Bird schistosomes: do they die in mammalian skin?

Molecular identification of Trichobilharzia species in recreational waters in North-Eastern Poland

Background

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.

Methodology

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.

Results

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.

Conclusions

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.

Zoonotic Threats: The (Re)emergence of Cercarial Dermatitis, Its Dynamics, and Impact in Europe

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.

Swimmer's itch control: Timely waterfowl brood relocation significantly reduces an avian schistosome population and human cases on recreational lakes

Swimmer's itch (SI) is a dermatitis in humans caused by cercariae of avian and mammalian schistosomes which emerge from infected snails on a daily basis. Mitigation methods for SI have long been sought with little success. Copper sulfate application to the water to kill the snail hosts is the historically employed method, but is localized, temporary, and harmful to many aquatic species. Here, we test an alternative method to control Trichobilharzia stagnicolae, a species well-known to cause SI in northern Michigan and elsewhere in North America. Summer relocation of broods of the only known vertebrate host, common merganser (Mergus merganser), greatly reduced snail infection prevalence the following year on two large, geographically separated lakes in northern Michigan. Subsequent years of host relocation achieved and maintained snail infection prevalence at ~0.05%, more than an order of magnitude lower than pre-intervention. A Before-After-Control-Intervention (BACI) study design using multiple-year snail infection data from two intervention lakes and three control lakes demonstrates that dramatic lake-wide reduction of an avian schistosome can be achieved and is not due to natural fluctuations in the parasite populations. The relevance of reducing snail infection prevalence is demonstrated by a large seven-year data set of SI incidence in swimmers at a high-use beach, which showed a substantial reduction in SI cases in two successive years after relocation began. In addition, data from another Michigan lake where vertebrate-host based intervention occurred in the 1980's are analyzed statistically and show a remarkably similar pattern of reduction in snail infection prevalence. Together, these results demonstrate a highly effective SI mitigation strategy that avoids the use of environmentally suspect chemicals and removes incentive for lethal host removal. Biologically, the results strongly suggest that T. stagnicolae is reliant on the yearly hatch of ducklings to maintain populations at high levels on a lake and that the role of migratory hosts in the spring and fall is much less significant.

Other Schistosomatoidea and Diplostomoidea

Trematodes of the order Diplostomida are well known as serious pathogens of man, and both farm and wild animals; members of the genus Schistosoma (Schistosomatidae) are responsible for human schistosomosis (schistosomiasis) affecting more than 200 million people in tropical and subtropical countries, and infections of mammals and birds by animal schistosomes are of great veterinary importance. The order Diplostomida is also rich in species parasitizing other major taxa of vertebrates. The "Aporocotylidae" sensu lato are pathogenic in fish, "Spirorchiidae" sensu lato in reptiles. All these flukes have two-host life cycles, with asexually reproducing larvae usually in mollusks and occasionally in annelids, and adults usually live in the blood vessels of their vertebrate hosts. Pathology is frequently associated with inflammatory reactions to eggs trapped in various tissues/organs. On the other hand, the representatives of Diplostomidae and Strigeidae have three- or four-host life cycles in which vertebrates often serve not only as definitive but also as intermediate or paratenic hosts. Pathology is usually associated with migration of metacercariae and mesocercariae within the host tissues. The impact of these trematode infections on both farm and wild animals may be significant.

Scratching the Itch: Updated Perspectives on the Schistosomes Responsible for Swimmer's Itch around the World

Although most studies of digenetic trematodes of the family Schistosomatidae dwell on representatives causing human schistosomiasis, the majority of the 130 identified species of schistosomes infect birds or non-human mammals. The cercariae of many of these species can cause swimmer's itch when they penetrate human skin. Recent years have witnessed a dramatic increase in our understanding of schistosome diversity, now encompassing 17 genera with eight more lineages awaiting description. Collectively, schistosomes exploit 16 families of caenogastropod or heterobranch gastropod intermediate hosts. Basal lineages today are found in marine gastropods and birds, but subsequent diversification has largely taken place in freshwater, with some reversions to marine habitats. It seems increasingly likely that schistosomes have on two separate occasions colonized mammals. Swimmer's itch is a complex zoonotic disease manifested through several different routes of transmission involving a diversity of different host species. Swimmer's itch also exemplifies the value of adopting the One Health perspective in understanding disease transmission and abundance because the schistosomes involved have complex life cycles that interface with numerous species and abiotic components of their aquatic environments. Given the progress made in revealing their diversity and biology, and the wealth of questions posed by itch-causing schistosomes, they provide excellent models for implementation of long-term interdisciplinary studies focused on issues pertinent to disease ecology, the One Health paradigm, and the impacts of climate change, biological invasions and other environmental perturbations.

Morphological, Behavioral, and Molecular Characterization of Avian Schistosomes (Digenea: Schistosomatidae) in the Native Snail Chilina dombeyana (Chilinidae) from Southern Chile

Avian schistosomes are blood flukes parasitizing aquatic birds and snails, which are responsible for a zoonotic disease known as cercarial dermatitis, a hypersensitive reaction associated to the cutaneous penetration of furcocercariae. Despite its worldwide distribution, its knowledge is fragmentary in the Neotropics, with most of data coming from Argentina and Brazil. In Chile, there are only two mentions of these parasites from birds, and one human outbreak was associated to the genus “Trichobilharzia”. However, the identity of such parasites is pending. The aim of this study was to identify the furcocercariae of avian schistosomes from Southern Chile using an integrative approach. Thus, a total of 2283 freshwater snails from different families were collected from three different regions. All snails were stimulated for the shedding of furcocercariae, but only Chilina dombeyana (Chilinidae) from the Biobío region was found to be parasitized. The morphology and phylogenetic analyses of 28S and COI genes stated two lineages, different from Trichobilharzia, shared with Argentina. This study provides new information on Neotropical schistosomes, highlighting the need for major research on these neglected trematodes, which are considered to be emerging/re-emerging parasites in other parts of the globe as consequence of anthropogenic disturbances and climatic change. Highlights: 1. Two different lineages (Lineage I and II) were described and molecularly characterized (28S and COI genes); 2. Cercaria chilinae I y II are proposed as a synonymous of Lineage II. Thus, a total of four different lineages of avian schistosomes are related to Chilina spp.; 3. Chilina spp. represents an important intermediate host for avian schistosomes in South America, constituting a reservoir de schistosomes with zoonotic potential; 4. Coinfection between the two different lineages was found, a finding previously not reported for avian schistosomes; 5. Expansion in the geographic distribution of Nasusbilharzia melancorhypha from its original record in Argentina, with Chilina dombeyana as an additional intermediate host.

Invaders as Diluents of the Cercarial Dermatitis Etiological Agent

Research on alien and invasive species focuses on the direct effects of invasion on native ecosystems, and the possible positive effects of their presence are most often overlooked. Our aim was to check the suitability of selected alien species (the snail Physa acuta, the bivalve Dreissena polymorpha, and the gammarid Dikerogammarus villosus) as diluents for infectious bird schistosome cercariae—the etiological factor of swimmer’s itch. It has been hypothesized that alien species with different feeding habits (scrapers, filterers and predators) that cohabit the aquatic environment with intermediate hosts of the schistosomatid trematodes are capable of feeding on their free-swimming stages—cercariae. In the laboratory conditions used, all experimental animals diluted the cercariae of bird schistosome. The most effective diluents were P. acuta and D. villosus. However, a wide discrepancy in the dilution of the cercariae between replicates was found for gammarids. The obtained results confirm the hypothesis that increased biodiversity, even when alien species are involved, creates the dilution effect of the free-living stages of parasites. Determining the best diluent for bird schistosome cercariae could greatly assist in the development of current bathing areas protection measures against swimmer’s itch.

Avian schistosome species in Danish freshwater lakes: relation to biotic and abiotic factors

Due to the increased prevalence of human infections with bird schistosome larvae (cercarial dermatitis) associated with bathing in Danish lakes, a nationwide survey of infected intermediate host snails was conducted in 2018-2020. Pulmonate snails (10,225 specimens) were collected from 39 freshwater lakes (in the four major geographic regions in Denmark) and subjected to shedding. Released schistosome cercariae were isolated and identified by polymerase chain reaction and sequencing whereby Trichobilharzia regenti, Trichobilharzia franki, Trichobilharzia szidati and Trichobilharzia anseri were recorded. Infections were primarily determined by biotic factors such as the presence of final host birds and intermediate host snails and water temperature was noted as an important abiotic parameter associated with the infection. No clear connection with other abiotic factors (conductivity, alkalinity, pH, nitrogen, phosphorous) was seen. The widespread occurrence of infected snails, when compared to previous investigations, suggests that climate changes at northern latitudes could be responsible for the increased risk of contracting cercarial dermatitis.

Dermatological and Molecular Evidence of Human Cercarial Dermatitis in North-Eastern Poland

Swimmer's itch or human cercarial dermatitis (HCD) appears as a skin rash caused by an allergic reaction to larval (cercariae) flatworm parasites of the family Schistosomatidae. In our study, two cases of HCD were analyzed; both of them were reported in people swimming in Lake Pluszne. In the summer of 2018, a sample of 397 snails was collected at swimming sites in that area. Five Lymnaea stagnalis (1.9%) were found to host cercariae of bird schistosomes. Positive samples were selected by amplification of the Internal Transcribed Spacers (ITS) gene region. Sequence analysis confirmed that they were homologous with European isolates of Trichobilharzia szidati. The cases reported in this article are the first confirmed cases of HCD in this lake. This study demonstrates that there is a rationale for conducting screening studies of regions with a high recreational potential.

The chemotactic swimming behavior of bird schistosome miracidia in the presence of compatible and incompatible snail hosts

No effective method has yet been developed to prevent the threat posed by the emerging disease-cercarial dermatitis (swimmer's itch), caused by infective cercariae of bird schistosomes (Digenea: Schistosomatidae). In our previous studies, the New Zealand mud snail-Potamopyrgus antipodarum (Gray, 1853; Gastropoda, Tateidae)-was used as a barrier between the miracidia of Trichobilharzia regenti and the target snails Radix balthica. Since the presence of non-indigenous snails reduced the parasite prevalence under laboratory conditions, we posed three new research questions: (1) Do bird schistosomes show totally perfect efficacy for chemotactic swimming behavior? (2) Do the larvae respond to substances emitted by incompatible snail species? (3) Do the excretory-secretory products of incompatible snail species interfere with the search for a compatible snail host? The experiments were carried out in choice-chambers for the miracidia of T. regenti and T. szidati. The arms of the chambers, depending on the variant, were filled with water conditioned by P. antipodarum, water conditioned by lymnaeid hosts, and dechlorinated tap water. Miracidia of both bird schistosome species chose more frequently the water conditioned by snails-including the water conditioned by the incompatible lymnaeid host and the alien species, P. antipodarum. However, species-specific differences were noticed in the behavior of miracidia. T. regenti remained more often inside the base arm rather than in the arm filled with water conditioned by P. antipodarum or the control arm. T. szidati, however, usually left the base arm and moved to the arm filled with water conditioned by P. antipodarum. In conclusion, the non-host snail excretory-secretory products may interfere with the snail host-finding behavior of bird schistosome miracidia and therefore they may reduce the risk of swimmer's itch.

Molecular characterization of liver fluke intermediate host lymnaeids (Gastropoda: Pulmonata) snails from selected regions of Okavango Delta of Botswana, KwaZulu-Natal and Mpumalanga provinces of South Africa

Lymnaeidae snail species are known to be intermediate hosts of human and livestock helminths parasites, especially Fasciola species. Identification of these species and their geographical distribution is important to better understand the epidemiology of the disease. Significant diversity has been observed in the shell morphology of snails from the Lymnaeidae family and the systematics within this family is still unclear, especially when the anatomical traits among various species have been found to be homogeneous. Although there are records of lymnaeid species of southern Africa based on shell morphology and controversial anatomical traits, there is paucity of information on the molecular identification and phylogenetic relationships of the different taxa. Therefore, this study aimed at identifying populations of Lymnaeidae snails from selected sites of the Okavango Delta (OKD) in Botswana, and sites located in the KwaZulu-Natal (KZN) and Mpumalanga (MP) provinces of South Africa using molecular techniques. Lymnaeidae snails were collected from 8 locations from the Okavango delta in Botswana, 9 from KZN and one from MP provinces and were identified based on phylogenetic analysis of the internal transcribed spacer (ITS-2). Analyses based on the ITS-2 marker identified the presence of a well-supported Radix clade containing Radix auricularia, R. natalensis and R. rubiginosa, which were not well resolved. Experimental samples from the OKD and KZN present in this clade were referable to these species. An unidentified experimental taxon from the OKD formed a well-supported sister clade to the Radix clade, although it was not possible to identify it. Galba truncatula was well supported in a sister relationship to a well-supported Pseudosuccinea columella clade which included samples from MP and KZN provinces of South Africa. We observed that P. columella shared the same habitats with R. natalensis and R. auricularia in KZN. Our study contributes new knowledge on the Lymnaeidae species present in Southern Africa and their phylogenetic relationships. The study further identifies the species which are likely to co-exist in the same environment and this information will be of use to those designing control programs for fasciolosis. This is the first study reporting the presence of R. auricularia in the OKD of Botswana and KZN province of South Africa.

Schistosomatoidea and Diplostomoidea

Trematodes of the order Diplostomida are well known as serious pathogens of man, and both farm and wild animals; members of the genus Schistosoma (Schistosomatidae) are responsible for human schistosomosis affecting more than 200 million people in tropical and subtropical countries, infections of mammals and birds by animal schistosomes are of great veterinary importance. The order Diplostomida is also rich in species parasitizing other major taxa of vertebrates. The Aporocotylidae are pathogenic in fish, Spirorchiidae in reptiles. All these flukes have two-host life cycles, with asexually reproducing larvae usually in molluscs and occasionally in annelids, and adults usually live in the blood vessels of their vertebrate hosts. Pathology is frequently associated with inflammatory reactions to eggs trapped in various tissues/organs. On the other hand, the representatives of Diplostomidae and Strigeidae have three- or four-host life cycles in which vertebrates often serve not only as definitive, but also as intermediate or paratenic hosts. Pathology is usually associated with migration of metacercariae and mesocercariae within the host tissues. The impact of these trematode infections on both farm and wild animals may be significant.

Cercarial dermatitis: a systematic follow-up study of human cases with implications for diagnostics

Cercarial dermatitis (CD) is an allergic skin disease that rises in consequence of infection by invasive stages (cercariae) of trematodes of the family Schistosomatidae. CD has been considered a re-emerging disease, human cases have been reported from all continents, and tourism-threatening outbreaks occur even in frequented recreational areas. Although the symptoms of CD are generally known, the data on immune response in human patients are sporadic and incomprehensive. In the present study, we attempted to correlate the symptoms, personal history, and time course of CD in human patients with differential cell counts, dynamics of selected cytokines, and dynamics and quality of antibody response. By a systematic follow-up, we obtained a uniquely complex dataset from ten persons accidentally and concurrently infected by the same parasite species in the same locality. The onset of CD was significantly faster, and the symptoms were heavier in participants with a history of CD if compared to naive ones, who, however, also developed some of the symptoms. The repeatedly infected persons had elevated proportion of eosinophils 1 week post exposure (p.e.) and a stronger specific IgG but not IgM response, whereas specific IgE response was not observed. Increased serum levels of IL-4 occurred 1 and 3 week(s) p.e. in all participants. There was high variability in individual immunoblot patterns of IgG response, and no antigen with a universal diagnostic potential was confirmed. The presented analyses suggested that a complex approach can improve the accuracy of the diagnosis of CD, but component data should be interpreted carefully.

Agents of swimmer's itch-dangerous minority in the Digenea invasion of Lymnaeidae in water bodies and the first report of Trichobilharzia regenti in Poland

Trichobilharzia spp. have been identified as a causative agent of swimmers’ itch, a skin disease provoked by contact with these digenean trematodes in water. These parasites have developed a number of strategies to invade vertebrates. Since we have little understanding of the behavior of these parasites inside the human body, the monitoring of their invasion in snail host populations is highly recommended. In our research, lymnaeid snails were collected from several Polish lakes for two vegetation seasons. The prevalence of bird schistosomes in snail host populations was significantly lower than that of other digenean species. We were the first to detect the presence of the snails emitted Trichobilharzia regenti (potentially the most dangerous nasal schistosome) in Poland. In addition, by sequencing partial rDNA genes, we confirmed the presence of the snails positive with Trichobilharzia szidati in Polish water bodies, showing that swimmer’s itch is more frequent during summer months and that large snails are more often infected with bird schistosomes than small ones.

Potamopyrgus antipodarum as a potential defender against swimmer's itch in European recreational water bodies-experimental study

Swimmer’s itch is a re-emerging human disease caused by bird schistosome cercariae, which can infect bathing or working people in water bodies. Even if cercariae fail after penetrating the human skin, they can cause dangerous symptoms in atypical mammal hosts. One of the natural methods to reduce the presence of cercariae in the environment could lie in the introduction of non–host snail species to the ecosystem, which is known as the “dilution” or “decoy” effect. The caenogastropod Potamopyrgus antipodarum—an alien in Europe—could be a good candidate against swimmer’s itch because of its apparent resistance to invasion by European bird schistosome species and its high population density. As a pilot study on this topic, we have carried out a laboratory experiment on how P. antipodarum influences the infestation of the intermediate host Radix balthica (a native lymnaeid) by the bird schistosome Trichobilharzia regenti. We found that the co–exposure of 200 P. antipodarum individuals per one R. balthica to the T. regenti miracidia under experimental conditions makes the infestation ineffective. Our results show that a non–host snail population has the potential to interfere with the transmission of a trematode via suitable snail hosts.

Changes in surface glycosylation and glycocalyx shedding in Trichobilharzia regenti (Schistosomatidae) during the transformation of cercaria to schistosomulum

The invasive larvae (cercariae) of schistosomes penetrate the skin of their definitive hosts. During the invasion, they undergo dramatic ultrastructural and physiological transitions. These changes result in the development of the subsequent stage, schistosomulum, which migrates through host tissues in close contact with host's immune system. One of the striking changes in the transforming cercariae is the shedding of their thick tegumental glycocalyx, which represents an immunoattractive structure; therefore its removal helps cercariae to avoid immune attack. A set of commercial fluorescently labeled lectin probes, their saccharide inhibitors and monoclonal antibodies against the trisaccharide Lewis-X antigen (LeX, CD15) were used to characterize changes in the surface saccharide composition of the neuropathogenic avian schistosome Trichobilharzia regenti during the transformation of cercariae to schistosomula, both in vitro and in vivo. The effect of various lectins on glycocalyx shedding was evaluated microscopically. The involvement of peptidases and their inhibitors on the shedding of glycocalyx was investigated using T. regenti recombinant cathepsin B2 and a set of peptidase inhibitors. The surface glycocalyx of T. regenti cercariae was rich in fucose and mannose/glucose residues. After the transformation of cercariae in vitro or in vivo within their specific duck host, reduction and vanishing of these epitopes was observed, and galactose/N-acetylgalactosamine emerged. The presence of LeX was not observed on the cercariae, but the antigen was gradually expressed from the anterior part of the body in the developing schistosomula. Some lectins which bind to the cercarial surface also induced secretion from the acetabular penetration glands. Seven lectins induced the shedding of glycocalyx by cercariae, among which five bound strongly to cercarial surface; the effect could be blocked by saccharide inhibitors. Mannose-binding protein, part of the lectin pathway of the complement system, also bound to cercariae and schistosomula, but had little effect on glycocalyx shedding. Our study did not confirm the involvement of proteolysis in glycocalyx shedding.

Having bird schistosomes in mind-the first detection of Bilharziella polonica (Kowalewski 1895) in the bird neural system

Nasal bird schistosomes can cause bilharziosis in birds and have the potential to cause swimmer's itch in humans. We determined the prevalence of bird schistosomes in 106 mallards (Anas plathyrhynchos) from 11 water sources in Germany from 2014. Dissections were performed focusing on parasitic infections of the neural system. Infections with Trichobilharzia regenti (Horák et al. 1998) were found in 21% of the birds (n = 22), whereas Bilharziella polonica (Kowalewski 1895) were found between the brain membranes (meninges) and the brain, in the spinal cord or in the intestine of 12% of the mallards (n = 13). No significant influence of sex, age, and body condition between infected and non-infected animals was observed. Our study provides the first description of B. polonica from the neural system of birds and provides an epidemiological understanding of a parasite of human health concern.

Surveillance of Amyloidosis and other Diseases at Necropsy in Captive Trumpeter Swans (Cygnus Buccinator)

The purpose of this study was to characterize the incidence and diagnostic features of amyloidosis and other diseases found at necropsy in captive trumpeter swans ( Cygnus buccinator). A search of Iowa State University's Department of Veterinary Pathology and Veterinary Diagnostic Laboratory databases yielded 31 trumpeter swan ( C. buccinator) necropsy cases from captive swans in protected habitats. Eleven of the 31 birds had amyloid deposition most commonly in the spleen (8 of 11), liver (7 of 11), and kidney (6 of 11) and less often in the pancreas (2 of 11) and adrenal gland (2 of 11). Amyloid deposition effaced normal tissue with adjacent necrosis and hemorrhage in severe cases. Amyloidosis was most often diagnosed in February and March. Other disease diagnoses in the trumpeter swans included aspergillosis (5 of 31, 16%); bacterial infection (5 of 31, 16%); lead toxicosis (3 of 31, 10%); gout (2 of 31, 6%); parasitic infection (2 of 31, 6%); vitamin E deficiency (1 of 31, 3%); trauma (1 of 31, 3%); and ventricular foreign body (1 of 31, 3%). Histopathologic, toxicologic, and microbiologic analyses did not define an etiologic diagnosis in the deaths of 9 trumpeter swans. In these cases, necropsy lesions included emaciation (5 of 9), enteritis (1 of 9), pulmonary hemorrhage (1 of 9), and no lesions (3 of 9). The number of trumpeter swan case submissions was greatest in January and February. This study provides a reference for veterinary diagnosticians concerning incidence and diagnostic features of amyloidosis and other diseases in captive trumpeter swans of the midwestern United States.

The real threat of swimmers' itch in anthropogenic recreational water body of the Polish Lowland

After numerous reports the local press about the “stinging water” in created on the Dzierżęcinka River—Water Valley reservoir and recognizing in bathers the symptoms of swimmers’ itch, environmental study on the presence of bird schistosome larvae in snail hosts was conducted. Snails belonging to Lymnaeidae and Planorbidae were collected at two sites: (i) part of anthropogenic reservoir (192 individuals) and (ii) the river part (37 individuals). Higher prevalence of Digenea was observed in snail populations living in Water Valley (29.8 %) compared to Dzierżęcinka River (21.3 %). The larvae of bird schistosomes were recorded in both localities in 1.8 % of collected snails. The prevalence of bird schistosomes reached 2.9 % in Planorbarius corneus, 2.8 % in Radix auricularia, and 5.9 % in Radix balthica/labiata. Laboratory tests have shown that at 19 °C the number of bird schistosome cercariae released from snail hosts significantly exceeded the number of cercariae of other identified Digenea species. It is worth underlining that despite the low prevalence of bird schistosomes, the high number of released cercariae was sufficient to create a real threat of swimmers’ itch in bathers. As indicated by the example presented, anthropogenic reservoirs create excellent conditions for Digenea species including bird schistosomes. In view of the real risk of people using the waters, tests on presence of the parasites in snail hosts should be included to the standard procedure of security control in bathing places.

The Early Worm Catches the Bird? Productivity and Patterns of Trichobilharzia szidati Cercarial Emission from Lymnaea stagnalis

Digenean trematodes are common and abundant in aquatic habitats and their free-living larvae, the cercariae, have recently been recognized as important components of ecosystems in terms of comprising a significant proportion of biomass and in having a potentially strong influence on food web dynamics. One strategy to enhance their transmission success is to produce high numbers of cercariae which are available during the activity peak of the next host. In laboratory experiments with 13 Lymnaea stagnalis snails infected with Trichobilharzia szidati the average daily emergence rate per snail was determined as 2,621 cercariae, with a maximum of 29,560. During a snail’s lifetime this summed up to a mass equivalent of or even exceeding the snail’s own body mass. Extrapolated for the eutrophic pond where the snails were collected, annual T. szidati biomass may reach 4.65 tons, a value equivalent to a large Asian elephant. Emission peaks were observed after the onset of illumination, indicating emission synchronizing with the high morning activities of the definitive hosts, ducks. However, high cercarial emission is possible throughout the day under favorable lightning conditions. Therefore, although bird schistosomes, such as T. szidati constitute only a fraction of the diverse trematode communities in the studied aquatic ecosystem, their cercariae can still pose a considerable risk for humans of getting cercarial dermatitis (swimmer's itch) due to the high number of cercariae emitted from infected snails.

Trichobilharzia anseri n. sp. (Schistosomatidae: Digenea), a new visceral species of avian schistosomes isolated from greylag goose (Anser anser L.) in Iceland and France

Parasitological investigations carried out on birds in Iceland and France highlight the presence of four species of avian schistosomes from greylag geese (Anser anser L.): the european nasal species Trichobilharzia regenti and three visceral species, among which an unknown species isolated from blood vessels of the large intestine and liver. Morphological and molecular analyzes of different parasite stages (eggs, adults) revealed new species of Trichobilharzia genus – Trichobilharzia anseri sp. nov. Studies on host-parasite relationship under natural conditions, showed that the life-cycle includes the snail Radix balthica (syn. R. peregra) as intermediate host. The cercariae, already isolated in Iceland from two ponds of the Reykjavik capital area – the Family park and Tjörnin Lake – are the same as those isolated in 1999 by Kolářová et al. during the first study on Icelandic parasitic agents of cercarial dermatitis.

Avian schistosomes and outbreaks of cercarial dermatitis

Cercarial dermatitis (swimmer's itch) is a condition caused by infective larvae (cercariae) of a species-rich group of mammalian and avian schistosomes. Over the last decade, it has been reported in areas that previously had few or no cases of dermatitis and is thus considered an emerging disease. It is obvious that avian schistosomes are responsible for the majority of reported dermatitis outbreaks around the world, and thus they are the primary focus of this review. Although they infect humans, they do not mature and usually die in the skin. Experimental infections of avian schistosomes in mice show that in previously exposed hosts, there is a strong skin immune reaction that kills the schistosome. However, penetration of larvae into naive mice can result in temporary migration from the skin. This is of particular interest because the worms are able to migrate to different organs, for example, the lungs in the case of visceral schistosomes and the central nervous system in the case of nasal schistosomes. The risk of such migration and accompanying disorders needs to be clarified for humans and animals of interest (e.g., dogs). Herein we compiled the most comprehensive review of the diversity, immunology, and epidemiology of avian schistosomes causing cercarial dermatitis.

Schistosoma mansoni tetraspanning orphan receptor (SmTOR): a new vaccine candidate against schistosomiasis

One approach to fight against schistosomiasis is to develop an efficient vaccine. Schistosoma mansoni tetraspanning orphan receptor (SmTOR) might be a vaccine candidate, as it is a tegument membrane protein expressed most highly in cercariae. In this study we characterized the recombinant first extracellular domain of SmTOR (rSmTORed1) as having the expected property to bind C2 of complement similarly to a smaller peptide of the same domain, and to produce specific and high-titre antibodies in BALB/c mice immunized using complete Freund's adjuvant/incomplete Freund's adjuvant (CFA/IFA). Immunization was protective against parasite infection, as demonstrated by a significant decrease in worm burden in immunized BALB/c mice versus the control groups over two independent trials [64 and 45% reduction for mean adult worm burden in immunized versus phosphate-bufferd saline (PBS) injected mice]. Interestingly, infection by itself did not lead to the generation of anti-rSmTORed1 antibodies, corresponding to the low frequency of specific anti-rSmTORed1 antibodies detected in the sera of patients infected with S. mansoni (2/20; 10%). These data suggest that, as opposed to the natural infection during which SmTOR induces antibodies only rarely, immunization with its smaller first extracellular domain might be more efficient.

Swimmer's itch: etiology, impact, and risk factors in Europe

This review summarizes current knowledge about the occurrence and distribution of swimmer's itch, with a focus on Europe. Although recent publications have reviewed the biology and systematics of bird schistosomes and their complex host-parasite interactions, the underlying ecological factors that create favorable conditions for the parasites and the way humans interact with infested water bodies require further attention. Relevant studies from the past decade were analyzed to reveal an almost complete set of ecological factors as a prerequisite for establishing the life cycle of bird schistosomes. Based on both records of the occurrence of the parasite infective agents, and epidemiological studies that investigate outbreaks of swimmer's itch, this review concentrates on the risk factors for humans engaged in recreational water activities.

Avian schistosomes and human cercarial dermatitis in a wildlife refuge in Mazandaran Province, northern Iran

Each year, hundreds of aquatic migratory birds migrate from northern hemisphere to the Mazandaran Province, northern Iran. Little information is available on prevalence and density of schistosomes in water birds in Iran and around the world. The objectives of this study were to determine definitive and intermediate hosts of avian schistosomes as well as to assess human cercarial dermatitis (HCD) in a wildlife refuge in Mazandaran Province. Of 1106 examined people, 589 (53.2%) had maculopapular rashes mainly on feet but also on hand. The majority of cases were adults and local residents. Of 260 ducks, 41 (15.8%) were found to be infected with Trichobilharzia spp. eggs or adult worms. Prevalence was highest in Anas clypeata and Anas platyrhynchos, 79% and 18.9%, respectively. A total of 1.2% snails, examined by both shedding and crushing methods, were infected with furcocercariae belonging to avian schistosomes. The most frequently infected snail was Lymnaea gedrosiana (5.9%). Our results showed that cercarial dermatitis and avian schistosomiasis is a common and yet neglected disease in this area. Anas clypeata played the most important role in exposing snails to miracidia in ponds and paddy fields. Moreover, because of the high prevalence in ducks and high prevalence of HCD in the region, it is considered as a new endemic focus in Iran.

Pathogenicity of Trichobilharzia spp. for Vertebrates

Bird schistosomes, besides being responsible for bird schistosomiasis, are known as causative agents of cercarial dermatitis. Cercarial dermatitis develops after repeated contact with cercariae, mainly of the genus Trichobilharzia, and was described as a type I, immediate hypersensitivity response, followed by a late phase reaction. The immune response is Th2 polarized. Primary infection leads to an inflammatory reaction that is insufficient to eliminate the schistosomes and schistosomula may continue its migration through the body of avian as well as mammalian hosts. However, reinfections of experimental mice revealed an immune reaction leading to destruction of the majority of schistosomula in the skin. Infection with the nasal schistosome Trichobilharzia regenti probably represents a higher health risk than infections with visceral schistosomes. After the skin penetration by the cercariae, parasites migrate via the peripheral nerves, spinal cord to the brain, and terminate their life cycle in the nasal mucosa of waterfowl where they lay eggs. T. regenti can also get over skin barrier and migrate to CNS of experimental mice. During heavy infections, neuroinfections of both birds and mammals lead to the development of a cellular immune response and axonal damage in the vicinity of the schistosomulum. Such infections are manifest by neuromotor disorders.

Two Cases of Cercarial Dermatitis in An Italian Lake

Cercarial dermatitis (also named Swimmer's itch or Clam-digger's disease) was first described about 80 years ago, but today its impact is increasing and it is considered an emerging disease. This represents a hypersensitive or allergic reaction to Trichobilharzia spp, clinically characterized by initial erythema and cutaneous itching, followed by macular or papular eruptions, itching and occasionally diffuse erythema or urticaria. We report two cases of patients affected by a cercarial dermatitis after bathing in Lake Vico (Central Italy). We highlight these cases for the rarity of this dermatitis and its increasing prevalence, also in previously non-affected geographical areas, associated to a high level of social discomfort.

Bridging gaps in the molecular phylogeny of the Lymnaeidae (Gastropoda: Pulmonata), vectors of Fascioliasis

Background

Lymnaeidae snails play a prominent role in the transmission of helminths, mainly trematodes of medical and veterinary importance (e.g., Fasciola liver flukes). As this family exhibits a great diversity in shell morphology but extremely homogeneous anatomical traits, the systematics of Lymnaeidae has long been controversial. Using the most complete dataset to date, we examined phylogenetic relationships among 50 taxa of this family using a supermatrix approach (concatenation of the 16 S, ITS-1 and ITS-2 genes, representing 5054 base pairs) involving both Maximum Likelihood and Bayesian Inference.

Results

Our phylogenetic analysis demonstrates the existence of three deep clades of Lymnaeidae representing the main geographic origin of species (America, Eurasia and the Indo-Pacific region). This phylogeny allowed us to discuss on potential biological invasions and map important characters, such as, the susceptibility to infection by Fasciola hepatica and F. gigantica, and the haploid number of chromosomes (n). We found that intermediate hosts of F. gigantica cluster within one deep clade, while intermediate hosts of F. hepatica are widely spread across the phylogeny. In addition, chromosome number seems to have evolved from n = 18 to n = 17 and n = 16.

Conclusion

Our study contributes to deepen our understanding of Lymnaeidae phylogeny by both sampling at worldwide scale and combining information from various genes (supermatrix approach). This phylogeny provides insights into the evolutionary relationships among genera and species and demonstrates that the nomenclature of most genera in the Lymnaeidae does not reflect evolutionary relationships. This study highlights the importance of performing basic studies in systematics to guide epidemiological control programs.

Confirmation of the presence of Trichobilharzia by examination of water samples and snails following reports of cases of cercarial dermatitis

In case of cercarial dermatitis after exposure to fresh water lakes, managers responsible for bathing water quality wish to confirm the presence of Trichobilharzia, which substantiates taking measures to protect bathers from further exposure. A novel approach, including concentration of suspected water samples by filtration and PCR detection of the parasite, is proposed. This approach has been applied to bathing sites with a history of cercarial dermatitis, sampled from 2005 to 2008. Examination of snails, the standard procedure for confirmation of the parasite's presence, and analysis of water samples, appear to be complementary procedures that enhance the chance of parasite detection in implicated bathing water. Water analysis is particularly valuable when snails cannot be found; it confirmed the presence of Trichobilharzia on 25% of sampling days with reported skin conditions and no snails found. PCR of the ToSau3A repeat directly confirmed the parasite in the water. The application of the combination of analysis of water samples and examination of snails is suggested when cases of (presumptive) cercarial dermatitis are reported or when lakes with a history of cercarial dermatitis are inspected prior to the bathing season, in order to guide interventions to prevent (further) cases of swimmers' itch.

Avian schistosomes in French aquatic birds: a molecular approach

The prevalence of human cercarial dermatitis (HCD) caused by bird schistosomes appears to be increasing in France, in light of the impact of tourism combined with high densities of wild aquatic hosts in freshwater areas. The present work expands our knowledge of schistosome systematics by including samples of bird schistosomes collected from their natural hosts in France. Heads (318) and viscera (81) of aquatic birds belonging to 16 species from five orders, collecting during the hunting seasons or found dead, were autopsied for nasal and visceral schistosomes. Eggs and/or adults were analysed by molecular methods using the D2 domain and the second internal transcribed spacer (ITS-2) region of rDNA to determine species. Even if nasal eggs were polymorphic according to the host, all haplotypes were similar to that of Trichobilharzia regenti. Marked diversity of visceral species was observed. Final hosts under natural conditions were reported. For the first time, Trichobilharzia franki is reported in its natural bird hosts, Anas platyrhynchos, Anas crecca, Aythya fuligula and Cygnus olor. We also identified T. szidati in A. crecca and Anas clypeata. Bilharziella polonica was found in six species of aquatic birds, including Grus grus. This finding is the first record of bird schistosomes in this aquatic bird. Three new taxa of visceral schistosomes in Anser anser are strongly suspected according to their haplotypes. Futhermore, a new haplotype of visceral schistosomes isolated in Cygnus olor and similar to Allobilharzia visceralis was identified.

In vitro cultivation of early schistosomula of nasal and visceral bird schistosomes (Trichobilharzia spp., Schistosomatidae)

Cercariae of bird schistosomes (Trichobilharzia szidati and Trichobilharzia regenti) were mechanically stimulated to transform to schistosomula and kept in different cultivation media supplemented with duck red blood cells and/or homogenized nervous tissue. The development under in vitro conditions was compared with that in vivo, using the following characters: emptying of penetration glands, surface changes, food uptake, and growth of early schistosomula. The results show that the cultivation medium routinely used for human schistosomes is also suitable for mass production of early schistosomula of bird schistosomes, including the unique nasal species-T. regenti. The changes observed resemble those present in worms developing in vivo; therefore, the in vitro produced early schistosomula might be used for further studies of host-parasite interactions.

Antibody responses induced by Trichobilharzia regenti antigens in murine and human hosts exhibiting cercarial dermatitis

Cercariae of bird schistosomes (genus Trichobilharzia) are able to penetrate the skin of mammals (noncompatible hosts), including humans, and cause a Th2-associated inflammatory cutaneous reaction termed cercarial dermatitis. The present study measured the antibody reactivity and antigen specificity of sera obtained after experimental infection of mice and natural infection of humans. Sera from mice re-infected with T. regenti showed a bias towards the development of antigen-specific IgM and IgG1 antibodies and elevated levels of total serum IgE, indicative of a Th2 polarized immune response. We also demonstrate that cercariae are a source of antigens triggering IL-4 release from basophils collected from healthy human volunteers. Analysis of sera from patients with a history of cercarial dermatitis revealed elevated levels of cercarial-specific IgG, particularly for samples collected from adults (> 14 years old) compared with children (8-14 years old), although elevated levels of antigen-specific IgE were not detected. In terms of antigen recognition, IgG and IgE antibodies in the sera of both mice and humans preferentially bound an antigen of 34 kDa. The 34 kDa molecule was present in both homogenate of cercariae, as well as cercarial excretory/secretory products, and we speculate it may represent a major immunogen initiating the Th2-immune response associated with cercarial dermatitis.

Cercarial dermatitis in the UK

'Swimmer's itch' or cercarial dermatitis (CD) results from an immunological reaction to free-swimming non-human schistosome parasites released from aquatic snails. Affected bathers develop a self-limiting, pruritic, macular or papular eruption shortly after leaving the water. The condition is well-recognized in continental Europe, Asia and America, but has not to date been recorded in the UK.

The functional expression and characterisation of a cysteine peptidase from the invasive stage of the neuropathogenic schistosome Trichobilharzia regenti

A transcriptional product of a gene encoding cathepsin B-like peptidase in the bird schistosome Trichobilharzia regenti was identified and cloned. The enzyme was named TrCB2 due to its 77% sequence similarity to cathepsin B2 from the important human parasite Schistosoma mansoni. The zymogen was expressed in the methylotropic yeast Pichia pastoris; procathepsin B2 underwent self-processing in yeast media. The peptidolytic activity of the recombinant enzyme was characterised using synthetic fluorogenic peptide substrates at optimal pH 6.0. Functional studies using different specific inhibitors proved the typical cathepsin B-like nature of the enzyme. The S(2) subsite specificity profile of recombinant TrCB2 was obtained. Using monospecific antibodies against the recombinant enzyme, the presence of cathepsin B2 was confirmed in extracts from cercariae (infective stage) and schistosomula (early post-cercarial stage) of T. regenti on Western blots. Also, cross-reactivity was observed between T. regenti and S. mansoni cathepsins B2 in extracts of cercariae, schistosomula or adults. In T. regenti, the antisera localised the enzyme to post-acetabular penetration glands of cercariae implying an important role in the penetration of host skin. The ability of recombinant TrCB2 to degrade skin, serum and nervous tissue proteins was evident. Elastinolytic activity suggests that the enzyme might functionally substitute the histolytic role of the serine class elastase known from S. mansoni and Schistosoma haematobium but not found in Schistosoma japonicum or in bird schistosomes.

The complete mitochondrial genome of the bird schistosome Trichobilharzia regenti (Platyhelminthes: Digenea), causative agent of cercarial dermatitis

The complete mitochondrial (mt) genome of the neuropathogenic bird schistosome Trichobilharzia regenti was fully sequenced in order to develop molecular markers for future diagnostic, molecular ecological, population, and phylogenetic studies. The genome was 14,838 bp in length, with a 68.4% AT bias in protein coding regions. A repeat element (3 x 184 bp) between trnV and trnW distinguished a single short noncoding region. As 9 of 14 genera of schistosomes parasitize birds, future characterization of their mt genomes is desirable for species-specific and strain- or population-specific diagnostic markers; this concerns not only the nasal representatives, e.g., T. regenti characterized in this study, but also numerous species within the predominant group of visceral (blood dwelling) bird schistosomes.

Cercarial dermatitis and lake eutrophication in south-central Chile

Cercarial dermatitis is caused by exposure to the cercariae of schistosome species which have birds or mammals as their definitive hosts. A public alarm was raised in the summer of 2004 when this parasitic disease was observed for the first time in south-central Chile at Laguna Chica de San Pedro (36 degrees 51'' S, 73 degrees 05'' W). Swimmers at this eutrophic lake were surveyed in order to estimate the amount of cercarial dermatitis in the area; participants were observed during the summers of 2004 and 2005 for clinical signs of cercarial dermatitis and 25 Chilina dombeyana snails were collected monthly from Laguna Chica de San Pedro during one year and then examined for animal schistosome cercariae. We found that 3% of the swimmers had pruritic maculopapular rashes on their legs, arms, necks, or other body parts and that between 9.1% (May 2006) and 52.4% (November 2004) of the snails examined were infected with the bird schistosome cercariae Trichobilharzia sp.

Comparison of cysteine peptidase activities inTrichobilharzia regentiandSchistosoma mansonicercariae

Cercariae of the bird schistosomeTrichobilharzia regentiand of the human schistosomeSchistosoma mansoniemploy proteases to invade the skin of their definitive hosts. To investigate whether a similar proteolytic mechanism is used by both species, cercarial extracts ofT. regentiandS. mansoniwere biochemically characterized, with the primary focus on cysteine peptidases. A similar pattern of cysteine peptidase activities was detected by zymography of cercarial extracts and their chromatographic fractions fromT. regentiandS. mansoni.The greatest peptidase activity was recorded in both species against the fluorogenic peptide substrate Z-Phe-Arg-AMC, commonly used to detect cathepsins B and L, and was markedly inhibited (>96%) by Z-Phe-Ala-CHN2at pH 4·5. Cysteine peptidases of 33 kDa and 33–34 kDa were identified in extracts ofT. regentiandS. mansonicercariae employing a biotinylated Clan CA cysteine peptidase-specific inhibitor (DCG-04). Finally, cercarial extracts from bothT. regentiandS. mansoniwere able to degrade native substrates present in skin (collagen II and IV, keratin) at physiological pH suggesting that cysteine peptidases are important in the pentration of host skin.

Helminth fauna of the yellow-legged gullLarus cachinnansin Galicia, north-west Spain

Thirty-six helminth species were found in 324 gulls examined during June 1994 to February 1996 from different localities of Galicia: 25 trematodes (Brachylaimasp.,Brachylecithum microtesticulatum,Cardiocephaloides longicollis,Cryptocotyle lingua,Cryptocotyle concavum,Diplostomum spathaceum,Echinostephilla virgula,Galactosomum phalacrocoracis,Gigantobilharzia acotylea,Gymnophallus deliciosus,Gynaecotyla longiintestinata,Himasthla elongata,Himasthla quissetensis,Knipowitschiatrema nicolai,Levinseniella(Levinseniella)propinqua,Maritrema gratiosum,Maritrema linguilla,Microphallus primas,Microphallus similis,Ornithobilharzia canaliculata,Parorchis acanthus,Phagicola minuta,Psilostomum brevicolle,Renicolasp. andStephanoprora denticulata), four cestodes (Alcataenia micracantha,Microsomacanthus ductilis,Tetrabothrius(Oriana)erostrisandWardium cirrosa), six nematodes (Anisakis simplex,Contracaecum rudolphii,Cosmocephalus obvelatus),Eucoleus contortus,Paracuaria aduncaandTetrameres(Tetrameres)skrjabini) and one acanthocephalan (Arhythmorhynchus longicollis).Tetrabothrius erostriswas the most prevalent species (79.6%), followed byC. obvelatus(47.8%),C. lingua(37.4%),G deliciosus(30.9%),G. longiintestinata(22.8%),P. adunca(21.9%),B. microtesticulatum(17.6%),E. contortus(14.5%) andM. similis(9.3%).Microphallus similiswas the dominant species, with a Berger-Parker index (BP) of 0.32, followed byT. erostris(BP=0.10). All species presented an aggregated dispersion exceptG. acotyleaandG. phalacrocoracis, which showed a random dispersion. Species that seem to have the greatest predilection for specific sites along the intestine are:C. longicollisandA. micracantha(first third),Brachylaimasp.,M. similisandG. longiintestinata(last third) andA. longicollis(second half). Eight species are known to be pathogenic to commercially important fish or molluscan species and several are pathogenic to humans.

Molluscan and vertebrate immune responses to bird schistosomes

There is a growing understanding of risks posed by human contact with the cercariae of bird schistosomes. In general, there are no fundamental biological differences between human and bird schistosomes in terms of their interactions with snail and vertebrate hosts. The penetration of host surfaces is accompanied by the release of penetration gland products and the shedding of highly antigenic surface components (miracidial ciliated plates and cercarial glycocalyx) which trigger host immune reactions. New surface structures are formed during transformation: the tegument of mother sporocysts and the tegumental double membrane of schistosomula. These surfaces apparently serve as protection against the host immune response. Certain parasite excretory-secretory products may contribute to immunosuppression or, on the other hand, stimulation of host immune reactions. Discovery of new species and their life cycles, the characterization of host-parasite interactions (including at the molecular level), the determination of parasite pathogenicity towards the host, the development of tools for differential diagnosis and the application of protective measures are all topical research streams of the future. Regularly updated information on bird schistosomes and cercarial dermatitis can be found at http://www.schistosomes.cz (web pages of Schistosome Group Prague).

Molluscicidal effect of nicotinanilide and its intermediate compounds against a freshwater snail Lymnaea luteola, the vector of animal schistosomiasis

The molluscicidal effect of nicotinanilide was evaluated and compared with niclosamide (2',5-dichloro-4'-nitrosalicylanilide, ethanolamide salt) against different stages of the freshwater snail Lymnaea luteola i.e., eggs, immature, young mature, and adults. Calculated values of lethal concentrations (LC50 and LC90) showed that both nicotinanilide and niclosamide as toxic against eggs, immature, and adults. The young mature stage of the snails was comparatively more tolerant to both molluscicides than the other stages. The toxicity of the intermediate compounds of nicotinanilide against the young mature stage of the snails showed them as ineffective. The mortality pattern of the snails exposed to LC90 concentration of these molluscicides showed niclosamide to kill faster (within 8 to 9 h) than nicotinanilide (26 to 28 h). In view of the above studies it may be concluded that both molluscicides are toxic against all the stages of the L. luteola snails.

Presence of Trichobilharzia szidati in Lymnaea stagnalis and T. franki in Radix auricularia in northeastern France: molecular evidence

A molecular approach was used to analyse a focus of cercarial dermatitis in northeastern France (Lake Der-Chantecoq), including both cercariae and snails,by sequencing the internal transcribed spacers (ITS1 for ocellate furcocercariae and ITS2 for snails). Lymnaea stagnalis were found infected with the furcocercariae of Trichobilharzia szidati, and T. franki furcocercariae were found in Radix auricularia. The record of these two visceral parasites of birds in northern France confirms strong host-parasite relationships. The use of these standardised markers will be of the highest significance for our understanding of the epidemiology of cercarial dermatitis in this recreational lake.