Cercarial dermatitis (CD), or “Swimmer’s itch” as it is also known, is a waterborne illness caused by a blood fluke from the family Schistosomatidae. It occurs when cercariae of trematode species that do not have humans as their definitive host accidentally penetrate human skin (in an aquatic environment) and trigger allergic symptoms at the site of contact. It is an emerging zoonosis that occurs through water and is often overlooked during differential diagnosis. Some of the factors contributing to the emergence of diseases like CD are related to global warming, which brings about climate change, water eutrophication, the colonization of ponds by snails susceptible to the parasite, and sunlight exposure in the summer, associated with migratory bird routes. Therefore, with the increase in tourism, especially at fluvial beaches, it is relevant to analyze the current epidemiological scenario of CD in European countries and the potential regions at risk.

Physella acuta is an invasive freshwater snail with a global distribution and a recognized role as an intermediate host for various trematodes, including Echinostoma spp. and Trichobilharzia physellae. In Portugal, P. acuta is commonly found in freshwater bodies such as Lake Alqueva, the largest artificial reservoir in Europe. The lake’s creation has altered local ecosystems, influencing freshwater snail populations and migratory bird activity, which may contribute to the dispersal of trematode parasites. While P. acuta is present in the region, its role in trematode transmission remains unclear. This study investigated P. acuta as a potential intermediate host for trematodes in Lake Alqueva. Freshwater snails were collected from 18 sites, with cercarial shedding induced under artificial light. Infected snails were found in 2 of the 18 snail populations surveyed. A sequence analysis of the amplified ITS2 rDNA region confirmed the presence of Posthodiplostomum sp., implicating, for the first time, P. acuta as an intermediate host for this parasite in Portugal. This study highlights the need for further research on P. acuta’s role in trematode transmission and potential impact on local ecosystems to assess parasitic risks to veterinary and public health.

Emerging parasites pose increasing challenges at the interface of human, animal, and ecosystem health. Among these, Trichobilharzia spp., schistosomatid of migratory and resident waterfowl, can cause cercarial dermatitis (CD) when its larval stage (cercaria) accidentally penetrate human skin. While this parasite has been documented in several European countries, its presence in Portugal has remained unreported, leaving a critical gap in understanding potential zoonotic risks in local freshwater ecosystems. Therefore, this study aimed to investigate the presence of Trichobilharzia spp. in freshwater snails from Lake Alqueva, providing the first insights into its potential ecological and public health implications in Portugal. Fieldwork was carried out on Lake Alqueva, considered the largest artificial reservoir in Western Europe, with ecological and public health relevance. Freshwater snails from shore locations in Lake Alqueva were examined for cercarial shedding, and molecular identification was performed by PCR amplification of the cytochrome c oxidase subunit 1 (COX1) gene and internal transcribed spacer (ITS) regions. Morphological and genetic analyses confirmed Trichobilharzia franki from Radix auricularia, showing high homology with European and Asian lineages. Moreover, human infections compatible with CD were also reported in the region. These findings highlight the need for continued malacological surveillance, particularly in recreational freshwater bodies, to assess risk areas and implement mitigation strategies. Furthermore, this study expands the known geographical distribution of T. franki in Europe and underscores the importance of integrating ecological and public health approaches to monitor emerging zoonotic parasites.