US-84A Final REPORT · Shin soreness in Thoroughbred racehorses A report for the Rural Industries...

Shin soreness

in Thoroughbred racehorses

A report for the Rural Industries Research

and Development Corporation

by N. Cogger, D.L. Evans, D.R.Hodgson, N.R.Perkins and S.W.J. Reid

December 2004

RIRDC Publication No 04/155

RIRDC Project No US-84A

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© 2004 Rural Industries Research and Development Corporation. All rights reserved. ISBN 1 74151 063 5 ISSN 1440-6845 Shin soreness in Thoroughbred racehorses Publication No. 04/155 Project No.US-84A The views expressed and the conclusions reached in this publication are those of the author and not necessarily those of persons consulted. RIRDC shall not be responsible in any way whatsoever to any person who relies in whole or in part on the contents of this report. This publication is copyright. However, RIRDC encourages wide dissemination of its research, providing the Corporation is clearly acknowledged. For any other enquiries concerning reproduction, contact the Publications Manager on phone 02 6272 3186. Researcher Contact Details Associate Professor David Evans Faculty of Veterinary Science University of Sydney NSW 2006 Australia Phone: +61 2 9351 2474 Fax:+61 2 9351 3957 Email: [email protected] In submitting this report, the researcher has agreed to RIRDC publishing this material in its edited form. RIRDC Contact Details Rural Industries Research and Development Corporation Level 1, AMA House 42 Macquarie Street BARTON ACT 2600 PO Box 4776 KINGSTON ACT 2604 Phone: 02 6272 4539 Fax: 02 6272 5877 Email: [email protected] Website: http://www.rirdc.gov.au Published in December 2004 Printed on environmentally friendly paper by Canprint

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Foreword Shin soreness is a training injury in Thoroughbred horses that is characterised by pain on palpation of the shin, or third metacarpal bone, and is often associated with an unwillingness to work at speed. Previous research funded by the Rural Industries Research and Development Corporation (RIRDC) identified shin soreness as a common and recurrent health problem. The study found that the problem affected 42% of two- and three-year-old Thoroughbred racehorses and recurred in 40% of affected horses. During the two-year-old racing season shin soreness accounted for the greatest number of modified training days and weeks spent resting at pasture. The aim of this study was to investigate the training-, track- and horse-related risk factors for shin soreness. The results demonstrate that training methods are a major risk factor for shin soreness. Risk of shin soreness was increased if the average weekly distance trained at speeds of greater than 890 metres per minute was higher during the first ten weeks of the training. Careful introduction of gallops at these speeds can reduce the risk of shin soreness. A gradual increase in the weekly distances at these speeds is the key to reducing the number of cases. Training horses in order to cause shin soreness does not reduce the risk of the disease in subsequent preparations. However, judicious use of gallops at speeds greater than 890 metres per minute can reduce the likelihood of shin soreness in subsequent preparations. The weekly distances of gallops at speeds between 800 and 890 metres per minute did not influence the risk of shin soreness. Shin soreness was also less likely in the group of horses with average age of 28 months at the commencement of the preparation, compared to horses with average age of 30 months. These results are very important because they demonstrate that wastage in the industry could be reduced and horse welfare can be promoted if training methods are refined. This project was funded from industry revenue matched by funds provided by the Australian Government. This report, an addition to RIRDC’s diverse range of over 1000 research publications, forms part of our Horse R&D program, which aims to assist in developing the horse industry and enhancing its export potential. Most of our publications are available for viewing, downloading or purchasing online through our website: downloads at www.rirdc.gov.au/reports/Index.htm purchases at www.rirdc.gov.au/eshop

Simon Hearn Managing Director Rural Industries Research and Development Corporation

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Acknowledgments The Rural Industries Research and Development Corporation provided the financial support for this study. AAP provided free access to their commercial database of racing and barrier trials for the duration of the study. In addition to these organisations the researchers would like acknowledging the individuals who provided assistance throughout the study. Dr Craig Suann helped to promote the study by publishing an article in the NSWTRB racing calendar. Sebastian Werner assisted in the design of the database that was used to store the training and injury data. Veterinarians at Randwick, Warwick Farm, Rosehill and Newcastle provided invaluable advice and assistance during the design of the study, and assisted with provision of introductions to trainers. Finally, we would like to acknowledge the participating trainers, and their staff. There were moments when the data collection was an intrusion in their busy work schedules, but they always found the time to complete the training and injury forms. The dedication and commitment of the trainers made the data collection process an enjoyable experience and was a major contributor to the success of the study.

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Contents

Foreword ............................................................................................................................................... iii Acknowledgments................................................................................................................................. iv Contents.................................................................................................................................................. v Executive Summary ............................................................................................................................. vi 1 Background ................................................................................................................................... 1 1.1 Introduction ................................................................................................................................ 1 1.2 Frequency and Impact of Shin Soreness .................................................................................... 1 1.3 Bone Loading and Adaptation.................................................................................................... 1 1.4 Epidemiological approaches to investigating shin soreness....................................................... 4 1.5 Intrinsic Risk Factors ................................................................................................................. 5 1.6 Extrinsic Risk Factors ................................................................................................................ 6 1.7 Conclusion.................................................................................................................................. 8 2 Materials and Method .................................................................................................................. 9 2.1 Study design and implementation .............................................................................................. 9 2.2 Data collection.......................................................................................................................... 11 2.3 Coding training data ................................................................................................................. 11 2.4 Coding of injury data................................................................................................................ 15 2.5 Data analysis ............................................................................................................................ 16 3 Results.......................................................................................................................................... 20 3.1 Study population ...................................................................................................................... 20 3.2 Frequency of musculoskeletal injures ...................................................................................... 25 3.3 Impact of shin soreness on training and racing ........................................................................ 27 3.4 Risk factor analysis .................................................................................................................. 29 4 Discussion .................................................................................................................................... 32 4.1 Frequency and impact of shin soreness.................................................................................... 32 4.2 Recommendations to industry .................................................................................................. 32 4.3 Further analysis ........................................................................................................................ 34 4.4 Adoption strategies................................................................................................................... 34 4.5 Future research ......................................................................................................................... 34 4.6 Conclusion................................................................................................................................ 35 5 References.................................................................................................................................... 36

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Executive Summary Shin soreness is a training injury that is characterised by pain on palpation of the shin bone and may be associated with an unwillingness to work at speed (Stover, Pool et al. 1988). Previous research has identified shin soreness as a common and recurrent problem that affects 42% of the horses during their two-year-old racing season and recurrs in 40% of cases. In two-year-old racehorses shin soreness accounted for the largest percentage of lost training days (20%) and weeks spent resting at pasture (23%). The causes of shin soreness are likely to involve a number of different factors (Caine, Caine et al. 1996; Brunker, Bennell et al. 1999). These factors are normally divided into intrinsic or extrinsic factors. Intrinsic risk factors are characteristics of the horse whilst extrinsic factors are characteristics of the environment. When examining an individual risk factor it is important to consider not only its independent effects but also how it interacts with other risk factors. At present, there is very little information relating to the inter-relationship and relative of individual risk factors. The aim of this study was to examine the training-, track- and horse related risk factors for musculoskeletal injuries in young Thoroughbred racehorses. Six hundred and six horses, trained by 18 trainers training were enrolled into a longitudinal study to investigate risk factors for shin soreness. The horses belonged to one of three cohorts, or groups, as outlined in Table 1.

Table 1: Entry requirements for three cohorts of horses

Cohort Requirement A Horses born in the 1998 foaling season that entered the study during their two-year-

old racing season B Horses born in the 1998 foaling season that entered the study during their three-year-

old racing season

C Horses born in the 1999 foaling season that entered the study during their two-year-old racing season

All participating stables were visited at approximately 14-day intervals to collect daily training and injury data. The training data included the following information:

• Training activity undertaken on each day • Distance worked at “evens” or “three quarter pace”, that is between 800 m/minute and 890

m/minute • Distance worked “home on the bit”, that is faster than 890 m/minute.

Every day that a horse was enrolled in the study and in the stable was referred to as a “training day”. Exercise for each training days was categorised as described in Table 2. The training days were grouped together into units referred to as preparations. If the preparation included one fast day it was referred to as a fast preparation. All fast preparations were divided into slow and fast portions. The slow portion was the period of time from the start of the preparation to the first fast day. The fast portion was the period from the first fast day until the end of the preparation.

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Table 2: Categories used to classify training days for two- and three-year-old Thoroughbred racehorses

Categories Definition

Rest day A training day where the only activity was walking and/or swimming or the horse was box rested

Slow day A training day where the maximum speed was less than 650 meters per minute.

Medium-speed fast day A fast day where the maximum speed was between 800 and 890 meters per minute, that is the horse did not work faster than “evens” or “three-quarter pace”.

High-speed fast day A training day where the maximal speed was greater than 890 meters per minute, that is the horse worked “home on the bit”

Data from 486 horses was analysed further. During the study period there were 442 musculoskeletal injuries in 263 (54.1%) horses. Shin soreness was the most common injury, with 187 cases reported in 113 horses, or just over 30% of the study population. In two-year-old racehorses in cohorts A and B the percentage of affected horses was 36 and 24% respectively. This is substantially higher than for three-year-olds in cohorts A (7%) and C (12%). During the study period shin soreness recurred in 23% of all horses, although no horses in cohort B suffered from a second episode of shin soreness. In cohorts A and B just over 45% of the time spent resting at pasture due to an injury was attributable to shin soreness. In contrast, for horses in cohort C only 20% of time spent resting at pasture was associated with an episode of shin soreness. Each case of shin soreness was associated with an average of 12 weeks resting at pasture. Examination of official racing records showed that 64% of horses had raced in the 12 months following an episode of shin soreness. The results demonstrate that training methods are a major risk factor for shin soreness. Risk of shin soreness was increased if the average weekly distance trained at speeds of greater than 890 metres per minute was higher during the first ten weeks of the training. Careful introduction of gallops at these speeds can reduce the risk of shin soreness. A gradual increase in the weekly distances at these speeds is the key to reducing the number of cases. Training horses in order to cause shin soreness does not reduce the risk of the disease in subsequent preparations. However, judicious use of gallops at speeds greater than 890 metres per minute can reduce the likelihood of shin soreness in subsequent preparations. The weekly distances of gallops at speeds between 800 and 890 metres per minute did not influence the risk of shin soreness. Shin soreness was also less likely in the group of horses with average age of 28 months at the commencement of the preparation, compared to horses with average age of 30 months.

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1 Background 1.1 Introduction Shin soreness is the most common musculoskeletal injury in two- and three-year-old Thoroughbred racehorses (Mason and Bourke 1973; Bailey 1998); (Buckingham and Jeffcott 1990). It is a low-grade training injury that affects the upper dorsal aspect of the shin or third metacarpal bone and is characterised by pain on palpation (Stover, Pool et al. 1988). Shin soreness is often associated with an unwillingness to work at high speeds. Continued training may lead to diffuse soft tissue swelling visible on the dorsum of the metacarpus. This chapter describes what is currently known about how bone adapts to exercise and how this may increase or decrease the risk of horses developing shin soreness. The review has been divided into five main sections: frequency and impact of shin soreness, bone loading, epidemiological approaches to investigating shin soreness, intrinsic risk factors and extrinsic risk factors for shin soreness. The chapter concludes by describing the aims of this research project.

1.2 Frequency and Impact of Shin Soreness A previous study funded by the Rural Industries Research and Development Corporation found that 85% of horses suffered from at least one illness or injury during their two- and three-year-old racing season (Bailey 1998). Shin soreness was the most common problem affecting 42% of the horses at least once. Forty percent of horses that were affected by shin soreness developed the problem for a second or third time. In two-year-old racehorses shin soreness accounted for the largest percentage of lost training days (20%) and weeks spent resting at pasture (23%). On average each case of shin soreness resulted in four lost training days and seven weeks resting at pasture.

1.3 Bone Loading and Adaptation The shape and quality of skeletal tissue is determined by a number of factors including nutrition, genetics and its mechanical load history (Brunker, Bennell et al. 1999). Mechanical loading occurs when the foal begins weight bearing and can be supplement by a training program. This load is caused by several different types of forces as outlined in Figure 1.1 and is commonly described in terms of stress and strain. In clinical terms, stress is a measure of the magnitude of the load and strain refers to the lengthening or deformation that occurs in a particular direction. It is also common to describe the rate and duration of the load. The adaptive response should be seen as a continuum with a number of factors affecting the way the bone is loaded or the bone’s respond. Figure 1.2 is a schematic representation of the interaction between these factors. The remainder of this section briefly describes the bone’s response to loading. More detail review of bone’s adaptive responses can be found in Bailey et al. (1996), Brunker et al. (1999), Burr (1997) and Lanyon (1990).

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Bending Forces: Bend the bone’s axis

Shear Forces: Are applied parallel to the bone surface

Torsion Forces: Twist the bone around its axis

Unloaded Compression Forces: Are directed toward the bone surface

Tension Forces: Are direct away from the surface of the bone

Figure1.1: Description and direction of forces that may act on a bone. (Adapted from Bruncker et al 1999)

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Bone Loading

Bone Response

Conformation Training

Fast Work Distance

Type of Fast Work

Muscle Fatigue

Muscle Strength

Surface

Rider

ImpactAttenuation Bone Health

Diet

Breeding

Disease

Training History

Total Distance

Normal Accelerated Stress Stress Stress Complete Remodeling Remodeling Reaction Injury Fracture Fracture

Continuum of clinical responses to bone loading

Figure 1. 2: Bone’s response to a load is determined by the health of the bone and the nature of the load. The type of training influences the total number of times a bone is loaded, the size and direction of the load. The size and direction of the load may also be changed by muscle strength and fatigue, the skill and weight of the rider and the training surface. In addition the conformation may affect how the load is distributed along the bone. Finally, bone health is a major factor affecting bone’s response to loading and is determined by many factors including diet, previous exercise history, breeding and the presences of disease. (Adapted from Brunker et al. 1999)

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1.3.1 Bone modeling Bone modeling involves the addition of bone to the outer surface (Pool 1991). During this process the rate of bone deposition on the outer surface exceeds the rate of bone resorption on the inner surface causing the bone to thicken. Studies have shown that when horse enter training that bone modeling will cause the third metacarpal cortical bone to thicken (Davies, Gale et al. 1999), allowing the bone to resist bending forces in the dorsal palmar direction (Nunamaker 1996). 1.3.2 Bone Remodeling Bone remodeling involves changing the internal architecture of the bone cortex without altering the shape of the bone (Pool 1991; Nunamaker 1996; Brunker, Bennell et al. 1999). The remodeling process allows the bone to adapt to mechanical loading. This adapation is thought to prevent the accumulation of micro-fractures. During the remodeling process a small packet of bone tissue is reabsorbed. Following reabsorption there is a period of one to two weeks before new bone is deposited (Riggs and Evans 1990; Nunamaker 1996). During this time period the bone density at the site of reabsorption is weaker than before the loading. Continued loading during this period may result in micro-damage accumulation and the beginning of overuse injuries, such as shin soreness. Research in Quaterhorses has shown that after the new bone is deposited it takes approximately three months for bone mineralization to occur (Nielsen, Potter et al. 1997). The application of load, below that required to cause a complete fracture, produces micro-damage (Brunker, Bennell et al. 1999). If the load is repeatedly applied the level of micro damage will increase and this will result in the remodeling process being accelerated. At a microscopic level the first signs of accelerated remodeling are vascular congestion, thrombosis and resorption of bone. In humans accelerated remodeling does not, initially, produce any symptoms. However as remodeling progresses, mild pain will occur during exercise. If loading of the bone does not cease then the pain will persist even after the completion of exercise. Continual loading of the bone during accelerated remodeling will increase the size of the resorptive cavities, resulting in the appearance of micro fractures that extend into the cortex (Riggs and Evans 1990; Nunamaker 1996). These cracks cause a marked reduction in bone strength. At some point there may be insufficient bone to withstand the load and a complete fracture results.

1.4 Epidemiological approaches to investigating shin soreness Epidemiology is the study of a health problem within a population (Reid 1998b). The initial process in any epidemiological investigation is to describe the extent of the problem (Robertson 1998). Descriptive studies and reviews of case histories can be used to identify variables that are associated with the onset of shin soreness. However, additional analytical studies should be conducted to determine if the variables are risk factors for the problem. Risk factors are variables that are associated with an increase or decreased risk of a horse developing shin soreness (Caine, Caine et al. 1996; Brunker, Bennell et al. 1999). These factors are commonly divided into intrinsic or extrinsic risk factors. Intrinsic factors are characteristics of the horse whilst extrinsic factors are characteristics of the environment. When examining an individual risk factor it is important to consider not only its independent effects but also how it interacts with other risk factors. The most appropriate study designs to investigate risk factors for injuries are case-control and cohort studies. Case-control studies are preferential when the health problem of interest is rare (Robertson 1998), such as fatal injuries to horses on race day. Case-control studies begin with the identification of a group of animals with the problem (cases) and a group without (controls) (Caine, Caine et al. 1996; Robertson 1998). Data relating to exposure to potential risk factors is then collected for both the cases and controls. This data is analysed to determine if there is an association between the factors and the health problem. The main disadvantage of a case-control study is that the health status is determined prior to collection of data relating to exposure. Therefore, recall or existing records are used to provide

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information about exposure to potential risk factors. It is possible that knowledge of the current health status might bias the recall. When investigating a relatively common health problem, such as shin soreness, a cohort study is most appropriate. A cohort study, otherwise known as a longitudinal or prospective study, involves the selection of a group (cohort) and following them over time (Caine, Caine et al. 1996; Robertson 1998). During the follow-up period horses are observed to determine exposure to risk factors and occurrence of the health problems. The observations are then analysed to determine if there is an association between the risk factors and the health problem of interest. The major advantage of a cohort study is that the exposure status for each subject is known before the health problem occurs (Thrusfield 1995). The main disadvantages of cohort studies are that they are expensive and take a relatively prolonged time for completion.

1.5 Intrinsic Risk Factors 1.5.1 Age At least two studies have reported that the incidence of shin soreness is highest in two-year-old racehorses (Moyer, Spencer et al. 1991; Bailey 1998). To date no prospective investigation of risk factors for shin soreness, or any other training injuries, if at the commencement of training older horses are at less risk of injury than younger horses. Anecdotal evidence suggests that older horses commencing training for the first time will develop shin soreness (Buckingham and Jeffcott 1990). More research is required to determine if age, the commencement of training or a combination of the two are risk factors for shin soreness. 1.5.2 Sex A prospective study found no significant difference in the proportion of male and female horses affected by musculoskeletal injuries (Bailey 1998). This is supported by studies in human athletes that have reported no significant difference in overall injury rates between males and females (Knutzen and Hart 1996). However, these studies have reported that male athletes were more prone to tendonitis and carpal problems, and female athletes were more likely to experience shin splints and stress fractures. Unfortunately, Bailey (1998) did not report on the distribution of different types of injuries between male and female horses. 1.5.3 Conformation There have been no studies in the horse to determine if conformation plays a role in the development of shin soreness. Studies in humans suggest that differences in structure and biomechanics of the leg may affect the incidence of shin splints (Viitasalo and Kvist 1983; Kaufman, Brodine et al. 1999). In the horse a study found that an eight-degree reduction in the angle between the axis of the tarsal and the third metatarsal bone was the only measure of conformation associated with an increased risk of degenerative joint disease. (Axelsson, Bjornsdottir et al. 2001). Although, the size of the difference does limit its clinical usefulness and the effect of factors such as this on the incidence of shin soreness is not clear. 1.5.4 Bone Strength at the Commencement of Training The strength of the bone will influence its ability to withstand repeated loading. The total strength of the bone is determined by its stiffness or elasticity, mineral density and shape. In Quarter horses greater cortical mass in the dorsal and medial metacarpal at the commencement of a training program has been associated with a lower injury rate (Nielsen, Potter et al. 1997), although the authors did not describe the types of injuries included. Davies and Larkin (1996) have developed an index to describe the shape of the bone by relating the width of the dorsal cortex to that of the palmar cortex in X-rays of Thoroughbred racehorses. It has been proposed that radiographic indices can be used to predict onset of shin soreness in racehorses (Davies 2003). Human studies have also indicated that measurements of bone shape can be used to predict individuals that are at an increased risk of developing stress

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fractures (Brunker, Bennell et al. 1999). However the cost of such a screening program could limit its clinical application. 1.5.5 Body Size and Composition Theoretically, body size could be a risk factor by affecting the forces applied to the bone. It is known from human studies that taller, heavier individuals are at greatest risk of injury (Knutzen and Hart 1996). However there have been no studies investigating the role of body size as a risk factor for shin soreness in the Thoroughbred racehorse. 1.5.6 Fatigue/Muscle Fatigue In humans, muscle activity has been shown to partially attenuate the large bending movement and reduces the tensile and compressive stress applied to the tibia (Brunker, Bennell et al. 1999). Laboratory studies in the horse have shown that an inability to attain a previous maximum exercise speed in a subsequent run, within the same exercise period, is associated with an increase in bone strain (Davies 1996). Therefore, the ability of the muscle to delay fatigue may play a role in the onset of shin soreness.

1.6 Extrinsic Risk Factors 1.6.1 Training Boston and Nunamaker (2000) suggested that changing the distance trained at different speeds could alter the risk of shin soreness. Specifically the risk of shin soreness increased as the weekly distance worked at speeds of 660 meters per minute increased and decreased as the weekly distance worked at speeds between 900 and 960 m/minute increased. Table 1.1 describes the average weekly distances trained at different speeds in horses that developed shin soreness and those that did not. The authors concluded that in order to avoid shin soreness, trainers should allocate more training time to regular short-distances at speeds 900-960 meters per minute and less time to exercise over long distances at speeds of 660 meters per minute.

Table 1.1: Average distance per week worked at 300 meters per minute, 660 meters per minute and 900-960 meters per minute at five stables enrolled in a study investigating risk factors for shin soreness.

Average weekly distance (meters) Speed (m/minute) Injury category Stable 1 Stable 2 Stable 3 Stable 4 Stable 5 900-960 Not shin sore 208 528 128 240 192 Shin sore 208 352 144 240 48 660 Not shin sore 7424 6384 7024 8928 4768 Shin sore 10064 5296 6112 9952 3376 300 Not shin sore 4512 1712 2272 5344 9968 Shin sore 5296 1232 1904 5856 8288 Adapted from Boston & Nunamaker 2000 The results of the Boston and Nunamaker (2000) study should be viewed with caution. The training and management of racehorses are probably very different in Australia, and the results may have been biased by the study design. Therefore research that is specific to training practices in Australia is needed. Specifically, the research should investigate the relationship between shin soreness and the distances and frequency of training velocities greater than approximately 800 meters per minute (sometimes referred to by trainers as “even time”). 1.6.2 Surface Training surfaces have long been considered a contributing factor to shin soreness (Buckingham and Jeffcott 1990). Hard tracks are a problem because the ground reaction forces are increased thereby

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increasing the strain on the bone (Brunker, Bennell et al. 1999). However, training on soft surfaces may hasten muscle fatigue (Brunker, Bennell et al. 1999), which could also increase the strain on the bone (Yoshikawa, Mori et al. 1994; Davies 1996). There have been no studies in Australia to determine which training surfaces increase or decrease the risk of developing shin soreness. A prospective study, in the USA, found that horses trained on wood fibre were at less risk of injury than those trained on dirt surfaces (Moyer, Spencer et al. 1991; Moyer and Fisher 1992). In this study 34% of horses that trained exclusively on dirt developed shin soreness whilst, only 13.5% of horses training on woodchip suffered the complaint. In addition survival time for horses was greater for horses trained on woodchip rather than dirt; horses training on woodchip accumulated 86 miles of fast work before the onset of shin soreness, whilst horses trained on dirt accumulated only 32 miles.

In Moyer et al. (1991) and Moyer et al. (1993) the the training surfaces were located at training centres. Consequently the observed difference between the two surfaces may have been due to factors such as the degree of banking in turns (Evans and Walsh 1997) and the track geometry (Fredricson, Dalin et al. 1975a; Fredricson, Dalin et al. 1975b). In addition Moyer and Fisher (1992) did not control for any aspect of training because the trainers appeared to train in an identical manner. However, no quantitative evidence was provided to support this conclusion. 1.6.3 Turns and Banking Most horses in Australia are trained on racetracks and must therefore exercise at high speeds around corners during training. Fredricson et al. (1975b) has suggested that because of centrifugal force acting on the limbs when travelling around turns there is an increased risk of injury. Studies in Standardbreds show that moving around a corner causes abnormal gait and increased the temperature in the fetlock joint (Crawford and Leach 1984). Research in Thoroughbreds reported that when moving around a turn the strain on the outside limb was consistently more than that on the inside limb (Davies 1996). The magnitude of the force and the difference in forces between the forelimbs is accentuated when speed is increased (Davies 1996) and the radius of corner is reduced (Fredricson, Dalin et al. 1975b). Increasing the banking of semicircular curves and introducing transitional curves at one Scandinavian Standardbred racetrack resulted in a marked reduction in gait asymmetry and heat in the fetlock joint (Fredricson, Dalin et al. 1975a; Fredricson, Dalin et al. 1975b). These results suggest that the strain on the limbs whilst negotiating the corners had been reduced and it was hypothesised that this would reduce injury rates. Subsequent surveys of trainers at the reconstructed racetrack found that the majority perceived that there had been a reduction in injuries (Fredricson, Dalin et al. 1976). However, there was no quantitative analysis of injury data pre and post track reconstruction. Analysis of injury data from a Standardbred racetrack in Sydney, Australia, pre and post reconstruction found a significant reduction in injury rates (Evans and Walsh 1997). The banking was increased from 4.8 to 5.7 degrees. Whilst the degree of banking was not optimal for an 800-metre track there was a significant reduction in total injury rate from 8.5 to 6.6 per 1000 starts. These studies are not specific for shin soreness but they do suggest that track geometry may be a contributing factor. Increasing the radius of corners and the degree of banking and placing inclines in straight stretches may be useful in reducing low grade lameness such as shin soreness. During the early stages of training it maybe advisable to reduce the speed with which a horse gallops around corners. 1.6.4 Diet Dietary deficiencies, especially in calcium, may influence bone density and remodelling. In young Quarter horses entering training those feed high levels of calcium (34.9 grams/day) and phosphorous (26.4 grams/day) have a higher bone density in the third metacarpal bone than those feed 28.3 grams/day of calcium and 21.9 g/day of phosphorous (Nielsen, Potter et al. 1997). It is not known if

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increasing the level of calcium and phosphorous will reduce the risk of shin soreness in Thoroughbred and Quarter horses. 1.6.5 Rider Increased load carried by a galloping horse results in an increase in the ground reaction forces. This increase in ground reaction forces could increase the load placed on the bone during exercise. Therefore it is possible that the use of heavier riders could increase the incidence of injuries. The skill of the rider may also affect the forces placed on the shin bone. Studies have shown that when horses are trotting the rider is able to redistribute weight from the front limbs to the hindlimbs. However, the overall differences where small (Schamhardt, Merkens et al. 1991). Training without a rider, for example, on a treadmill, may help reduce lameness rates. 1.6.6 Size of stable and level of veterinary and farrier involvement A study examining all types of lameness in a number of different equine operations found that lameness decreased in larger operations and increased in stables with higher levels of veterinary and farrier involvement (Ross and Kaneene 1996). These results could reflect differences in rates of injury detection, rather than rates of lameness. The effect of these factors on the incidence of shin soreness is not clear. 1.6.7 Rearing in first 12 months A study comparing pasture-raised foals to those raised in boxes with and without additional exercise found that pasture-raised foals appeared to have a “stronger” musculoskeletal system (van Weeren and Barneveld 1999). It is not possible to draw conclusions regarding the consequences for later performance, as horses in this study were not followed beyond 11 months of age.

1.7 Conclusion The causes of shin soreness are likely to involve a number of different factors (Caine, Caine et al. 1996; Brunker, Bennell et al. 1999). Some risk factors will affect the way in which the skeletal tissue is loaded or its ability to respond to the loading. When examining individual risk factors it is important to consider not only their independent effects but also how they interact with other risk factors. At present, there is very little information relating to the inter-relationship and relative importance of the training-, track- and horse-related risk factors. The aim of this study was to examine the training-, track- and horse related risk factors for musculoskeletal injuries in young Thoroughbred racehorses. The research focused on the relationships between exposure to speeds in excess of 800 meters per minute and the onset of shin soreness.

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2 Materials and Method 2.1 Study design and implementation The risk factors for injuries were investigated using a longitudinal study design. Data was collected from trainers in NSW, Australia over two consecutive racing seasons. In Australia the racing season begins on the 1st August. 2.1.1 Recruiting and enrolling trainers Consultations were held with racetrack veterinarians and racing officials. The aim of these meetings was to identify trainers with metropolitan and provincial licenses that may have 10 or more two-year-old racehorses entering the stable in the 2000/01 racing season. The perceived willingness of these trainers to participate in the study was also discussed. The trainers that were identified during these meetings were then approached with the assistance of their regular veterinarian. The initial meeting took place during the veterinarian’s routine visit and involved the veterinarian, trainer or his nominated representative and the investigator. At least one follow-up visit was made to discuss the details of the data collection process and to invite the trainer to participate in the study. The date of commencement of data collection varied between stables. Data collection continued until the end of the second racing season, or until the trainer resigned or was removed from the study. 2.1.2 Enrolling the horse population Three cohorts of horses were enrolled in the study. The eligibility requirements for each cohort are described in Table 2.1. Eligible horses were enrolled at the time of their initial entry into the stable. Participating trainers enrolled all eligible horses in cohort A. Not all trainers participating in the study enrolled horses in cohort B. Furthermore, the trainers that enrolled horses in cohort B did not enroll all eligible horses. Similarly, only some trainers entered horses in Cohort C, however, these trainers enrolled all eligible horses. The enrolment of horses in cohort A, B and C is illustrated in Figure 2.1.

Table 2.1: Eligibility requirements for three cohorts enrolled in a 27-month longitudinal study.

Cohort Requirement A All horses born in the 1998 foaling seasona that entered the study as two-year-old

racehorses B All horses born in the 1998 foaling seasona that entered the study as three-year-old

racehorse

C All horses born in the 1999 foaling seasona that entered the study as two-year-old racehorses

a The foaling season begins on the 1st August.

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Figure 2.1: Study design for a 27-month longitudinal study investigating risk factors for musculoskeletal injuries in two- and three-year-old Thoroughbred horses.

Start of first racing season

Start of second racing season

Enrolled two-year-old horses (cohort A) trained by 18 trainers at 5 race tracks

Data collection discontinued in 7 trainers and 1 race track

End of second racing season

Enrolled three-year-old horses (cohort B) trained by 11 trainers at 4 race tracks

Enrolled two-year-old horses (cohort C) trained by 9 trainers at 3 racetracks

11

At the time of enrolment, information relating to previous training, previous injuries, gender and either the breeding or racing name of the horse was recorded. The breeding or racing name was used to search the Australian and New Zealand studbooks to determine the exact date of birth and to confirm the horse’s gender. Horses were followed from the time of enrolment until the completion of the study, or until the horse was lost to follow up. A horse was lost to follow up if the trainer resigned from the study or if the horse changed trainers, was sold overseas, sent to stud, retired from racing or died.

2.2 Data collection One investigator (NC) visited all participating stables at approximately 14-day intervals to collect training and injury data. The data was collected using a standardized form. This form was maintained by stable staff on a daily basis or completed at the time of the visit. The training data included the following information:

• Training activity undertaken on each day • Distance worked at “evens” or “three quarter pace”, that is between 800 meters per minute

and 890 meters per minute • Distance worked “home on the bit”, that is faster than 890 meters per minute.

During the visit the trainer, or nominated person, was interviewed to confirm the injury status of all horses that were in the stable at the time of the last visit. If the horse had a problem involving the musculoskeletal system the anatomical location of the problem and the affected leg(s) were recorded. If possible additional information relating to the specific nature of the problem was collected. The trainer or nominated person also informed the investigator if any horses either previously enrolled in the study, or eligible for enrolment in the study, had entered the stable between visits. For each horse the official racing and barrier trial records for the 2000/01 and 2001/02 racing seasons were downloaded from a commercial database. The racing, training and injury data was then entered into a customised relational database designed using Access 2000 ™.

2.3 Coding training data A training day was defined as a day that the horse was enrolled in the study and in the stable. Each training day was categorised depending on the activity undertaken, as described in Table 2.2. The training days were then grouped into units referred to as preparations. A preparation was defined as a period of time in which the horse did not leave the stable for more than seven days (Figure 2.2). The preparations were categorised as slow or fast preparations, depending on the activity undertaken, as depicted in Figure 2.3.

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Table 2.2: Categories of training days in a 27-month longitudinal study of two- and three-year-old Thoroughbred racehorses

Categories Definition

Rest day A training day in which the only activity was walking and/or swimming or the horse was box rested

Slow day A training day in which the maximum speed was less than 650 meters per minute.

Medium-speed fast day A fast day in which the maximum speed was between 800 and 890 meters per minute, that is the horse did not gallop faster than “evens” or “three-quarter pace”.

High-speed fast day A training day in which the maximal speed was greater than 890 meters per minute, that is, the horse galloped “home on the bit”

13

Figure 2.2: Definition of preparations. A preparation was defined as a period of time in which the horse did not leave the stable (“spell”) for more than seven days

Enter stable Enter stable

Exit stable

Exit stable

Enter stable Enter stable

Exit stable

Exit stable

Preparation 2

Spell <7 days

Spell >7 days

Legend

Preparation 1

14

Figure 2.3: Schematic representation of slow and fast preparations

Slow preparation

Fast preparation

Slow interval

Fast Interval

Legend

End

First fast day

Start

Start End

15

2.4 Coding of injury data A musculoskeletal injury was defined as any problem involving the musculoskeletal system that resulted in the horse being removed from the stable for a period of more than seven days. Injuries were recorded as shin soreness, fetlock problem, knee problem or tendon/ligament problem. Table 2.2 provides more details of the injury categories. Problems involving the feet were not recorded.

Table 2.3: Categories used to describe the anatomical location and nature of problems involving the musculoskeletal system that resulted in the horse leaving the stable for a period of more than seven days during a 27-month longitudinal study.

Location Nature of injury Shin Shin sore

Complete fracture Fetlock Swelling and/or heat

Degenerative changes Fracture or chip Sesamoiditis Unidentified or unknown

Knee Swelling and/or heat Degenerative changes Fracture or chip Splint Unidentified or unknown

Soft tissue Tendon injury Ligament injury

Other a Complete fracture in an area other than the shin or fetlock and carpal joints Stress fracture in an area other than the shin or fetlock and carpal joints Unidentified/not localised problem involving the musculoskeletal system Back problem Muscle problem other than tying up Musculoskeletal problem other than a complete or stress fracture involving the hind limb Swelling and/or heat in a joint other than the fetlock or knee

a Foot problems were not record as a musculoskeletal injury

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2.5 Data analysis 2.5.1 Frequency of musculoskeletal injuries The frequencies of musculoskeletal injuries were calculated using data from horses that were enrolled in the study for more than 14 days. The frequencies described the percentage of horses that sustained an injury, the percentage of affected horses that sustained a second injury, and the rate of injury. The rate of injury was calculated by dividing the number of injuries by number of training days. The measures of frequency were determined for all musculoskeletal injuries, and for shin soreness, fetlock and knee problems individually. 2.5.2 Impact of musculoskeletal injuries The average, minimum and maximum number of days spent resting at pasture with a musculoskeletal injury, including shin soreness, were determined for all horses and separately for each cohort. These values were also calculated for shin soreness. Official racing and barrier trial results were examined to determine if the horse raced or barrier trailled in the three-, six-, or 12 months following injury. 2.5.3 Risk factor analysis An analysis was conducted to determine the combined effects of training-, track and horse-related variables considered to be risk factors for shin soreness. The training variables that were considered as risk factors are described in Tables 2.4 and 2.5. Equation 2.1 was used to determine the average distance trained at different speeds and Equation 2.2 was used to determine the average number of different types of training days per week in each section of the preparation.

17

7TNd

×=disA

Adis is the average distance per week

Nd Total distance trained at a nominated speed during the fast interval of a preparation.

T Total number of days in the fast interval

Equation 2.1 : Equation used to determine the average distance trained at speeds between 800 and 890 meters per minute, in excess of 890 meters per minute and during races and barrier trials in the fast interval of the preparation.

7×=TNRate td

Ntd Number of rest days, slow days, medium-speed fast days or high-speed fast days in the interval or the section

in the fast interval.

T Total number of days in the slow or fast interval

Equation 2.2 : Equation used to determine the rate of rest days, slow days in the slow interval and the rate of rest days, slow days, medium-speed fast days, high-speed fast days in the fast interval and the sections in the fast interval

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Table 2.4: Variables describing exposure to training during slow and fast intervals of a preparation.

Variable group Variable

Duration Number of days in the slow intervala

Number of days in the fast intervalb

Average number of rest daysc in the slow intervala

Average number of slow daysd in the slow intervala

Average number of rest days in the fast intervalb

Average number of slow days in the fast intervalb

Average number of training days medium-speed fast dayse in the fast

intervalb

Average number of high-speed fast daysf in the fast intervalb

Average number of

training days

(days per week)

Average number of barrier trials and races in the fast intervalb

Average distance trained at speeds between 800 and 890 meters per minute

in the fast intervalb

Average distance trained at speeds greater than 890 meters per minute in the

fast intervalb

Average distance

(m per week)

Average distance raced and barrier trailed in the fast intervalb a Interval from the start of the preparation to the training day in which the maximal speed was greater than 800 meters per minute b Interval from the first day with a maximum speed during training greater than 800 meters per minute until the end of the preparation. c Training day in which the horse was box rested, walked and/or swum only d Training day with a maximum speed not greater than 660 meters per minute. e Training day with a maximum speed between 800 and 890 meters per minute. f Training day with a maximum speed greater than 890 meters per minute.

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Table 2.5: Variables used to describing exposure to training.

Variable group Variable Total distance (m) Total distance trained at speeds between 800 and 890 meters

per minute Total distance trained at speeds greater than 890 meters per

minute Total distance raced or barrier trialled Average distance (m per week)

Average distance trained at speeds between 800 and 890 meters per minute in the fast intervala

Average distance trained at speeds greater than 890 meters per minute during the fast intervala

Average distance of races and barrier trails during the fast intervala

Spell Number of days from the end of the preceding preparation to the start of the current preparation

aInterval from the first training day with a maximum speed greater than 800 m/minute until the end of the preparation

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3 Results 3.1 Study population Tracks and trainers Data were collected from trainers located at three metropolitan racetracks and two provincial racetracks. All racetracks had a grass track available for training. However, use of the track was dependent on a number of factors including condition of the track and day of the week. There were other training tracks available and the surface varied between tracks as shown in Table 3.1. The availability of swimming facilities at each of the five racetracks is also described in Table 3.1. Eighteen trainers were enrolled in the study between April and October 2000 (Table 3.2).

Table 3.1: Training surfaces and availability of additional swimming facilities at three metropolitan and two provincial racetracks.

Track type Track Training surfaces Swimming facilities Provincial 1 Grass and sand River

Provincial 2a Grass and cinders Pool and beach

Metropolitan 3 Grass, dirt and sand Pool and beach

Metropolitan 4 Grass, dirt, sand and cinders Dam

Metropolitan 5 Grass and sand Pool

a Due to logistical constraints data collection was discontinued at this track in June 2001.

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Table 3.2: Dates of commencement of collection of data and the duration of follow-up for 18 trainers in a 27-month study.

Track type Track Trainer

Date data collection commenced

Duration of follow-up (days)

Provincial 1 1 10/07/2000 431 Provincial 1 2 24/07/2000 188 Provincial 1 3 26/07/2000 732 Provincial 2 4 11/10/2000 277 Provincial 2 5 28/10/2000 257 Provincial 2 6 26/07/2000 354 Metropolitan 3 7 23/05/2000 372 Metropolitan 3 8 30/04/2000 826 Metropolitan 3 9 7/08/2000 728 Metropolitan 4 10 27/06/2000 693 Metropolitan 4 11 6/09/2000 798 Metropolitan 4 12 23/05/2000 760 Metropolitan 4 13 27/06/2000 743 Metropolitan 5 14 27/06/2000 760 Metropolitan 5 15 30/06/2000 743 Metropolitan 5 16 14/08/2000 167 Metropolitan 5 17 30/06/2000 737 Metropolitan 5 18 30/06/2000 741

3.1.1 Horses Six hundred and six horses were enrolled in the study; 426 in Cohort A, 37 in Cohort B and 141 in Cohort C (Figure 3.1). The year of birth and therefore the cohort was unknown for two horses enrolled in the study. Table 3.3 describes the number of horses enrolled at each racetrack and by each trainer. At the completion of cohort A’s two-year-old racing season, 215 horses, or 50% of horses had been lost to follow-up (Table 3.4).

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Figure 3.1: Study design for a 27-month longitudinal study investigating risk factor for musculoskeletal injuries in two- and three-year-old Thoroughbred horses.

Start of first racing season

Start of second racing season

Enrolled 426 two-year-old Thoroughbred racehorses, trained by 18 trainers at 5 racetracks (cohort A)

Data collection discontinued in 7 trainers and at 1 race track.

211 horses in cohort A lost to follow-up

End of second racing season

Enrolled 37 three-year-old Thoroughbred racehorses (cohort B) trained by 11 trainers at 4 racetracks

Enrolled 141 two-year-old Thoroughbred racehorses (cohort C) trained by 9 trainers at 3 racetracks

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Table 3.3: Total number of horses and the number of horses enrolled in each cohort during a 27-month longitudinal study investigating risk factors for shin soreness.


Total number of horses

Number of horses in Cohort Aa

Number of horses in cohort Bb

Number of horses in cohort Cc

Provincial 1 1 10 10 0 0 Provincial 1 2 7 7 0 0 Provincial 1 3 21 19 2 0 Provincial 2 4 4 4 0 0 Provincial 2 5 8 8 0 0 Provincial 2 6 17 17 0 0 Metropolitan 3 7 40 40 0 0 Metropolitan 3 8 45 42 3 0 Metropolitan 3 9 88 62 4 22 Metropolitan 4 10 18 7 2 9 Metropolitan 4 11 44 20 1 23 Metropolitan 4 12 29 14 5 9 Metropolitan 4 13 18 12 1 5 Metropolitan 5 14 42 24 3 15 Metropolitan 5 15 110 69 9 31 Metropolitan 5 16 53 53 0 0 Metropolitan 5 17 22 9 2 11 Metropolitan 5 18 30 9 5 16

Combined total 606 426 37 141

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Table 3. 4: Number of two-year old Thoroughbred racehorses that entered the study during the 2000/01 racing season (Cohort A) and the percentage that were lost to follow-up at the end of the two-year-old racing season.


Number of horses enrolled at the end of the two-year-old

racing season

% of horses lost to follow-up at the end of the two-year-old

racing season Provincial 1 1 0 60.0 Provincial 1 2 0 100.0 Provincial 1 3 12 36.8 Provincial 2 4 0 100.0 Provincial 2 5 0 100.0 Provincial 2 6 0 100.0 Metropolitan 3 7 0 100.0 Metropolitan 3 8 34 19.0 Metropolitan 3 9 41 33.9 Metropolitan 4 10 7 0.0 Metropolitan 4 11 85 15.0 Metropolitan 4 12 6 57.1 Metropolitan 4 13 10 16.7 Metropolitan 5 14 17 29.2 Metropolitan 5 15 55 20.2 Metropolitan 5 16 0 100.0 Metropolitan 5 17 7 22.2 Metropolitan 5 18 9 0.0

Combined total 215 49.5

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3.2 Frequency of musculoskeletal injures The data from 486 horses enrolled in the study for more than 14 days were analysed. During the study period there were 442 injuries in 263 (54.1%) horses. The location or type of musculoskeletal injury is described in Table 3.4. Shin soreness was the most common injury, with 187 cases reported in 140 horses, or approximately 30% of the study population (Table 3.5). In two-year-old racehorses in cohorts A and B the percentage of affected horses was 36 and 24% respectively. This is substantially higher than for three-year-olds in cohorts A (7%) and C (12%). A similar pattern was observed when comparing the rate of injury during the study period (Table 3.6). During the study period shin soreness recurred in 23% of all horses, although no horses in cohort B suffered from a second episode of shin soreness (Table 3.7). Tables 3.4 to 3.7 also describe the frequencies of all musculoskeletal injuries, fetlock problems and knee problems.

Table 3.5: The location or type of 442 musculoskeletal injuries in 263 two- and three-year-old Thoroughbred racehorses.

Cohort Age group Fetlock joint Knee joint Other Shin Tendon or ligament

Aa 2 48 32 22 116 19

3 21 21 28 26 26

2 and 3 69 53 44 144 45

Bb 3 1 2 2 4 3

Cc 2 12 13 10 39 1

Combined total 82 68 56 187 49 a horses born in the 1998 foaling season that entered the study during their two-year-old racing season. b horses born in the 1998 foaling season that entered the study during their three-year-old racing season. c horses born in the 1999 foaling season that entered the study during their two-year-old racing season.

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Table 3.6: The percentage of 486 two- and three-year-old Thoroughbred racehorses that sustained a musculoskeletal injury (MSI) or an injury classified as a fetlock or knee problem, or shin soreness.

Cohort Age group All MSI Fetlock problem Knee problem Shin soreness

Aa 2 60.2 16.4 12.3 35.8

3 13.3 6.0 6.0 7.3

2 and 3 51.2 12.3 8.3 30.9

Bb 3 33.3 3.0 6.1 12.1

Cc 2 44.9 9.4 7.9 24.4

Combined total 54.1 13.6 10.7 31.1 a b c see Table 3.5 for legend

Table 3.7: The rate of injury (injuries per 100 horse days) for all musculoskeletal injuries (MSI) and injuries classified as fetlock problems, carpal problems, other, shin problems and soft tissue injuries in 486 two- and three-year-old Thoroughbred racehorses.

Cohort Age group All MSI Fetlock problem Knee problem Shin problem

A a 2 0.65 0.13 0.09 0.32

3 0.38 0.07 0.07 0.09

2 and 3 combined 0.53 0.10 0.08 0.22

B b 3 0.31 0.03 0.05 0.10

Cc 2 0.64 0.10 0.11 0.33


Table 3.8: Percentage of 263 injured two- and three-year-old Thoroughbred racehorses that horses that sustained a second musculoskeletal injury (MSI) or a developed a fetlock or knee problem or shin soreness for a second time.

Cohort All MSI Fetlock problem Knee problem Shin soreness

A a 50.8 24.5 30.0 23.9

B b 9.1 0.0 0.0 0.0

Cc 28.1 0.0 30.0 25.8


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3.3 Impact of shin soreness on training and racing A musculoskeletal injury of some kind, including shin soreness, was associated with nearly 30% of all days spent resting at pasture. Shin soreness alone accounted for 16% of all days spent resting at pasture (Table 3.8). In cohorts A and B just over 45% of the time spent resting at pasture due to an injury was attributable to shin soreness. In contrast, for horses in cohort C only 20% of time spent resting at pasture was associated with an episode of shin soreness. Each case of shin soreness was associated with an average of 12 weeks resting at pasture, and this duration did not vary considerably between cohorts.

Examination of official racing and barrier trial records showed that 24% of the horses that suffered from shin soreness raced or barrier trailed in the three months after an injury (Table 3.9). Twelve months after an episode of shin soreness 78% of horses had raced or barrier trailed and 64% had raced. For the purposes of comparison Table 3.9 describes the impact of shin soreness and all musculoskeletal injuries on completion of a race or barrier trial within 3, 6 and 12 months of the injury.

Table 3.9: The percentage days spent resting at pasture that were associated with a musculoskeletal injury (MSI) or shin soreness and the average (standard error of the mean), median and range of weeks spent resting at pasture following a MSI or shin soreness.

Percentage of time resting at pasture

Average number of weeks spent resting at pasture (s.e.)

Weeks spent resting at pasture (Range)

Cohort All MSI Shin soreness All MSI Shin soreness All MSI Shin soreness

Aa 32.4 15.6 13.0 (0.5) 12.4 (0.7) 1.6-70.7 1.6-70.7

B b 41.6 8.8 13.6 (2.4) 10.0 (1.1) 8.1-20.1 8.9-11.1

C c 41.7 19.5 11.6 (1.0) 10.0 (0.9) 3.9-33.9 5.1-21.6

Combined total 33.7 15.9 13.0 (0.5) 11.9 (0.6) 1.6-70.7 1.6-70.7 a b c see Table 3.5 for legend

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Table 3.10: The percentage of two- and three-year-old Thoroughbred racehorses that raced or completed a barrier trial in the three, six and twelve months following a musculoskeletal injury (MSI) of any kind and shin soreness

Percentage of horses that raced or barrier trailed after injury

Activity Type of injury 3 months 6 months 12 months Race or barrier trial All MSI 16.0 39.3 59.9 Shin 21.4 59.1 77.6 Race All MSI 6.6 24.3 47.4 Shin 7.5 37.0 63.8 Barrier trial All MSI 13.2 36.7 58.2 Shin 17.3 55.8 75.0

3.4 Risk factor analysis Table 3.10 describes the average age of the horse at the commencement of preparation. It also lists the average weekly distance trained at speeds between 800 and 890 meters per minute and greater than 890 meters per minute in preparations that ended in shin soreness and in those that did not end in a MSI. After controlling for the effects of other variables, the risk of shin soreness increased as the average distance trained at speeds in excess of 890 m/minutes, or “home on the bit”, increased. In preparations of less than 10 weeks duration, horses that did not develop shin soreness galloped, on average, approximately 200 meters per week at speeds greater than 890 m/minute. In contrast, horses that developed shin soreness in preparations of the same duration galloped, on average, almost 400 m per week at speeds in excess of 890 m/minute. Interestingly, the association between risk of shin soreness and distance of gallops greater than 890 m/minute differed between trainers, calendar year and racetracks. The analysis found no association between shin soreness and the average weekly distance trained at speeds between 800 and 890 m/minute, or “evens”. Table 3.11 lists the training variables that were not associated with risk of shin soreness. The risk of shin soreness was influenced by the average distance trained at speeds in excess of 890 meters per minute in the previous preparation. Horses that had galloped more than 300 meters per week at speeds greater than 890 meters per week in the previous preparation were at less risk of injury than those that had lower distances of such gallops. None of the other variables used to describe training in the previous preparation were found to be associated with shin soreness (Table 3.12). Horses that were older at the commencement of the preparation had lower risk of shin soreness. On average, horses that suffered from shin soreness were 28 months old at the start of the preparation. The horses that did not suffer from any injury were 30 months old at the start of the preparation. The analysis also showed that horses that suffered from a musculoskeletal injured in the previous preparation were at an increased risk of shin soreness in their next preparation. Interestingly the risk of shin soreness was not increased, or decreased, if the horse suffered from an episode of shin soreness in the previous preparation. Table 3.12 lists the other horse-related variables that were not associated with shin soreness.

30

Table 3.11: The average and ranges of age at the start of the preparation, and variables related to average distances trained at speeds greater than 890 m per minute and between 800 and 890 m per minute in preparations that did and did not not end with any injury, and ended in shin soreness.

Variable Preparations that did not end with any injury

Preparations that ended with shin soreness

Age at the start of the preparation (months) 30 (20-44) 28 (21-45) Average distance per week trained at speeds greater than 890 meters per minute (meters) Preparation duration:

Less than seven weeks Eight to nine weeks Ten to twelve weeks More than twelve weeks

209 (0-933) 248 (0-618) 368 (0-1213) 487 (142-1029)

422 (0-1303) 351 (0-811) 430 (0-888) 407 (106-819)

Average distance per week trained at speeds between 800 and 890 meters per minute (meters) Preparation duration:

Less than seven weeks Eight and nine weeks Ten to twelve weeks More than twelve weeks

866 (0-2477) 1157 (0-2888) 1045 (0-2030) 1015 (313-1526)

1106 (0-4200) 1112 (210-2130) 886 (343-1470) 974 (276-2161)

Table 3.12: Training-related variables for the preparation that were not associated with either an increased or decreased risk of shin soreness.

Variable group Variable Duration Number of days in the slow intervala

Number of days in the fast intervalb Average number of rest daysc in the slow intervala Average number of slow daysd in the slow intervala Average number of rest days in the fast intervalb Average number of slow days in the fast intervalb Average number of training days medium-speed fast dayse in the fast

intervalb Average number of high-speed fast daysf in the fast intervalb

Average number of training days (days per week)

Average number of barrier trials and races in the fast intervalb Average distance (meters per week)

Average distance trained at speeds between 800 and 890m/minute in the fast intervalb

Average distance raced and barrier trailed in the fast intervalb a Interval from the start of the preparation to the training day were maximal speed >800 m/minute. b Interval from the first day were the maximum speed during training was >800 m/minute and the end of the preparation. c Training day were the horse was box rested, walked and/or swum only d Training day were the maximum speed did not exceed 660 m/minute. e Training day were the maximum speed was between 800 and 890 m/minute. f Training day were the maximum speed was >890 m/minute.

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Table 3.13: Training variables that describing exposure training in the previous preparation that were not associated with either an increased or decreased risk of shin soreness.

Variable group Variable Total distance (meters) Total distance trained at speeds between 800 and 890

m/minute Total distance trained at speeds >890 m/minute Total distance raced or barrier trailed Average distance (meters per week)

Average distance trained at speeds >890 m/minute during the fast intervala

Average distance raced and barrier trailed during the fast intervala

Spell Number of days from the end of the preceding preparation to the start of the current preparation

a Interval from the first training day where maximum speed was >800 m/minute and the end of the preparation

Table 3.14: Horse-related variables that are were not associated with either and increased or decreased risk of shin soreness.

Variable Sex of the horse

Age at the commencement of the two-year-old racing season Age at the commencement of training Shin soreness in the previous preparation

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4 Discussion This study was undertaken in order to investigate risk factors for shin soreness in Thoroughbred racehorses. Previous studies had identified shin soreness as a major cause of wastage in young Thoroughbred horses in training. However, during the design phase of the study feedback from trainers indicated their preference for widening the scope of the study so as to include investigation of a larger range of causes of lameness. From a practical point of view, these changes did not increase the burden of data collection for either the investigator or individual responsible for maintaining records. Furthermore, Bailey (1998) showed that whilst shin soreness was the most common type of health problem, fetlock and knee problems resulted in more time resting at pasture per case. Also, fetlock problems recurred in a greater percentage of affected horses than shin soreness. Therefore the scope of the study was expanded to better meet industry needs. This report describes the frequency of knee and fetlock injuries but does not include results for risk analysis of these types of lameness. The results of epidemiological studies need to be interpreted with respect to possible bias in sampling procedures. In this study, selection of trainers was based on two criteria. These were the number of horses trained, and the perceived willingness of the trainer to participate. Given the large number of horses that were enrolled in the study it was perceived that enrolment of trainers with 10 or more horses would facilitate efficient data collection. The willingness of the trainers to participate in the study was important, as data collection required a high level of compliance. Reluctance to continue with provision of training and injury records could have impacted on the quality of the data and results.

4.1 Frequency and impact of shin soreness This study confirmed that shin soreness is a common problem, affecting 30% of two-and three-year-old Thoroughbred racehorses. Previous research in Australia reported than 18% of two-year-olds (Mason and Bourke 1973) and 42% of two- and three-year-old Thoroughbreds (Bailey 1998). A description of the percentage of horses affected is termed period prevalence (Cumming, Kelsey et al. 1990). Period prevalence is affected by the duration of study and the losses to follow-up. Therefore, when comparing the frequency of injuries between studies, it is better to use incidence because it measures the number of new cases per unit of time (Reid 1998a). Bailey (1998a) is the only study to use incidence and rate of injury to describe the frequency of shin soreness. There was no difference in the frequency of shin soreness in this study compared with that in the study by Bailey (1998a). To determine the impact of shin soreness on racing outcomes, the percentages of horses that raced or barrier trailed within certain time periods after an injury were determined. This is the first time that such index has been used to assess the affect of a training-related injury on participation in future races. The results showed that in the 12-months following an episode of shin soreness approximately 75% of horses completed an official barrier trial and nearly 70% of horses raced. These figures were based on Australian and New Zealand racing histories and were not adjusted for horses that were sold overseas. If these horses had been excluded, or racing histories obtained, the percentage of horses that raced or completed a barrier trial may have been higher.

4.2 Recommendations to industry This is the first study in Australia to investigate the relationship between daily exercise training and shin soreness. When examining risk factors for shin soreness the analysis was conducted at the level of the preparation. The preparation was selected because discussion with industry showed that training is viewed in terms of preparations. Furthermore, horses often change trainers and/or owners so information relating to exposure to training in previous preparations is not always available. Therefore, the analysis was conducted at the level of the preparation because it was believed that it would be easier for industry to adopt recommendations.

33

4.2.1 Training The study found that increasing the average weekly distance trained at speeds greater than 890 meters per minute increases the risk of shin soreness. The results suggest that during the first nine weeks of a preparation the average distance worked at speeds greater than 890 meters per minute should be limited to 200 meters per week. After week nine is should be possible for horses to complete 400 meters per week of speeds greater than 890 meter per minute. The distance trained at “evens”, or speeds between 800 and 890 meters per minute was not found to be associated with shin soreness. However, this finding does not mean that horses are able to work unlimited distances at this speed without risk of shin soreness. Rather, that within the range of values examined in this analysis, there was no effect. Consequently, during the first seven weeks of the preparation the fast work may also include 800 meters per week of speeds between 800 and 890 meters per minute. From week seven the average weekly distance trained at speeds between 800 and 890 meters per minute can be safely increased to 1000 meters per week. These findings may not be suitable for every horse, and trainers should take into account individual horse or track factors that could also affect the likelihood of shin soreness. The duration of the slow interval of the preparation was not associated with shin soreness. This finding may reflect lack of wide variation in the duration of the slow interval in the horses in this study. Future research could investigate whether or not greater or reduced durations of slow portions of preparations could be protective. Consequently, more research is required before recommending the early introduction of fast gallops. 4.2.2 Previous exposure to training The number of previous preparations that a horse had experienced did not alter the risk of shin soreness. However, horses that had trained at speeds of greater than 890 m/minute in the previous preparation were at less risk of shin soreness in their next preparation. This suggests that there may be benefits to horses having a short preparation that included gallops at speeds greater than 890 meters per second over a distance of not more than 200 m per week. It is likely that such preparations promote adaptation of bone, increasing its ability to cope with the demands of exercise tat those speeds. The aim of these preparations is to encourage the bone to adapt to high speed exercise. 4.2.3 Previous injury It is a widely held view in the industry that allowing horses to develop shin soreness will protect the horse against another episode of shin soreness in a subsequent preparation. An important finding in this study is that horses that developed shin soreness did not have reduced risk of shin soreness in their subsequent preparation. The industry view that there is a protective effect of shin soreness probably reflects the protective effects of exposure to gallop speeds greater than 890 m/minute in the previous preparation. It is the high speed gallops that is protective, not the occurrence of shin soreness. Therefore, training in a manner that encourages a horse to develop shin soreness should not be undertaken because it has no beneficial protective effect. Furthermore, training in manner that encourages shin soreness may increase the risk of other injuries including fatal fractures (Estberg, Gardner et al. 1995; Estberg, Stover et al. 1996a; Estberg, Stover et al. 1996b; Estberg, Gardner et al. 1997; Estberg, Gardner et al. 1998). Training methods that promote the onset of shin soreness are also obviously not in the interests of the welfare of the horse. 4.2.4 Age Horses with greater age at the start of a preparation were at less risk of shin soreness and other injuries. However, the differences in age were not large in the two groups. On average, horses that developed shin soreness were 28 months of age at the start of the preparation whilst those that did not were 30 months of age. This suggests that the owners and trainers should delay the commencement of training for horses born later in the foaling season. Dealy for as little as two months could reduce the likelihood that a horse will develop shin soreness.

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4.2.5 Sex There was no association between sex and the shin soreness. The results of previous research is contradictory, with some studies reporting no association between injury and sex (Mohammed, Hill et al. 1991; Bailey, Reid et al. 1997; Bailey, Reid et al. 1998) and others reporting that males are more likely to suffer from a fatal fracture (Carrier, Estberg et al. 1998; Estberg, Gardner et al. 1998; Estberg, Stover et al. 1998; Hernandez, Hawkins et al. 2001). However, the previous studies did not examine daily training thus the difference could be because the male and female horses were trained differently. 4.2.6 Racetrack There was no difference in the proportion of horses that suffered from shin soreness at each of the five racetracks. However, the effect of average weekly distance trained at speeds in excess of 890 meters per minute differed between tracks. This suggests that there are factors associated with tracks, such as hardness, that could be risk factors for shin soreness. Future studies may consider investigating the relationship between track-related factors, training and training injuries such as shin soreness.

4.3 Further analysis The association between shin soreness and average weekly distance galloped at speeds greater than 890 m per minute differed between trainers. This suggests that there are other factors relating to the training that were not accounted for that may be risk factors for shin soreness. Such factors could include that rate that average distance worked at speeds greater than 890 m per minute was increased during the preparation. Further analyses of the data will be conducted to investigate the association between injuries and the rate of increase in average distances trained per week. Further analysis of the data collected during this study will also be conducted with the aim of identifying risk factors for fetlock problems, knee problems, and tendon and ligament injures. The results will be published in scientific papers and industry magazines.

4.4 Adoption strategies Full analysis of the data collected during this study is likely to result in number of training and management recommendations. To increase the likelihood that these recommendations are adopted it will be necessary to ensure that the results are discussed in a number of different forums including racing industry magazines, scientific journals, newspapers and radio. This will ensure that the results of this study are conveyed to owners, trainers, veterinarians, racing administrators and the general public. To assist trainers in implementing any training-related recommendations consideration should be given to the development of software that would identify when horses are entering high-risk training periods.

4.5 Future research Future studies should investigate risk factors for injuries in horses of all ages. Possible areas of study include case-control investigations of risk factors for tendon injuries, stress fractures and complete fractures. There would also considerable merit in a study that describes the frequency of musculoskeletal injuries in horses four or more year of age. The results suggest that factors associated with racetracks play a role in the onset of shin soreness. There is limited information describing the relationship between training track geometry, track surface, track condition and training injuries. The first stage in an investigation of the relationship between track-related factors and training injuries should be to describe how track condition changes over time, and how different track surfaces compare in hardness. This next stage would describe the association between these factors and training injuries whilst control for aspects of daily training. The

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information from such studies would assist racetrack managers in the preparation of training tracks and allow trainers to make informed choices regarding track usage.

4.6 Conclusion Shin soreness is a common cause of lameness in young Thoroughbred racehorses. Age of the horse at the commencement of training and training methods are both important factors that influence the likelihood of the disease. The impact of these factors also depends on racetrack factors, but further studies are needed to identify those factors. The common industry practice of training in manner that induces shin soreness should be discouraged because occurrence of shin soreness does not reduce the risk of the disease in a subsequent preparation. However, appropriate use of gallops at speeds greater than 890 metres per minute can reduce the risk of shin soreness, both in current and subsequent preparations. If trainers wish to reduce the number of cases of shin soreness in horses in their care, it is recommended that records of weekly distances of gallops completed at speeds greater than 890 metres per minute should be used. The distances of such gallops at each stage of the preparation can be compared with the results in this study to assist with reduction of the risk of shin soreness. Introduction of high weekly distances of gallops at speeds greater than 890 metres per minute early in a preparation will increase the risk of shin soreness. Commencement of training of young Thoroughbred racehorses should be delayed until the horse is 30 months of age.

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US-84A Final REPORT · Shin soreness in Thoroughbred racehorses A report for the Rural Industries...

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