Dairy calves exposed to ambient fine particulate matter have increased bovine respiratory disease risk

Objective: To evaluate the association between ambient fine particulate matter (PM2.5) exposure and bovine respiratory disease (BRD) in northern Colorado dairy calves. Methods: Retrospective data were analyzed from 45 preweaned dairy calves. Calves were screened for BRD twice weekly from May 23 through August 7, 2023. Daily PM2.5 concentrations were obtained from Environmental Protection Agency monitors. Logistic mixed-effects models, including distributive-lag nonlinear models, were used to estimate associations between PM2.5 and BRD. Results: There were 27 clinical and 190 subclinical BRD occurrences over 900 screenings. Mean daily PM2.5 during the study was 4.5 μg/m3, with peaks up to 54.3 μg/m3 coincident with a wildfire smoke event preceding screening. A 5 μg/m3 increase in daily PM2.5 was associated with 66% higher odds of incident BRD (OR, 1.66; 95% CI, 1.08 to 2.53). Temperature-humidity index (OR, 1.17; 95% CI, 1.11 to 1.22) and male sex (OR, 1.65; 95% CI, 1.08 to 2.53) were also significant predictors. Lagged analyses showed variable associations across 0- to 30-day lag, with increased odds of BRD observed at PM2.5 concentrations between 4.8 and 8 μg/m3. Conclusions: PM2.5 was significantly associated with increased BRD risk in dairy calves. Even relatively low PM2.5 concentrations were linked to higher BRD odds, highlighting impacts below regulatory thresholds established for human health. Future prospective studies across multiple dairies are warranted to characterize disease progression and potential mitigation strategies. Clinical Relevance: Veterinarians should consider ambient air quality when assessing disease risk, guiding preventive strategies, and advising clients. Awareness of PM2.5 and implementation of adaptation strategies may improve respiratory health outcomes in dairy calves.