Distribution of Clinostomum complanatum in two commercially important freshwater fish, perch and rudd, in France

Over the last two decades, the popularization of new eating habits and the increase in fish products demand led to a raising risk for consumers due to food-borne parasitic zoonoses. Species of Clinostomum Leidy, 1856 are cosmopolitan zoonotic digenetic trematodes. They are present, at the juvenile stage, in numerous freshwater fish. Among them, Clinostomum complanatum may induce pharyngitis or laryngitis in humans following consumption of raw fish infected by the metacercariae. In France, the first mention of Clinostomum spp. metacercariae on freshwater fish was made in the Durance River in 2008, with almost no data and very obscure conclusions. However, recently, the infection of a batch of wild perch in the Jura in December 2019 was described. The present study provides the first evidence of the extent of the distribution of C. complanatum across continental France based on sampling of freshwater fish (European perch and rudd) from different departments. A survey of infection levels was realized on 14 batches of fish collected in eight departments. Fish were sampled by recreational anglers or pond fish farmers. They were dissected for the presence of C. complanatum metacercariae. All metacercariae were identified based on molecular analyses on mtDNA cox1 gene fragment and rDNA 28S gene fragment. Encysted metacercariae were found in five departments and in 113 out of 526 sampled fish. Intensity of infection ranged from 4 to 25 parasites per fish. All metacercariae were identified as C. complanatum. Phylogenetic and haplotype network analyses reported low genetic diversity in French C. complanatum individuals on mtDNA cox1 marker indicating the absence of distinct populations in the French territory. French specimens shared common mtDNA haplotypes with C. complanatum specimens from Italy. This study confirmed the presence of this zoonotic trematode species in freshwater consumed fish, and expanded the known geographical distribution area in France. Moreover, the high intensity and prevalence recorded suggested it could represent a hazard both to human and animal health. We discuss how future research should address the zoonotic risk of this parasite and ensure the health safety of fish-based products and new consumer habits.