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4 Fracture Epidemiology

       T.D.H. Parkin

       Bristol Veterinary School, University of Bristol, Bristol, UK

State of Knowledge

      In the last 20 or 30 years, epidemiology research groups, around the world, have focussed efforts on addressing one of the primary welfare concerns associated with equine sports, i.e. how to minimize the risk of injury in horses competing for human pleasure. Most sports‐horse‐related injuries that result in euthanasia affect the musculoskeletal system, in particular the bony structures of the distal limb. For this reason, the majority of work has focussed on identification of risk factors and preventive measures to mitigate such injuries. Equine fractures that have been the subject of epidemiological analyses are predominantly non‐traumatic and related to exercise, principally during racing and training but also in other forms of equestrian competition. There is a dearth of published information on the epidemiology of fractures in the non‐sports horse. As a result, the majority of this chapter focuses on work conducted in competition horses.

      The chapter is structured to firstly describe the relative incidence of different fracture types in different disciplines or horse populations in different geographical locations in both racing (competition) and training. Secondly, it summarizes work conducted on the identification of risk factors for different types of fracture, in competition and during training. In recent years, research in this field has attempted to better predict which horses are most likely to sustain a fracture. In other words, we have moved from the identification of risk factors to trying to identify the ‘at‐risk’ horse. A description of how these efforts are proceeding is included. The chapter concludes with a short commentary on predictability and the potential for pre‐fracture screening methods that could be employed in order to minimize the risk of injury in equine athletes.

      It is worth noting that over the years different case definitions have been used for studies in this area which genuinely reflect differences in outcome, but it is also the case that the same outcome (in particular fatal injury) has been referred to in several different ways by different authors. For example, authors have referred to a fatal injury as a ‘catastrophic injury’ or a ‘fatal musculoskeletal injury’. I have retained the original terminology used when referencing individual papers, but readers should be aware that all three definitions relate to the same outcome.

Geographic, Discipline and Horse Level Incidence

      The measure of incidence used in different studies largely depends on the focus of the study. For example, studies that focus on competition (in the main racing) ideally quote figures that describe the number of fractures as ‘X’ per 1000 starts, whereas those that focus on training need to describe the number of fractures as ‘X’ per 100 horse years or months at risk. The success of interventions cannot genuinely be assessed unless appropriate measures of pre‐intervention risk are used. A small reduction in the number of fractures or injuries at a particular racecourse/racing jurisdiction/yard/competition venue should not be regarded as good evidence of effective intervention unless the number of starts/horse months at risk at that location is also reported.

      Incidence of Fractures Sustained During Competition

      Thoroughbred Racing

A number of epidemiological studies of racehorse injury or fatality have identified the structures or tissues affected. Some of these studies have relied upon racecourse veterinary reports [1–6] or clinical records [7, 8], whereas others have initiated or used post‐mortem examinations to ensure accurate fracture classification [9–13]. The cost of racecourse post‐mortem programmes often restricts their implementation, but recent work suggests that, provided these are accurate and use precise case definitions, they are invaluable [14]. At the very least, it is important that racecourse veterinary surgeons, reporting to injury databases, attempt to identify which bone is fractured. As the majority of fatal fractures sustained during racing are associated with the metacarpophalangeal joint [3, 4, 6, 7, 10, 11], this can often simply involve opening the joint from the dorsal aspect to confirm which and how many bones are involved.

       Thoroughbred Racing in North America

      Studies of races held at New York racecourses between 1983 and 1985 reported 2.1 fractures per 1000 starts on dirt tracks and 1.1 per 1000 starts on turf tracks [15]. In Kentucky, between 1992 and 1993, 1.4 catastrophic injuries per 1000 starts were reported [16]. The majority of these injuries affected bones in the forelimb. In 1990, the California Horse Racing Board (CHRB) requested that the California Veterinary Diagnostic Laboratory System carry out post‐mortem examination of all horses that die at racetracks under the jurisdiction of the CHRB. Subsequently, the risk of fatal fracture on these tracks during the 1990s was reported as 1.5 per 1000 starts [17, 18].

      More recently, the Jockey Club in the North America initiated the routine collection of (fatal) injury data from North American racetracks, known as the Equine Injury Database (EID). Currently, racetracks that stage 96% of all race starts in North America contribute to the database. Between January 2009 and December 2014, there were 2.0 fatal and non‐fatal fractures per 1000 starts on reporting racetracks [19]. Annual statistics relating to the risk of fatal injury are supplied by the Jockey Club, which in the last four years have shown a significant reduction in the risk of fatal injury from 1.89 per 1000 starts (2014) to 1.61 per 1000 starts (2017) [20]. The majority of these injuries involve a distal limb fracture, so it is fair to assume that the figure of 2.0 fractures per 1000 starts reported between 2009 and 2014 [19] will have dropped by a few decimal points since that time. All in all, there has been a 20% drop in the incidence of equine fatal injury during racing in North America between 2009 and 2017. On the assumption that the number of starts had remained constant, this is equivalent to approximately 150 fewer horses dying on North American racetracks in 2017 compared with 2009.

       Thoroughbred Racing in the UK

      Between 1987 and 1993, reports indicated that catastrophic fractures occurred with frequencies of 0.33 per 1000 flat, 1.4