Sequential changes in bronchoalveolar cytology after autologous blood inoculation

Six horses, free of recent exercise-induced pulmonary haemorrhage (EIPH), each had 8 bronchial segments inoculated with 40 ml of autologous blood lavaged on Days 3, 5, 7, 10, 14 and 21 after inoculation. The bronchoalveolar lavage (BAL) fluid samples were analysed, and total leucocyte counts and differential cell percentages determined. Statistical analysis of the differences between the 8 post inoculation and 1 pre-inoculation (control) lavage sites demonstrated an early neutrophilic response, rising from < 5% of alveolar leucocytes to 10% by 24-48 h post inoculation. As this response waned, macrophage numbers increased and persisted at mildly elevated levels for the remainder of the experiment. Small numbers of erythrophagocytes were observed prior to 3 days post inoculation, but their numbers increased 10-fold by Day 10. In the first 3 days, blood was observed endoscopically streaming from inoculated lung regions, suggesting that mucociliary clearance is a major mechanism of early erythrocyte clearance. However, both free and phagocytosed whole erythrocytes were recovered in lavages 14 days after inoculation with small numbers of erythrocytes still present at 21 days. We conclude that intrapulmonary blood provokes a mild, but prolonged inflammatory reaction and that the removal of alveolar haemorrhage is dependent on a combination of mucociliary clearance and phagocytic mechanisms. Erythrophagocytosis is slow to commence and the removal of blood prolonged, suggesting that horses in fast work, sufficient to induce EIPH, are unlikely to recover fully if intervals between strenuous work are less than 14 days.