A newly detected parasite in Portugal hints at an emerging and little-understood risk in freshwater ecosystems.
Researchers in Portugal have identified parasites from the Trichobilharzia group, including T. franki, in Lake Alqueva. This is the first confirmed record of these avian schistosomes in the country and highlights a major recreational freshwater site as a potential location for cercarial dermatitis risk 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.
