EXERTIONAL rhabdomyolysis or exercise-induced myopa- thy is the most common muscle disorder that affects athletic horses (Freestone and Carlson 1991). Its clinical signs can vary from stiffness of gait and pain on palpation of muscles

EXERTIONAL rhabdomyolysis or exercise-induced myopathy is the most common muscle disorder that affects athletic horses (Freestone and Carlson 1991). Its clinical signs can vary from stiffness of gait and pain on palpation of muscles after exercise in mild cases, to myoglobinuria, recumbency and death in severe cases (Hodgson 1993). The aetiopathogenesis of the condition is complex and the biochemical and pathophysiological changes leading to the clinical signs are poorly understood. Several causes and risk factors have been proposed to explain why an individual horse may suffer an episode or repeated episodes. They include a dietary excess of carbohydrates (Valentine and others 1998, MacLeay and others 1999a, b), a day of rest before exercise or other variations in exercise routine (Frauenfelder and others 1986, MacLeay and others 1999a), vitamin E deficiency (Roneus and Hakkarainen 1985, Petersson and others 1991,) being young (Frauenfelder and others 1986, Harris 1991), stress (MacLeay and others 1999a, c), being female (Harris 1991, MacLeay and others 1999a), hypothyroidism (WaldronMease 1979), electrolyte imbalance (Harris and Colles 1988, Harris and Snow 1991), defects in metabolism (Valberg and others 1993a, 1997, Valentine and others 1997), and dysfunction in muscle contraction and calcium regulation (Beech and Lindborg 1993, Hodgson 1993, Lentz and others 1999a, b, Ward and others 2000). As a result of the failure to find a unifying cause or satisfactory treatment for exertional rhabdomyolysis, it has been proposed that the condition may be due to several different underlying muscle diseases that induce similar clinical signs (Lentz and others 1999a, Valberg and others 1999). Particular myopathies in horses that show clinical signs consistent with exertional rhabdomyolysis have recently been identified in certain breeds of horses, including a polysaccharide storage myopathy in quarter horses (Valberg and others 1996, 1997, 1999) and some draught breeds (Valentine and others 1997), and a NADH reductase deficiency in an Arabian horse (Valberg and others 1997). In thoroughbreds, the clinical signs have been suggested to be due to a heritable defect in calcium regulation (Lentz and others 1999b, MacLeay and others 1999a), and other studies have demonstrated a familial basis for the disease in quarter horses (Valberg and others 1996) and standardbreds (Collinder and others 1997). In view of the complicated, multifactorial nature of the condition, it is unlikely that the cause(s) will be identified solely by laboratory-based research involving a small number of affected horses. Multifactorial diseases require a population-based approach if the risk factors are to be successfully identified (Reeves 1997). There have been quantitative studies of the condition in the UK and the USA (Harris 1991, MacLeay and others 1999a, McGowan and others 2002a, b). In the UK, a retrospective analysis of laboratory data found that female horses were more likely to be affected than males (Harris 1991). In racehorses in training, the incidence over one racing season has been estimated to be 4·9 per cent in the USA (MacLeay and others 1999a) and 6·7 per cent in the UK (McGowan and others 2002a). These studies also reported that females and horses assessed as being nervous or very nervous were significantly more likely to be affected. In the USA, lameness was also a significant risk factor. The incidence in polo horses in the USA and the UK has been reported to be 7·3 per cent (McGowan and others 2002b). Although they were not specifically designed to investigate the condition, other epidemiological studies have indicated that exertional rhabdomyolysis is a common cause of disability in athletic horses. In a study of wastage in thoroughbred racehorses in the UK, 11·4 per cent of 314 twoto four-year-old thoroughbreds had had one or more episodes of the condition (Jeffcott and others 1982), whereas less than 1 per cent of pleasure horses in Scotland were estimated to have been affected (Mellor and others 2001). Whether this lower prevalence was due to usage, breed or geographical differences is unclear. The prevalence in a population of premier thoroughbreds in Sydney was estimated to be 6 per cent (Bailey and others 1999). There is no other population-based information about the prevalence of exertional rhabdomyolysis among racehorses in Australia, and its prevalence in all other breed and activity groups of horses is unknown. As a result of this absence of information, compounded by the multifactorial nature and physiological complexity of the condition, it continues to affect horses repeatedly (Bailey and others 1999) despite many methods having been used in attempts to treat it. In order to reduce the occurrence and recurrence of exertional rhabdomyolysis in horses in Australia, detailed population-based data are required. This paper describes a questionnaire-based investigation designed to estimate the prevalence of exertional rhabdomyolysis among different types of horses in Australia; it is an initial step in a larger investigation of the epidemiology of the condition in Australian horses. In this preliminary study, univariable analyses were applied to identify the demographic characteristics that were associated with the condition, so that these characteristics could be further investigated by the analysis of more comprehensive data obtained later.