Nasal shedding of bovine coronavirus during commingling is associated with reduced average daily gain and lung lesions in weaned seropositive calves.

Bovine coronavirus (BCV) is a common calf pathogen and has been proposed as a potential contributor to bovine respiratory disease (BRD); however, little is known about its epidemiology and genetic diversity on British farms. The aim of this study was to model the relationship between mixing duration and intensity, BCV shedding, BRD development, and production outcomes during a controlled commingling event designed to mimic conventional farming conditions. Three cohorts of weaned Angus × Holstein calves (8-10 wk old; n = 120) sourced from 10 British source farms (SF) were enrolled and quasirandomly assigned to mixing groups of varying intensity. Mixing intensity (MI) was defined by richness, (i.e., the number of SF contributing to each pen and evenness-the relative proportion of calves from each SF). Mixing intensity ranged from low richness and low evenness (all calves in a pen originating from a single SF) to high richness and high evenness (2 calves from 5 different SF in a single pen). Nasal swabs and fecal samples were collected on source farms on animal collection day (SF collection), d 0, 3, 7 and 21 after mixing, and analyzed by BCV quantitative PCR and Sanger sequencing. Serum samples were taken upon SF collection, d 0 and 21 postmixing, and tested for BCV-specific IgG using ELISA. Calves were monitored daily for signs clinical respiratory disease using a modified University of Wisconsin-Madison system, and presence of lung lesions was assessed by thoracic ultrasonography on SF collection and d 0 and 21 after mixing. Calves were weighed on SF, and at the beginning and end of mixing to determine ADG. Associations between MI, BCV shedding, clinical respiratory disease, lung lesions, and ADG were investigated using mixed-effect models appropriate to the outcome type, with calf defined as the experimental unit. High serum BCV IgG titers were detected in all calves at enrollment and declined steadily over the study period. Seropositivity did not prevent BCV shedding, and overall genetic diversity among isolates was low (∼2%). However, distinct genetic variants were identified between source farms and, in one case, within the same animal at the beginning and end of the study period. Nasal BCV shedding was not associated with clinical disease within 7 d of recorded symptoms. Conversely, high shedders were significantly more likely (∼4.5 times; 95% CI: 1.21, 6.8) to have concurrent lung lesions and grew slower (-0.38 kg/d; 95% CI: -0.62, -0.14) when compared with nonshedders. Concurrently, the odds of clinical BRD increased with both commingling duration and MI; however, clinical BRD was not meaningfully associated with ADG. Overall, these findings suggest that in BCV seropositive commingled calves, BCV infection is widespread but largely subclinical; importantly, high levels of virus shedding are associated with measurably worse production outcomes whereas BRD risk is primarily influenced by the duration and intensity of mixing.