Reducing injuries in racehorses: Mission impossible?

Some 25 years ago, a UK animal welfare charity commissioned research into racecourse fatalities with particular emphasis on determining whether racing 2-year-olds was detrimental to equine welfare. Retrospectively available data on all flat, hurdle and steeplechase races run on UK racecourses from 1990 to 1999 (over 700,000 starts made by nearly 64,000 Thoroughbred racehorses) were analysed using multivariable statistical methods. Rather unexpectedly at the time, results showed that, in any race type, horses that had started racing as 2-year-olds were less likely to die on the racecourse subsequently compared with those that started their racing careers at an older age 1. Further experimental studies that investigated effects of exercise on bone, cartilage and tendons found supporting evidence of beneficial effects of early race training on musculoskeletal tissues 2-5. This is consistent with findings in humans that exercising the growing skeleton, which is more responsive than mature skeletal tissue, has beneficial effects on long-term fracture resistance 6, 7. Racing 2-year-olds therefore does not seem to be a bad thing, but do these horses consequently have shorter careers than those that start racing when they are older? And how successful are they on the racecourse? A limited number of earlier studies showed that starting racing at an earlier age was also associated with a longer racing career 8-10, and more recent studies from Australia and New Zealand equally provide evidence of beneficial effects of racing as a 2-year-old on career longevity and success in Standardbreds and Australian Thoroughbreds 11-13. The paper by Tanner et al. in this issue of the Equine Veterinary Journal 14 complements these recent studies by investigating the racing careers of the foal crop (n = 4683) registered with New Zealand Thoroughbred Racing in 2001–2002. Associations between the age of attaining certain training milestones (registered with a trainer, first trial and first race) with career longevity and racecourse performance were determined. The study found that horses that first raced as 2-year-olds had longer racing careers and were more successful on the racecourse compared with those that started racing when older, further supporting results from earlier studies indicating beneficial effects of an early racing career start. However, as the authors rightly point out, associations do not imply causation, and it may well be that horses that start racing earlier can do so because they show maturity for racing at an early age and have the potential, i.e. they are ‘good horses’ no matter when they start their racing career. Furthermore, data analysis in the present and earlier epidemiological studies did not adjust for confounding factors that may weaken the strength of associations found. For example, in the present paper 14, the authors note that it was not possible to adjust their findings for a potential effect of ‘trainer’, a variable that has been shown to be associated with racing performance in other studies 15, 16. Trainer is likely to be a direct determinant of racecourse performance (because the trainer is responsible for entering horses in races that maximise their potential of winning or being placed) but also serves as a proxy measure for other factors that affect racing performance, e.g. management practices and the exercise that horses undertake during training. Training exercise has been associated with performance in racehorses 15, 16 and is also a determinant of injury risk, which in turn is likely to affect a horse's potential for success on the racecourse. Musculoskeletal injury is the main cause of morbidity and mortality in the racing industry worldwide and, as such, the past few decades have seen considerable research efforts into the epidemiology of such injuries, including identification of modifiable risk factors 17-20. Epidemiological studies in this area can broadly be classified as those investigating risk factors for injury/fatality on the racecourse vs. those studying determinants of training injuries. The majority of publications report on the former, because routine recording of injuries diagnosed on the racecourse and public availability of horse-, racecourse- and race-related information make data collection relatively easy. Well-established risk factors for racing injury include race type, race distance, track surface and firmness of the going, horse age and age at first race, previous racing/exercise history and prior injury 17-19. Two recent studies found previous tendon injury to be a major risk factor for racecourse tendon injury diagnosis 21, 22. This highlights an important issue with racing injury studies: by and large, they do not take account of the horse's training preparation prior to appearing on the racecourse and/or any injuries it may have sustained during training. Moreover, a considerable proportion of injuries sustained on the racecourse are only diagnosed when the horse returns home after racing (23 and personal observations) and are therefore not centrally recorded in databases used by researchers, resulting in potentially biased findings from racecourse injury studies. Moving forward, an integrated approach is needed whereby reliable training data, including data on injuries sustained during training, can be combined with racecourse data to provide a more detailed picture of how these two research areas relate to each other. Although it has been shown that racing poses a higher risk of sustaining an injury compared with spending a day on the training gallops at home, proportionally, most injuries occur during training rather than on the racecourse, in particular in flat racing 23, 24. Studying injuries in training is more difficult owing to lack of readily available and reliably recorded training exercise information. Nonetheless, a number of purpose-designed prospective cohort studies have been conducted to estimate the incidence of different types of musculoskeletal injury sustained during training 23-30 and identify associated risk factors, in particular in relation to exercise regimens. Findings from these studies reveal complex associations between exercise and injury, depending on exercise speed, time periods considered and type of injury 17-19. Results generally suggest that greater amounts of exercise in short time periods are associated with increased injury risk and that accumulation of canter exercise since entering training induces skeletal microdamage resulting in fracture. Effects of high-speed exercise differ according to the time period considered and the injury under study. For example, greater amounts of high-speed exercise in short time periods (up to 1 month) increase overall fracture risk, although accumulation of high-speed exercise in the longer term promotes fracture resistance 29. However, accumulation of high-speed exercise since entering training has been associated with increased risk of fetlock joint injury Reed, S.R., Jackson, B.F., Wood, J.L.N., Price, J.S. and Verheyen, K.L.P., unpublished data). Because of sample size requirements and time constraints, prospective cohort studies have been limited in their ability to study specific outcomes, such as specific fracture types (e.g. lateral condylar fracture of the third metacarpal bone) or selected joint pathologies (e.g. subchondral bone sclerosis of metatarsophalangeal joint condyles associated with ‘bone pain’). A case–control approach to investigate exercise-related factors for specific injury types may overcome this limitation, although this could be at the expense of reliability of retrospectively collected training information. In this respect, methods for collection of training information also require further refinement to increase their accuracy and level of detail. With advancement of modern technology, routine recording of training speeds and distances using global position systems techniques may well become possible in the future 31. Such developments would allow objective monitoring of racehorse fitness, which would be of benefit to trainers, as well as facilitate epidemiological research of training exercise in relation to injury risk and performance. While training injury studies conducted thus far can provide some guidance on training regimens that could reduce injury occurrence, the complexity of associations between exercise patterns and different types of injuries means that there are no black-and-white answers to the question of how we should train racehorses to reduce injury risk while optimising racecourse performance. This poses difficulties in communicating findings to trainers, because no clear-cut advice can be formulated that will undoubtedly have a positive impact on all aspects of training racehorses. Accumulating evidence from sustained research efforts generally points towards beneficial effects of small amounts of high-speed exercise, introduced at an early age, and detrimental effects of accumulating large amounts of slower-speed canter exercise. Practical implementation of training regimens that take account of research findings, at population level, may well reduce the burden of disease in the racehorse population. It would be ideal to be able to implement intervention studies to test training protocols informed by current research evidence and measure injury incidence and race performance 17. This is, however, likely to be time consuming and expensive, and difficult to achieve in a commercial race training setting owing to requirements for long-term compliance and adherence to exercise protocols, which is likely to be beyond the control of investigators. Such intervention studies are therefore unlikely to be a realistic possibility in the near future. In the absence of purpose-designed intervention studies, there need to be clear communication channels between researchers, industry stakeholders and those ‘on the ground’ who can voluntarily implement interventions (i.e. modify training regimens). It is currently not clear how much and what type of information reaches racehorse trainers, what they make of it and whether changes in knowledge result in a change in behaviour. Without communication channels and sustained interaction between all interested parties, it will be difficult to achieve the common goal of reducing injury incidence. In addition, for both training and racing injuries, there is a clear need to continue to monitor trends over time and evaluate the effect of interventions, to assess whether progress is being made and whether the objectives of intervention programmes are being achieved. Recent research suggests that, despite considerable advances in diagnostic imaging modalities and improvements in veterinary care, little progress seems to have been made in reducing wastage in the UK racing industry over the past 20 years 32. This clearly suggests a need for further action and continued monitoring and evaluation of this situation, in particular where specific interventions have been implemented. Without such continued scrutiny, findings from research projects in this area will have little impact on future policy directions, and reducing injuries in racehorses may well become a mission impossible.