Seasonal particulate matter exposure is associated with upper respiratory microbiota restructuring in dairy heifers

BACKGROUND: Particulate matter (PM), particularly PM2.5 and PM10, is a major environmental health concern linked to respiratory diseases in humans and animals. Northern Thailand, especially Chiang Mai, experiences recurrent seasonal air pollution from biomass burning, exposing outdoor-housed livestock to elevated ambient PM levels. The bovine upper respiratory tract (URT) harbors both commensal and opportunistic microorganisms, and disruption of this microbiota may influence susceptibility to bovine respiratory disease (BRD). However, the impact of natural PM exposure on the bovine URT microbiota remains poorly understood. RESULTS: Nasopharyngeal swabs from 25 clinically healthy dairy heifers were analyzed during low-PM and high-PM periods. During the high-PM period, peak PM2.5 levels exceeded 30 times the WHO 24-hour guideline. Alpha diversity, including observed features, Shannon diversity, and Simpson index, was significantly higher during the high-PM period than during the low-PM period. Beta diversity analysis showed significant differences in Bray-Curtis dissimilarity and Jaccard distance, indicating changes in both relative abundance-based community structure and presence-absence-based community membership. The high-PM period was characterized by altered taxonomic profiles, including higher proportions of Proteobacteria, Firmicutes, Gammaproteobacteria, and Bacilli, and lower proportions of Actinobacteriota, Bacteroidota, Actinobacteria, and Bacteroidia. Among selected dominant genera, Moraxella and Fusobacterium were significantly reduced during the high-PM period. BRD-associated genera, including Mycoplasma, Pasteurella, Mannheimia, and Histophilus, showed higher average relative abundances during the high-PM period; however, paired comparisons were not statistically significant. CONCLUSIONS: Seasonal high-PM exposure in Chiang Mai was associated with measurable changes in the nasopharyngeal microbiota of clinically healthy dairy heifers, including increased alpha diversity, altered beta diversity, changes in taxonomic profiles, and reductions in selected dominant genera. These findings suggest that ambient air pollution may contribute to respiratory microbiota restructuring in dairy heifers. Further longitudinal studies integrating microbiota composition, host immune responses, farm-level environmental monitoring, and clinical respiratory outcomes are needed to clarify whether PM-associated microbiota changes contribute to BRD susceptibility.