A Combined Intranasal and Parenteral Vaccination Strategy Reduces Morbidity and Antimicrobial Treatments Associated with Bovine Respiratory Disease in Danish Commercial Veal Calves

Background: Bovine respiratory disease (BRD) is a major driver of antimicrobial use in veal calf production systems. Although vaccination may reduce disease incidence and antimicrobial treatments, field evidence supporting clinically relevant reductions in antimicrobial use remains limited. This study evaluated the clinical efficacy of a combined intranasal (IN) and parenteral (subcutaneous/intramuscular; SC/IM) vaccination strategy targeting bovine respiratory syncytial virus, Histophilus somni, and Mannheimia haemolytica under commercial conditions. Methods: A randomised controlled field trial was conducted on two Danish veal calf farms between November 2019 and April 2020, including 880 calves allocated to a vaccinated group (n = 445) or an unvaccinated control group (n = 435). Health outcomes included morbidity, antimicrobial use, mortality, growth performance, and slaughterhouse pulmonary and pleural lesions and were analysed using mixed-effects models. Results: Vaccination significantly reduced BRD-associated morbidity across all evaluation periods (from day 5 after arrival to slaughter), with relative reductions of 18.8–24.6% and 30% fewer antimicrobial treatment days. Pulmonary and pleural lesions were significantly less frequent in vaccinated calves (≈36% reduction). No statistically significant differences were observed in growth performance or mortality. Conclusions: Combined IN + SC/IM vaccination was associated with lower BRD-associated morbidity, antimicrobial treatments, and slaughterhouse respiratory lesions under commercial conditions.