Lake Alqueva Parasite Discovery
A significant parasitic flatworm, identified as belonging to the Trichobilharzia group, specifically T. franki, has been detected for the first time in Portugal
within the waters of Lake Alqueva. This finding is particularly noteworthy as it identifies a major recreational freshwater body as a potential area where humans could be exposed to cercarial dermatitis, commonly known as swimmer's itch. These avian schistosomes typically complete their life cycle between waterfowl and freshwater snails. The free-swimming larvae of these parasites can inadvertently penetrate human skin. While the resulting dermatitis is usually not severe, it can manifest as intense itching, painful red welts, and blister-like reactions, with symptoms potentially escalating after repeated exposure. The presence of T. franki is significant because this species is frequently implicated in human infections throughout Europe, making its confirmed presence in Portugal a development that warrants attention for public health and ecological monitoring.
Uncharted Territory in Portugal
Prior to this recent study, there was no official record of Trichobilharzia species existing within Portugal. This absence was considered unusual, especially given Lake Alqueva's prominence as the largest artificial reservoir in Western Europe and its increasing popularity among people engaging in various recreational activities like swimming, fishing, and birdwatching. The researchers also highlighted that swimmer's itch has not been a condition that has been systematically tracked or monitored within the country. However, preliminary data gathered from local healthcare professionals and pharmacists had already indicated occurrences consistent with symptoms of swimmer's itch in the vicinity of the Alqueva region, suggesting that the issue might have been present but undocumented.
Investigating Lake Alqueva
To systematically investigate the presence of these parasites, a dedicated research team conducted a comprehensive survey across 25 different locations along the shoreline of Lake Alqueva. This fieldwork took place over an extended period, spanning from May 2023 to October 2024. During these eight separate field excursions, the researchers meticulously collected a substantial number of freshwater snails. In total, they gathered 7,125 snails, comprising 6,414 individuals identified as Physella acuta, 660 belonging to the Radix genus, and 51 specimens of Gyraulus species. Following collection, 325 of these snails were deemed suitable for detailed molecular analysis. The results of this analysis revealed that four Radix auricularia snails were actively releasing larvae that aligned with the characteristics of Trichobilharzia. Notably, all these infected snails were discovered concentrated at a single specific site named Campinho, during an examination in September 2024, indicating a localized hotspot for the parasite.
Confirming T. franki
Following the initial identification of larvae, rigorous morphological examination and advanced genetic testing were performed. These analyses definitively confirmed that the detected parasites were indeed T. franki. The DNA sequences obtained from the larval samples exhibited a high degree of similarity, exceeding 95%, when compared to known COX1 sequences of T. franki found in other studies. Furthermore, the ITS1 region of the DNA showed a complete match with established T. franki profiles. The snail species identified as the carrier was confirmed to be Radix auricularia, reinforcing its established role as an intermediate host for this particular parasite within the Lake Alqueva ecosystem. Across the entire vast expanse of the lake basin, the infection rate within the Radix snail population was recorded at a low 0.6%. However, at the specific Campinho site where the infected snails were found, the infection rate surged dramatically to 13.8%, highlighting a significantly concentrated presence of the parasite in that localized area. It is important to note that the parasite was only detected in snails that had already begun releasing larvae, with no evidence of earlier-stage infections being found in other collected samples. The study also took into account existing reports of human cases exhibiting symptoms consistent with swimmer's itch in the same geographic vicinity.
Bird-Borne Spread Evidence
The genetic fingerprint of the T. franki samples obtained from Portugal shows a close evolutionary relationship to strains of the parasite that have previously been identified in other European countries, including Austria and Hungary. This genetic linkage strongly suggests that the parasite may have been introduced and subsequently spread through the movement of migratory waterfowl. These birds are capable of carrying the parasite over considerable distances as they travel. During the course of the research, scientists observed a variety of bird species in the vicinity of Lake Alqueva. These included domestic ducks, other members of the Anatidae family (which also includes ducks, geese, and swans), and the invasive Egyptian geese, all of which are potential carriers or hosts for the parasite.
Public Health Implications
The confirmation of T. franki in Portugal not only extends the known geographical distribution of this parasite across Europe but also designates Lake Alqueva as a potential new hotspot for this zoonotic organism. This discovery serves as a stark reminder of the intricate connections between wildlife populations, aquatic environments, and human activities, all falling under the 'One Health' framework. Lake Alqueva is a hub of recreational activity, particularly during the warmer months when snail populations are most active and human interaction with the water is at its peak. While swimmer's itch is generally considered a mild ailment, its prevalence can still negatively impact tourism and the accessibility of public beaches if not properly addressed. The study authors advocate for continuous monitoring of recreational water bodies, enhanced public awareness campaigns targeting both the general population and healthcare providers, and the adoption of advanced techniques like environmental DNA analysis for early detection of such risks. Integrating ecological surveillance with proactive public health strategies is crucial for identifying and managing emerging parasitic threats before they become widespread public health issues.
