Migratory hosts are key vectors of long-distance parasite dispersal. In the North Atlantic, migratory connectivity of avian hosts suggests that diplostomid trematodes with broad host specificity could potentially exhibit widespread distributions. However, host migration does not necessarily result in successful parasite dispersal. We investigated the diversity and distribution of Diplostomum spp. infecting freshwater salmonids in Greenland and the Faroe Islands using a metabarcoding approach. In Greenland, infection prevalence in Arctic char and Atlantic salmon ranged from 0% to 100% across locations. Molecular analysis revealed four lineages, including the first records of Diplostomum sp. 6, 7, and 9 (Locke et al. 2010a; 2010b), and a novel lineage/species showing 92.6–94.2% sequence identity to its closest relatives. In contrast, no evidence of diplostomid infection was found in brown trout and Atlantic salmon collected from sixteen streams in the Faroe Islands. The presence of a novel, locally abundant (28.9% prevalence) parasite lineage/species in Greenland, the affinity of these parasite assemblages with those in North America, the lack of overlap with Icelandic and Norwegian communities, and the absence of diplostomids in Faroese salmonids indicate limited bird-mediated dispersal of diplostomids across the North Atlantic. We propose that a ‘short transmission window’ at high latitudes during the breeding season, combined with specific avian migration routes, may limit long-distance dispersal and promote regional isolation between Nearctic and Palearctic diplostomid communities across the North Atlantic.

The biogeographic histories of parasites and pathogens are infrequently compared with those of free-living species, including their hosts. Documenting the frequency with which parasites and pathogens disperse across geographic regions contributes to understanding not only their evolution, but also the likelihood that they may become emerging infectious diseases. Haemosporidian parasites of birds (parasite genera Plasmodium, Haemoproteus and Leucocytozoon) are globally distributed, dipteran-vectored parasites. To date, over 2000 avian haemosporidian lineages have been designated by molecular barcoding methods. To achieve their current distributions, some lineages must have dispersed long distances, often over water. Here we quantify such events using the global avian haemosporidian database MalAvi and additional records primarily from the Americas. We scored lineages as belonging to one or more global biogeographic regions based on infection records. Most lineages were restricted to a single region but some were globally distributed. We also used part of the cytochrome b gene to create genus-level parasite phylogenies and scored well-supported nodes as having descendant lineages in regional sympatry or allopatry. Descendant sister lineages of Plasmodium, Haemoproteus and Leucocytozoon were distributed in allopatry in 11, 16 and 15% of investigated nodes, respectively. Although a small but significant fraction of the molecular variance in cytochrome b of all three genera could be explained by biogeographic region, global parasite dispersal likely contributed to the majority of the unexplained variance. Our results suggest that avian haemosporidian parasites have faced few geographic barriers to dispersal over their evolutionary history.