Introduction
There is an extensive range of helminthic parasites of fish, but only a moderate number of species are capable of producing foodborne diseases in humans (WHO 1995). These diseases are either caused by an infection following ingestion of viable parasites, or by an allergic reaction against parasite antigens which occurs, so far, for nematodes of the family Anisakidae. Anisakis simplex is the species most frequently associated with human disease, followed by Pseudoterranova decipiens (Adams, Murrell et al. 1997) and Contracaecum osculatum (Strom, Haarder et al. 2015). In fact, A. simplex is the second most predominant biological hazard, constituting 33% of the reported hazards and in recent years a blip is observed in the notifications for this nematode (Kleter, Prandini et al. 2009). This increase can be explained for several reasons: (i) the development of new and improved clinical diagnosis tests, (ii) the increase in raw fish consumption and (iii) the growth in international market in fish products. For these main reasons in 2010, EFSA recommended studies to evaluate the effects of different farming practices on the prevalence of anisakids in aquaculture (EFSA 2010). Very recently a survey detected anisakids larvae (A. simplex and Hysterothylacium aduncum) in Atlantic salmon runts (Mo, Gahr et al. 2014) which contrast with a number of previous studies that confirmed no anisakids larvae in sea-farmed salmonids. These discrepancies remark the necessity of new surveys to evaluate another possible infection route such as invertebrates and/or "sneakers" in open cages that can be eaten by farmed fish and subsequently infect them (Mo, Gahr et al. 2014) especially if the farmed fish is not properly fed. Other fish helminths of zoonotic concern are cestodes and trematodes (especially Diphyllobothriid and Opisthorchioidea, respectively). They have received less attention despite of the reported human cases in Europe caused by these freshwater fish-borne zoonotic parasites Thus far, the available epidemiological data for farmed freshwater fish are scarce.
Concluding, it is generally assumed that farmed fish products have a very low or null prevalence of these helminths. However, this assumption has not been demonstrated scientifically and even less globally for the main European farmed fish species. The objective of these systematic surveys is to clarify the potential sources/routes of infection and design management strategies to decrease the occurrence of zoonotic helminths in farm fish food to reinforce competitiveness of EU aquaculture producers by development and implementation of good practices to obtain safe, high-quality fish food products.
Material and Methods
From spring 2016 to spring 2017 a total of 7000 farmed fish have been examined from the following species: Gilthead Sea Bream (GSB), European Sea Bass (ESB), Turbot (T), Atlantic Salmon (AS), Marine Rainbow Trout (MRBT), Rainbow Trout (RBT) and Common Carp (CC); obtained from a representative number of fish farms located in Spain, Italy, Greece, Denmark, Norway and Hungary. A random polietapic and stratified sampling plan was selected with a confidence level of 99%. In addition, primary processed products (fresh ESB and GSB) and secondary products (smoked AS), from supermarkets in France, Italy and Spain were also included in the study. Identification of parasites were carried out with different methodologies depending on the type of sample (viscera, fillet etc…) and fish species including visual inspection, UV-press method, artificial digestion, candling, muscular compression/artificial digestion followed by microscopic examination and PCR. Species identification of parasites were done by sequencing.
Results
The findings showed that no zoonotic parasites have been found in all the examined marine fish at the level of confidence of 99% with a margin of error of 4-8%. Only one specimen of Hysterothylacium fabri L4 encapsulated on the surface of the liver in one sea bass from one Italian farm has been found. Similarly, no zoonotic parasites have been found in freshwater RBT. However, we have already found several metacercarial infected muscle samples (Holostephanus sp.) of CC with a preliminary estimated prevalence of 13.9% (36/258) from Hungarian fish farms.
Discussion and conclusion
In conclusion, we have not detected any zoonotic parasites in marine samples and freshwater rainbow trout, thus the prevalence is zero. However, some digenean metacercariae (Holostephanus sp) infecting muscle samples of common carp were identified and one specimen of H. fabri were also identified in one European sea bass. These findings will be confirmed in a second round focused survey that will be run to identify the critical points in the farming production chain which are responsible for the entrance of these parasites.
Acknowledgments
Our study was carried out within the framework of ParaFishControl (n º 634429), an EU H2020-funded project that aims at increasing the sustainability and competitiveness of the European aquaculture industry through control and mitigation of the most harmful parasites affecting the main European farmed fish species.
References
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