Fracture incidence rates and the association of rest with bone remodelling in Thoroughbred racehorses at the Hong Kong Jockey Club

Abstract

Fractures in Thoroughbred racehorses are an omnipresent welfare issue. Previous studies have often failed to adequately address the issue of fractures in racing and training because of the limitations associated with record keeping. Racehorses being rested from intensive exercise are also at greater risk of fracture on re-introduction to high intensity work. This thesis aims to accurately report first event fractures and determine racing and training fracture incidence rates in the highly controlled Thoroughbred racehorse population at the Hong Kong Jockey Club (HKJC) and to identify differences in bone remodelling (as defined in this thesis) between racehorses, which have been rested compared to horses in current high intensity exercise. Hong Kong to date of fracture, retirement, or end of the study period, whichever was first. Incidence rates for racing were expressed as catastrophic or non-catastrophic fracture events per 1000 race starts (Chapter Three). Training incidence rates were expressed as catastrophic or non-catastrophic fracture events per 10,000 horse days at risk (Chapter Four). The incidence rate for catastrophic fractures during racing was 0.6 per 1000 race starts (95% CI 0.4 -0.8) and for non-catastrophic fractures it was 2.2 per 1000 race starts (95% CI 1.8 -2.6). The incidence rate for catastrophic fractures in training was 0.08 per 10,000 horse days at risk (95% CI 0.05 -0.11), and for non-catastrophic fractures was 0.85 per 10,000 horse days at risk (95% CI 0.75 -0.96). The incidence rate of catastrophic racing fractures at the HKJC were lower when compared to the results of studies conducted in the UnitedStates (Estberg, Stover et al. 1996b) and higher than in the United Kingdom (Parkin, Clegg et al. In another study, bones were obtained post-mortem from horses in Exercised (n = 6) and Rested (n = 6) Groups who died for reasons unrelated to fracture or the fracture study. Exercised horses had been euthanized within seven days of high intensity exercise while Rested horses had been retired for one to four months and whose work level was much reduced before retirement. Six bone blocks were cut from each horse at the following locations; right third metacarpal bone (MCIII) where samples were collected from the mid-diaphysis, distal lateral metaphysis and medial condyle; right third metatarsal bone (MTIII) where a sample was collected from the lateral condyle; distal left tibia, and middiaphysis of the left tenth rib. Each bone block was cut to 250 µm thick specimens using a diamond 3 annular saw. Microradiographs were obtained using point projection digital microradiography (Faxitron) and analysed to identify radiolucent spaces indicative of recently formed resorption canals as a proxy for active bone remodelling. This study identified that resorption canals of all sections from MCIII and the tibia were significantly greater in the Rested Group compared with the Exercised Group (P < 0.05). There was no significant difference in the resorption canal density of the rib between the two g...

Training practices, speed and distances undertaken by Thoroughbred racehorses in Victoria, Australia

“…Rest period frequency and duration depend on trainer and horse factors, but also on climate, racing seasons and availability of spelling facilities. For example, the Hong Kong racing season includes a mandatory 2-week break with otherwise year-round racing [23]. In previous studies, it is difficult to distinguish between voluntary and involuntary (e.g.…”