Effect of hydrocortisone on circulating lymphocyte numbers and their mitogen-induced blastogenesis in lambs.

The effect of hydrocortisone on the number of circulating lymphocytes and their blastogenic response was studied in 20 feedlot lambs given combinations of 3 treatments: hydrocortisone (25 mg/kg of body weight, 4 times a day, IM), feed changes (100% roughage to 90% concentrate over a 6-day period), and oral inoculation of Pasteurella haemolytica biotype T (10(9) to 10(11) bacteria/day via stomach tube) to develop a model for reproduction of septicemic pasteurellosis. Hydrocortisone caused lymphopenia and inhibited the blastogenic response of peripheral blood lymphocytes to phytohemagglutinin and concanavalin A mitogens. A synergistic effect was observed between hydrocortisone injections and feed changes resulting in higher than expected serum hydrocortisone concentrations and lower circulating lymphocyte counts. Seemingly, stress-induced increases in serum hydrocortisone concentrations cause suppression of the immune response of feedlot lambs. The combined effect of feed changes and stress on the immune response of lambs may explain the role of these 2 factors in the pathogenesis of septicemic pasteurellosis.