Seasonal modulation of platelet indices in mares

Platelet indices provide valuable information on platelet activation, heterogeneity, and production kinetics, yet their physiological variability remains poorly characterized in horses. Although seasonal changes in platelet count have been described, the seasonal modulation of derived platelet indices in mares remains poorly understood. The aim of this study was to evaluate the seasonal variation of platelet count and derived platelet indices in clinically healthy mares from the Valencia region (Spain), and to assess their interrelationships. Forty clinically healthy Spanish Purebred mares (6-12 y) were monitored over one year under natural environmental conditions. Monthly blood samples were collected and analyzed for platelet count (PLT) and derived indices using an automated hematology analyzer. Seasonal effects were evaluated using non-parametric repeated-measures analysis. Significant seasonal variation was observed in most platelet indices (p < 0.05), while PCT and aggregation index remained unchanged. Platelet count was lowest in autumn, whereas indices reflecting platelet size and heterogeneity increased during both warm and cold seasons. Correlation analysis revealed consistent associations among platelet size, mass, and heterogeneity, as well as inverse relationships between platelet density and activation-related variables. These findings demonstrate a clear seasonal modulation of platelet dynamics in mares, likely reflecting adaptive responses to environmental conditions. The results highlight the importance of considering seasonal variation when interpreting platelet indices and support the need for season-adjusted reference intervals in clinical practice.