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. All fracture events from 1st July 2004 to 30th June 2011 were retrieved from veterinary and racing databases of the HKJC. Only first fracture events were included in this analysis. Fractures were classified as catastrophic if euthanasia occurred within five days of the fracture and non-catastrophic if euthanasia was not the outcome. Horses were considered to be at risk from time of importation into 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 United States (Estberg, Stover et al. 1996b) and higher than in the United Kingdom (Parkin, Clegg et al. 2004a) and Australia (Boden, Anderson et al. 2006). The incidence rate for training fractures indicate that in a population of 1,000 horses in training, there would be approximately three catastrophic fractures and 31 non-catastrophic fractures every 365 training days at risk. 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 mid-diaphysis of the left tenth rib. Each bone block was cut to 250 µm thick specimens using a diamond 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 groups. Resorption canals were seen to be at sites that were predisposed to fatigue fracture. Racehorses that have been rested for one to four months showed significantly greater resorption canals of the tibia and third metacarpal bone compared to exercised horses, and are therefore potentially more susceptible to fracture at predilection sites. This has important ramifications for the management of racehorses returning to high intensity exercise. This thesis presents very detailed information about racehorse fractures at the HKJC. It emphasises the need for more stringent monitoring for fractures in horses in training and the importance of detecting non-catastrophic fractures before they become catastrophic. Highlighted in this thesis is the immediate need to introduce a monitoring program in horses that are returning to high intensity exercise after a period of rest. More research is needed to determine the safest way to reintroduce horses to exercise after a rest period.