Exercise-induced pulmonary haemorrhage in the horse: results of a detailed clinical, post mortem and imaging study. V. Microscopic observations.

The research article investigates the microscopic elements of exercise-induced pulmonary haemorrhage (EIPH) in racehorses, through inspection of the lungs, categorizing the kind of lesions and provides hypotheses about the cause of EIPH.

Methods for Examination

• The researchers studied lungs from 19 Thoroughbred racehorses with a history of EIPH. Different forms of microscopy, including light microscopy of paraffin sections, Transmission electron microscopy and scanning electron microscopy were used in the study.

Microscopic Observations

• Upon examination, it was observed that there were three lesions in the caudodorsal region of the lungs from each horse. The placement of these lesions corresponded with the location of blue to brown stains observed at necropsy.
• The lesions observed include sequelae of bronchiolitis, hemosiderophages and increased connective tissue.
• The researchers noticed that with a few exceptions most of the other areas of the lungs, especially the more cranial or ventral portions, appeared to be normal.
• The researchers did observe eosinophil infiltration in seven of the 19 lungs they examined, these foci had different distributions than the three lesions.
• Vascular lesions characteristic of hypertension were also found occasionally in areas of severe bronchiolar changes and fibrosis.
• Transmission electron microscopy confirmed cell types observed by light microscopy and was also used to examine arterioles for changes characteristic of neovascularisation, the process of forming new blood vessels.
• Scanning electron microscopy was used to examine enlarged airspace areas – these areas were found to have varying balance between fibrosis and destruction, but none were as extensive as those seen in chronic obstructive pulmonary disease (COPD).

Conclusions

• The researchers hypothesize that bronchiolitis and related neovascularisation could be the fundamental components in the aetiology of EIPH. This suggests that inflammation and damage in the bronchioles leading to the formation of new blood vessels could be the prime reason for the occurrence of EIPH in racehorses.