Dibothriocephalosis in salmonids from Iceland: A more complex taxonomic problem than assumed until now?

The diphyllobothriid tapeworm Dibothriocephalus ditremus, one of the three Dibothriocephalus species native to Europe, parasitises exclusively piscivorous birds and has not yet been detected in mammals. This is probably the reason why there is much less molecular data on this tapeworm. The aim of our study was to determine the genetic structure of the D. ditremus populations from Europe, namely Iceland, for the first time. To exclude any possible misidentifications between sympatrically occurring D. ditremus and D. dendriticus, D. dendriticus from Iceland was also analysed. Great genetic diversity of D. ditremus, displayed by a large number of cytochrome c oxidase subunit 1 (cox1) haplotypes and three distant clusters, contrasted sharply with the lower genetic variation in D. dendriticus. Previously published cox1 sequences of D. ditremus from different localities in Europe (UK - Scotland), Asia (Russia and Japan) and North America (USA - Oregon) were also included in the analysis in order to determine the genetic architecture of D. ditremus at a broader geographical scale. While the sequences of tapeworms from Scotland and Russia were placed in D. ditremus Clusters 2 and 3, the sample from USA (Oregon) displayed a unique position distant from the Icelandic tapeworms. Japanese samples of D. ditremus and Diphyllobothrium hottai formed a common clade, indicating their conspecificity. The unexpected output of the analysis was a unique position of the currently detected Haplotype 31 of a tapeworm from Iceland, which was distant from all other D. ditremus individuals from Iceland, but showed close relationships with the Japanese D. ditremus/D. hottai cluster. Further studies are needed to reveal if D. ditremus represent a complex of genetically diversified populations, or more species occur in salmonids in the Northern Hemisphere.