Exercise-induced haemorrhagic lesions in the dorsocaudal extremities of the caudal lobes of the lungs of young thoroughbred horses.

The dorsocaudal extremities of the caudal lobes of the lungs of racehorses are vulnerable to exercise-induced pulmonary haemorrhage (EIPH). The morphology of the lungs at these sites was studied in 13 Thoroughbred horses aged 18 to 22 months. These animals, which had been performing low-intensity exercise on a track at maximum running speeds of approximately 5-8.5 metres/second (m/s), were withdrawn from the racehorse training programme for reasons of unsuitability. Lung lesions observed in the dorsocaudal lung extremities in 10 of the 13 horses were not found in the craniodorsal or cranioventral portions of the lungs. The lesions, which resembled those previously found in Thoroughbred racehorses aged 5 to 11 years with a history of EIPH, were of two main types, namely, multifocal bronchiolar distortion and alveolar epithelialization. EIPH lesions were found only in horses that had been trained at maximum speeds greater than approximately 7.0 m/s. It would seem, therefore, that exercise intensity is an important factor in the pathogenesis of EIPH and that running speeds greater than approximately 7.0 m/s may be sufficient to generate the pulmonary vascular pressures necessary to cause EIPH lesions in young Thoroughbreds. Copyright 1999 Harcourt Publishers Ltd.