Effects of dexamethasone on shedding of Listeria monocytogenes in dairy cattle

Ten lactating Holstein cows that had been given multiple injections of Listeria monocytogenes (serotype 4B, Scott A strain) via the intramammary route were allotted to 2 groups: group 1 (n = 5) was treated with the synthetic glucocorticoid, dexamethasone (0.04 mg/kg of body weight), for 3 consecutive days, and group 2 (n = 5) served as controls. Two days after the initial dexamethasone injection, the number of L monocytogenes in the milk had increased nearly 15-fold (1.16 log10) over pretreatment values. On day 3, Listeria numbers in the milk had increased by 1.83 log10, compared with pretreatment values. By day 4, Listeria numbers in the milk were approximately 100-fold (2.03 log10) greater than pretreatment numbers. Numbers remained high through day 7 and, by day 11, approached pretreatment numbers. Dexamethasone administration was accompanied by high total WBC and milk somatic cell counts and decreased eosinophil and lymphocyte numbers, and decreased milk production. The increase in shedding of L monocytogenes in the milk may reflect impairment of cell-mediated immune mechanisms and phagocytic cell functions that are critical for sustaining listerial immunity.