Heat sensitivity of first host and cercariae may restrict parasite transmission in a warming sea.

To predict global warming impacts on parasitism, we should describe the thermal tolerance of all players in host-parasite systems. Complex life-cycle parasites such as trematodes are of particular interest since they can drive complex ecological changes. This study evaluates the net response to temperature of the infective larval stage of Himasthla elongata, a parasite inhabiting the southwestern Baltic Sea. The thermal sensitivity of (i) the infected and uninfected first intermediate host (Littorina littorea) and (ii) the cercarial emergence, survival, self-propelling, encystment, and infection capacity to the second intermediate host (Mytilus edulis sensu lato) were examined. We found that infection by the trematode rendered the gastropod more susceptible to elevated temperatures representing warm summer events in the region. At 22 °C, cercarial emergence and infectivity were at their optimum while cercarial survival was shortened, narrowing the time window for successful mussel infection. Faster out-of-host encystment occurred at increasing temperatures. After correcting the cercarial emergence and infectivity for the temperature-specific gastropod survival, we found that warming induces net adverse effects on the trematode transmission to the bivalve host. The findings suggest that gastropod and cercariae mortality, as a tradeoff for the emergence and infectivity, will hamper the possibility for trematodes to flourish in a warming ocean.

Trematodes coupled with neonicotinoids: effects on blood cell profiles of a model amphibian.

Habitat loss, climate change, environmental contaminants, and parasites and pathogens are among the main factors thought to act singly or together in causing amphibian declines. We tested for combined effects of neonicotinoid pesticides and parasites (versus parasites-only) on mortality, growth, and white blood cell profiles of a model amphibian: the northern leopard frog (Rana pipiens). We first exposed infectious stages of frog trematodes (cercariae of Echinostoma spp.) to low and high concentrations of thiamethoxam or clothianidin versus water-only controls. There were no differences in survival of trematode cercariae between treatments. For the main experiment, we exposed tadpoles to clean water versus high concentrations of clothianidin or thiamethoxam for 2 weeks and added trematode cercariae to all tanks after 1 week. Exposure of tadpoles and parasites to high concentrations of thiamethoxam or clothianidin did not affect parasite infection success. Tadpole survival was not different between treatments before or after parasite addition and there were no significant differences in tadpole snout-to-vent lengths or developmental stages between treatments. Tadpoles exposed to thiamethoxam + parasites had smaller widths than parasite-only tadpoles, whereas tadpoles exposed to clothianidin + parasites had higher eosinophil to leukocyte ratios compared to parasite-only tadpoles. Tadpoles of both neonicotinoid + parasite treatments had significantly lower monocyte to leukocyte ratios relative to parasite-only tadpoles. High concentrations of neonicotinoid combined with parasites appear to influence tadpole immune function important for further defense against parasites and pathogens. This work highlights the need for more holistic approaches to ecotoxicity studies, using multiple stressors.

Parvatrema duboisi (Digenea: Gymnophallidae) Life Cycle Stages in Manila Clams, Ruditapes philippinarum, from Aphae-do (Island), Shinan-gun, Korea.

Life cycle stages, including daughter sporocysts, cercariae, and metacercariae, of Parvatrema duboisi (Dollfus, 1923) Bartoli, 1974 (Digenea: Gymnophallidae) have been found in the Manila clam Ruditapes philippinarum from Aphaedo (Island), Shinan-gun, Jeollanam-do, Korea. The daughter sporocysts were elongated sac-like and 307-570 (av. 395) µm long and 101-213 (av. 157) µm wide. Most of the daughter sporocysts contained 15-20 furcocercous cercariae each. The cercariae measured 112-146 (av. 134) µm in total length and 35-46 (av. 40) µm in width, with 69-92 (av. 85) µm long body and 39-54 (av. 49) µm long tail. The metacercariae were 210-250 (av. 231) µm in length and 170-195 (av. 185) µm in width, and characterized by having a large oral sucker, genital pore some distance anterior to the ventral sucker, no ventral pit, and 1 compact or slightly lobed vitellarium, strongly suggesting P. duboisi. The metacercariae were experimentally infected to ICR mice, and adults were recovered at day 7 post-infection. The adult flukes were morphologically similar to the metacercariae except in the presence of up to 20 eggs in the uterus. The daughter sporocysts and metacercariae were molecularly (ITS1-5.8S rDNA-ITS2) analyzed to confirm the species, and the results showed 99.8-99.9% identity with P. duboisi reported from Kyushu, Japan and Gochang, Korea. These results confirmed the presence of various life cycle stages of P. duboisi in the Manila clam, R. philippinarum, playing the role of the first as well as the second intermediate host, on Aphae-do (Island), Shinan-gun, Korea.

Oncomelania hupensis retains its ability to transmit Schistosoma japonicum 13 years after migration from permissive to non-permissive areas.

BACKGROUND: The East Route Project (ERP) of the South-to-North Water Diversion Project (SNWDP) stretches across schistosomiasis endemic and non-endemic areas in China, which may lead to the dispersal of Oncomelania hupensis, the intermediate host of Schistosoma japonicum, from permissive areas along the Yangtze River Basin to non-permissive areas in northern China. A previous survey demonstrated that O. hupensis could survive and breed for 13 years (12 generations) after being transferred to a non-permissive area, and could be infected by S. japonicum. However, it is not clear if the migrated snails will change their ability to transmit S. japonicum. METHODS: We infected mice with the cercariae released from the infected transferred snails bred in Jining city of Shandong Province (non-permissive areas) for 13 years. The mice in the control group were infected with cercariae derived from the snails collected in their original habitat (Jiangdu county of Jiangsu Province, permissive areas). Then, we explored the pathogenicity to mice including worm burden, liver egg count and pathology. Additionally, the gene expression profiles of the adult male and female worms recovered from the infected mice were analyzed by RNA sequencing. RESULTS: The worm burden, liver egg count and pathology of the mice infected with cercariae released from transferred snails bred in non-permissive areas for 13 years showed no significant differences, when compared with the control cercariae. Slight changes occurred at the transcription level between adult male and female worms recovered from mice infected with cercariae derived from snails bred in permissive and non-permissive areas. Only fourteen genes were significantly differentially expressed in the comparison of adult female worms, and no significantly differentially expressed gene was found in the comparison of adult male worms. CONCLUSIONS: Our findings strongly suggest that transferred snails did not change their schistosomiasis transmission ability and the worms derived from them retained the original pathogenicity, even after migrating from permissive to non-permissive areas for 13 years. Therefore, a long-term surveillance system of snails along the SNWDP is urgently needed to prevent the diffusion of O. hupensis and reduce the risk of transmission of schistosomiasis.

A rotifer-derived paralytic compound prevents transmission of schistosomiasis to a mammalian host.

Schistosomes are parasitic flatworms that infect over 200 million people, causing the neglected tropical disease, schistosomiasis. A single drug, praziquantel, is used to treat schistosome infection. Limitations in mass drug administration programs and the emergence of schistosomiasis in nontropical areas indicate the need for new strategies to prevent infection. It has been known for several decades that rotifers colonizing the schistosome's snail intermediate host produce a water-soluble factor that paralyzes cercariae, the life cycle stage infecting humans. In spite of its potential for preventing infection, the nature of this factor has remained obscure. Here, we report the purification and chemical characterization of Schistosome Paralysis Factor (SPF), a novel tetracyclic alkaloid produced by the rotifer Rotaria rotatoria. We show that this compound paralyzes schistosome cercariae and prevents infection and does so more effectively than analogous compounds. This molecule provides new directions for understanding cercariae motility and new strategies for preventing schistosome infection.

Molecular evidence for distinct modes of nutrient acquisition between visceral and neurotropic schistosomes of birds.

Trichobilharzia species are parasitic flatworms (called schistosomes or flukes) that cause important diseases in birds and humans, but very little is known about their molecular biology. Here, using a transcriptomics-bioinformatics-based approach, we explored molecular aspects pertaining to the nutritional requirements of Trichobilharzia szidati ('visceral fluke') and T. regenti ('neurotropic fluke') in their avian host. We studied the larvae of each species before they enter (cercariae) and as they migrate (schistosomules) through distinct tissues in their avian (duck) host. Cercariae of both species were enriched for pathways or molecules associated predominantly with carbohydrate metabolism, oxidative phosphorylation and translation of proteins linked to ribosome biogenesis, exosome production and/or lipid biogenesis. Schistosomules of both species were enriched for pathways or molecules associated with processes including signal transduction, cell turnover and motility, DNA replication and repair, molecular transport and/or catabolism. Comparative informatic analyses identified molecular repertoires (within, e.g., peptidases and secretory proteins) in schistosomules that can broadly degrade macromolecules in both T. szidati and T. regenti, and others that are tailored to each species to selectively acquire nutrients from particular tissues through which it migrates. Thus, this study provides molecular evidence for distinct modes of nutrient acquisition between the visceral and neurotropic flukes of birds.

Thermal preferences of bird schistosome snail hosts increase the risk of swimmer's itch.

Ambient temperature strongly affects host parasite interactions, especially when both are ectothermic. Bird schistosomes, which cercariae are known as agents of swimmer's itch and their snail hosts can be a good example of this phenomenon. The snails of these parasites play the key role, as the source of harmful larvae. Cercarial dermatitis is noted even in areas when prevalence of parasites in snail populations is very low. The main question is what adaptation in snail-fluke association can lead to a sufficient number of cercariae causing swimmer's itch in lake water? The influence of ambient temperature on snail survival and cercarial production as well as the thermal preferences of two host species naturally infected with bird schistosomes were studied. The 24-h preferences of Lymnaea stagnalis infected with Trichobilharzia szidati, and Planorbarius corneus infected with Bilharziella polonica were recorded using an oblong thermal gradient set (OTGS). Both cercariae releasing hosts of bird schistosomes preferred a significantly lower temperature than non-infected snails. Additionally, at a higher temperature, the survival of snail hosts was shortened as a result of the increase in daily cercarial expulsion. An especially interesting result concerns the release of a significantly larger total cercariae number by L. stagnalis at lower than at higher temperatures. These data indicate that preferences of infected snails to low temperature microhabitats can increase host survival and parasite success, as well as affecting the increase in the number of invasive larvae in the environment increasing the risk of swimmer's itch.

Host Specificity and Ecology of Digenean Parasites of Nassariid Gastropods in Central Queensland, Australia, with Comments on Host-Parasite Associations of the Nassariidae.

Nassariid gastropods are well represented in Australia, and Nassarius dorsatus (Röding), Nassarius olivaceus (Bruguière), and Nassarius pullus (Linnaeus) are common in the intertidal mangroves of the Capricornia region in Central Queensland. A survey of the digenean trematode fauna of nassariids of the region revealed cercariae of 12 species from six trematode families associated with these gastropods. Six species of Acanthocolpidae were found, including three putative species of the genus Stephanostomum. Additionally, two species of Himasthlidae and one species each of Microphallidae, Opecoelidae, Zoogonidae, and Lepocreadiidae were identified. The majority of the trematode species identified were found in only one host species; only four infected more than one species of nassariid. Sixteen new host-parasite combinations were added to the fauna known to infect the Nassariidae. Broader examination of host-parasite associations shows that families using nassariids commonly as first intermediate host are the Acanthocolpidae, Lepocreadiidae, and Zoogonidae, as well as Himasthla spp. from the Himasthlidae. Nassariid gastropods are also the dominant marine first intermediate hosts for the Acanthocolpidae, Lepocreadiidae, and Zoogonidae and one of the three most common first intermediate host families for marine Himasthla spp.

Larval digenean communities in Biomphalaria species in two contrasting wetlands from South America.

A key factor in the colonization probabilities of parasites is the host's characteristic habitat. Considering that rice fields are simple habitats compared with natural wetlands, we comparatively analyzed the composition of the cercariae communities in Biomphalaria snails in an agricultural and a nonagricultural habitat of Corrientes Province, Argentina. We collected 2998 Biomphalaria snails from November 2011 to May 2012: B. tenagophila, B. occidentalis, B. peregrina, and B. orbignyi in the nonagricultural habitat and B. straminea in the agricultural habitat. Nine cercaria species were found in the nonagricultural habitat and 12 in the agricultural habitat, with overall parasite prevalence of 9.27 and 7.69%, respectively. The overall prevalence and mean species richness of cercariae showed no significant differences between habitats. The cercariae communities of the 2 habitats showed low similarity (38%), sharing 4 species. The mean species diversity of cercariae was higher in the nonagricultural habitat. Our results suggest that rice fields provide conditions for the hosts and cercariae that are somewhat similar to those of nonagricultural wetlands, although the contrasting characteristics of habitats can influence the presence of different intermediate and definitive hosts and consequently impact parasite diversity.

Hsp70 May Be a Molecular Regulator of Schistosome Host Invasion.

Schistosomiasis is a debilitating disease that affects over 240 million people worldwide and is considered the most important neglected tropical disease following malaria. Free-swimming freshwater cercariae, one of the six morphologically distinct schistosome life stages, infect humans by directly penetrating through the skin. Cercariae identify and seek the host by sensing chemicals released from human skin. When they reach the host, they burrow into the skin with the help of proteases and other contents released from their acetabular glands and transform into schistosomula, the subsequent larval worm stage upon skin infection. Relative to host invasion, studies have primarily focused on the nature of the acetabular gland secretions, immune response of the host upon exposure to cercariae, and cercaria-schistosomulum transformation methods. However, the molecular signaling pathways involved from host-seeking through the decision to penetrate skin are not well understood. We recently observed that heat shock factor 1 (Hsf1) is localized to the acetabular glands of infectious schistosome cercariae, prompting us to investigate a potential role for heat shock proteins (HSPs) in cercarial invasion. In this study, we report that cercarial invasion behavior, similar to the behavior of cercariae exposed to human skin lipid, is regulated through an Hsp70-dependent process, which we show by using chemical agents that target Hsp70. The observation that biologically active protein activity modulators can elicit a direct and clear behavioral change in parasitic schistosome larvae is itself interesting and has not been previously observed. This finding suggests a novel role for Hsp70 to act as a switch in the cercaria-schistosomulum transformation, and it allows us to begin elucidating the pathways associated with cercarial host invasion. In addition, because the Hsp70 protein and its structure/function is highly conserved, the model that Hsp70 acts as a behavior transitional switch could be relevant to other parasites that also undergo an invasion process and can apply more broadly to other organisms during morphological transitions. Finally, it points to a new function for HSPs in parasite/host interactions.

The influence of trematode parasite burden on gene expression in a mammalian host.

BACKGROUND: Parasites can profoundly impact their hosts and are responsible for a plethora of debilitating diseases. To identify global changes in host gene expression related to parasite infection, we sequenced, assembled, and annotated the liver transcriptomes of Balb/cj mice infected with the trematode parasite Schistosoma mansoni and compared the results to uninfected mice. We used two different methodologies (i.e. de novo and reference guided) to evaluate the influence of parasite sequences on host transcriptome assembly. RESULTS: Our results demonstrate that the choice of assembly methodology significantly impacted the proportion of parasitic reads detected from the host library, yet the presence of non-target (xenobiotic) sequences did not create significant structural errors in the assembly. After removing parasite sequences from the mouse transcriptomes, we analyzed host gene expression under different parasite infection levels and observed significant differences in the associated immunologic and metabolic responses based on infection level. In particular, genes associated with T-helper type 1 (Th-1) and T-helper type 2 (Th-2) were up-regulated in infected mice whereas genes related to amino acid and carbohydrate metabolism were down-regulated in infected mice. These changes in gene expression scale with infection status and likely impact the evolutionary fitness of hosts. CONCLUSIONS: Overall, our data indicate that a) infected mice reduce the expression of key metabolic genes in direct proportion to their infection level; b) infected mice similarly increase the expression of key immune genes in response to infection; c) patterns of gene expression correspond to the pathological symptoms of schistosomiasis; and d) identifying and filtering out non-target sequences (xenobiotics) improves differential expression prediction. Our findings identify parasite targets for RNAi or other therapies and provide a better understanding of the pathology and host immune repertoire involved in response to S. mansoni infections.

Infectivity and development of Opisthorchis viverrini metacercariae in immunosuppressed Barbonymus gonionotus fingerlings (Cyprinidae).

Opisthorchiasis caused by Opisthorchis viverrini infection, is a serious public health problem in Lao PDR, Cambodia, Vietnam and Thailand. Cyprinid fish are required for the transmission of O. viverrini metacercariae to fish eating-mammal hosts including humans. The transmission of infective metacerariae is an important event, potentially resulting in the development of cholangiocarcinoma in cases of chronic opisthorchiasis. For a better understanding of the development of O. viverrini metacercariae in the second intermediate host, Barbonymus gonionotus (Cyprinidae), the immune response of infected B. gonionotus through fibrous tissue formation and superoxide dismutase (SOD) activity were investigated. The immunosuppressant (prednisolone) was administered to B. gonionotus infected with O. viverrini cercariae to study infectivity and development of the metacercariae. Additionally, the effects of prednisolone were studied by the investigation of fibrous tissue formation around the metacercariae, as well as SOD activity. In our study, prednisolone administered prior to infection in B. gonionotus had higher metacercarial recovery when compared to the non-administered control group. On the other hand, the administration of prednisolone after the O. viverrini infection allowed metacercariae to develop to the infective stage. Energy consumption from encystation to the infective stage required 443.9-463.7degree-days. The results show that immunosuppression decreases fibrous tissue and SOD activity in infected fish, and also affects the ability of O. viverrini metacercariae to develop to the infective stage. The immune response to metacercarial development of O. viverrini in cyprinid fish should be further studied to gain a greater understanding of this process, and hence, a better understanding of the mechanism(s) involved in metacercarial wall formation.

Molecular diversity of avian schistosomes in Danish freshwater snails.

Avian schistosomes are widespread parasites of snails and waterfowl and may cause cercarial dermatitis (swimmer's itch) in humans, a disease that is frequently reported in European countries. These parasites are known to occur in Denmark, but here, we applied a new approach using molecular tools to identify the parasites at species level. In order to do that, 499 pulmonate freshwater snails (Radix sp., Lymnaea stagnalis, Stagnicola sp. and Planorbarius corneus) were sampled from 12 lakes, ponds, and marshes in the greater Copenhagen area. Avian schistosome cercariae were identified by microscopy and subjected to molecular investigation by sequencing and phylogenetic analysis of the 5.8S and ITS2 ribosomal DNA for species identification. Additionally, snail hosts belonging to the genus Radix were identified by sequencing and phylogenetic analysis of partial ITS2 ribosomal DNA. Three out of 499 snails shed different species of Trichobilharzia cercariae: Trichobilharzia szidati was isolated from L. stagnalis, Trichobilharzia franki from Radix auricularia and Trichobilharzia regenti from Radix peregra. In the light of the public health risk represented by bird schistosomes, these findings are of concern and, particularly, the presence of the potentially neuro-pathogenic species, T. regenti, in Danish freshwaters calls for attention.

Structural and Population Polymorphism of RT-Like Sequences in Avian Schistosomes Trichobilharzia szidati (Platyhelminthes: Digenea: Schistosomatidae).

Recently we developed the genus-specific markers of the avian schistosomes of the genus Trichobilharzia, the causative agents of human cercarial dermatitis. The 7 novel genome sequences of T. franki, T. regenti, and T. szidati revealed similarity with genome repeat region of African schistosome Schistosoma mansoni. In the present work we analyzed the 37 new T. szidati sequences to study intragenome variability and host specificity for the parasite from three localities of East Europe. DNAs were isolated from cercariae or single sporocysts obtained from 6 lymnaeid snails Lymnaea stagnalis and L. palustris from Belarus and Russia. All sequences formed three diverged groups, one of which consists of the sequences with multiple deletions; other groups involved two paralogous copies with stop codons and frameshift mutations. Strong association between geographical distribution and snail host specificity cannot be established. All studied sequences have homology with the reverse transcriptase domain (RT) of Penelope-like elements (PLE) of S. mansoni and S. japonicum and new members of RT family were identified. We proposed that three diverged groups RT sequences of T. szidati are results of duplication or transposition of PLE during parasite evolution. Implications of the retroelement dynamics in the life history of avian schistosomes are discussed.

Host life history and host-parasite syntopy predict behavioural resistance and tolerance of parasites.

There is growing interest in the role that life-history traits of hosts, such as their 'pace-of-life', play in the evolution of resistance and tolerance to parasites. Theory suggests that, relative to host species that have high syntopy (local spatial and temporal overlap) with parasites, host species with low syntopy should have lower selection pressures for more constitutive (always present) and costly defences, such as tolerance, and greater reliance on more inducible and cheaper defences, such as behaviour. Consequently, we postulated that the degree of host-parasite syntopy, which is negatively correlated with host pace-of-life (an axis reflecting the developmental rate of tadpoles and the inverse of their size at metamorphosis) in our tadpole-parasitic cercarial (trematode) system, would be a negative and positive predictor of behavioural resistance and tolerance, respectively. To test these hypotheses, we exposed seven tadpole species to a range of parasite (cercarial) doses crossed with anaesthesia treatments that controlled for anti-parasite behaviour. We quantified host behaviour, successful and unsuccessful infections, and each species' reaction norm for behavioural resistance and tolerance, defined as the slope between cercarial exposure (or attempted infections) and anti-cercarial behaviours and mass change, respectively. Hence, tolerance is capturing any cost of parasite exposure. As hypothesized, tadpole pace-of-life was a significant positive predictor of behavioural resistance and negative predictor of tolerance, a result that is consistent with a trade-off between behavioural resistance and tolerance across species that warrants further investigation. Moreover, these results were robust to considerations of phylogeny, all possible re-orderings of the three fastest or slowest paced species, and various measurements of tolerance. These results suggest that host pace-of-life and host-parasite syntopy are powerful drivers of both the strength and type of host defence strategies against parasites. Future research should evaluate how often and how strongly host pace-of-life and host-parasite syntopy are correlated and which is the better predictor of the strength and type of host investments in anti-parasite defences.

In vitro encystment of Himasthla elongata cercariae (Digenea, Echinostomatidae) in the haemolymph of blue mussels Mytilus edulis as a tool for assessing cercarial infectivity and molluscan susceptibility.

Infectivity of Himasthla elongata cercariae to mussels, their second intermediate hosts, and resistance by these hosts to infection were assessed on the basis of the cercariae's ability to encyst in mussel haemolymph in vitro. A series of experimental in vivo infections of mussels with batches of cercariae, each batch released from a different single infected mollusc and referred to as a clone (due to their shared genotype), demonstrated that the results of the in vitro tests corresponded to the actual indices of infectivity/susceptibility of the parasites and their hosts. Most cercarial clones had high infectivity, with a few clones having very high or, at the other extreme, very low infectivity. A similar pattern was revealed in mussel resistance to cercarial infection. Most of the molluscs tested were moderately susceptible to cercarial infection, but at each extreme a small fraction (less than 10%) displayed very high or very low susceptibility. It was shown that there were no totally compatible or totally incompatible 'cercaria clone/mussel' combinations. Results obtained are compared with the data on intra-population variability using the characters parasite infectivity/host compatibility for trematode/mollusc-first intermediate host associations. Results are made relevant to actual infection levels in mussel settlements at the White Sea.

[Surveillance and forecast system of schistosomiasis in Jiangsu Province. IV. Establishment of Schistosoma japonicum cercaria-killing method by spraying niclosamide suspension on water surface].

OBJECTIVE: To investigate the effect of suspension concentrate of niclosamide (SCN) on killing cercariae of Schistosoma japonicum on water surface, optimization and impact on fish, so as to establish an emergency-treatment intervention for rapidly killing cercariae and eliminating water infectivity. METHODS: SCN was formulated into different concentrations of solutions, and then the solutions were sprayed on the surface of water containing S. japonicum cercariae. The water infectivity was determined by using mice at 0, 10, 30 min after spraying SCN. SCN was formulated into a solution of 100 mg/L and then sprayed on the surface of the water by using the spraying values of 0.01, 0.02, 0.03 g/m2 and 0.04 g/m2. At 30 min and 60 min after spraying, the water infectivity was determined by using mice. Zebra fish were transferred into the static water, then 100 mg/L SCN (s), using spraying values of 0.01, 0.02, 0.03 g/m2 and 0.04 g/m2, were sprayed on water surface. At 0, 10, 30, 60 min after spraying, the samples were collected at water depths of 0, 10, 20, 30, 40 cm, and niclosamide was determined by using high-performance liquid chromatography. The death of zebra fish was continually observed within 96 h after spraying SCN. RESULTS: At 0, 10, 30 min after spraying 1 000, 100, 10, 1, 0.1 mg/L SCN on water surface, the infectivity of water significantly decreased. At 30 min after spraying 1 000 mg/L and 100 mg/L SCN, no schistosome infectivity was detected in the water. At 30 min after spraying 100 mg/L SCN, with spraying values of 0.01, 0.02, 0.03, 0.04 g/m2, the water infectivity significantly reduced, and no infectivity was found 60 min after spraying SCN. After the surface of static water was sprayed with 100 mg/L SCN, the peak concentration was found at 0 min, and the solution diffused to site with a water depth of 10 cm after 10 min, and 30 min later, SCN diffused to the whole water body, and distributed evenly. After spraying 100 mg/L SCN on the surface of water with a volume of (3.14 x 20(2) x 50) cm3, by using the spraying value of 0.02 g/m2, 96 h later, no death of zebra fish was found. CONCLUSIONS: From 30 to 60 min after spraying 100 mg/L SCN, with the value of 0.02 g/m2, on the surface of S. japonicum-infested water, the water infectivity can be eliminated, and there is no evident toxicity to fish. This cercaria-killing method, as an emergency-treatment intervention for infested water, can be applied in those surveillance and forecast sites.

Cercarial dermatitis transmitted by exotic marine snail.

Cercarial dermatitis (swimmer's itch) is caused by the penetration of human skin by cercariae of schistosome parasites that develop in and are released from snail hosts. Cercarial dermatitis is frequently acquired in freshwater habitats, and less commonly in marine or estuarine waters. To investigate reports of a dermatitis outbreak in San Francisco Bay, California, we surveyed local snails for schistosome infections during 2005-2008. We found schistosomes only in Haminoea japonica, an Asian snail first reported in San Francisco Bay in 1999. Genetic markers place this schistosome within a large clade of avian schistosomes, but do not match any species for which there are genetic data. It is the second known schistosome species to cause dermatitis in western North American coastal waters; these species are transmitted by exotic snails. Introduction of exotic hosts can support unexpected emergence of an unknown parasite with serious medical or veterinary implications.