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

Abstract

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.