Repeated intravenous dexamethasone administration causes transient changes in leukocyte parameters in healthy warmblood horses

Objective: To determine the effect of repeated IV administration of dexamethasone (0.06 mg/kg, q 24 h, twice) on WBC parameters in healthy adult horses. Methods: This prospective, longitudinal study was conducted from October 7 through November 5, 2024. Five warmblood mares received an IV dexamethasone sodium phosphate injection (DMI) at 0 and 24 hours and an IV injection with 0.9% sodium chloride solution at 48 and 72 hours. Blood samples were collected at 0, 6, 12, 24, 30, 36, 48, 72, 96, and 168 hours, and WBC parameters were evaluated using linear mixed-effects models. Results: Leukocyte count, neutrophil count, and neutrophil-to-lymphocyte ratio were increased at 6 hours after DMI. Mean leukocyte count peaked 6 hours after DMI and exceeded the reference interval; however, not all horses had peaks above the reference interval. Neutrophilia was observed in all horses, with peak values 6 hours after DMI. Lymphocyte, monocyte, and eosinophil counts were decreased at 6 hours after DMI, with complete suppression of eosinophils. Myeloperoxidase index was increased only at 6 hours after the second DMI. Neut-X was decreased at 12 hours after DMI, and Neut-Y did not change up to 156 hours after the first DMI. Most parameters returned to baseline at 48 to 72 hours. Conclusions: Repeated IV dexamethasone administration induces recognizable changes in leukograms in healthy horses with transient neutrophilia combined with increased leukocyte counts and neutrophil-to-lymphocyte ratios and decreased lymphocyte, monocyte, and eosinophil counts. Clinical Relevance: Recognition of glucocorticoid-induced changes is important for accurate interpretation of hematologic findings in horses receiving glucocorticoids.