Mapping of Ligula plerocercoids in the freshwater bream Abramis brama in Lake Syamozero and some other lakes of Northwestern Russia

The tapeworm Ligula intestinalis (Linnaeus, 1758) is a broadly distributed parasite with high epidemiological significance. Its first intermediate hosts are crustaceans, its second intermediate hosts are fish, while the definitive hosts are various fish-eating birds and, as recently shown, ringed seals. The stages of Ligula intestinalis in fish are called plerocercoids. In this study we obtained new data on the occurrence of plerocercoids of L. intestinalis in freshwater bream Abramis brama in six lakes in Karelia (Northwestern Russia). Infection of freshwater bream with L. intestinalis in Lakes Onego, Ladoga, Svyatozero and Konchezero was found for the first time. In Lake Syamozero, where long-term parasitological monitoring has been conducted, infection indices of bream with L. intestinalis were similar to those recorded in 1975. Using two mitochondrial genes (cytb and COI), we examined the genetic structure of L. intestinalis from different localities. Phylogenetic analysis showed that all specimens of of L. intestinalis examined in our study belonged to “Lineage A” identified by Nazarizadeh et al. (2023). We found 40 haplotypes of concatenated cox1+ cytb in freshwater bream from different water bodies in Northwestern Russia and Europe. Plerocercoids from Karelian lakes were represented by 22 haplotypes, of which 21 were unique. Our molecular analysis of L. intestinalis from freshwater bream in European part of the Palearctic shows that this parasite lineage is genetically diverse and rapidly expanding.