Fracture rate in Thoroughbred racehorses is affected by dam age and parity
Introduction
In humans, the Barker hypothesis proposes fetal origins of adult diseases such as diabetes, coronary heart disease and stroke (Barker, 2000). Evidence has accumulated that small and low birth weight human babies are more prone to these common adult diseases and it has been proposed that an adverse intra-uterine environment leads to cardiovascular, endocrine and metabolic adaptations that can permanently change the structure and function of different organ systems. These concepts have recently been extended to studies of intra-uterine, neo- and early postnatal influences on adult peak bone mass and the risk of osteoporosis in later life (Cooper et al., 2000, Cooper et al., 2002). Although results of these studies have been inconsistent, evidence suggests that low birth weight may have adverse effects on adult bone mineral density and bone mineral content; and associations can only partly be explained by adult height and/or weight (Antoniades et al., 2003, Gale et al., 2001). To our knowledge, no studies have been undertaken to explore potential relationships between intra-uterine and neonatal parameters and the risk of adult disease in the horse.
Studies of the association between parity and birth weight in Thoroughbred horses have shown that first foals are lighter than subsequent foals and that those from aged, multiparous mares are lighter than those born from younger mares that have only had a few foals (Bhuvanakumar and Satchidanandam, 1989, Hintz et al., 1979, Wilsher and Allen, 2003). Furthermore, there is evidence of an association between parity, dam age and performance in Thoroughbred racehorses, with first foals and those of older dams tending to be less successful on the racecourse (Barron, 1995, Finocchio and Rosenzweig, 1995, O’Sullivan, 1980).
Extrapolating from human studies, we hypothesised: (a) that first foals would have a higher fracture rate than subsequent foals and (b) that fracture rate in offspring would increase with increasing dam age.
Genetic contributions to adult bone mass and risk of osteoporosis, and hence osteoporotic fracture, have been well documented in humans (Baldock and Eisman, 2004, Ralston, 2005). The genetic basis of exercise-induced injury is less clear, although several reports suggest a genetic contribution to stress fracture predisposition (Burr and Milgrom, 2001). We therefore also explored the variations in fracture rates with sire, dam and maternal grandsire as indirect evidence of a genetic basis for predisposition to skeletal injury.
Section snippets
Study design
Data were drawn from a large prospective cohort study of fractures in Thoroughbred racehorses in training for flat racing, the design of which has been described previously (Verheyen and Wood, 2004). Briefly, horses from 13 trainers throughout England were monitored for up to two years, with daily recording of exercise information (type of exercise and, for all canter and high-speed exercise, distance and surface) and injury occurrence. A case was defined as any animal with a confirmed
Descriptive results
A total of 335 horses joined the study when they started training as yearlings. There were 62 first foals in the dataset and the dams ranged from 5-year-old maiden mares to 22-year-old mares having delivered their 16th foal. Numbers of study horses by dam age and parity are presented in Table 1. Table 2 presents descriptive statistics on numbers of fractures, horse months at risk and fracture rates by dam age and parity.
The study horses were born out of 323 dams, by 155 sires and 209 maternal
Discussion
This report described associations between dam age and parity and the rate of fracture in young Thoroughbred racehorses. Being a first foal had a protective effect on the rate of fracture, which disproved our hypothesis and does not seem to be consistent with human studies that have shown an association between low birth weight and low peak bone mass and osteoporosis (Cooper et al., 2000, Cooper et al., 2002, Dennison et al., 2001, Gale et al., 2001). We did not have data on the birth weight of
Conclusion
Fracture rates were lower in first foals than in subsequent ones and decreased with increasing dam age. These findings were unexpected and further research is required to elucidate the potential mechanisms involved. Better understanding of factors that influence early skeletal development in the horse and how this relates to the risk of injury in later life can inform management practices both at the stud and in training yards that can aid in reducing injury occurrence.
Acknowledgements
Racehorse trainers, their staff and veterinarians are gratefully acknowledged for their prolonged collaboration with the study. The study was funded by the Horserace Betting Levy Board and a scholarship from the Home of Rest for Horses.
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