Epidemiology of health and performance in New Zealand racehorses A thesis presented In partial fulfilment of the requirements for the degree of Doctor of Philosophy in Veterinary Epidemiology at Massey University, Palmerston North, New Zealand Nigel Ross Perkins 2005 ii iii iv v Abstract: The aim of this research was to describe training and racing patterns, and causes of wastage in New Zealand Thoroughbred racehorses. Two separate studies were performed. The first involved analysis of data from before and after construction of a new training track at the Matamata Racing Club. Comparisons of measures of performance failed to detect any adverse impact that could be attributed to the new track. The second involved a longitudinal study over a 34-month period, and that involved 1,571 horses. Duration of training preparations and spell periods were associated with horse age, and with the reason for ending a training preparation. Most horses began a training preparation doing slow work and then progressively advanced to a first start. Incidence rates were estimated for starts per 100 training-days, and other summary measures were estimated including training-days to first start, and between successive starts. A total of 834 musculoskeletal injuries (MSI) were observed, resulting in either a spell period, retirement, or death of the horse. There were 165 respiratory disease events, and 58 conditions involving other body systems. Multivariate statistical models were used to explore risk factors for different types of MSI. Older horses were at higher risk of lower limb MSI, and injury to either the superficial digital flexor tendon (SDFT) or suspensory apparatus (SA), while they were at lower risk of shin soreness and other conditions. Male horses had higher risk of tendon and ligament injury than females. Lower risk of injury was observed in Autumn and Winter months compared with December, and in the 1999-2000 year relative to 1997-1998. Measures of cumulative exercise intensity showed a complex relationship with risk of injury that varied with type of injury. The risk of SDFT and SA injury was higher for those preparations without starts, while that for shin soreness was increased after the first start. For lameness conditions other than shin soreness or injury to the SDFT, the hazard was increased after the first start in a preparation, but the magnitude of effect was dependent on the number of days from the beginning of a preparation to the first start. vi vii Acknowledgements A large number of people contributed towards various stages of this project and I am grateful to them all for their support. The New Zealand Equine Research Foundation provided funding and encouragement. New Zealand Thoroughbred Racing staff and regional racetrack management staff, provided assistance. A special thank you is due to Ms Dagmar Fikken who was employed as a technician to assist in the collection of data, and to Glenda who helped so much with the Matamata end of the project. Thank you to the large number of veterinarians, trainers, and stable staff, who helped in so many ways to make this project possible, not the least of which was giving up their time on a regular basis to allow myself, Dagmar, or Glenda to ask questions and fill out forms. A special thank you to my supervisors, Stuart Reid and Roger Morris, for their guidance and advice, and to colleagues from the Institute of Veterinary, Animal and Biomedical Sciences for their general support. Finally to Kim, Emily and Caroline, and my extended family, your love and support has been wonderfully sustaining throughout this process. viii ix Table of Contents Abstract: ..........................................................................................................................v Acknowledgements........................................................................................................vii List of Tables .................................................................................................................xv List of Figures.............................................................................................................xxiii Introduction.....................................................................................................................1 Literature Review............................................................................................................5 1. Introduction...............................................................................................................6 2. New Zealand racing industry ....................................................................................7 3. Review of epidemiological study design ................................................................14 3.1. Experimental studies ........................................................................................14 3.2. Observational studies .......................................................................................14 4. Sources of bias in epidemiological studies .............................................................18 4.1. Selection bias ...................................................................................................18 4.2. Information bias ...............................................................................................20 4.3. Confounding bias .............................................................................................21 5. Matching .................................................................................................................21 6. Measures of frequency and association...................................................................22 7. Chronological development of research methods to investigate racehorse health and performance..........................................................................................................24 8. Data sources and Outcomes used............................................................................29 9. Events interfering with training and racing.............................................................34 9.1. Classification by location and system affected ................................................34 9.2. Frequency estimates.........................................................................................40 10. Specific risk factors for musculoskeletal injury....................................................45 x 10.1. Age.................................................................................................................45 10.2. Gender............................................................................................................46 10.3. Quality of horse and class of race ..................................................................46 10.4. Leg: left vs right.............................................................................................47 10.5. Race type........................................................................................................48 10.6. Race number ..................................................................................................49 10.7. Barrier position ..............................................................................................49 10.8. Race distance..................................................................................................50 10.9. Jockey.............................................................................................................50 10.10. Racetrack and racing surface .......................................................................50 10.11. Surface condition .........................................................................................52 10.12. Season ..........................................................................................................54 10.13. Exercise intensity .........................................................................................54 10.14. Pre-existing conditions.................................................................................59 10.15. Shoeing ........................................................................................................61 10.16. In-race activity and interaction ....................................................................61 10.17. Trainer..........................................................................................................63 11. Conclusion ............................................................................................................63 Effect of training location and time period on racehorse performance in New Zealand. I. Descriptive analysis * ................................................................................65 Abstract .......................................................................................................................66 Introduction.................................................................................................................67 Materials and methods ................................................................................................68 Study design............................................................................................................68 Variables .................................................................................................................68 xi Statistical analysis.......................................................................................................70 Results.........................................................................................................................70