Effects of exposure of dairy cows to a mixture of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs), dioxin-like (dl) polychlorinated biphenyls (PCBs), and non-dl-PCBs during the period of negative and positive energy balance on health traits and on aryl hydrocarbon receptor (AHR)-signaling

This study investigated the impact of oral dioxin and PCB exposure on Holstein cows during negative (NEB) and positive (PEB) energy balance. Nine cows were divided into control (CON, n = 4) and exposed (EXP, n = 5) groups. The EXP group received 278/266 pg dioxin-like (dl)-PCB TEQ/kg BW/day and 75/72 ng non-dl-PCBs/kg BW/day for 28 days during both NEB and PEB. Inner exposure and health parameters were assessed. Cholesterol levels increased faster in the EXP group during NEB (p = 0.049). During PEB, CD4⁺ cell proportion and mean fluorescence intensity (MFI), along with CD8⁺ cell MFI, decreased in the EXP group while increasing in the CON group (p = 0.004, p = 0.05, and p = 0.023, respectively). AHR-signaling mRNA expression was unaffected in subcutaneous adipose tissue, spleen, and peripheral blood mononuclear cells. However, approximately 100 days post-exposure, hepatic AHR and ARNT mRNA expression remained elevated in the EXP group (p = 0.023 each) and correlated positively with liver dioxin and PCB residues (r² = 0.43–0.75). Overall, dioxin and PCB effects on the examined endpoints were minor. The long-term implications of the observed T-cell and hepatic AHR-signaling changes require further investigation.