Sixty Standardbred horses, aged 3 to 10 years, were examined endoscopically for exercise-induced pulmonary haemorrhage (EIPH) 1 h after racing, on at least 3 occasions. Racing time, finishing position and post exercise venous lactate concentrations were also recorded. Horses positive for tracheal blood on endoscopy were classified as having either grade 1 EIPH (one or a few spots of blood in the trachea), or grade 2 (stream of blood). Air temperature, relative humidity and air pollutant levels were recorded on each examination day.
Of the 60 horses, 52 (87%) were EIPH-positive on at least one evaluation out of 3 and 40 of the 52 (77%) showed grade 2 EIPH on at least one occasion. Of the 52 EIPH-positive horses, 52% bled 3 times out of 3, 21% 2 out of 3 and 27% once. When only the first examination was considered, 37 of the 60 horses (62%) were EIPH-positive. There was no significant relation detected between either presence or frequency of EIPH and age, sex, or gait. Horses that showed grade 2 EIPH bled significantly (P = <0.001) more frequently (mean: 83% of examinations) than horses having shown no more than grade 1 EIPH (47% of examinations).
No significant differences could be detected between EIPH-positive and EIPH-negative races in the average racing times, finishing positions and blood lactate concentrations of the 29 horses which showed intermittent EIPH. The proportion of horses showing EIPH on each day where at least 5 horses were examined was correlated with air temperature, relative humidity, and the levels of several air pollutants on those days. There was a significant negative correlation between air temperature and proportion of EIPH-positive horses (P = 0.0007, r2= 0.39). No significant relations were detected with relative humidity or air pollutant levels.