Pulmonary bleeding in racehorses: A gross, histologic, and ultrastructural comparison of exercise-induced pulmonary hemorrhage and exercise-associated fatal pulmonary hemorrhage.
Authors: Rocchigiani Guido, Verin Ranieri, Uzal Francisco A, Singer Ellen R, Pregel Paola, Ressel Lorenzo, Ricci Emanuele
Journal: Veterinary pathology
Summary
# Pulmonary Bleeding in Racehorses: Understanding Two Distinct Conditions Exercise-induced pulmonary haemorrhage (EIPH) and exercise-associated fatal pulmonary haemorrhage (EAFPH) represent two ends of a spectrum in Thoroughbred racing, yet their underlying pathophysiology remains poorly understood. Rocchigiani and colleagues examined postmortem lung tissue from 10 horses with EIPH, 10 with EAFPH, and 5 controls using gross examination, histology, and electron microscopy to determine whether these conditions share a common mechanism or represent fundamentally different pathological processes. Counter-intuitively, EAFPH lungs showed acute, severe pulmonary oedema and haemorrhage primarily in the cranial lobes but lacked the chronic adaptive changes characteristic of EIPH—specifically, minimal vascular remodelling, haemosiderophage accumulation, iron-encrusted collagen and elastin fibres, and type II pneumocyte hyperplasia. Horses with EIPH displayed evidence of repeated bleeding and tissue repair through these chronic inflammatory markers, particularly in the caudal lung regions, whilst EAFPH lungs contained collagen fibrils with abnormally large diameters, suggesting a structural defect in the perivascular matrix rather than repeated microtrauma. These findings have significant clinical implications: EAFPH may result from an acute vascular or connective tissue failure unrelated to the cumulative stress underlying EIPH, which could explain why performance interventions effective for EIPH horses may not prevent sudden fatal haemorrhage and suggests practitioners should consider EAFPH a distinct syndrome requiring investigation of potential underlying vascular or genetic predisposition.
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Practical Takeaways
- •EAFPH and EIPH appear to be distinct pathological entities: EAFPH represents acute catastrophic bleeding in previously unaffected lungs, while EIPH reflects chronic recurrent microhemorrhage with progressive tissue remodeling
- •The presence of hemosiderophages and iron-stained fibers on necropsy indicates previous EIPH episodes; their absence doesn't rule out predisposition to fatal bleeding
- •Current diagnostic and preventive strategies for EIPH may not address the underlying vascular structural defect that predisposes some horses to sudden fatal hemorrhage during exercise
Key Findings
- •EAFPH lungs showed significantly greater cranial lobe reddening and edema compared to EIPH and control horses, indicating acute severe bleeding rather than chronic adaptation
- •EIPH horses demonstrated higher hemosiderophages, iron encrustation, and vascular remodeling scores compared to EAFPH horses, suggesting chronic recurrent bleeding and tissue adaptation
- •EAFPH horses exhibited larger perivascular collagen fibril diameters on ultrastructural analysis, indicating structural differences in vascular support architecture
- •Caudal lung locations showed significantly higher vascular remodeling and iron accumulation across all groups, suggesting preferential bleeding distribution in exercising horses