectively) or treatment x day interaction (P = 0.29 and P = 0.44, respectively) was evident. Results indicate that revaccination with an intranasal MLV in feedlot heifers did not impact health or growth, and intranasal vaccination on arrival resulted in less G:F and REA concomitant with increased liver abscessation. Furthermore, the frequency of carriage of H. somni was abundant on d 60, suggesting important prevalence of this bacterial pathogen in the southern U.S. cattle population.