A newly detected parasite in Portugal hints at an rising and little-understood threat in freshwater ecosystems.
Researchers in Portugal have recognized parasites from the Trichobilharzia group, together with T. franki, in Lake Alqueva. That is the primary confirmed report of those avian schistosomes within the nation and highlights a serious leisure freshwater website as a possible location for cercarial dermatitis threat in Europe.
Trichobilharzia species are parasitic flatworms that cycle between waterfowl and freshwater snails. Their free-swimming larvae can accidentally penetrate human skin, leading to cercarial dermatitis, also known as swimmer’s itch. While not typically dangerous, the condition can cause intense itching, red bumps, and blister-like lesions, with symptoms sometimes worsening after repeated exposure. Several species in this group have been linked to human infections across Europe, and T. franki is among the most commonly involved.
A Notable Gap in Portugal
Until this study, Trichobilharzia species had not been formally reported in Portugal. This absence was notable given Lake Alqueva’s status as the largest artificial reservoir in Western Europe and its growing popularity for recreation, including swimming, fishing, and birdwatching.
The researchers also point out that swimmer’s itch has not been systematically monitored in the country. However, early survey data from local clinicians and pharmacists had already suggested cases consistent with the condition in the Alqueva region.
To investigate, the research team surveyed 25 shoreline sites around Lake Alqueva between May 2023 and October 2024. Over eight field visits, they collected 7,125 freshwater snails, including 6,414 Physella acuta, 660 Radix species, and 51 Gyraulus species.
Out of 325 snails suitable for molecular testing, four Radix auricularia individuals released larvae consistent with Trichobilharzia. All infected snails were found at a single location, Campinho, in September 2024.
Molecular Confirmation of T. franki
Detailed morphological and genetic testing confirmed that the parasites belonged to T. franki. The larval DNA showed over 95% similarity to known COX1 sequences of T. franki and a complete match for the ITS1 region.
The snail host was also confirmed as Radix auricularia, reinforcing its role as an intermediate host for this parasite in Lake Alqueva.
Across the entire lake basin, the infection rate among Radix snails was 0.6%. However, at the specific site where infected snails were found, the rate reached 13.8%, indicating a concentrated local presence.
The parasite was detected only in snails that had already released larvae, with no signs of earlier-stage infections in other samples. The study also notes reports of human cases consistent with swimmer’s itch in the same area.
Evidence of Bird-Mediated Spread
Genetic analysis showed that the Portuguese samples are closely related to T. franki strains previously identified in countries such as Austria and Hungary.
This pattern suggests the parasite may have spread through migratory waterfowl, which can transport it over long distances. During the study, researchers observed several bird species near the lake, including domestic ducks, other members of the Anatidae family, and invasive Egyptian geese.
Public Health and One Health Implications
The discovery expands the known range of T. franki in Europe and identifies Portugal as a potential hotspot for this zoonotic parasite. It also underscores the interconnected roles of wildlife, aquatic ecosystems, and human activity.
Lake Alqueva experiences heavy recreational use during warmer months, when both snail activity and human water contact are highest. Although swimmer’s itch is usually mild, it can still affect tourism and access to public beaches if not managed.
The authors recommend ongoing monitoring of recreational waters, increased awareness among healthcare providers and the public, and the use of environmental DNA methods for early detection. Combining ecological data with public health strategies could help identify and control emerging parasite risks before they become more widespread.
Reference: “Emerging zoonotic risk: molecular detection of Trichobilharzia franki in Western Europe’s largest artificial lake, Alqueva, Portugal” by Maria Teresa Bispo, Isabel Larguinho Maurício, Pedro Manuel Ferreira, Silvana Belo and Manuela Calado, 21 January 2026, Science in One Health.
DOI: 10.1016/j.soh.2025.100143
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