Sudden death in racing Thoroughbreds in Victoria, Australia.

Abstract: Racing fatalities are an important welfare concern and a source of economic loss to racing industries. A fatality can be categorised as sudden (a death occurring while racing/training or within minutes of finishing) or as a euthanasia if a horse is destroyed after incurring a catastrophic injury while racing or training. Sudden deaths of racehorses have been attributed to respiratory and cardiovascular disease (Platt 1982; Gelberg et al. 1985; Gunson et al. 1988; Johnson et al. 1994a,b). Respiratory causes of sudden death include exercise-induced pulmonary haemorrhage (EIPH) and pre-existent airway disease (Platt 1982; Gelberg et al. 1985; Johnson et al. 1994a). Cardiovascular causes of sudden death include myocarditis, rupture of chordae tendineae, aorta or other large arteries (Platt 1982; Gelberg et al. 1985; Gunson et al. 1988; Johnson et al. 1994a), aneurysm (Gelberg et al. 1985), atrial dysrhythmia (Platt 1982), valvular lesions (Platt 1982; Johnson et al. 1994a), cardiomyopathy (Johnson et al. 1994a), myocardial necrosis (Johnson et al. 1994a), sclerosing coronary arteropathy (Johnson et al. 1994a) and massive disseminated haemorrhage (Johnson et al. 1994a). Sudden deaths without detectable lesions have also been reported (Vaughan and Mason 1976; Platt 1982; Gelberg et al. 1985; Johnson et al. 1994a,b). Proportions of sudden death while racing or training attributed to particular causes vary substantially (Gelberg et al. 1985; Gunson et al. 1988; Suann 1992; Bourke 1994; Johnson et al. 1994a,b). For example, 17 of 25 horses (68%) died from acute exercise-induced cardiovascular failure in Illinois (Gelberg et al. 1985), whereas EIPH was the cause of sudden death in 9 of 11 cases (82%) in another survey (Gunson et al. 1988). In California, 32 of 64 horses (50%) had no abnormalities that could account for sudden death (Johnson et al. 1994a). In 3 of 15 horses (20%) in the UK, post mortem examination failed to reveal the cause of sudden death (Vaughan and Mason 1976). One possible explanation for this variability may be different criteria for case selection. In 2001, the Victorian racing authority (Racing Victoria Ltd, Australia) introduced mandatory post mortem examinations for all racing and training fatalities on racetracks in Melbourne. This allowed the first study of racetrack fatalities in Australia with negligible potential sampling bias and comprehensive pathological examination of cases. Prior to this, racetrack fatalities were classified on the basis of observations by racetrack veterinarians or stewards without corroboration by necropsy, and it is probable that cases of sudden death prior to 2001 were misclassified with respect to cause in racing records. The aim of this study was to determine the proportion of racetrack fatalities that are sudden deaths, to describe lesions in such deaths and to establish whether sampling from a subset of fatalities can affect the estimated proportional mortality due to sudden death.