EQUINE EXERCISE PHYSIOLOGY 6 Equine vet. J., Suppl. 34 (2002) 375-378 375 Summary The incidence and severity of exercise-induced pulmonary haemorrhage (EIPH) in the 2 most commonly raced horse breeds, Thoroughbreds (TB) and Standardbreds (STD), were studied, with particular interest in the possible influence of frusemide (F) and/or the breed (or running gait) on EIPH. The appearance of blood within the trachea was semi-quantified using a published 5-point system, with zero assigned when no blood was observed, and numbers 1–4 assigned with increasing amounts of blood. Considering each endoscopic examination as a separate event, approximately 75% of the postrace endoscopic examinations had blood-scores of 1, 2, 3, or 4, regardless of breed or F administration. For horses examined twice, the chances of finding blood-scores of 1 or greater in either of the examinations increased to approximately 95%. All horses examined 3 or more times had endoscopic blood-scores of 1 or greater following one or more races, again, irrespective of the breed or F administration. Mean ± s.e. ‘blood scores’ were 1.5 ± 0.1 and 1.8 ± 0.2 for TB, and 1.4 ± 0.2 and 1.2 ± 0.1 for STD racing with and without prerace F, respectively. Therefore, there was no apparent effect of breed (or possibly racing gait) on EIPH, and no differences in the incidence or severity of EIPH were observed between horses with or without prerace frusemide administration. Introduction Studies concerning various aspects of exercise-induced pulmonary haemorrhage (EIPH) abound (Sweeney 1991; Erickson and Lowe 1994; Erickson 2000). Investigations into the pathophysiological basis of EIPH have shown that the blood originates primarily in the dorsal-caudal aspects of the lungs (O’Callaghan et al. 1987a,b; Pascoe 1997) and results from stress failure of pulmonary capillaries (West et al. 1993; Birks et al. 1994, 1997). Research into the mechanisms of the failure of the capillaries has been varied, focusing on possibilities such as vessel strength (Birks et al. 1994), exercise-related pulmonary hypertension (Birks et al. 1991; West et al. 1993; Erickson 2000), left heart function (Hiraga et al. 1999), rheological alterations (Manohar et al. 2000), and hoof-impact forces (Schroter et al. 1998, 1999), among others. Recommendations for treatments to prevent or limit EIPH depends largely upon which of the mechanisms a particular clinician favours; however, the most commonly utilised therapy in the horseracing industry is the diuretic frusemide. Reports on the efficacy of frusemide as a treatment for EIPH are equivocal. Most direct research indicates little or questionable benefit (Pascoe et al. 1985; Soma et al. 1985; Sweeney et al. 1990) while, anecdotally, many veterinary practitioners and trainers indicate the opposite (Bowen 1990). In North America, horses to be treated with frusemide prior to racing must be certified by racing authorities to have had evidence of EIPH, and the fact that frusemide is given to a horse must be disclosed to the wagering public. Because of these stringent requirements and the commonly held opinion that EIPH adversely affects performance, it would be reasonable to assume that the relative proportion of horses being treated with frusemide prior to racing would reflect the proportion of horses with demonstrated EIPH. In the Commonwealth of Pennsylvania for the years 1999 and 2000, the percentage of horses racing with frusemide pre-medication was approximately 90% for Thoroughbreds at Philadelphia Park and approximately 30% for Standardbreds at Pocono Downs (Commonwealth of Pennsylvania Racing Commission records, unpublished data). With these observations in mind, the present study was designed to address 2 questions: (1) Whether the incidence/severity of EIPH in horses administered frusemide prior to racing different from that of horses racing without frusemide; and (2) Whether the incidence and/or severity of EIPH are affected by breed (or racing gait), i.e. do Standardbreds (symmetrical gait: trotting or pacing), and Thoroughbreds (asymmetrical gait: gallop) have the same incidence and/or severity of EIPH? Materials and methods Horses Horses from 2 racetracks located in the Commonwealth of Pennsylvania, USA (Philadelphia Park, Bensalem, and Pocono Downs, Wilkes Barre) participated in this study from April–November 1999. Philadelphia Park features Thoroughbred races, while Pocono Downs races Standardbreds. The Thoroughbreds raced at various distances, on dirt or turf. All Standardbred races were on an identical surface over a distance of approximately 1600 m (one US mile). As the procedures involved were not considered experimental or perceived by trainers/owners to have any potential negative impact on their horses, virtually all horses racing at each of the 2 racetracks were made available for EIPH: postrace endoscopic evaluation of Standardbreds and Thoroughbreds E. K. BIRKS*, K. M. SHULER, L. R. SOMA, B. B. MARTIN, L. MARCONATO † , F. DEL PIERO † , D. C. TELEIS, D. SCHAR, A. E. HESSINGER and C. E. UBOH ‡ Departments of Clinical Studies and † Pathobiology, New Bolton Center, University of Pennsylvania, 382 W. Street Road, Kennett Square, Pennsylvania 19348 and ‡ Pennsylvania Equine Toxicology and Research Laboratory, Department of Chemistry,West Chester University, West Chester, Pennsylvania 19382, USA. Keywords: horse; performance; pulmonary haemorrhage *Author to whom correspondence should be addressed.