Changes in hemopoietic tissue of rainbow trout under influence of stress

As in higher vertebrates, stress increases the susceptibility of fishes to disease.This effect is accompanied by changes in circulating white blood cells: the number of lymphocytes decreases while the number of granulocytes increases.Investigations were therefore undertaken to determine in what ways stress induces changes in hemopoietic organs, particularly spleen and head kidneys of rainbow trout Salmo gairdneri Handling and social conflict were used as stressors.These psychic stimuli are typically encountered in aquaculture and under laboratory conditions.Under the influence of both stressors hemopoiesis in spleen and head kidney is disturbed.Blast cells become rare or disappear completely from the tissue.The numbers of mature, intact PMN cells and of small lyrnphocytes also decrease.Hypertrophic macrophage-like cells with vacuolized cytoplasm appear in increasing numbers.They develop mainly from immature PMN and reticuloendothelial cells.They swell continuously while undergoing evident disintegration.At the same time, destruction of erythrocytes proceeds at an increased rate.The high numbers of macrophage-like cells may indicate that the stresson initially stimulate the defense mechanisms and thereby serve an adaptive function.Possibly, under continued stress, their proteolytic enzymes begin to damage the body's own blood cells.The decreased production of new blood cells and the increased destruction of immune-competent cells still present seem finally to contribute to disease susceptibility under stress.