Exercise-induced pulmonary haemorrhage in Thoroughbred racehorses: a longitudinal study

Background Exercise-induced pulmonary haemorrhage (EIPH) is considered a progressive disease based on histopathology, but it is unknown if tracheobronchoscopic EIPH severity worsens over time. Objectives Study design The aim of this study was to examine tracheobronchoscopic EIPH changes over time in a population of Thoroughbred racehorses. A secondary aim was to identify factors that affect changes in tracheobronchoscopic EIPH severity between observations. Prospective, longitudinal, observational cross-sectional study. Methods Results Thoroughbred racehorses were examined with tracheobronchoscopy no earlier than 30 min after racing. Examinations were recorded and graded blindly by experienced veterinarians using a 0-4 scale. Horses with 2 or more observations were included in the analysis. The association between the previous and current EIPH score was investigated using a linear mixed effect model. Factors associated with transitioning from a lower to a high EIPH grade and vice versa were examined using multiple ordinal regression. A semi-parametric regression model was used to examine progression using the number of career starts as a marker for time. Models were adjusted for potential confounding variables. There were 2974 tracheobronchoscopic examinations performed on 747 horses. Blood was detected in over half of all examinations (55.6%). The population prevalence of EIPH increased as the number of examinations for each horse increased. The preceding EIPH score was significantly associated with the current EIPH score. Significant variables associated with moving between EIPH grades were the number of days since last racing, ambient temperature and weight carried. Tracheobronchoscopic EIPH is mildly progressive over the first thirty career starts. Main limitations Conclusion Enrolment was voluntary. Horses were not followed for their entire career. Limiting the number of days in the current racing preparation and spacing races for horses with moderate to severe EIPH may be beneficial for reducing tracheobronchoscopic EIPH severity. The association between ambient temperature and EIPH warrants further investigation.