Modulation of nonspecific host defenses in fish.

Seasonal, nutritional and hormonal effects on fish nonspecific protective mechanisms have been reviewed, together with the influence of some environmental pollutants. In the plaice (Pleuronectes platessa L.), the serum concentration of lysozyme and the acute phase reactant, C-reactive protein (CRP) and in vitro neutrophil migration have been used as indicators of the ability of test substances to modulate nonspecific defense mechanisms. Cortisol and adrenalin appear to be implicated in the production of CRP, and cortisol, at concentrations found in naturally stressed plaice, significantly reduces the migration of peritoneal neutrophils. Exposure to sub-lethal concentrations of inorganic mercury reduce circulating levels of lysozyme. The results provide further support for an association between environmental stressors and the disease susceptibility of fish populations. Phagocytic cell function, as measured by chemiluminescence, migration or phagocytic index, emerges as a useful test system.