Bronchial vessel density is correlated with airway smooth muscle cell proliferation in horses with mild and moderate asthma.

Objective: In severe equine asthma, structural remodeling of the airways ultimately leads to bronchial wall thickening and airflow obstruction. Increased bronchial vascularization has been described in horses affected by the severe form of the disease, but whether it contributes to bronchial remodeling in milder forms of asthma remains to be determined. In a blinded, retrospective case-control study, we evaluated the presence of bronchial angiogenesis in horses with mild and moderate equine asthma (MEA) and its correlation to airway smooth muscle remodeling.

Methods: Endobronchial biopsies from the Equine Respiratory Tissue Biobank collected between August 14, 2014, and May 31, 2019, from 9 horses with MEA and 7 healthy controls were studied. The vascular basement membrane was identified by immunohistochemistry, allowing the measurement of the number of bronchial vessels, vascular area, and mean vessel size by histomorphometry. The correlations between angiogenic parameters, airway smooth muscle (ASM) remodeling features, and airway neutrophilia were studied.

Results: No differences between groups were observed for the angiogenic parameters evaluated. The number of vessels was correlated to ASM cell proliferation in MEA horses (Spearman r = 0.73) but not in controls. Airway neutrophilia correlated negatively with mean vessel size in horses with MEA (Pearson r = -0.83) but not in control horses.

Conclusions: Major changes in bronchial vascularization do not occur in central airways in MEA.

Clinical relevance: Contrary to previous findings in horses with severe equine asthma, angiogenesis is not a prominent feature of MEA, but it might be associated with ASM remodeling.