Integrating molecular biomarkers and ectoparasitic infestation to assess water quality.

Fish and their ectoparasites serve as model organisms for evaluating aquatic ecosystem health and can thus function as bioindicators of environmental pollution. This study, therefore, investigates the gene expression of biomarkers of effect in Clarias gariepinus inhabiting a polluted drain called the Kitchener Drain and the Nile River (a reference habitat). Ectoparasites and metacercariae of digenean endoparasites on the collected fish were examined to further clarify the impact of pollution. Compared with the Nile River, the Kitchener Drain contained significantly elevated concentrations of heavy metals, bicarbonates, sulfates, and minerals. Interestingly, the prevalence, mean intensity, and abundance of ectoparasitic monogeneans from C. gariepinus collected from the Nile River were higher compared with those from the Kitchener Drain, whereas the drain had higher levels of metacercaria of the digenean endoparasite Centrocestus formosanus. Fish from the Kitchener Drain had significantly elevated hepatic expressions of genes related to metal toxicity, oxidative stress, inflammation, heat shock response, and xenobiotic metabolism. The results indicate that C. gariepinus exposed to metals and other pollutants triggers an adaptive response involving multiple stress-response pathways. The findings also highlight the impact of pollution on parasitic infestation in this fish population. The study supports the utilization of molecular biomarkers and ectoparasitic infestation as early warning signals for water pollution.