Circadian Oscillation of Leukocyte Subpopulations and Inflammatory Cytokines over a 24-H Period in Horses

: The objective of the present study was to investigate the influence of daily rhythms on the immune and inflammatory systems in horses, considering white blood cell count (WBCs), leukocyte subpopulations (neutrophils, basophils, eosinophils, lymphocytes, and monocytes), CD4+, and CD8+ lymphocyte populations, interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α). Ten Italian Saddle horses (7-12 years old, body weight 480 ± 30 kg) underwent blood sampling every 4 h over a 24-h period. The COSINOR method was used to identify rhythms and their parameters. A one-way analysis of variance (ANOVA) was applied to identify the differences in acrophase and robustness, and a multiple correlation analysis model (Pearson) was used to evaluate the relationships among the investigated parameters. WBCs, leukocyte subpopulations, CD4+, CD8+, IL-1β, IL-6, and TNFα exhibited daily rhythmicity. In particular, white WBCs, lymphocytes, IL-1β, and IL-6 reached their acrophases during the dark phase, while neuthrophils, CD4+, CD8+, and TNFα showed a diurnal acrophase. One-way ANOVA showed a statistical difference in the acrophase among the investigated parameters (p < 0.0001). The Pearson correlation matrix showed positive and negative relationships among the parameters. Circadian rhythms should be taken into consideration with the daily fluctuations in immune and inflammatory biomarkers to develop good management practices and improve welfare in horses.