Unexplained underperformance syndrome in sport horses: classification, potential causes and recognition.
Authors: Rivero J L L, van Breda E, Rogers C W, Lindner A, van Oldruitenborgh-Oosterbaan M M Sloet
Journal: Equine veterinary journal
Summary
# Editorial Summary: Unexplained Underperformance Syndrome in Sport Horses Rivero and colleagues examined overtraining syndrome (OTS) in equine athletes, distinguishing it from the milder over-reaching condition that resolves within 1–2 weeks; OTS is defined as sustained performance loss exceeding two weeks in heavily trained horses without obvious clinical pathology. Through literature review and clinical assessment frameworks, the authors identified an imbalance between training load and recovery as the primary aetiological factor, whilst acknowledging secondary contributors including transport stress, nutritional inadequacy, subclinical disease and management practices. Current evidence refutes red cell hypervolaemia as a mechanism, with hypothalamic-pituitary-adrenocortical axis dysfunction emerging as the only mechanistically implicated pathway, though the underlying physiology remains incompletely understood and no validated objective biomarkers exist. Diagnosis currently relies on thorough history-taking—evaluating duration of performance deficit, unexplained weight loss despite adequate feeding, concurrent subclinical problems, failed treatments and behavioural changes—supplemented by standardised exercise testing to detect subtle hormonal response variations. For practitioners, this means recognising OTS as a management-related syndrome requiring systematic evaluation of training-recovery ratios and ancillary stressors rather than pursuing symptomatic treatment; longitudinal physiological profiling may eventually enable earlier detection, but clinicians should currently prioritise comprehensive case history assessment and exercise testing protocols as diagnostic tools.
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Practical Takeaways
- •Differentiate OTS from over-reaching: sustained performance loss >2 weeks indicates OTS requiring intervention, whereas recovery within 1-2 weeks suggests over-reaching with spontaneous resolution
- •Evaluate training load holistically—address not just exercise intensity but also transport, feeding adequacy, subclinical health issues, and management stress when horses underperform without obvious clinical signs
- •Use standardized exercise tests and hormonal response assessment to detect early overtraining before performance loss becomes severe, though diagnosis currently relies on thorough history and clinical presentation
Key Findings
- •Overtraining syndrome is distinguished from over-reaching by performance loss lasting >2 weeks versus recovery within 1-2 weeks
- •Imbalance between training and recovery is the primary cause, with contributing factors including transport, feeding, subclinical disease, and management
- •Red cell hypervolaemia is not a mechanism for OTS in horses, while hypothalamo-pituitary-adrenocortical axis dysfunction is the only mechanism currently linked to syndrome development
- •Thorough history assessment including weight loss despite adequate feed intake, unspecific subclinical problems, and behavioral changes is the most effective diagnostic method in absence of objective biomarkers