Peripheral leukocyte transcriptomic changes in preweaned Holstein heifer calves with varying stages of Bovine Respiratory Disease.

Reduction of morbidity and mortality attributable to Bovine Respiratory Disease (BRD) requires a nuanced approach rooted in understanding associated pathophysiological transcriptomic changes. This study aimed to identify 1) transcriptomic changes in peripheral leukocytes indicative of the onset and duration of BRD in preweaned Holstein dairy calves, and 2) potential predictive features (i.e., genes) associated with the progression or resolution of BRD underlying disease susceptibility and animal robustness. Neonatal Holstein dairy calves were enrolled across two years (May 2021: n = 60; May 2022 n = 61) on two conventional dairies located in the central region of the state of Washington, United States. Calves were followed for the first 12 weeks of life with sequential thoracic ultrasonographic and clinical assessments. Calves were retrospectively categorized as healthy (2021: n = 8; 2022: n = 15), onset lobar consolidation (2021: n = 16; 2022: n = 15), chronic lobar consolidation (2021: n = 8; 2022: n = 9), or resolved lobar consolidation (2021: n = 7; 2022: n = 11) based upon thoracic ultrasonography. Leukocytes were isolated from whole blood samples via modified Ficoll-Paque separation followed by RNA extraction and sequencing. Differentially expressed genes (DEGs; FDR < 0.05 and |logFC| > 1) were identified via edgeR for each BRD stage compared to healthy: onset (DEG = 163), chronic (DEG = 27) and resolved (DEG = 0). Random forest classification modeling was then conducted to identify gene features with potential to be predictive of disease progression or resolution. Functional enrichment of associated DEGs suggested a shift in resource allocation from primary growth activity to immune response. Identification of key molecular constituents such as interleukins, integrins, serine protease inhibitors and matrix-associated-remodeling proteins highlight potential intrinsic factors driving differences in disease phenotypes, susceptibility, and animal robustness.