Abstract
Bovine respiratory disease (BRD) affects millions of cattle in the US annually. This study sought to identify commensal pathogens in animals without clinical signs that were predictive of clinical BRD using the Wisconsin Health Scoring (WHS) system. Cases were healthy cattle that were later diagnosed with BRD (WHS ≥ 5.0), positive controls (PC) were cattle that were diagnosed with BRD at the time of initial sampling and negative controls (NC) were those that remained healthy throughout the study (WHS ≤ 4.0). Beef feedlot cattle were sampled in Colorado (CO; cases = 13, NC = 500, PC = 488) and Washington (WA; cases = 5, NC = 499, PC = 506). Preweaned Holstein dairy calves were sampled from California (CA; cases = 73, NC = 1017, PC = 1003) and New Mexico (NM; cases = 25, NC = 381, PC = 376). For each animal, mid-nasal and deep pharyngeal swabs were used to test for nine BRD pathogens. ANOVA testing (P< 0.05) and odds ratios (OR) were calculated for each population. In CO, bovine viral diarrhea virus (P = 0.0001, OR = 13.1) and Histophilus somni (P = 0.008, OR = 4.2) were predictive of cattle that would be diagnosed with BRD while bovine respiratory syncytial virus (BRSV; P = 1.1 × 10(-6), OR = 40.8) was most predictive in WA. In NM, BRSV (P = 0.005, OR = 5.8) was also predictive of clinical BRD. No pathogen was predictive of BRD risk in CA. Prevalence of the predictive pathogen was higher (P < 0.05) in cases than in PC cattle suggesting that the incidence of pathogens taken at the time of BRD diagnosis may not reflect the pathogen initiating disease. These results also highlight geographical differences in the incidence of BRD pathogens which should be considered when managing to reduce disease.