Inhaled Corticosteroids Influence Pulmonary Microbiota in Severe Equine Asthma

The use of inhaled corticosteroids (ICs) could influence the respiratory microbiota. In animals with asthma it is, however, difficult to separate the immunomodulatory effects of ICs from their indirect effects via improvement of ventilation. Our objective was to determine if ICs alter the pulmonary microbiota independently from their effects on lung function, using a blinded, controlled trial in an experimental model of asthma exacerbation in horses. We treated horses with severe asthma with either bronchodilators alone, or in combination with ICs. Twelve horses in exacerbation received long-acting β2-agonist (LABA, salmeterol) or ICs/LABA (fluticasone/salmeterol) by inhalation, for 2 weeks. Lung function and bronchoalveolar lavages (BAL) were performed before and after treatment. 16S rRNA gene quantification and sequencing were performed on BAL fluid, using digital droplet PCR and the Illumina MiSeq platform. Data were processed using the software package mothur v. 1.44.2. In the LABA group, pulmonary bacterial load and the relative abundance of Actinobacteria and Verrucomicrobia phyla decreased with treatment (p < 0.05 for both), and β-diversity differed from baseline (p = 0.007). The relative abundance of families and genera belonging to the Bacteroidetes phylum increased with ICs/LABA (p < 0.05). Lung function significantly improved with both treatments, suggesting that treatment-related differences in pulmonary microbiota could be attributed in part to medication, not solely to change in ventilation. However, it is not clear if these changes are positive or detrimental to the lung environment. Furthermore, lung function following treatment was not perfectly identical between groups.