**1. Introduction**

The ability of persistent organic pollutants (POPs) to spread through long distances and remain in the environment has resulted in their presence worldwide. They have a tendency to accumulate within the food chain and, as a result, pose a high risk to human and animal health. The risks caused by industrial POPs are best illustrated by polychlorinated biphenyls (PCBs), extensively used in a wide variety of applications from the early 20th century. PCBs became ubiquitous contaminants of various biotic and abiotic environments worldwide due to their massive and uncontrolled use in industry and agriculture [1, 2].

PCBs are highly persistent compounds in the environment, especially in aquatic sediments, which act as a stable reservoir from which PCBs can continue to be released over a long period of time, because of their low solubility in water and low volatility [3]. The involvement of PCBs in the food chain occurs through the incorporation of suspended particles in phytoplankton and zooplankton, at the base of the food chain. Bottom feeders and other aquatic organisms ingest, accumulate and pass PCBs upward in the food chain [4, 5]. Fish near the top of the aquatic food web have a relatively long life span and concentrate high amounts of PCBs [6]. The concentration of poorly metabolized chemicals accumulated in fish can thus reflect the degree of pollution in an aquatic system [7, 8].

Biomonitoring is a vital and rapidly growing field that uses several biological groups, such as phytoplankton, macrophytes, invertebrates and fish, as bioindicators [9]. This ecological methodology is increasingly used to assess the level of pollution in aquatic environments [10– 13]. Numerous studies have also documented the close relationship between aquatic pollution and parasitism [e.g. 13, 14]. Certain parasitic organisms have the ability to concentrate high quantities of pollutants in their tissues and organs and thus can provide information about the chemical state of the environment [15, 16]. These studies have mainly focused on intestinal parasites, mostly acanthocephalans, as indicators of heavy-metal pollution [14, 17, 18], but data on organic pollutants, including PCBs and their bioaccumulation in parasitic organisms, are still scarce.

The Zemplínska šírava water reservoir (eastern Slovakia; Fig. 1) is one of the most PCBcontaminated sites in Europe [19, 20]. Large amounts of PCB compounds have been released to the reservoir from a chemical factory in the nearby town of Strážske without treatment (decontamination) or preventive measures, leading to heavy contamination of soil, superficial and underground water and subsequently food chains in this area [21]. The PCBs in the reservoir and the inflowing Šíravský canal thus belong to the so-called old environmental burdens, and their amount varies from tens to hundreds of milligrams per kilogram of sediment dry weight. At least 40, 000 tons of PCB-contaminated sediments are assumed to be still present in the reservoir, effluent canal and Laborec River [22]. Studies over the past 20 years have indicated that elevated concentrations of PCBs in the reservoir pose a potential hazard to human and aquatic health [19, 23].
