UNASSIGNED: In Europe, avian schistosomes of the genus Trichobilharzia are the most common etiological agents involved in human cercarial dermatitis (swimmer\'s itch). Manifested by a skin rash, the condition is caused by an allergic reaction to cercariae of nonhuman schistosomes. Humans are an accidental host in this parasite\'s life cycle, while water snails are the intermediate, and waterfowl are the final hosts. The study aimed to conduct a molecular and phylogenetic analysis of Trichobilharzia species occurring in recreational waters in North-Eastern Poland.
UNASSIGNED: The study area covered three water bodies (Lake Skanda, Lake Ukiel, and Lake Tyrsko) over the summer of 2021. In total, 747 pulmonate freshwater snails (Radix spp., Lymnaea stagnalis) were collected. Each snail was subjected to 1-2 h of light stimulation to induce cercarial expulsion. The phylogenetic analyses of furcocercariae were based on the partial sequence of the ITS region (ITS1, 5.8S rDNA, ITS2 and 28SrDNA). For Radix spp. phylogenetic analyses were based on the ITS-2 region.
UNASSIGNED: The prevalence of the Trichobilharzia species infection in snails was 0.5%. Two out of 478 (0.4%) L. stagnaliswere found to be infected with Trichobilharzia szidati. Moreover, two out of 269 (0.7%) snails of the genus Radix were positive for schistosome cercariae. Both snails were identified as Radix auricularia. One of them was infected with Trichobilharzia franki and the other with Trichobilharzia sp.
UNASSIGNED: Molecular identification of avian schistosome species, both at the intermediate and definitive hosts level, constitutes an important source of information on a potential threat and prognosis of local swimmer\'s itch occurrence, and helps to determine species diversity in a particular area.

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.

BACKGROUND: Swimmer\'s itch, an allergic contact dermatitis caused by avian and mammalian blood flukes, is a parasitic infection affecting people worldwide. In particular, avian blood flukes of the genus Trichobilharzia are infamous for their role in swimmer\'s itch cases. These parasites infect waterfowl as a final host, but incidental infections by cercariae in humans are frequently reported. Upon accidental infections of humans, parasite larvae will be recognized by the immune system and destroyed, leading to painful itchy skin lesions. However, one species, Trichobilharzia regenti, can escape this response in experimental animals and reach the spinal cord, causing neuroinflammation. In the last few decades, there has been an increase in case reports across Europe, making it an emerging zoonosis.
METHODS: Following a reported case of swimmer\'s itch in Kampenhout in 2022 (Belgium), the transmission site consisting of a private pond and an adjacent creek was investigated through a malacological and parasitological survey.
RESULTS: Six snail species were collected, including the widespread Ampullaceana balthica, a well-known intermediate host for Trichobilharzia parasites. Shedding experiments followed by DNA barcoding revealed a single snail specimen to be infected with T. regenti, a new species record for Belgium and by extension the Benelux. Moreover, it is the most compelling case to date of the link between this neurotropic parasite and cercarial dermatitis. Additionally, an Echinostomatidae sp. and Notocotylus sp. were isolated from two other specimens of A. balthica. However, the lack of reference DNA sequences for these groups in the online repositories prevented genus- and species-level identification, respectively.
CONCLUSIONS: The presence of T. regenti in Belgium might have severe clinical implications and its finding highlights the need for increased vigilance and diagnostic awareness among medical professionals. The lack of species-level identification of the other two parasite species showcases the barcoding void for trematodes. Overall, these findings demonstrate the need for a Belgian framework to rapidly detect and monitor zoonotic outbreaks of trematode parasites within the One Health context.

BACKGROUND: Human cercarial dermatitis (HCD) is a clinical disease typically caused by skin-penetrative larvae of avian schistosomes. Its geographical epidemiology is firmly tied with that of infected freshwater intermediate snail hosts. To better understand the current distribution of HCD and its level of nuisance in the UK, we undertook a systematic literature review.
METHODS: Following PRIMSA guidelines, PubMed and Scopus databases were searched with keywords \"human cercarial dermatitis\" OR \"swimmer\'s itch\" AND \"United Kingdom\". Articles about imported cases of HCD, or HCD outside the UK, were not formally included.
RESULTS: A total of 30 articles were initially identified. A further two were gained by inspection of all citations. After screening, eight publications were analysed where the location, number of cases and putative avian schistosome species incriminated were tabulated. HCD is mainly found in the south of England, though gaps in evidence and reporting remain across the UK.
CONCLUSIONS: Despite its noted recent rise in open water swimmers, published literature on HCD across the UK is sparse; this condition is both overlooked and under-reported. We therefore recommend establishing a national database that raises awareness and encourages self-reporting of this nuisance disease.

Lymnaeids are aquatic snails playing an important role in the transmission of many parasitic trematode species of veterinary and medical importance. In this study, we assessed the presence of cercarial flukes in naturally infected lymnaeid snails from Phayao province, Thailand, and determined the species diversity of both the intermediate snail hosts and parasite larvae. A total of 3,185 lymnaeid snails were collected from paddy fields at 31 sites in eight districts of Phayao province between October 2021 and December 2022. Larval fluke infection was assessed using the cercarial shedding method. The collected snails as well as emerging cercariae were identified at the species level via morphological and molecular methods. The sequences of snail internal transcribed spacer region 2 (ITS2) and cercarial 28S ribosomal RNA gene (28S rDNA) and cytochrome C oxidase1 (Cox1) were determined by PCR amplification and sequencing. Three species of lymnaeid snails were detected in this study, including Radix (Lymnaea) rubiginosa (Michelin, 1831), Radix (Lymnaea) swinhoei (Adams, 1866) and Austropeplea viridis (Quoy & Gaimard, 1832), of which R. rubiginosa was the most abundant, followed by A. viridis and R. swinhoei. The overall rate of trematode cercarial infection in the lymnaeid snails was 2.8% (90/3,185); the cercarial infection rate in R. rubiginosa and A. viridis was 3.5% (60/1,735) and 3.1% (30/981), respectively. No larval fluke infection was observed in the studied R. swinhoei (0/469). Nine morphotypes of cercariae were detected at 15 sites from four districts. The emerging cercariae were molecularly identified as Clinostomum sp., Aporocotylidae sp., Apharyngostrigea sp., Trichobilharzia sp., Apatemon sp., Pegosomum sp., Petasiger sp., Echinostoma revolutum and Plagiorchis sp. These findings emphasize the occurrence and diversity of trematode cercariae among naturally infected lymnaeid snails in Phayao province and could contribute to broadening our understanding of the host-parasite relationships between trematodes and their first intermediate hosts as well as developing effective interventions to control trematode parasitic diseases.

Cercarial dermatitis (‘swimmer\'s itch’; SI), characterized by small itchy bumps caused by schistosome parasites of birds and mammals, is a common problem in Michigan. Research on avian schistosomes began nearly 100 years ago in Michigan inland lakes, yet scientists are still uncovering basic biological information including the identification of local snail and parasite species that cause SI. Previous research primarily focused on lakes in the northern half of Michigan\'s lower peninsula, although SI occurs throughout the state. We surveyed snails and snail-borne trematodes in lakes across Michigan\'s lower peninsula and used quantitative polymerase chain reaction analysis of filtered water samples to identify parasites to the species level, including a recently discovered parasite species that uses the snail Planorbella (Helisoma) trivolvis as its intermediate host. Most SI mitigation efforts have focused on a parasite species hosted by the snail Lymnaea catescopium ( = Stagnicola emarginata); however, lymnaeid snails and their associated schistosome species were largely restricted to northern lakes. In contrast, P. trivolvis and its associated parasite species were common in both northern and southern Michigan lakes. A third schistosome species associated with physid snails was also present at low levels in both northern and southern lakes. These results indicate that the recently discovered parasite species and its planorbid snail intermediate host may be more important drivers of Michigan SI than previously thought, possibly due to increased definitive host abundance in recent decades. These results have potentially important implications for SI mitigation and control efforts.

A novel schistosome from Planorbella snails currently known as avian schistosomatid sp. C (ASC) was recently described as being capable of causing the papules associated with swimmer\'s itch. We conducted a paired study with 24 human volunteers, exposing each of their forearms to five drops of water containing cercariae of ASC or Trichobilharzia stagnicolae, and examined the skin for papules 1-3 days later. A mixed effects model showed that only the parasite species significantly affected the number of papules, while prior experimental exposure, swimming history, and swimmer\'s itch experience did not. The total number of papules produced by the two species were very different: ASC produced a total of 2 papules from the 298 cercariae used, compared to 49 papules from 160 T. stagnicolae cercariae, a difference factor of more than 43X, which was comparable to the odds ratio of 45.5 computed using the statistical model. A well-known agent of swimmer\'s itch, T. stagnicolae, is able to penetrate human skin more frequently than ASC, likely meaning that ASC is only a minor cause of swimmer\'s itch where T. stagnicolae is present. We also completed limited experiments that compared the cercarial behavior of the two species in vitro and in situ. A known stimulant of schistosome cercarial penetration, α-linolenic acid, did not stimulate ASC cercariae to initiate penetration-associated behaviors as frequently as T. stagnicolae. However, when placed on esophageal tissue of the known vertebrate host for ASC, Canada goose (Branta canadensis), ASC cercariae were observed penetrating the esophageal epithelium quickly, whereas T. stagnicolae cercariae did not exhibit any penetration behaviors.

The emergence of cercariae from infected mollusks is considered one of the most important adaptive strategies for maintaining the trematode life cycle. Short transmission opportunities of cercariae are often compensated by periodic daily rhythms in the cercarial release. However, there are virtually no data on the cercarial emergence of bird schistosomes from freshwater ecosystems in northern latitudes. We investigated the daily cercarial emergence rhythms of the bird schistosome Trichobilharzia sp. \"peregra\" from the snail host Radix balthica in a subarctic lake under both natural and laboratory seasonal conditions. We demonstrated a circadian rhythm with the highest emergence during the morning hours, being seasonally independent of the photo- and thermo-period regimes of subarctic summer and autumn, as well as relatively high production of cercariae at low temperatures typical of northern environments. These patterns were consistent under both field and laboratory conditions. While light intensity triggered and prolonged cercarial emergence, the temperature had little effect on cercarial rhythms but regulated seasonal output rates. This suggests an adaptive strategy of bird schistosomes to compensate for the narrow transmission window. Our results fill a gap in our knowledge of the transmission dynamics and success of bird schistosomes under high latitude conditions that may serve as a basis for elucidating future potential risks and implementing control measures related to the spread of cercarial dermatitis due to global warming.

Cercarial dermatitis in humans is a re-emerging zoonotic disease caused by infectious larvae of avian blood flukes within the Schistosomatidae family. Upon water contact, these avian schistosome larvae directly penetrate human skin and cause irritation. Between September 2018 and September 2020, carcasses of 94 mallards (Anas platyrhynchos), two green-winged teals (Anas crecca) along with one ferruginous duck (Aythya nyroca) were examined. Birds were collected within 12 regions of Hungary, representative of 9 different counties. Inspecting both morphological characters and molecular data, the cytochrome oxidase subunit I (COI) and large subunit ribosomal DNA (LSU rDNA) sequences, Bilharziella polonica and Dendritobilharzia pulverulenta were each identified. Most importantly, Trichobilharzia franki was found for the first time in Hungary and in 5 dispersed counties in conjunction with of 3 counties where other avian schistosomes were found. Of note, these avian blood flukes were predominately encountered in wild mallards (∼50%) and not in birds reared for hunting (∼1%). In total, 245 European ear snails (Radix auricularia), a known intermediate host of Trichobilharzia spp., were collected from an urban pond in Eger, Hungary. Five snails (∼2%) consistently shed numerous furcocercariae of T. franki that were confirmed by molecular methods. Our findings help to pinpoint a contemporary life cycle of this avian schistosome within an urban environment, a location regularly visited by various wild waterfowl and the possible species of avian schistosomes responsible for cercarial dermatitis in Hungary. Taken as a whole, we demonstrate the actual and potential risk zone for cercarial dermatitis, particularly with reference to R. auricularia distributions, within Hungary and across Central Europe.

Phenotypic polymorphism is a commonly observed phenomenon in nature, but extremely rare in free-living stages of parasites. We describe a unique case of somatic polymorphism in conspecific cercariae of the bird schistosome Trichobilharzia sp. \"peregra\", in which two morphs, conspicuously different in their size, were released from a single Radix balthica snail. A detailed morphometric analysis that included multiple morphological parameters taken from 105 live and formalin-fixed cercariae isolated from several naturally infected snails provided reliable evidence for a division of all cercariae into two size groups that contained either large or small individuals. Large morph (total body length of 1368 and 1339 μm for live and formalin-fixed samples, respectively) differed significantly nearly in all morphological characteristics compared to small cercariae (total body length of 976 and 898 μm for live and formalin samples, respectively), regardless of the fixation method. Furthermore, we observed that small individuals represent the normal/commonly occurring phenotype in snail populations. The probable causes and consequences of generating an alternative, much larger phenotype in the parasite infrapopulation are discussed in the context of transmission ecology as possible benefits and disadvantages facilitating or preventing the successful completion of the life cycle.