A novel controlled wood smoke exposure model to evaluate physiological responses to smoke-derived air pollution in dairy calves.

Fine particulate matter (PM2.5) from wildfire smoke is a major air pollutant in dairy-producing regions of the United States. However, knowledge of direct causative effects of wildfire-derived pollutants on calf health is lacking because of a reliance on observational studies from natural wildfire smoke exposures. Calves represent an important population in a dairy herd and improving their health is especially critical during the vulnerable pre-weaning window, as respiratory diseases during this period can affect performance later in life. The objective of this study was to evaluate the acute effects of controlled short-term wood smoke exposure on respiratory function, immune responses, and clinical health measures in pre-weaned dairy calves using novel smoke enclosures we developed. Intact male Holstein calves (n = 24; 37 ± 2 d of age [mean ± SD]) were randomly assigned to control (CON; n = 12) or wood smoke (WS; n = 12) treatments. Calves were kept for 6 h in the experimental smoke enclosures where they were either exposed to wood combustion smoke (WS; smoke inlet open) or not exposed (CON; smoke inlet closed). Environmental conditions within the enclosures including PM2.5, carbon dioxide, carbon monoxide, ambient air temperature, and relative humidity were recorded during the experiment trials. Fine particulate matter concentrations (mean ± SD) were 9.94 ± 4.50 µg/m3 for CON and 238.44 ± 62.97 µg/m3 for WS. All other environmental conditions were similar between CON and WS groups. Immediately after treatment, pulmonary function and health scores were evaluated, and blood samples were collected. Calves were humanely euthanized, and bronchoalveolar lavage was performed. Wood smoke exposure altered pulmonary function, as WS calves had a lower respiratory rate (17.50 vs. 21.52 breaths/min) and higher tidal volume (0.90 vs. 0.75 L) compared with CON calves, while minute ventilation did not differ between groups. Bronchoalveolar lavage fluid (BALF) immunophenotyping revealed increased proportions of granulocytes, CD68-positive macrophages, and natural killer cells in WS compared with CON calves, suggesting activation of pulmonary innate immune responses. Circulating neutrophil counts were reduced in WS calves, whereas total white blood cell counts, and other hematological variables did not differ between groups. Plasma concentrations of acute phase proteins, including serum amyloid A, haptoglobin, and albumin, did not differ statistically between treatments. Phagocytosing granulocytes (%) in whole blood was greater in the WS group compared with CON (97.03 vs. 93.89%). Oxidative stress, assessed by concentrations of thiobarbituric acid reactive substances (TBARS) in BALF and plasma, was not affected by treatment. These findings demonstrate that short-term wood smoke exposure at concentrations reflective of natural wildfire conditions induces an acute pulmonary inflammatory response and alters respiratory physiology in pre-weaned dairy calves. A systemic inflammatory response and oxidative stress were not detected, which could be attributed to the timing of sample collection immediately after exposure. This novel controlled exposure model provides a foundation for future studies investigating the progression of air pollution-induced health effects in dairy cattle and other large animals.