Black spot diseases in seven commercial fish species from the English Channel and the North Sea: infestation levels, identification and population genetics of Cryptocotyle spp

Fish are often speckled with "black spots" caused by metacercarial trematode infection, inducing a host response. Cryptocotyle spp. (Opisthorchiidae) are among the parasites responsible for this phenomenon. So far, the impact on human health is still unknown. In addition, few publications dealing with black spot recovery, identification, distribution and diversity among commercially important fish are available. Moreover, "black spots" have been observed by fishermen on marine fish, revealing an appreciable but unquantified presence in consumed fish. An epidemiological survey of 1,586 fish from seven commercial species (herring, sprat, whiting, pout, dab, flounder, and plaice) was conducted in the Eastern English Channel and the North Sea in January 2019 and 2020. Encysted metacercariae were found in 325 out of 1,586 fish, with a total prevalence of 20.5%. Intensity of infection varied from 1 to 1,104 parasites. The recorded encysted metacercariae were identified either by microscopic examination or with molecular tools. Partial sequences of the mtDNA cox1 gene and of the rDNA ITS region were obtained. Two species of Cryptocotyle, Cryptocotyle lingua (Creplin, 1825) and Cryptocotyle concava (Creplin, 1825) were found. Metacercariae belonging to other trematode families were also identified. Molecular phylogenetic analysis and haplotype network construction were performed to confirm the identification and to study the potential presence of different populations of Cryptocotyle spp. This survey enabled us to describe the distribution of two species of Cryptocotyle in the English Channel and North Sea ecosystems. The observed differences in infestation levels between fish species and geographical areas will contribute to better understanding of the ecology of these parasites.

Optimization of tools for the detection and identification of Cryptocotyle metacercariae in fish: Digestion method and viability studies

Some trematode metacercariae, including marine digeneans belonging to the genus Cryptocotyle, induce black spots in target tissues due to the attraction of fish host melanophores. To promote precise quantification of infection, the counting of black spots has to be confirmed by reliable quantification of metacercariae after tissue digestion. This process ensures the isolation of undamaged parasites for morphological and molecular identification. The aim of this work was to optimize the pepsin digestion protocol and to assess the duration of viability of Cryptocotyle metacercariae in fish post-mortem (pm). Four digestion protocols were compared by measuring the viability rate of metacercariae. The present study shows that the orbital digestion method was the least destructive for metacercariae and allowed better quantification of Cryptocotyle infection. Moreover, morphological identification seemed reliable up to 8 days pm for Cryptocotyle infection.