Serum Osteocalcin and CTX-I in Fit-to-Race Thoroughbred Racehorses: Reference Intervals and Associations with Demographic, Training, and Lameness-Related Factors.

Overview

• This study examined two blood biomarkers related to bone turnover in Thoroughbred racehorses to establish reference values and investigate how these markers relate to factors like age, training, and lameness.
• The goal was to assess whether these markers could help predict short-term soundness issues (lameness) in racehorses to improve animal welfare and performance management.

Background and Purpose

• Musculoskeletal injuries are a significant welfare concern in Thoroughbred horse racing.
• Early identification of horses at risk for injury could improve management and performance outcomes.
• Biomarkers measurable in the blood, specifically markers indicating bone turnover, may help with early risk stratification.
• The two biomarkers studied are:
  • Osteocalcin (OC or BGLAP), a protein linked with bone formation.
  • C-terminal telopeptide of type I collagen (CTX-I), a marker of bone resorption (breakdown).
• The study intended to establish reference ranges for these markers in fit-to-race Thoroughbreds and to explore how demographic factors, training conditions, and lameness relate to biomarker levels.

Study Design and Population

• This was a multicenter observational cohort study involving 1,359 fit-to-race Thoroughbred racehorses.
• Horses were sampled from three Australian states: New South Wales (NSW), Victoria, and Western Australia.
• Sampling focused on horses considered healthy enough to race, aiming to establish baseline biomarker levels and detect any relationships to specific factors.

Key Findings: Distribution and Reference Intervals

• Both OC and CTX-I biomarker levels in serum showed positive skew (more low values with a tail toward higher values).
• Population-wide percentile ranges (from 2.5th to 97.5th percentile) were:
  • CTX-I: 0.13 to 9.44 ng/mL
  • OC: 0.02 to 7.25 ng/mL
• CTX-I levels varied substantially depending on geographical location (jurisdiction), indicating environmental or management differences.
• Because of this variation, the combined population reference range for CTX-I should not be used as a clinical reference for individual horses without considering location.

Associations of Biomarkers with Demographic and Training Variables

• CTX-I:
  • Varied significantly by age, with older or younger horses showing different CTX-I levels.
  • Varied by jurisdiction (with notably higher levels in NSW horses).
  • Varied by racing venue and training surface; training on polytrack surfaces was associated with lower CTX-I levels.
• OCs:
  • Higher in horses that were lame at the time of sampling.
  • Showed some ability to discriminate/preempt which horses would develop lameness later, especially persistent lameness at 7 and 28 days.
  • This predictive ability was modest and weakened after adjusting for age, sex, location, and training surface.

Predictive Value for Lameness Outcomes

• OC, at an unadjusted threshold around 1.24 ng/mL, showed a high negative predictive value for persistent lameness—meaning horses with OC levels below this cutoff were unlikely to develop long-term lameness.
• This suggests OC might be useful as a “rule-out” biomarker in screening fit racehorses to identify those unlikely to develop short-term lameness.
• CTX-I did not predict lameness outcomes effectively, indicating it may be better suited to understanding population-level bone turnover rather than individual clinical risk.

Implications and Future Directions

• OC could be valuable in clinical or training environments to screen horses for soundness and possibly prevent injury by early identification of risk.
• More research is needed to validate OC as a screening tool, particularly addressing how adjustments for demographic and training factors affect its predictive performance.
• CTX-I may serve more as a research tool to characterize overall population skeletal turnover influenced by environment and management rather than as a clinical marker.
• The findings underscore the importance of considering location, age, and training surface when interpreting bone turnover markers in racehorses.