The effect of the nematode Contracaecum (Ascaris) on fish of the cooling reservoir of the Zaporizhzhzia nuclear power plant

Purpose: To determine species composition and localization of anisakidosis pathogens, infestation of fish species in the Zaporizhzhia NPP cooling reservoir, and identify trends in their infestation compared to monitoring epizootic studies of the last year. Methodology. In the course of the work, we used the generalized results of the studies conducted in the summer and autumn of 2021. The fish species composition in the ZNPP cooling reservoir as well as the species composition of parasites of Anisakidae family and their localization were determined. The studies were carried out using the method of incomplete parasitological dissection of fish at the laboratory. Nematode larvae were detected by visual inspection. The parasites were identified using identification keys. The detected parasites were washed with a saline and stored in 70% ethanol. For the quantitative analysis of the parasitofauna, we used such parameters as prevalence (%) and intensity (ind./ fish). Findings. The parasitological analysis showed that Contracaecum (Ascaris) larvae were found in European, channel and redtail catfishes, which were localized as cysts or free-living in the abdominal cavity. The infestation intensify reached 86 ind./fish, and prevalence reached 100%. The parasites caused hyperemia (redness) and inflammation of internal organs. At the same time, no Contracaecum larvae were found in common and silver carps. Compared to 2020, the infestation of fish was less pronounced, namely: no monogenean Dactylogirus were found on the gills of carps as well as ciliates including Trichodina and Apiosoma, which were found in small amounts on the body surface and gills of common and silver carps. The Contracaecum larvae invasion intensity in catfish decreased by 17 %. Originality. For the first time, a large tendency to the spread of Contracaecum larvae among fish of the ZNPP cooling reservoir was detected, namely in European, channel and redtail catfishes. The importance of the threat of this invasion for fish was established. Practical value. The obtained results can be used to identify timely trends in the infestation of fish by Anisakidae and to develop sanitary and preventive measures in the reservoir.