Health effects of some aquatic pollutants in European flounder : Laboratory experiments with emphasis on histopathological and immunological aspects

Abstract

Triggered by the concern over relative high prevalences of liver tumors, skin ulcers and the viral lymphocystis disease in European flounder (Platichthys flesus) living in Dutch coastal and estuarine waters, an integrated study was initiated to investigate a possible causal relationship between chemical environmental pollution and these diseases. This thesis describes laboratory experiments in which we focused on histopathological and immunological aspects. The following working hypothesis was formulated from the results of previously conducted field and semi-field experiments: Exposure to major environmental chemical contaminants cause adverse health effects in European flounder including: - immunodeficiency, making individuals more susceptible to infectious diseases - induction of (pre)neoplastic liver lesions. European flounder proved to be a good choice for the main purpose of our laboratory experiments: filling the gap between correlative relationships (field surveys) and circumstantial evidence (semi-field study) on one side and causal relationships (laboratory studies) on the other side, linking chemical pollution and diseases in fish. Exposure to bis (tri-n-butyltin)oxide (TBTO), an organotin compound used as a biocide in anti-fouling paints, resulted in a number of effects such as a decrease in percentage of lymphocytes and total lymphocyte number in the spleen and a reduction of the absolute and relative thymus size. Also a decrease of the non-specific cytotoxic cell (NCC) activity was recorded that might facilitate the surviving of virus-infected cells and tumor cells due to diminished or absent lysis of such cells by NCCs. In the experiments with 2,3,7,8-tetra-chlorodibenzo-p-dioxin (TCDD) and the structurally related 3,3',4,4',5 pentachlorobiphenyl (PCB-126) also effects with possible repercussions for the effectiveness of the immune system were noted. Oral exposure to TCDD resulted in a trend in thymus size reduction, and oral exposure to PCB-126 resulted in a statistically significant reduction of relative thymus size. In both the TCDD and PCB-126 experiments strong induction of cytochrome P4501A (CYP1A) was recorded in several organs and cells, including cells of the hematopoietic system and circulating leukocytes. In literature an association between CYP1A induction and both immunotoxicity and carcinogenesis is suggested. Results from our experiments with TBTO under controlled laboratory conditions using levels that are comparable to high levels in the field indicate that TBTO is indeed a risk factor that may contribute to an increased occurrence of infectious and non-infectious diseases in wild populations. The relevance of the effects recorded in our experiments with TCDD and PCB-126 are more difficult to interpret due to aspects related to the experimental design (e.g. exposure levels, frequency and time), but a contribution of these substances to chemical-induced diseases in the real world is likely. Experiments using juvenile animals or even eggs, that are generally more susceptible to toxic effects, and long-term exposure in the case of carcinogenesis might offer a different perspective. Establishing a functional infection model is an essential prerequisite for validation of the immune status. Unfortunately, our attempts to develop a reproducible and practical infection model with the lymphocystis virus were unsuccessful.