Toxicology of environmental chemicals in the flounder (Platichthys flesus)with emphasis on the immune system: field, semi-field (mesocosm) and laboratory studies

European flounder (Platichthys flesus) has shown an increased prevalence of liver tumors and lymphocystis disease (a viral infection) that correlated with pollution in field research in Dutch coastal and estuarine waters. Sem i-field or mesocosm experiments confirmed the supposed causality. Although these types of research are highly relevant for the feral population, labor atory experiments are necessary to establish causal relationships between s pecific chemical pollutants and disease. Therefore, the effects on flounder of some of the potentially causative chemicals such as benzo[a]pyrene (BaP ), dimethyl-benz[a]anthracene (DMBA), 2,3,7,8 tetrachlorodibenzo-p-dioxin ( TCDD), 3,3',4,4',5 pentachlorobiphenyl (PCB-126), and bis(tri-n-butyltin)ox ide (TBTO) were examined in several laboratory experiments. These effects w ere evaluated using general toxicological parameters and histopathology. Fo r immune function assessment, attempts to develop an infection model with t he lymphocystis virus were made, but appeared unsuccessful and immune funct ion tests are not fully operational at the moment. Flounder has been succes sfully maintained and exposed to toxic substances in captivity in our labor atory. Short-term aqueous exposure to high levels of BaP or DMBA did not in duce marked effects under our experimental conditions. Results of oral expo sure of flounder to low levels of TCDD, PCB-126 or harbor sludge extract sh ow significant induction of cytochrome P4501A (CYP1A) in hepatocytes. Oral exposure to high levels of TCDD or PCB-126 also significantly induced CYP1A immunoreactivity in epithelium in mesonephros and digestive tract and in e ndothelium in several organs. Remarkable was the induction of CYP1A in a di stinct population of mononuclear cells in the mesonephros. Moreover, oral e xposure to TCDD resulted in an increased mitotic activity and an increase o f the hepatosomatic index in the 20 and 500 mu g TCDD/kg group respectively . Therefore, exposure to TCDD and related substances may promote the develo pment of liver tumors in the held. Exposure to PCB-126 also significantly r educed the relative thymus volume, but other results indicate that flounder is relatively insensitive to this type of chemicals. Short-term aqueous ex posure of flounder to TBTO, in concentrations that were in the same order o f magnitude as upper TBT levels measured in the field, caused mortality aft er 7-12 days associated with gill lesions, and induced reduction of the non -specific resistance and decrease of the relative thymus volume. From these results we therefore conclude that TBTO might play a causal role in, for i nstance, increased prevalence of lymphocystis virus infections in the field (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.