Pre-transport risk factors for severity of respiratory associated symptoms in unweaned calves following long-distance transport

An essential component of protecting calf welfare during transport is identifying those most likely to develop transport-related respiratory disease. We aimed to associate pre-transport risk factors with post-transport respiratory disease based on the highest recorded value for thoracic ultrasound score, clinical respiratory score, neutrophil, monocyte, white blood cell count, neutrophil: lymphocyte ratio, and lowest recorded value for lymphocyte count and total immunoglobulins between arrival and 3-weeks post-arrival, in calves deemed legally fit for transport. The highest or lowest point of an individual variable within 3 weeks post-arrival was chosen to reflect the point at which the disease was perceived to be at its worst, and the direction of change (highest vs. lowest) was based on the reported direction of change in individual variables in calves diagnosed with respiratory disease in prior research. We studied two transport cohorts (in April and May 2022) between Ireland and The Netherlands and used generalized linear mixed models to analyze associations between precipitating health factors [pre-transport thoracic ultrasound (TUSpre), pre-transport clinical respiratory score (CRSpre)] and predisposing genetic and environmental factors (breed, sex, transport cohort, source, age, and body weight) on post-transport respiratory disease signs. Calves with a favorable TUSpre of 0 had lower post-transport ultrasound scores, neutrophil counts, and neutrophil: lymphocyte ratios than calves with a TUSpre of 2 (<i>p</i> < 0.01, <i>p</i> = 0.04, and <i>p</i> = 0.02 respectively), but CRSpre did not affect post-transport respiratory disease signs (all <i>p</i> > 0.2). Among predisposing factors, Holstein-Friesian calves developed higher ultrasound and clinical respiratory scores (showing more signs of pathology), as well as neutrophilia, monocytosis and higher neutrophil: lymphocyte ratios post-transport than Holstein-Friesian*Beef calves (<i>p</i> = 0.02, <i>p</i> = 0.01, <i>p</i> = 0.04, <i>p</i> = 0.03, and <i>p</i> = 0.01 respectively). Lighter calves experienced post-transport neutrophilia and higher neutrophil: lymphocyte ratios than heavier calves (both <i>p</i> < 0.01). We found slight increases in respiratory disease signs in cohort 1 calves, females, and calves originating from livestock marts, but these were not consistent across indicator variables. In conclusion, we should consider a high ultrasound score, dairy breed, and low body weight as risk factors for respiratory disease signs post-transport.