The chicken embryo bioassay as a tool to assess the possible toxic effects of persistent organic pollutants (POPs) : a study with special reference to thiamine deficiency, EROD induction and the bursa of Fabricius

Since the introduction of man made chemicals into the environment, the reproductive success of bird species all over the world has been compromised. Since the ban on the use of several chemicals, pollution has decreased, but residues still remain, and reproductive and physiological are still reported to be associated with contaminants. In the Baltic Sea, one of the most contaminated areas in the world, fish suffer from massive mortality during the early life stages. These fish contain high levels of contaminants and low levels of thiamine; thus, it has been hypothesised that contaminants induce thiamine deficiency. As fish eating birds occupy the same position in the food chain as the affected fish, one of the aims of the presented research was to establish the role of contaminant induced thiamine deficiency in bird embryotoxicity. Although the contamination levels in Baltic guillemots have decreased since the 1960s, the birds still contain appreciable amounts of contaminants. The second aim of the research was therefore to assess the potential toxic effects of these contaminants, at the current levels, in these birds.

Several experiments were conducted to study both hypotheses, using an optimised chicken embryo bioassay. The thesis describes the development and validation of the bioassay, as well as its use in studies on the effects of contaminants on embryonal transketolase activity, which is a biomarker for thiamine deficiency. In addition, extracts from Baltic guillemots as well as from oystercatcher eggs from the Zeehavenkanaal in the Netherlands (which is contaminated with hexachlorobenzene) were tested for their toxic potencies. None of the selected model compounds or extracts resulted in decreased transketolase activity, indicating that contaminant induced thiamine deficiency probably does not play a role in avian embryotoxicity. Levels of PCBs in guillemot extracts were high enough to enhance the incidence of malformed embryos in fish eating birds. Both extracts resulted in decreased lymphocyte density in the bursa of Fabricius, and increased activity of EROD. Since EROD induction was found to be a very sensitive biomarker for exposure to contaminants that bind to the Ah receptor, this biomarker was studied during embryonic development; it was found to be a conservative predictor for toxicity in birds. Results were discussed in the framework of species differences in sensitivity towards toxic actions of chemicals. It was concluded that under the present contamination levels, toxic effects could not be excluded in Baltic guillemots and oystercatchers from the Zeehavenkanaal.