A new study published in the Journal of Helminthology by researchers from the Estonian University of Life Sciences and the Swedish University of Agricultural Sciences together with collaborators from Greenland and the Faroe Islands, has revealed surprisingly limited dispersal of Diplostomum parasites across North Atlantic islands. The findings challenge the common assumption that migratory birds readily transport parasites over large geographic distances.
Diplostomum is a genus of trematodes (parasitic flatworms) ubiquitous in freshwater ecosystems. They are characterised by a complex life cycle involving aquatic snails, fish, and fish-eating birds as their definitive host. Because these birds typically undertake annual migrations from southern wintering areas in the south to Arctic breeding grounds, they serve as an ideal model system for studying long-distance dispersal and biological connectivity. To test the role of the avian host as a potential parasite dispersal vector, the international research team investigated the diversity and distribution of Diplostomum parasites infecting freshwater salmonids in Greenland and the Faroe Islands. They utilized modern DNA metabarcoding, a next-generation sequencing, method that allows the simultaneous characterisation of complex communities using short DNA fragments.
The researchers found striking differences between the North-Atlantic island systems. In Greenland, infections were common in Arctic char and Atlantic salmon and the genetic analyses revealed four parasite lineages, including a potentially undescribed new species. In contrast, no Diplostomum infections were detected in brown trout or Atlantic salmon sampled from sixteen streams across the Faroe Islands.
The findings suggest that migratory birds are not always effective vectors of parasite dispersal, and other factors may limit parasite spread across ecosystems. Consequently, parasite communities in Greenland were more closely related to those found in North America than to those reported from Iceland or northern Europe. Despite being potentially connected by the migration of the avian definitive hosts, the results indicate a limited exchange of parasites across the North Atlantic.
“Given the extensive movements of migratory birds across the North Atlantic, we initially expected much greater overlap in parasite communities among North-Atlantic islands,” said the first author, Alfonso Díaz-Suarez, a postdoctoral researcher at the Estonian University of Life Sciences, “Instead, we found striking differences between regions, indicating that Diplostomum parasites have a more limited distribution despite the presence of highly mobile hosts.”
The researchers suggest that this limited distribution may result from a short transmission season, with parasite transmission occurring only during the breeding season of the avian definitive host in the Arctic and not in the southern wintering areas. This temporal limitation of transmission together with specific migration routes and host distribution, may substantially reduce opportunities for successful parasite colonization between island systems.
“Many people assume that migratory birds freely transport parasites across vast geographic distances,” added Professor Anti Vasemägi. “Our findings suggest that successful parasite dispersal is much more restricted and depends on a combination of host movements, environmental conditions, and the complex life cycles of the parasites themselves.”
One of the most intriguing discoveries was the identification of a potentially new parasite species in Greenland. The finding suggests that North Atlantic and Arctic ecosystems may harbour unique parasite biodiversity that has remained undocumented.
Such hidden diversity may provide valuable insights into evolutionary processes, host–parasite interactions, and the historical colonization of northern freshwater ecosystems. The study also demonstrates the power of modern DNA-based methods for uncovering biodiversity that would be difficult to detect using traditional approaches alone, which can be an essential tool to explore the diversity of a changing ecosystem.
Beyond advancing the understanding of parasite ecology, the findings highlight a broader lesson about biological connectivity. While migratory birds are often viewed as powerful agents of dispersal, complex life cycles and ecological constraints can strongly limit the movement of associated organisms.
Journal of Helminthology
Observational study
Animals
Not-so-frequent flyers? Restricted avian dispersal of Diplostomum parasites across North Atlantic islands
9-Jun-2026
There are no conflicts-of-interest related.