Parasite behind swimmer’s itch found for the first time in Portugal

A study in Portugal has identified parasites from the Trichobilharzia spp. group, including T. franki, in Lake Alqueva. The finding marks the first confirmed record of these avian schistosomes in Portugal and places a major recreational freshwater site on the map of potential cercarial dermatitis risk in Europe.

Background

Trichobilharzia spp. are avian schistosomes that circulate between waterfowl and freshwater snails. Their free-swimming larval stage can also penetrate human skin accidentally, causing cercarial dermatitis, commonly known as swimmer’s itch. Although the condition is usually not life-threatening, it can cause intense itching, erythematous papules, and vesicular eruptions, and symptoms may become more severe or prolonged after repeated exposure. Across Europe, several species in this genus have been linked to human cases, with T. franki among the species most frequently associated with cercarial dermatitis.

Until now, however, Trichobilharzia spp. had not been documented in Portugal. That gap was particularly relevant for Lake Alqueva, the largest artificial reservoir in Western Europe and an increasingly popular site for water sports, fishing, and birdwatching. The authors note that cercarial dermatitis has not been systematically monitored in Portugal, even though preliminary local questionnaire data from clinicians and pharmacists had already suggested human infections compatible with the condition in the Alqueva region.

Snail survey detected Trichobilharzia spp. in Lake Alqueva

The research team surveyed 25 shoreline locations around Lake Alqueva between May 2023 and October 2024. Across eight field trips, they collected 7,125 freshwater snails, including 6,414 Physella acuta, 660 Radix spp., and 51 Gyraulus spp. Among 325 lymnaeid snails suitable for molecular analysis, four individuals identified as Radix auricularia shed furcocercariae consistent with Trichobilharzia spp. All four infected snails were collected at Campinho in September 2024.

Molecular analysis identified the detected parasite as T. franki

Morphological and genetic analyses showed that the detected Trichobilharzia spp. belonged to T. franki. The cercarial sequences showed more than 95% similarity to reference COX1 sequences of T. franki and 100% homology for ITS1. The infected snail host was also molecularly confirmed as Radix auricularia, supporting its role as an intermediate host for T. franki in Lake Alqueva.

A low overall prevalence but a stronger local signal

At the basin scale, the infection rate among Radix snails was 0.6%. At the specific habitat and sampling event where infection was detected, however, the rate reached 13.8%. The parasite was detected only in the hepatopancreas of the four snails that had already shed cercariae, and no evidence of pre-patent infection was found in the other analyzed samples. The paper also notes reports of human infections compatible with cercarial dermatitis in the same region.

European lineages point to possible bird-mediated dispersal

Phylogenetic analysis placed the Portuguese Trichobilharzia spp. isolates within T. franki lineages previously reported from other parts of Europe, including Austria and Hungary. The authors interpret this pattern as compatible with long-distance dispersal through migratory waterfowl, which are known to contribute to the geographic spread of these parasites. During fieldwork, the team observed aquatic birds around the lake, including domestic ducks, other Anatidae species, and invasive Egyptian geese.

Significance and implications

The findings extend the known European distribution of T. franki and establish Portugal as a zoonotic hotspot. They reinforce a One Health perspective: birds, snails, aquatic environments, and human recreation intersect in a shared system. Lake Alqueva's heavy recreational use during warm months—when snail activity and human water contact peak—raises practical concerns for public health monitoring. Although cercarial dermatitis is often regarded as minor, it can affect tourism and public beach access if unmanaged. The study calls for continued surveillance in recreational freshwater systems, stronger awareness among residents and clinicians, and use of environmental DNA approaches for early detection. Integrating ecological surveillance with public health action can help detect and manage emerging zoonotic parasites before they become a larger problem.