Prevalence and distribution of trematodes hosted by Bithynia siamensis in Southern Thailand.

This study aimed to investigate the occurrence of larval trematode infections in bithyniid snails across five provinces in southern Thailand. A total of 1,413 Bithynia siamensis snails were collected between October 2021 and October 2022 using handpicking and scooping methods. Among these, 844 were identified as B. s. siamensis and 569 as B. s. goniomphalos. The snail samples were examined for parasitic infections in the laboratory using shedding and crushing methods. Among the 27 sampling sites, snails from 6 sites were infected with various trematode species (infection rate: 4.95%, 70/1,413). Evaluation of the morphological and internal organ characteristics of the cercariae allowed for the categorization of the trematodes into five species of trematodes (belonging to four families). The identified species included Echinochasmus pelecani (family: Echinostomatidae), Echinostoma revolutum (family: Echinostomatidae), Haematoloechus similis (family: Haematoloechidae), Loxogenoides bicolor (family Lecithodendriidae), and Stictodora tridactyla (family: Heterophyidae), and the infection rates for these species were 3.26% (46/1,413), 0.35% (5/1,413), 0.42% (6/1,413), 0.78% (11/1,413), and 0.14% (2/1,413), respectively. The cercariae from the identified trematode species were studied for DNA analysis. The phylogenetic lineage reveals relationships among the species, confirming the morphological distinctions.

Alaria alata mesocercariae prevalence and predilection sites in amphibians in Latvia.

Alaria alata is known as a trematode with a complex life cycle. The trematode Alaria alata infects amphibians as second intermediate hosts. In the present study, we examined 390 amphibians-European water frogs Pelophylax esculentus complex (n = 335), common frogs Rana temporaria (n = 19), moor frogs Rana arvalis (n = 3), and common toads Bufo bufo (n = 30) collected from randomly selected wetland habitats in Latvia. Out of all examined specimens, 80 were tadpoles and 310 were adult amphibians. Mesocercariae of A. alata was detected in 108 specimens from all examined amphibian species, except the common toad, reaching the overall prevalence of 27.7%. Tadpoles were found to be more frequently infected with A. alata, when compared with adults, 58.8% and 22.4%, respectively. The results showed that mesocercariae accumulate in visceral membranes, different internal organs, and muscles in the head area. This is a comprehensive study to identify A. alata mesocercariae predilection sites in amphibians.

Molecular evidence of new freshwater turtle blood flukes (Digenea: Spirorchiidae) in the intermediate snail host Biomphalaria occidentalis Paraense, 1981 in an urban aquatic ecosystem in Brazil.

In this study, two potentially new species of turtle blood flukes (TBFs) (Digenea: Spirorchiidae) have been recorded from South Brazil. The spirorchiid parasites infect the vascular system of turtles, thereby compromising their health. The life cycle of these parasites is not well studied. The larval stage of cercaria is found in intermediate gastropod hosts, with some species presenting similar morphological characteristics, which can result in misinterpretations when using only morphological taxonomy for species identification. In this study, we recorded a single morphotype belonging to the family Spirorchiidae in Biomphalaria occidentalis in an urban aquatic ecosystem in Brazil. However, molecular data (28S rDNA and cytochrome oxidase subunit I) confirmed the presence of two species of Spirorchiidae in the sampled environment; both phylogenetically close to genera previously studied in freshwater turtles from the Peruvian Amazon. In this study, species characterization was possible because of molecular tools. We recommend using more than one molecular marker in future studies focusing on TBFs, which need attention about their evolutionary history and ecology to understand their distribution in South America.

Insights on foodborne zoonotic trematodes in freshwater snails in North and Central Vietnam.

Foodborne zoonotic trematode (FZT) infections are common neglected tropical diseases in Southeast Asia. Their complicated life cycles involve freshwater snails as intermediate hosts. A cross-sectional study was conducted in Yen Bai and Thanh Hoa provinces in North and Central Vietnam, to investigate the diversity of cercariae of potential FZT and to construct the phylogenetic relationship of trematode cercariae based on the Internal Transcribed Spacer 2 (ITS2) region. Among 17 snail species collected from various habitats, 13 were infected by 10 cercarial groups among which parapleurolophocercous, pleurolophocercous, and echinostome cercariae were of zoonotic importance. The monophyletic tree separated cercarial sequences into different groups following the description of the cercariae families in which Haplorchidae, Opisthorchiidae, Echinochasmidae, and Echinostomatidae are important families of FZT. The overall prevalence was different among snail species and habitats and showed a seasonal trend. Parapleurolophocercous and echinostome cercariae emerged as the most common cercariae in snails in Yen Bai, while monostome, echinostome, and megalura cercariae were most common in Thanh Hoa. Using a molecular approach, we identified Parafossarulus striatulus as the first intermediate snail host of Clonorchis sinensis in Thac Ba Lake. Melanoides tuberculata and Bithynia fuchsiana were we identified preferred intermediate snail hosts of a diverse range of trematode species including intestinal flukes (i.e., Haplorchis pumilio and Echinochasmus japonicus) in Yen Bai and Thanh Hoa, respectively.

Cattle as natural host for Schistosoma haematobium (Bilharz, 1852) Weinland, 1858 x Schistosoma bovis Sonsino, 1876 interactions, with new cercarial emergence and genetic patterns.

Schistosomiasis remains a parasitic infection which poses serious public health consequences around the world, particularly on the African continent where cases of introgression/hybridization between human and cattle schistosomiasis are being discovered on a more frequent basis in humans, specifically between Schistosoma haematobium and S. bovis. The aim of this paper is to analyze the occurrence of S. bovis in cattle and its relationship with S. haematobium in an area where cattle and humans share the same site in Benin (West Africa). We used the chronobiology of cercarial emergence as an ecological parameter and both molecular biology (COI mtDNA and ITS rDNA) of the larvae and morphology of the eggs as taxonomic parameters. The results showed a chronobiological polymorphism in the cercarial emergence rhythm. They showed for the first time the presence of S. bovis in Benin, the presence of introgressive hybridization between S. bovis and S. haematobium in domestic cattle, and the presence of atypical chronobiological patterns in schistosomes from cattle, with typical S. haematobium shedding pattern, double-peak patterns, and nocturnal patterns. Our results showed that the chronobiological life-history trait is useful for the detection of new hosts and also may reveal the possible presence of introgressive hybridization in schistosomes. Our results, for the first time, place cattle as reservoir host for S. haematobium and S. bovis x S. haematobium. The consequences of these results on the epidemiology of the disease, the transmission to humans, and the control of the disease are very important.

Infection patterns of trematodes across size classes of an invasive snail species using field and laboratory studies.

In the Upper Mississippi River Region, invasive faucet snails (Bithynia tentaculata) and their trematode parasites have been implicated in more than 182 000 waterfowl deaths since 1996. Estimating transmission potential depends on accurate assessments of susceptible host population size. However, little is known about the mechanisms underlying snail-host susceptibility in this system. Prior field studies suggest that very small, likely young, faucet snails are less suitable secondary intermediate hosts. Here, we test whether the patterns observed in the field are because small snails (1) are refractory to infection by cercariae, (2) die from infection and are removed from sampled populations, and/or (3) are not preferred by cercariae. Our own field collections were consistent with the observation that smaller faucet snails exhibit lower metacercarial infection prevalence and abundance than larger snails. However, laboratory-based experiments show that smaller snails were actually more susceptible to infection than larger snails. Moreover, the smallest snail size class had significantly higher mortality than larger snails following infection, which may explain their reduced infection levels observed in the field. Our study demonstrates the importance of pairing field and laboratory studies to better understand mechanisms underlying patterns of infection.

Molecular detection of natural infection of Lymnaea (Pseudosuccinea) columella (Gastropoda: Lymnaeidae) with Fasciola gigantica (Digenea: Fasciolidae) from two provinces of South Africa.

The main intermediate host of Fasciola gigantica in sub-Saharan Africa is Lymnaea (Radix) natalensis. Lymnaea (Pseudosuccinea) columella is capable of transmitting both F. gigantica and F. hepatica and has been reported to be present in South Africa. To date, no natural infection with F. gigantica has been reported despite the wide distribution of the snail. The aim of this study was to confirm whether L. (P.) columella was transmitting F. gigantica and/or F. hepatica in selected locations of KwaZulu-Natal and Eastern Cape provinces of South Africa. Lymnaea (Pseudosuccinea) columella snails were collected from two locations in two provinces of South Africa and screened for cercariae shedding. This was followed by humanely sacrificing the screened snails, and whole tissue of each individual snail was homogenized and amplified using primers designed to amplify the ITS-1 region of Fasciola spp. No cercariae were shed from the screened snails and molecular analysis showed that snails from the two locations were infected with F. gigantica. This study confirms natural infection of L. (P.) columella with F. gigantica in South Africa, where F. gigantica and F. hepatica have already been reported to coexist. Although L. (P.) columella is able to transmit the two species, surprisingly no infection with F. hepatica was detected from the screened snails. The natural intermediate host of F. gigantica in southern Africa, including South Africa, is Lymnaea (Radix) natalensis and comparative studies are needed to determine the competence of the two snail species in the transmission of F. gigantica.

Five new morphological types of virgulate and microcotylous xiphidiocercariae based on morphological and molecular phylogenetic analyses.

The phylogenetic position of most xiphidiocercariae from subgroups Cercariae virgulae and Cercariae microcotylae remains unknown or unclear, even at the family level. In this paper, we studied the morphology and molecular phylogeny of 15 microcotylous and virgulate cercariae (11 new and four previously described ones). Based on morphological and molecular data, we suggested five distinct morphological types of xiphidiocercariae, which are a practical alternative to Cercariae virgulae and Cercariae microcotylae subgroups. Four of these types correspond to actual digenean taxa (Microphallidae, Lecithodendriidae, Pleurogenidae and Prosthogonimidae), while the fifth is represented by Cercaria nigrospora Wergun, 1957, which we classified on the basis of molecular data for the first time. We reassessed the relative importance of morphological characters used for the classification of virgulate and microcotylous cercariae, and discussed the main evolutionary trends within xiphidiocercariae. Now stylet cercariae can be reliably placed into several sub-taxa of Microphalloidea on the basis of their morphological features.

Pronocephaloid cercariae (Platyhelminthes: Trematoda) from gastropods of the Queensland coast, Australia.

The superfamily Pronocephaloidea Looss, 1899 comprises digeneans occurring in the gut and respiratory organs of fishes, turtles, marine iguanas, birds and mammals. Although many life cycles are known for species of the Notocotylidae Lühe, 1909 maturing in birds and mammals, relatively few are known for the remaining pronocephaloid lineages. We report the cercariae of five pronocephaloid species from marine gastropods of the Queensland coast, Australia. From Lizard Island, northern Great Barrier Reef, we report three cercariae, two from Rhinoclavis vertagus (Cerithiidae) and one from Nassarius coronatus (Nassariidae). From Moreton Bay, southern Queensland, an additional two cercariae are reported from two genotypes of the gastropod worm shell Thylacodes sp. (Vermetidae). Phylogenetic analysis using 28S rRNA gene sequences shows all five species are nested within the Pronocephaloidea, but not matching or particularly close to any previously sequenced taxon. In combination, phylogenetic and ecological evidence suggests that most of these species will prove to be pronocephalids parasitic in marine turtles. The Vermetidae is a new host family for the Pronocephaloidea.

Patterns of Clinostomum marginatum infection in fishes and amphibians: integration of field, genetic, and experimental approaches.

Digenetic trematodes of the genus Clinostomum are cosmopolitan parasites infecting fishes, amphibians, reptiles, and snails as intermediate hosts. Despite the broad geographical distribution of this genus, debate about the number of species and how they vary in host use has persisted. To better understand patterns of infection among host species and across life stages, we used large-scale field surveys and molecular tools to examine five species of amphibians and seven species of fishes from 125 California ponds. Among the 12,360 examined hosts, infection was rare, with an overall prevalence of 1.7% in amphibians and 9.2% in fishes. Molecular evidence indicated that both groups were infected with Clinostomum marginatum. Using generalized linear mixed effects models, host species identity and host life stage had a strong influence on infection status, such that Lepomis cyanellus (green sunfish) (49.3%) and Taricha granulosa (rough skinned newt) (9.2%) supported the highest overall prevalence values, whereas adult amphibians tended to have a higher prevalence of infection relative to juveniles (13.3% and 2.5%, respectively). Experimentally, we tested the susceptibility of two amphibian hosts (Pseudacris regilla [Pacific chorus frog] and Anaxyrus boreas [western toad]) to varying levels of cercariae exposure and measured metacercariae growth over time. Pseudacris regilla was 1.3× more susceptible to infection, while infection success increased with cercariae exposure dose for both species. On average, metacarcariae size increased by 650% over 20 days. Our study highlights the importance of integrating field surveys, genetic tools, and experimental approaches to better understand the ecology of host-parasite interactions.

Occurrence of Schistosoma bovis on Pemba Island, Zanzibar: implications for urogenital schistosomiasis transmission monitoring.

The causative agent of urogenital schistosomiasis, Schistosoma haematobium, was thought to be the only schistosome species transmitted through Bulinus snails on Unguja and Pemba Island (Zanzibar, United Republic of Tanzania). For insights into the environmental risk of S. haematobium transmission on Pemba Island, malacological surveys collecting Bulinus globosus and B. nasutus, two closely related potential intermediate hosts of S. haematobium were conducted across the island in November 2016. Of 1317 B. globosus/B. nasutus collected, seven B. globosus, identified through sequencing a DNA region of the mitochondrial cytochrome oxidase subunit 1 (cox1), were observed with patent infections assumed to be S. haematobium. However, when the collected cercariae were identified through sequencing a region of the cox1 and the nuclear internal transcribed spacer (ITS1 + 2), schistosomes from five of these B. globosus collected from a single locality were in fact S. bovis. The identified presence of S. bovis raises concerns for animal health on Pemba, and complicates future transmission monitoring of S. haematobium. These results show the pertinence for not only sensitive, but also species-specific markers to be used when identifying cercariae during transmission monitoring, and also provide the first molecular confirmation for B. globosus transmitting S. bovis in East Africa.

Trematode infections of the freshwater snail Biomphalaria pfeifferi from a south-east Nigerian community with emphasis on cercariae of Schistosoma.

Knowledge of trematode infections in Biomphalaria pfeifferi from south-eastern Nigeria is scarce, due to the absence of Schistosoma mansoni infection in the region. Therefore, the present study sought to describe trematode infections in B. pfeifferi from the River Uzuru in the Nigeria Cement Factory area, Nkalagu, south-eastern Nigeria. Four hundred and sixty snails were checked for trematode infections, and mice were exposed to the Schistosoma cercariae shed from the snails. Adult worms were harvested from the mice 13 weeks post-infection, while sections of the liver and spleen were examined. Primary school children living in the area were screened for S. mansoni infection and assessed for activities involving water contact. The edges of the river were also searched for burrows and rodents. The five cercaria morphotypes found were armatae xiphidiocercariae, echinocercariae, Schistosoma cercariae, cystophorous cercariae and cercariaeum cercariae. The overall prevalence and mean intensity of trematode infections were 39.78% and 195.46, with the prevalence and mean intensity of most cercaria morphotypes higher in the hot-dry than in the cool-dry season. The infected mice showed S. mansoni-like characteristics but the stool samples of the schoolchildren were negative for S. mansoni eggs. Water-contact activities in the River Uzuru were minimal. Burrows were seen at the river edges but no Schistosoma eggs were recovered from captured rats. This is the first report of Schistosoma cercariae and other cercaria morphotypes in B. pfeifferi from south-eastern Nigeria. Additional molecular investigations are needed to identify correctly these Schistosoma cercariae, due to their public health implication.

Aptitude of Lymnaea palustris and L. stagnalis to Fasciola hepatica larval development through the infection of several successive generations of 4-mm-high snails.

Bimiracidial infections of Lymnaea palustris and Lymnaea stagnalis (shell height at exposure, 4 mm) with Fasciola hepatica were carried out during six successive snail generations to determine if prevalence and intensity of snail infection increased over time through descendants issuing from eggs laid by parents already exposed to this digenean. Controls were constituted by a French population of Galba truncatula (a single generation) infected according to the same protocol. In a first experiment performed with the F1 to F5 generations of L. palustris, the prevalence and intensity of F. hepatica infection in snails progressively increased. Immature rediae and a few cercariae-containing rediae of the digenean were observed in L. stagnalis from the F3 generation, but no free cercaria was noted in the bodies of this lymnaeid from the F4 to F6 generations. In another experiment carried out with the F6 generation of L. palustris, the prevalence of F. hepatica infection and the number of shed cercariae were significantly lower in L. palustris than in G. truncatula. This mode of snail infection suggests an explanation for cases of human fasciolosis occurring in central France after the collection of wild watercress from beds where L. palustris was the sole lymnaeid.

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 Dzierzecinka 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 Dzierzecinka 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.

New detection method in experimental mice for schistosomiasis: ClinProTool and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Oncomelania hupensis snails along the Yangtze River and the low positive rate and infectiosity of human and livestock schistosomiasis still pose a threat to public health in China. Adult blood flukes were recognized as Schistosoma japonicum, which are found in the portal system of the sentinel mice bred in the laboratory for 35 days after contact with the water. However, 35 days was too long from the field test to dissection, and the dissection in the laboratory was also time-consuming and labor-intensive. Serum peptides in mice at different times after infection were measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. ClinProTool was used to establish the proteomic detection pattern (PDP), based on the differentially expressed peptide between the infection and healthy control groups. Under experimental conditions, characteristic PDP were detected in 5 % (3/60), 35 % (21/60), 75 % (45/60), 87.93 % (51/58), and 98.15 % (53/54) of infected mice from weeks 1 to 5 post-infection, whereas ELISA and dissection examination for adult blood flukes missed the first 2 weeks. At 35 days post-infection, the infectiosity assay showed 40 % (4/10), 50 % (5/10), and 80 % (8/10) positivity with the PDP test in mice infected with 4, 6, and 10 cercariae, respectively, as well as 100 % (10/10) positivity in mice infected with 14, 18, and 22 cercariae. Five stored sera of positive sentinel mice with parasite detection were verified correctly in the PDP test. The results confirm that PDP can be used as a rapid and early detection method for S. japonicum infection in experimental mice, which are expected to apply in early surveillance for schistosomiasis.

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.

TREMATODE INFECTION OF FRESHWATER SNAIL, FAMILY BITHYNIIDAE IN THAILAND.

Opisthorchis viverrini is restricted to and requires for its aquatic life cycle only Bithynia snail as first intermediate host but many species of cyprinid fish as second intermediate hosts. A survey in Thailand of trematode infection in freshwater snails of the family Bithyniidae carried out during October 2008 - July 2009 found a total of 5,492 snails, classified into ten species distributed in various geographic areas. Bithyniafuniculata and Gabbia pygmaea were localized to the north, B. s. goniomphalos, Wattebledia siamensis and W. crosseana to northeast and B. s. siamensis, Hydrobioides nassa and G. wykoffi to central region. W. baschi and G. erawanensis was found only in the south and Erawan waterfall, Kanchanaburi Province, respectively. Trematode infection rate was 3.15%. Cercariae were identified as belonging to six types, namely, amartae , monostome, mutabile, O. viverrini, virgulate, and unknown. The prevalence of cercarial infection in B. s. goniomphalos of amartae, mutabile, O. viverrini, virgulate, and unknown type cercaria was 0.55%, 0.74%, 1.07%, 2.87%, and 0.37%, respectively, and in B. s. siamensis monostome (1.10%) and virgulate (0.55%). Only virgulate cercariae were shed from W. crosseana (3.85%) and W. siamensis (5.19%). Cercariae of the unknown type were found in G. wykoffi (1.69%). No infection of O. viverrini cercariae was detected in the other species.

Trematode-associated morbidity and mortality of tadpoles in Israel.

Amphibians stand at the forefront of the global biodiversity crisis. The causes of their decline are diverse and include a rise in amphibian malformations due to various factors, especially trematode infection. However, linking amphibian mortality and morbidity with trematode infection has proven to be challenging due to the complex life cycle of the trematodes and the fact that trematodes are nonfastidious in their choice of definitive hosts. In Israel, the decline in local amphibian populations has been mostly attributed to the loss and degradation of wetlands and riparian habitats. Recently, however, there have been several reports of morbidity and mortality of tadpoles with signs of edema and malformations from various localities in Israel. We collected dead and morbid tadpoles and metamorphs of Hyla savignyi and Pelophylax bedriagae, and we showed that the morbidity and the deformations observed in the field are the result of infection by trematodes. We also isolated an echinostomatid trematode from the malformed and edematous tadpoles and from the freshwater snail Bulinus truncatus, all from the same site. We further succeeded in experimentally infecting H. savignyi tadpoles by echinostomatid cercariae that were shed from the snails, and we showed that infection had significantly increased the mortality rates of these tadpoles. The combination of high trematode prevalence and their pathogenic effects suggests that in nature, the effect of echinostome infection on amphibians may be substantial and could become an emerging disease in Israel.

A review of the currently recognised zoogonid cercariae, including the identification and emergence ecology of Cercaria capricornia XI (Digenea: Zoogonidae) from Nassarius olivaceus (Gastropoda: Nassariidae) in Central Queensland, Australia.

In the present study, we review the known zoogonid cercariae, summarise their life-cycles and first intermediate host distributions, and present a new cercaria, Cercaria capricornia XI (Digenea: Zoogonidae), which was found in one of three nassariid gastropods, Nassarius olivaceus (Bruguière), surveyed in the intertidal zone in the Capricornia region of Central Queensland, Australia. Morphological data and molecular analysis of the ITS2 rDNA region support placement of this cercaria in the family Zoogonidae but do not allow any further resolution of its identity. There are now fifteen cercariae described as belonging to the Zoogonidae; thirteen of these, including the present species, infect neogastropods as first intermediate hosts and two use vetigastropods. This study reinforces the pattern that the Nassariidae is by far the most commonly reported family for the Zoogonidae. Given its richness we predict that the Nassariidae will prove to harbour many more zoogonid species.

An evaluation of three sampling methods to monitor a digenetic trematode Centrocestus formosanus in a spring-fed ecosystem.

Centrocestus formosanus is a digenetic trematode from Asia that parasitizes multiple hosts and is a concern in the Comal River, Texas, USA, because of its negative effects on the endangered fountain darter Etheostoma fonticola. To determine a practical sampling method to monitor C. formosanus in the Comal River, we evaluated three sampling methods using wild-caught fish, caged fish reared in the laboratory, and cercariometry. Cercariometry detected significant spatial and temporal patterns of cercarial density in river water that were similar with metacercarial intensity in caged fish, but inconsistent with metacercarial intensity in wild-caught fish. Our results also showed a positive correlation between cercarial density in river water and metacercarial intensity in caged fish. Conversely, the relationship was not significant between cercarial density and metacercarial intensity in wild-caught fish. Because cercariometry predicted similar trends with the caged fountain darter sampling method, cercariometry was useful in predicting C. formosanus gill infections, infection rate, and longevity in infected fountain darters. Although trends from cercariometry and caged fish sampling methods were similar, we recommend cercariometry because it was less expensive to use given the amount of sampling effort required and provides trends that can be used to make pro-active management decisions in C. formosanus-infested aquatic ecosystems.