A previously unrecorded parasite has been identified in Portugal’s Lake Alqueva, raising new questions about public health risks in popular recreational waters.
Researchers have confirmed the presence of Trichobilharzia franki, a parasitic flatworm, in Lake Alqueva, the largest artificial reservoir in Western Europe. This marks the first official detection of this specific avian schistosome in Portugal, signaling a potential emerging health risk for millions of visitors who swim, fish, and relax along its shores.
While the parasite does not cause serious disease in humans, it triggers cercarial dermatitis, commonly known as “swimmer’s itch.” This condition causes intense itching, red bumps, and blister-like lesions that can persist for days or weeks. The discovery highlights a gap in surveillance for a condition that has likely been present but undiagnosed in the region.
A Hidden Threat in a Popular Destination
Lake Alqueva is a major tourist hub, attracting visitors for water sports, birdwatching, and leisure. Until this study, Trichobilharzia species had never been formally reported in Portugal. This absence was surprising given the lake’s size and popularity, suggesting that previous cases of swimmer’s itch may have been overlooked or misdiagnosed.
Early informal surveys from local clinicians and pharmacists had already hinted at cases consistent with the condition in the Alqueva region. The new study provides the scientific evidence needed to connect these symptoms to a specific biological source.
Why this matters: The detection transforms a vague health complaint into a identifiable environmental risk. It forces a re-evaluation of water safety protocols in a region that relies heavily on tourism.
How the Research Unfolded
To investigate the suspected outbreak, a research team conducted a comprehensive survey between May 2023 and October 2024. They visited 25 shoreline sites around the lake, collecting a total of 7,125 freshwater snails.
The analysis revealed a critical pattern:
– The vast majority of snails tested negative.
– However, at a single location called Campinho, researchers found infected snails.
– Out of 325 snails suitable for molecular testing, four individuals of the species Radix auricularia released larvae consistent with Trichobilharzia.
The infection rate across the entire lake basin was low at 0.6%, but at the specific Campinho site, it spiked to 13.8%. This indicates that the parasite is not evenly distributed but exists in concentrated “hotspots” linked to specific environmental conditions.
Confirming the Culprit: T. franki
Using detailed morphological and genetic testing, scientists confirmed the parasite as T. franki. The larval DNA showed over 95% similarity to known sequences of this species.
This identification is significant for two reasons:
1. Host Confirmation: Radix auricularia was confirmed as the intermediate host, meaning this specific snail species is essential for the parasite’s life cycle in this ecosystem.
2. Geographic Spread: Genetic analysis showed that the Portuguese samples are closely related to strains found in Austria and Hungary. This suggests the parasite was likely transported by migratory waterfowl, which act as definitive hosts for the parasite.
Researchers observed several bird species near the lake, including domestic ducks, wild Anatidae, and invasive Egyptian geese. These birds carry the adult worms and shed eggs into the water, which then infect snails, completing the cycle.
Public Health and the “One Health” Approach
The discovery expands the known geographic range of T. franki in Europe and identifies Portugal as a potential hotspot for this zoonotic parasite. While swimmer’s itch is rarely dangerous, it can significantly impact quality of life and deter tourists from using public beaches.
The study underscores the importance of a “One Health” approach, recognizing that human health is interconnected with wildlife and aquatic ecosystems. Key recommendations include:
– Ongoing Monitoring: Regular surveillance of recreational waters, particularly in warm months when snail activity is highest.
– Early Detection: Using environmental DNA (eDNA) methods to detect parasite presence before outbreaks occur.
– Public Awareness: Educating healthcare providers and the public to recognize symptoms and understand the source of infection.
Conclusion
The detection of Trichobilharzia franki in Lake Alqueva is a wake-up call for environmental health monitoring in Europe. It demonstrates how migratory birds can introduce parasites to new regions, creating hidden risks in popular recreational areas. Proactive surveillance and public awareness are essential to manage this emerging threat and ensure the safety of lake users.
