A review on swimmer's itch and the occurrence of bird schistosomes in Iceland

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.

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.

Parasite effects on receivers in animal communication: Hidden impacts on behavior, ecology, and evolution

Parasites exert a profound effect on biological processes. In animal communication, parasite effects on signalers are well-known drivers of the evolution of communication systems. Receiver behavior is also likely to be altered when they are parasitized or at risk of parasitism, but these effects have received much less attention. Here, we present a broad framework for understanding the consequences of parasitism on receivers for behavioral, ecological, and evolutionary processes. First, we outline the different kinds of effects parasites can have on receivers, including effects on signal processing from the many parasites that inhabit, occlude, or damage the sensory periphery and the central nervous system or that affect physiological processes that support these organs, and effects on receiver response strategies. We then demonstrate how understanding parasite effects on receivers could answer important questions about the mechanistic causes and functional consequences of variation in animal communication systems. Variation in parasitism levels is a likely source of among-individual differences in response to signals, which can affect receiver fitness and, through effects on signaler fitness, impact population levels of signal variability. The prevalence of parasitic effects on specific sensory organs may be an important selective force for the evolution of elaborate and multimodal signals. Finally, host-parasite coevolution across heterogeneous landscapes will generate geographic variation in communication systems, which could ultimately lead to evolutionary divergence. We discuss applications of experimental techniques to manipulate parasitism levels and point the way forward by calling for integrative research collaborations between parasitologists, neurobiologists, and behavioral and evolutionary ecologists.

Cercariae of a Bird Schistosome Follow a Similar Emergence Pattern under Different Subarctic Conditions: First Experimental Study

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.

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.

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.

Temperature and light effects on Trichobilharzia szidati cercariae with implications for a risk analysis

Background

Cercarial dermatitis (swimmer’s itch) caused by bird schistosome cercariae, released from intermediate host snails, is a common disorder also at higher latitudes. Several cases were observed in the artificial Danish freshwater Ringen Lake frequently used by the public for recreational purposes. The lake may serve as a model system when establishing a risk analysis for this zoonotic disease. In order to explain high risk periods we determined infection levels of intermediate host snails from early spring to late summer (March, June and August) and elucidated the effect of temperature and light on parasite shedding, behavior and life span.

Results

Field studies revealed no shedding snails in March and June but in late summer the prevalence of Trichobilharzia szidati infection (in a sample of 226 pulmonate Lymnaea stagnalis snails) reached 10%. When investigated under laboratory conditions the cercarial shedding rate (number of cercariae shed per snail per day) was positively correlated to temperature raising from a mean of 3000 (SD 4000) at 7 °C to a mean of 44,000 (SD 30,000) at 27 °C). The cercarial life span was inversely correlated to temperature but the parasites remained active for up to 60 h at 20 °C indicating accumulation of cercariae in the lake during summer periods. Cercariae exhibited positive phototaxy suggesting a higher pathogen concentration in surface water of the lake during daytime when the public visits the lake.

Conclusion

The only causative agent of cercarial dermatitis in Ringen Lake detected was T. szidati. The infection risk associated with aquatic activities is low during spring and early summer (March-June). In late summer the risk of infection is high since the release, behavior and life span of the infective parasite larvae have optimal conditions.

Confirmation of the presence of zoonotic Trichobilharzia franki following a human cercarial dermatitis outbreak in recreational water in Slovakia

Human cercarial dermatitis is a parasitic disease that causes an allergic reaction in the skin (swimmer's itch) as a consequence of contact with cercariae of bird schistosomes present in water, mainly of the genus Trichobilharzia Skrjabin et Zakarow, 1920. The main objective of the study was to confirm the presence of the zoonotic disease agent following reports of human infections in recreational water in Slovakia. We identified two species of freshwater snails at Košice Lake, Radix auricularia (Linnaeus, 1758) and Physa acuta (Draparnaud, 1805). Trematode infections were observed only in R. auricularia. Of the 62 snails collected, 11 (17.7%) were infected with 5 different species of larval stages of trematodes. The blood fluke Trichobilharzia franki was found in 2 (3.2%) of the examined snails. The present record provides the first evidence that T. franki from the pulmonate snail R. auricularia represents a source of human cercarial dermatitis in recreational water in Slovakia. Our finding complements the easternmost records of both swimmer's itch and the confirmed occurrence of a bird schistosome in a waterbody in Europe. The present work suggests that the health risks associated with trichobilharziasis need to be further studied by detailed monitoring of the occurrence of the major causative agent of human cercarial dermatitis, T. franki.

Global prevalence status of avian schistosomes: A systematic review with meta-analysis

Objectives

Human cercarial dermatitis (HCD) is a water-borne zoonotic parasitic disease. Cercariae of the avian schistosomes of several genera are frequently recognized as the causative agent of HCD. Various studies have been performed regarding prevalence of bird schistosomes in different regions of the world. So far, no study has gathered and analyzed this data systematically. The aim of this systematic review and meta-analysis study was to determine the prevalence of avian schistosomes worldwide.

Methods

Data were extracted from six available databases for studies published from 1937 to 2017. Generally, 41 studies fulfilled the inclusion criteria and were used for data extraction in this systematic review. Most of studies have been conducted on the family Anatidae.

Results

The overall prevalence of avian schistosomes was estimated to be 34.0% (95%CI, 28%-41%) around the world. Furthermore, results displayed that, Allobilharzia visceralis and Trichobilharzia spp. had the highest frequency and their prevalence in the birds was 50.0% (95% CI, 3.0%-97.0%) and 32.0% (95% CI, 21.0%-0.36%), respectively. The results showed that the prevalence of avian schistosomes was 43.0% (95% CI, 29% - 56%) in the US and 38.0% (27.0% -50.0%) in Europe, which were higher than other continents, respectively.

Conclusions

The prevalence of 34% shows that the bird schistosomes are very common zoonotic worms among aquatic birds in the world. Also, this study shows the importance of avian schistosome research when facing animal and human health of the future.

An efficient method for collecting the full-length adults, fragments, and eggs of Trichobilharzia spp. from the liver of definitive hosts

Precise identification of avian schistosomes in the genus Trichobilharzia at the species level is difficult and requires both traditional morphological and molecular techniques. To obtain satisfactory results by traditional methods, the characteristics of the intact adults or large fragments of male and females are necessary. The present study aimed to introduce a more efficient method for collecting eggs and both fragments and intact worms for morphological identification of visceral Trichobilharzia spp. Thirty-eight domestic ducks (twenty-eight fresh and ten frozen) were studied. For fresh samples, warm saline (40-45 °C) was injected into the portal vein or liver tissue, followed by slicing of the liver to small pieces in a large Petri dish. All materials were then transferred into the laboratory sieves arranged from the largest to the smallest mesh size and while crushed with the hand, washed, and filtered using a trigger water sprayer. The collected materials were studied under a stereomicroscope for parasite eggs, fragments, and full-length worms. Out of 28 freshly killed ducks, 19 (67.9%) and of 10 frozen ducks 6 (60%) were positive for visceral Trichobilharzia spp. The full-length worms and large fragments of male worms were mostly recovered with the mesh no. 150 (diameter of 106 μm) and small fragments, especially of females, and eggs with the mesh no. 270 (diameter of 53 μm). In addition to large numbers of fragments, 15 full-length adults were obtained from fresh and 2 from frozen ducks. The number of collected full-length adults was related to the worm burden. Since morphological description of different species of the genus Trichobilharzia is primarily based on the availability of adult worms, the application of methods that provide a higher number of intact males and females will result in better characterization of the species and deposition of appropriate voucher specimens. These results show the present method as a suitable tool for the collection of quality adults of visceral Trichobilharzia spp. in ducks.

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.

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.

Cercarial dermatitis, a neglected allergic disease

Cercarial dermatitis (swimmer's itch) is a common non-communicable water-borne disease. It is caused by penetration of the skin by larvae (cercariae) of schistosomatid flukes and develops as a maculopapular skin eruption after repeated contacts with the parasites. The number of outbreaks of the disease is increasing, and cercarial dermatitis can therefore be considered as an emerging problem. Swimmer's itch is mostly associated with larvae of the bird schistosomes of Trichobilharzia spp. Recent results have shown that mammalian infections (including man) manifest themselves as an allergic reaction which is able to trap and eliminate parasites in the skin. Studies on mammals experimentally infected by bird schistosome cercariae revealed, however, that during primary infection, parasites are able to escape from the skin to the lungs or central nervous system. This review covers basic information on detection of the infectious agents in the field and the clinical course of the disease, including other pathologies which may develop after infection by cercariae, and diagnosis of the disease.

New insight in lymnaeid snails (Mollusca, Gastropoda) as intermediate hosts of Fasciola hepatica (Trematoda, Digenea) in Belgium and Luxembourg

BACKGROUND

The present study aims to assess the epidemiological role of different lymnaeid snails as intermediate hosts of the liver fluke Fasciola hepatica in Belgium and Luxembourg.

METHODS

During summer 2008, 7103 lymnaeid snails were collected from 125 ponds distributed in 5 clusters each including 25 ponds. Each cluster was located in a different biogeographic area of Belgium and Luxembourg. In addition, snails were also collected in sixteen other biotopes considered as temporary wet areas. These snails were identified as Galba truncatula (n = 2474) (the main intermediate host of F. hepatica in Europe) and Radix sp. (n = 4629). Moreover, several biological and non-biological variables were also recorded from the different biotopes. DNA was extracted from each snail collected using Chelex® technique. DNA samples were screened through a multiplex PCR that amplifies lymnaeid internal transcribed spacer 2 gene sequences (500-600 bp) (acting as an internal control) and a 124 bp fragment of repetitive DNA from Fasciola sp.

RESULTS

Lymnaeid snails were found in 75 biotopes (53.2%). Thirty individuals of G. truncatula (1.31%) and 7 of Radix sp. (0.16%) were found to be positive for Fasciola sp. The seven positive Radix sp. snails all belonged to the species R. balthica (Linnaeus, 1758). Classification and regression tree analysis were performed in order to better understand links and relative importance of the different recorded factors. One of the best explanatory variables for the presence/absence of the different snail species seems to be the geographic location, whereas for the infection status of the snails no obvious relationship was linked to the presence of cattle.

CONCLUSIONS

Epidemiological implications of these findings and particularly the role of R. balthica as an alternative intermediate host in Belgium and Luxembourg were discussed.

Trichobilharzia mergi sp. nov. (Trematoda: Digenea: Schistosomatidae), a visceral schistosome of Mergus serrator (L.) (Aves: Anatidae)

Parasitological investigations on red-breasted mergansers (Mergus serrator L.) in Iceland revealed digenean flukes of the family Schistosomatidae. Adult worms were detected in blood vessels of the large intestine and eggs were deposited in the mucosa and surrounded by granulomatous reactions. Traditional morphological methods showed that the flukes have very slender filiform bodies, males are equipped with a short gynaecophoric canal and both suckers and spatulate ends are present on each sex. Among characteristics of the flukes which render them morphologically distinct from other Trichobilharzia species are: i) males-well developed vesicula seminalis (v.s.) consisting of a short v.s. externa and a significantly longer (approx. 3 times) v.s. interna, unusually well developed genital papilla and localization of the first testis a relatively long distance posterior to the gynaecophoric canal; ii) eggs-small and elongated with slightly rounded poles and a short terminal spine. DNA taxonomic techniques confirmed that a new species had been identified, Trichobilharzia mergi sp. n. The sequence data were deposited in GenBank under the accession numbers JX456151 to JX456172. Comparison of the results with our previously published data on characterization of DNA of cercariae isolated from freshwater lymnaeid snails showed that larval development of T. mergi is associated with Radix balthica L. (=Radix peregra Müller, 1774;=Radix ovata Draparnaud, 1805).

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.

Morphological features of the nasal blood fluke Trichobilharzia regenti (Schistosomatidae, Digenea) from naturally infected hosts

The first author detected the nasal bird schistosome Trichobilharzia regenti in Iceland in Anas platyrhynchos in Landmannalaugar in autumn of 2003. Since then, measurements and morphological studies have been performed on fresh worms (fragments) obtained in the area from naturally infected ducks, A. platyrhynchos and Aythya marila. In the present study, we compare our findings to the original description of T. regenti by Horák et al. (Parasite 5:349-357, 1998) that relies upon worms obtained by experimental infections of A. platyrhynchos f. domestica and Cairina moschata f. domestica ducklings. Fragments obtained from naturally infected birds are markedly larger than those obtained in the experimental infection. Also, indistinct sex-related size difference was confirmed; males were more abundant in the material than females. Previously unknown morphological features detected in the present study include, e.g. subterminal spines on the oral sucker directed to the oral opening, spine pattern on the apical part of acetabulum and long tegumental spines (up to 16 μm) in the gonad and tail regions of both sexes. In males, we evaluated for the first time the morphology of the cirrus sac enclosing ejaculatory duct, prostata and the entire prostatic region, the course of vas deferens and the position of genital papilla. In females, the posterior part of the reproductive system was studied for the first time. The presence of the Laurer's canal was confirmed and its course from the oviduct to the surface, where spermatozoa were noticed to leave the canal, was described.

Cathepsins B1 and B2 of Trichobilharzia SPP., bird schistosomes causing cercarial dermatitis

Trichobilharzia regenti and T. szidati are schistosomes that infect birds. although T. regenti/T. szidati can only complete their life cycle in specific bird hosts (waterfowl), their larvae-cercariae are able to penetrate, transform and then migrate as schistosomula in nonspecific hosts (e.g., mouse, man). Peptidases are among the key molecules produced by these schistosomes that enable parasite invasion and survival within the host and include cysteine peptidases such as cathepsins B1 and B2. These enzymes are indispensable bio-catalysts in a number of basal biological processes and host-parasite interactions, e.g., tissue invasion/migration, nutrition and immune evasion. Similar biochemical and functional characteristics were observed for cathepsins B1 and B2 in bird schistosomes (T. regenti, T. szidati) and also for their homologs in human schistosomes (Schistosoma mansoni, S. japonicum). Therefore, data obtained in the research of bird schistosomes can also be exploited for the control of human schistosomes such as the search for targets of novel chemotherapeutic drugs and vaccines.

Geographical distribution of cercarial dermatitis in Norway

Bird schistosomes can cause a disease called cercarial dermatitis, or swimmer's itch, in humans. The disease occurs when people have direct contact with fresh water or sea water containing the free-swimming cercariae of the flukes. The symptoms are well known, and include intense itching, maculae, papulae, urticariae and, in some cases, local oedema with enlarged lymph nodes and fever. In this study, we present the geographical distribution of freshwater cercarial dermatitis in Norway. The study is based on random reports obtained from both individuals and physicians treating patients with itching skin rash after freshwater bathing. The first case of cercarial dermatitis in Norway was reported in 1980 and was traced to a lake near Trondheim in the central part of Norway. In the following years, an increasing number of cases were reported, especially in southern Norway. However, case reports are distributed almost all over the country, even from lakes in northern Norway. As far as we know, these are the northernmost case reports in Europe. So far, only one fluke species (Trichobilharzia franki) from a single infected snail (Radix auricularia) has been identified in Norway. However, unidentified schistosomatid ocellate cercariae have been found on several occasions in snails collected from six lakes where swimmer's itch is frequently reported. Future studies should be performed to identify the fluke species, as well as the most important snail and bird hosts, in Norwegian lakes.

Molecular diversity of Trichobilharzia franki in two intermediate hosts (Radix auricularia and Radix peregra): a complex of species

Recently, the systematic use of the molecular approach as a complement to the other approaches (morphology, biology, life cycle) has brought help for the identification of species considered as different in the past to be regrouped and synonymised, and distinctions to be drawn between species similar at the morphological level. Among these species, we tried to clarify the situation of Trichobilharzia frankiMüller and Kimmig, 1994, species that today include more than 50 haplotypes notably coming from larval stages isolated from intermediate hosts belonging to gastropods of the Radix genus. Cercariae were isolated in France and Iceland from various molluscs, before being analyzed, with their hosts, by molecular analysis of various fields such as the D2 and ITS of the ribosomal DNA and the COX1 of mitochondrial DNA. We thus show the presence of two clades depending on the specificity of their intermediate host in which they were isolated (Radix auricularia or Radix peregra), thus allowing separation of the species T. franki that had been described in the past as a probable new species.

Final hosts and variability of Trichobilharzia regenti under natural conditions

In Europe, despite of intensive work on avian schistosomes in the past, only one representative of nasal schistosomes has been detected so far, Trichobilharzia regenti, a species that remained undescribed to the scientific community until 1998. Since then, however, T. regenti has gained a considerable attention, not only due to the induction of serious lesions concomitant with the migration of the schistosomulae through the central nervous system of their natural hosts (birds) but also due to their implication in cercarial dermatitis and their neurotrophic behaviour in mammals (mice, experimental) where the parasite migrate to the CNS. From 2001 to 2010, by a parasitic control of various potential final hosts (aquatic birds) living in natural condition in France and in Iceland, we detected T. regenti in six species: Anas platyrhynchos, Cygnus olor, Aythya fuligula, Aythya ferina, Mergus merganser and Anser anser. Although morphological features, mainly of eggs, showed some variations with the original description of the parasite, molecular analysis confirmed their membership to the T. regenti species. Nevertheless, specific molecular and morphological variations were especially observed for nasal parasite isolated from Anas clypeata. For this species, we discuss the possibility for this nasal schistosome to belong to a nasal Trichobilharzia species already described in Africa or even an undescribed nasal species.

Methodical approaches in the identification of areas with a potential risk of infection by bird schistosomes causing cercarial dermatitis

Larval stages (cercariae) of schistosomatid flukes represent the causative agents of swimmer's itch (cercarial dermatitis), a waterborne allergic disease. Cercariae of bird schistosomes are the most frequently reported agent. Recent studies on parasite behaviour in mammals showed that infections by cercariae can be linked to more than skin syndromes. Despite the failure of complete development in mammals, bird schistosomes can escape from the skin and migrate transitorily in the hosts. These findings brought novel insights into the fate and potential pathogenic effect of the parasites in non-compatible hosts, including humans. Cercarial dermatitis occurs globally and recently is considered to be re-emerging; however, there are no data on the number of afflicted persons per year. This might be explained by a relatively low interest in human skin infections arising after bathing in fresh water. In addition, the real occurrence of bird schistosomes in the field is known only for a few areas. The paucity of epidemiological/biogeographical data is probably caused by difficulties associated with detection of the parasites in intermediate and definitive hosts. Therefore, based on personal experience and data available in the literature, we have summarized methodological approaches enabling the detection of bird schistosomes in various hosts and environments.

Bird schistosome diversity in Iceland: molecular evidence

During the years 2002-2007, Icelandic freshwater snails and birds from different orders were examined for bird schistosomes. Only the snail Radix peregra and anatid birds proved to be infected. In total, 32 samples of bird schistosome cercariae from seven localities and four samples of adults of Anas platyrhynchos and Mergus serrator from two localities were used for sequencing of the internal transcibed spacer (ITS) region of rDNA. Based on the sequence and position in the phylogenetic tree, five species/genotypes of bird schistosomes were identified. Three of them correspond to the sequences in GenBank for Trichobilharzia franki, Trichobilharzia sp. JR-2007 from Anas penelope (isolates Pl7 and Pl10) and schistosomatid sp. JR-2004 from Physa fontinalis. The other two species/genotypes belong to the genus Trichobilharzia, but their further determination was not possible due to lack of data. All of these species/genotypes develop in R. peregra, probably the only suitable intermediate host species in Iceland. As T. franki and schistosomatid sp. JR-2004 were previously reported from other snail species, the findings from Iceland proved that, under some circumstances, bird schistosomes are able to adapt to a different snail species; some of them can even adapt to a snail belonging to a different gastropod family. High diversity of bird schistosomes in Iceland is probably linked with migratory flyways of birds, as several bird species from Iceland overwinter in western Europe as well as in North America.