Changes in leukocyte indices of Holstein cows under prolonged heat stress conditions.

Leukocyte indices integrate shifts in innate and adaptive immune function, making them sensitive indicators of physiological stress in dairy cows. This study evaluated how prolonged heat stress affects immune function in Holstein cows using composite leukocyte indices. Eighteen multiparous cows were studied: one group (n = 8) was exposed to 45 days of heat stress (THI ≥ 72), while the control group (n = 10) stayed under thermal comfort. Heat-stressed cows had a 2.2-fold increase in band neutrophils (p = 0.0035) and a 79% rise in the Nuclear Shift Index (NSI; p = 0.0246). Lymphocyte and monocyte counts dropped by 24% (p = 0.0404) and 42% (p = 0.0183), with a notable decrease in adaptive immune indices such as the Lymphocyte–Granulocyte Index (LGI) and Index of Adaptation by Garkavi (IAG). These results indicate a shift towards innate immune defence and partial suppression of adaptive immunity under chronic heat load. Leukocyte indices proved sensitive to thermal stress and may serve as practical biomarkers for monitoring herd health. Their use could help inform early interventions and support dairy herd management as heat events become more common.