Update on the prevelance of EIPH and its effect on performance in racehorses

Exercise-induced pulmonary haemorrhage (EIPH) is a disease without geographical boundaries as it occurs in equine populations world-wide. Horses with EIPH may be presented for investigation of poor performance, or less commonly, epistaxis. While the development of epistaxis shortly after exercise is almost inevitably secondary to EIPH, many other respiratory and non-respiratory conditions may result in poor performance. Thus, EIPH can only be considered as an explanation for poor performance in an individual animal after other causes have been excluded by appropriate physical and ancillary testing.

Prevalence of EIPH

Accurate determination of prevalence of EIPH is problematic as it is dependent on the methodology by which the condition is diagnosed, frequency of examination and when in relation to exercise horses are examined. Methods of diagnosis of EIPH clinically include:
1. Epistaxis post-exercise; while epistaxis is very specific for EIPH, sensitivity is poor as the majority of horses with EIPH do not develop this sign. Epistaxis has been reported to occur in up to 9% of racehorses in Hong Kong (Mason et al. 1983), however it is rare in UK racehorses with 0.33 cases/1000 starts (Newton et al. 2005)
2. Tracheobronchoscopy; endoscopic examination of the trachea and large bronchi is a reasonably sensitive and very specific technique to detect evidence of EIPH, particular when performed at a time after racing to allow for rostral movement of blood from the distal airways (Pascoe et al. 1981; Hinchcliff et al. 2005). Most horses with EIPH will have blood in the large airways 30-120 minutes after racing, however blood may occasionally be detectable for up to 7 days. Methods of scoring the amount of blood present in trachea have been developed and have been determined to have good inter-observer reliability, however the relationship between the score and actual severity of EIPH has not been determined. If performed immediately after exercise, tracheobronchoscopy may provide false negative results in some horses with EIPH.
3. Examination of bronchoalveolar lavage fluid (BALF); the presence of free erythrocytes and/or erythrophages/haemosiderophages in BALF is highly sensitive and specific for EIPH. The high sensitivity is due to the ability of this technique to detect small amounts of blood or erythrocyte breakdown products for a protracted period after pulmonary haemorrhage (up to 3 weeks with haemosiderophages). While enumeration of erythrocytes in samples of BALF has been advocated as a quantitative reflection of the severity of EIPH, validation of this technique has not been reported.

Prevalence values for EIPH are also dependant on the frequency that diagnostic techniques are used to examine horses. Prevalence in racehorses when a single tracheobronchoscopy is performed range between 40 and 75% (Pascoe et al. 1981; Raphael and Soma 1982); however the prevalence may increase to over 90% when multiple examinations are performed (Lapointe et al.1994). Approximately 62% of racing Quarter horses have EIPH as evidenced by tracheobronchoscopic findings, while only 11% of polo ponies have evidence of EIPH. When examination of BALF is used as a diagnostic tool, the prevalence of EIPH in thoroughbred racehorses may approach 100% (McKane et al. 1993).

Epidemiological information available indicates that EIPH is very common in thoroughbred and standardbred racehorses, and prevalence may approach 100%, while the prevalence in horses used for less strenuous athletic use is lower, consistent with the greater magnitude of transmural pressure involving the pulmonary capillaries and greater likelihood of capillary rupture with increasing intensity of exercise.

EIPH: a performance limiting disorder or an incidental finding?

While EIPH is clearly a pathophysiological process and it is tempting to attribute poor performance of racehorses to pulmonary haemorrhage when the disorder is detected, any such relationship has not been elucidated fully.

After experimental inoculation of autologous blood into the lungs of horses, and in spontaneous EIPH, altered lung function and lower arterial oxygen tension may occur during exercise, suggestive of a performance-limiting effect of EIPH. However, several studies of thoroughbred and/or standardbred racehorses have found no association between EIPH and poor performance (Pascoe et al 1981; Raphael and Soma 1982). In contrast, Rohrbach (1990) documented a trend for an association between EIPH and superior performance, suggesting greater racing effort as a reason for this finding.

Certain racing jurisdictions worldwide permit administration of frusemide on the day of racing to reduce the likelihood/severity of EIPH. Frusemide has been shown to have positive effects on performance and earnings and possibly reduce the severity of EIPH as defined by RBC numbers in BALF, leading to the suggestion that this provides indirect evidence of the performance-limiting effects of EIPH. However, improved performance may be a result of frusemide-induced weight loss (via diuresis) rather than improved pulmonary function.

Recently, an association between EIPH and reduced performance in a large population of thoroughbred racehorses (n=744) has been determined (Hinchcliff et al. 2005). In that study, horses were not treated with frusemide or nasal dilator strips and tracheobronchoscopic examination was used to diagnose and grade EIPH. This study provides evidence that EIPH impairs performance and possibly in a dose-dependant manner (subjectively on the basis of grading of tracheal blood). While the results of this study and those of experimental studies suggest EIPH is performance-limiting, further investigation of the effects of EIPH, including the dose-response relationship and means of quantifying pulmonary haemorrhage and effect on performance is required, to improve understanding of both the disease and efficacy of treatment and preventative measures.

References

Hinchcliff, K.W. et al (2005) Association between exercise-induced pulmonary hemorrhage and performance in thoroughbred racehorses. J Am Vet Med Assoc. 227, 268-774.

Lapointe, J.M. et al (1994) A survey of exercise-induced pulmonary haemorrhage in Quebec standardbred racehorses. Equine Vet J. 26, 482-485.

McKane, S.A., Canfield, P.J. and Rose, R.J. (1993) Equine bronchoalveolar lavage cytology: survey of thoroughbred racehorses in training. Aust Vet J. 70, 401-404.

Mason, D.K. et al (1983) Exercise-induced pulmonary haemorrhage in horses. In: Snow, D.H., Persson, S.G.B., Rose, R.J. eds. Equine exercise physiology. Cambridge: Granta Publications, 57-63.

Newton, J.R., Rogers, K., Marlin, D.J., Wood, J.L. and Williams, R.B. Risk factors for epistaxis on British racecourses: evidence for locomotory impact-induced trauma contributing to the aetiology of exercise-induced pulmonary haemorrhage. Equine Vet J. 37, 402-411.

Pascoe, J.R. et al (1981) Exercise-induced pulmonary hemorrhage in racing thoroughbreds: a preliminary study. Am J Vet Res. 42, 703-707.

Raphael, C.F. and Soma, L.R. (1982) Exercise-induced pulmonary hemorrhage in Thoroughbreds after racing and breezing. Am J Vet Res. 43, 1123-1127.

Rohrbach, B.W. (1990) Exercise-induced pulmonary hemorrhage, chronic obstructive pulmonary disease, and racing performance J Am Vet Med Assoc. 196,1563-1564.