Two for the price of one: A case report of a neglected trematode of predatory fish, Azygia lucii (Digenea: Azygiidae), and an ignored problem of commercial fisheries

Our investigation aimed to determine the prevalence of Azygiidae (Platyhelminthes: Digenea) firstly in snail hosts (Planorbis planorbis) from a lake widely used for fishing purposes (Lake Drawsko located in the West Pomerania Voivodeship, Poland) and secondly in fish – northern pike (Esox lucius) – from trade (obtained from a fish wholesaler located in the Warmian-Masurian Voivodeship, Poland). Additionally, our molecular identification aimed to document the exact species identity of azygiids. Our analysis of DNA sequences for the mitochondrial COI marker obtained for adults and cercariae confirmed that the checked azygiids belong to Azygia lucii. Additionally, we provided sequences for the first time for the ITS2 marker, which was previously not available for this species. The prevalence of azygiids from the planorbid snail population was 22.58 %, which accounts for 87.5 % of detected digenean trematode infections. All purchased pike were infected with gastrointestinal helminths. We found digeneans in 53.97 % of the fish examined, with A. lucii being the only species recorded. The obtained DNA sequences from both larval and adult stages of A. lucii are the first molecular confirmation of its systematic affiliation in Central Europe. The high prevalence of the trematode in both intermediate and definitive hosts reported in our investigation indicates that more attention should be paid to the epidemiology of fish parasitic diseases, especially in areas where they are obtained for trade. This result raises concerns about its possible adverse economic effects, which A. lucii could have when occurring at high intensities in fish definitive hosts.