Changes in Leukocyte Indices of Holstein Cows Under Prolonged Heat Stress Conditions

This study assessed the effects of prolonged heat stress (HS) on leukocyte indices in Holstein cows. Blood samples from 18 multiparous Holstein cows were analyzed: a hyperthermia group (HYP, n = 8) exposed to THI ≥ 72 and a control group (CON, n = 10) under thermal comfort. Integral leukocyte indices, including the Nuclear Shift Index (NSI), Neutrophil-to-Monocyte Ratio (NMR), and Lymphocyte-Granulocyte Index (LGI), were calculated using validated clinical methodologies. Results revealed a 2.2-fold increase in band neutrophils (p=0.0035) and a 78.7% elevation in the NSI (p=0.0246) in response to HS. In contrast, lymphocyte and monocyte counts decreased by 23.7% (p=0.0404) and 42.1% (p=0.0183), respectively, accompanied by significant declines in adaptive capacity indices such as the LGI and the Index of Adaptation by Garkavi (IAG). These findings highlight the physiological trade-offs in immune responses under HS, emphasizing the utility of leukocyte indices as biomarkers for assessing thermal stress impacts. Developing strategies to mitigate HS-induced effects is crucial for enhancing the welfare and productivity of dairy cows.