Seasonal heat load is more potent than the degree of body weight loss in dysregulating immune function by reducing white blood cell populations and increasing inflammation in Holstein dairy cows.

We investigated the effects of seasonal heat stress and the degree of body weight (BW) loss postpartum (PP) on immune and inflammatory markers in transition dairy cows. Blood sampled twice weekly during the transition period (3 wk prepartum to 3 wk PP) from 12 peripartum Holstein cows in summer and 12 in winter was analyzed for plasma nonesterified fatty acids, the oxidative stress marker malondialdehyde (MDA), haptoglobin, and tumor necrosis factor-α (TNF-α). Complete blood counts and white blood cell (WBC) subpopulations (CD4, CD8, CD25, WC1, and CD335) were examined weekly during the transition period. Adipose tissue biopsies were taken at 7 d PP from summer-calving cows. In cows calving during seasonal heat stress, plasma TNF-α concentrations were 4.9-fold higher prepartum and 5.1-fold higher PP, and MDA concentrations were higher prepartum and PP than in winter. Hematocrit was lower prepartum and tended to be lower PP in summer compared with winter, and after adjusting to hematocrit values fewer WBC prepartum (-17%) and PP (-22%), lower monocytes prepartum (-37%) and PP (-49%), and fewer neutrophils (-25%) PP were found in summer compared with winter-calving cows. The percentages of cytotoxic T cells (CD8) and natural killer cells (CD335) were lower, as well as prepartum gamma delta-T cells (WC1) in summer compared with winter-calving cows. Moreover, lower red blood cell counts prepartum and mean platelet volume and red blood cell distribution width prepartum and PP, as well as higher hemoglobin prepartum and PP, were found in summer than winter-calving cows. In summer, cows that lost more BW PP had higher plasma MDA and TNF-α concentrations, fewer basophil numbers prepartum, a decrease in the percentage of cells expressing the IL-2 receptor (CD25), increased ratio of T helper cells (CD4):CD8, and a tendency for increased WC1 percentages in blood prepartum, as well as a higher percentage of WC1 PP and higher abundance of TNF-α and lower lipopolysaccharide-binding protein in adipose tissue compared with cows that lost less BW PP. In conclusion, seasonal heat stress during the peripartum period was associated with lower WBC counts coupled with increased levels of subacute inflammation, indicating dysregulation of immune function, whereas the degree of BW loss had minor effects. This suggests that heat load is a more potent stressor associated with immune function than the variation in the degree of BW loss PP in high-yielding peripartum dairy cows.