Effect of aerosolized bacterial lysate on development of naturally occurring respiratory disease in beef calves

BACKGROUND: Bovine respiratory disease (BRD) is a major problem affecting beef cattle after arrival to feedlots. Alternatives to antibiotics are needed for prevention. HYPOTHESIS: Stimulation of pulmonary innate immune responses at the time of arrival to a feedlot reduces the occurrence and severity of BRD. ANIMALS: Sixty beef steers at high risk of BRD. METHODS: Randomized, double‐blinded, placebo‐controlled study. Calves received saline or a lysate of Staphylococcus aureus and Escherichia coli by aerosol, at 16 hours after feedlot arrival. Calves were monitored for 28 days for disease outcomes and levels of Mycoplasma bovis and Mannheimia haemolytica in nasal swabs. RESULTS: Death from M bovis pneumonia was significantly greater in lysate‐treated animals (6/29, 24%) compared to controls (1/29, 3%; odds ratio = 10.2; 95% confidence interval [CI] = 1.1‐96.0; P = .04). By 28 days after arrival, 29/29 lysate‐treated calves had ultrasonographic pulmonary consolidation compared to 24/29 control calves (P = .05). Lysate‐treated calves had lower weight gain compared to control calves (−8.8 kg, 95% CI = −17.1 to −0.5; P = .04), and higher body temperatures on days 4, 7, and 21 (0.19°C; 95% CI = 0.01‐0.37; P = .04). Nasal M bovis numbers increased over time and were higher in lysate‐treated calves (0.76 log CFU, 95% CI = 0.3‐1.2; P = .001). CONCLUSIONS AND CLINICAL IMPORTANCE: Aerosol administration of a bacterial lysate exacerbated BRD in healthy high‐risk beef calves, suggesting that respiratory tract inflammation adversely affects how calves respond to subsequent natural infection with M bovis and other respiratory pathogens.