Pathogenic infections confounds induction of the estrogenic biomarker vitellogenin in rainbow trout

In their environment, aquatic organisms are exposed to multiple rather than single stressors. 26 While during recent years, considerable progress has been achieved in assessing combination 27 effects of chemical mixtures, less knowledge is available on the combined effects of chemical 28 and physical and/or biological stressors. Here, we use the well-established biomarker 29 vitellogenin (VTG), which is induced under exposure of fish to estrogen-active environmental 30 compounds, to investigate the combined impact of a chemical and a biological stressor. 31 Juvenile rainbow trout, Oncorhynchus mykiss, were treated with two doses of the prototypic 32 estrogen-active compound, 17-beta-estradiol (E2), alone and in combination with the fish-33 pathogenic parasite Tetracapsuloides bryosalmonae,. This parasite is the etiological agent of 34 the proliferative kidney disease (PKD) and can cause severe losses in wild salmonid 35 populations. The hepatic VTG response of rainbow trout was assessed at the mRNA level 36 using qRT-PCR; in addition, microarray analysis was used to examine the hepatic 37 transcriptome response to the combined stressor impact. The results of the mixture 38 experiment showed that both rate and magnitude of the VTG response were significantly 39 reduced in fishes exposed to both stressors compared to fishes exposed to E2 only. 40 Microarray analyses revealed a comparable response pattern of most other E2-inducible 41 genes. The observed depression of the VTG biomarker induction in diseased fish may be 42 particularly relevant in situations with low concentrations of environmental estrogens, as it is 43 the typical exposure scenarios in many rivers.