Growth of preweaned, group-housed dairy calves diagnosed with respiratory disease using clinical respiratory scoring and thoracic ultrasound-A cohort study.

The objectives of this cohort study were to identify a cut point on a previously described 6-level ultrasound score (USS6) at which average daily gain (ADG) is affected and to determine whether there is any additional benefit of using a clinical respiratory score. Calves from a commercial herd in Ohio were enrolled at entry to an automated calf feeder barn at (mean ± standard deviation) 21 ± 6 d of age (n = 308). Calves that survived until 50 d (n = 233) were included in the analyses. Twice-weekly health exams included a clinical respiratory score (CRS), USS6 (0-5, based on lung mass involved), and body weight. For the CRS, the nose, eyes, ears, cough, and rectal temperature were assigned a score (0-3), and calves were considered positive (CRS+) when at least 2 areas scored ≥2. For analysis, USS6 and CRS status were based on a calf's first bovine respiratory disease event identified during the study period. The first multivariable linear model was fit to determine whether USS6 was associated with ADG and controlled for CRS. We detected no difference in ADG among calves with USS6 scores of 2, 3, 4, and 5. Based on this finding, we proposed a simplified 2-level ultrasound score (USS2; without lung consolidation or with lung consolidation ≥1 cm2). A second multivariable model was fit to assess the association between USS2 and ADG; this model controlled for CRS, birth weight category, breed, and cohort. Calves with lung consolidation (n = 169) had lower ADG than calves without lung consolidation (n = 64; 0.73 vs. 0.85 kg/d, respectively). Calves that were CRS+ (n = 61) had lower ADG than calves that were CRS- (n = 172; 0.74 vs. 0.84 kg/d, respectively). Although CRS did not affect the relationship between USS2 and ADG, both CRS and USS2 are necessary to explain variation in ADG. We simplified USS6 and proposed USS2 based on how lung consolidation affected ADG. A simplified 2-level ultrasound score may be more practical for veterinarians to identify calves that may be at risk for poor growth. The effect on ADG was similar between calves with lung consolidation and calves identified as CRS+. Therefore, both thoracic ultrasonography and CRS should be used to identify calves with all types of respiratory disease that affect growth. However, this study represents calves in group housing from 21 to 50 d of age on 1 farm with high disease incidence. We encourage studies that investigate the effects of lung consolidation and CRS on ADG in different management systems.