Eleanor Mackie

Background Musculoskeletal injuries (MSI) are common in racehorses and have been of increasing concern in horses travelling internationally to compete. Understanding the differences in bone turnover between local horses and international horses following long‐distance air transportation may inform MSI prevention strategies. Objectives To understand the differences in bone turnover markers and risk of MSI between local horses and international horses following long‐distance air transportation. Study design Prospective cohort. Methods The concentrations of bone turnover markers (OCN and CTXI), markers of stress (cortisol), inflammation (serum amyloid A) and circadian rhythm (melatonin), and bisphosphonates were determined in blood samples collected twice (14–17 days apart), from horses following international travel ( n = 69), and from local horses ( n = 79). The associations between markers, long‐distance travel and MSI were determined using multivariable generalised linear regression models. Results Within 3–5 days post‐transport, concentrations of cortisol in international horses were higher than those of local horses (main effect, Coef. 0.39; 95% CI 0.24, 0.54; p < 0.001) but they decreased and were not different to those of local horses at the second timepoint (interaction effect, Coef. −0.27; 95% CI −0.46, −0.07; p = 0.007). After adjusting for age and sex, OCN and CTXI were not significantly different between international and local horses; however, OCN was lower in international horses at timepoint 2 (interaction effect, Coef. −0.16; 95% CI −0.31, −0.01; p = 0.043). The prevalence of MSI was higher in the international (26%; 95% CI 16, 38%) compared with local horses (8%; 95% CI 3, 16%; p < 0.001), with all severe MSI sustained by the international horses. At the second timepoint compared with the first timepoint post‐transport, cortisol remained high or increased (interaction effect, Coef. 0.43; 95% CI 0.24, 0.61; p < 0.001) and OCN increased (interaction effect, Coef. 0.26; 95% CI 0.08, 0.44; p = 0.006) in the horses that sustained severe MSI. Main limitations Horse population and racing career parameters differed between groups. Bone turnover markers have low sensitivity to detect local bone changes. Conclusions Most horses showed minimal effects of long‐distance air transport within 2 weeks relative to local horses as assessed by stress and bone turnover markers. Screening for persistent high cortisol and evidence of net bone formation after long‐distance air transportation may help to identify racehorses at high risk of catastrophic MSI.