Lethal Avian Parasite Behind Swimmer’s Itch Detected in Portugal for the First Time at Lake Alqueva

Trichobilharzia franki detected in Lake Alqueva, Portugal. Researchers warn of rising swimmer's itch risks at Europe's largest artificial reservoir.

By: AXL Media

Published: Apr 29, 2026, 4:14 AM EDT

Source: Information for this report was sourced from EurekAlert

Discovery of Zoonotic Pathogens in Western Europe’s Largest Reservoir

A significant shift in the European distribution of waterborne parasites has been confirmed following the discovery of Trichobilharzia franki in Portugal. This avian schistosome, which was previously undocumented in the country, was identified within Lake Alqueva, a major hub for recreational activities. Researchers note that while the parasite primarily circulates between waterfowl and snails, its free-swimming larvae can inadvertently penetrate human skin. This biological interaction results in cercarial dermatitis, a condition characterized by intense itching and skin eruptions that can escalate in severity with repeated exposure. The findings establish the reservoir as a new focal point for zoonotic monitoring in the region.

Documenting the Arrival of Trichobilharzia Through Snail Surveys

The presence of the parasite was confirmed during an extensive environmental survey conducted between May 2023 and October 2024. During this period, a research team sampled 25 different shoreline locations, collecting over 7,000 freshwater snails for analysis. Molecular testing specifically identified the Radix auricularia snail as the intermediate host for the parasite at a site in Campinho. While the infection rate across the entire basin remained low at 0.6 percent, localized signals were significantly stronger. In specific habitats during peak collection periods, the infection rate among susceptible snails surged to 13.8 percent, indicating concentrated clusters of potential human exposure.

Migratory Birds and the Mechanism of Parasitic Dispersal

Phylogenetic analysis suggests that the Portuguese isolates of the parasite are closely related to lineages previously identified in Central European countries such as Austria and Hungary. This genetic link points toward a dispersal mechanism driven by migratory waterfowl, which transport the pathogens across vast distances. During the study, investigators observed various aquatic bird species, including invasive Egyptian geese and domestic ducks, frequenting the lake's shores. This avian presence creates a continuous cycle of infection, where birds deposit the parasite into the water, which then infects local snail populations before seeking out new hosts.