Shin soreness in Thoroughbred racehorses
by N. Cogger, D.L. Evans, D.R.Hodgson, N.R.Perkins and S.W.J. Reid

December 2004 RIRDC

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-yearold 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.

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.