Trematode infections in freshwater snails and cattle from the Kafue wetlands of Zambia during a period of highest cattle-water contact

A total of 984 snails, comprising nine species, were collected from six areas in the Kafue wetlands between August and October 2003 to assess larval trematode infections. Of these, 135 (13.7%) were positive. Most trematode infections were recorded from Lymnaea natalensis (42.8%), which harboured four of the five morphologically different cercariae found. No trematodes were recovered from Bellamya capillata, Biomphalaria pfeifferi, Melanoides tuberculata, Physa acuta and Cleopatra nswendweensis. One snail (0.2%) of 416 Bulinus snails shed brevifurcate-apharyngeate distome cercariae while three (0.7%) shed amphistomes. Gymnocephalous and longifurcate-pharyngeate distome were the commonest types of cercariae recorded while xiphidiocercaria was the least common. The highest prevalence rates of F. gigantica (68.8%) and amphistomes (50.0%) in cattle (n = 101) were in Chiyasa while those in Kaleya had the lowest (9.1 and 18.2%, respectively). In most habitats, infections were recorded in both cattle and snails. Critical determinants of infection may have been the distance of settlements and/or cattle kraals, the number of animals in nearby homesteads and the presence of susceptible host snails. This study suggests that fascioliasis and amphistomiasis could be major constraints of cattle production in the Kafue wetlands because favourable factors were available to introduce and maintain the infections. It further provides a starting point for some comprehensive studies on snail-related aspects of transmission and snail host ecology in Zambia.     

Larval trematode infections and spatial distributions of snails

Abstract. The hypothesis that infecting trematodes influence the spatial distribution of the estuarine snail Ilyanassa obsoleta was tested. This work was conducted in the Savages Ditch habitat, Rehoboth Bay, DE, USA, which has an essentially flat, sandy-mud bottom bordered by saltmarsh shorelines and many infected snails. In 1996, two groups of snails were individually marked and released from one location after being screened for trematode infections. One group, transplanted from sites where snails tended not to be infected, consisted of snails that tested as uninfected. The other group consisted of snails native to Savages Ditch. Species of trematode carried by each snail was recorded. Marked snails were found and their positions were recorded until 2001. Snails were in five infection categories: (1) not infected, and infected with (2) Himasthla quissetensis , or (3) Lepocreadium setiferoides or (4) Zoogonus rubellus , or (5) with both H. quissetensis and Z. rubellus . The results show that the spatial distributions of snails depended on whether or not they were infected and, if infected, on which trematode species they carried. To complete life cycles, these parasites must accomplish transmission from the first (the snail) to the second intermediate hosts by short-lived, swimming cercariae. These data do not allow resolution of why snails distributed as they did, but sighting distributions of infected snails can be related to distributions of second hosts and it is proposed that parasites engender host snail distributions that improve chances of transmission.     

洞庭湖外睾吸虫新种及其生活史

本文报告洞庭湖区鲶鱼肠道寄生的洞庭湖外睾吸虫Exorchis dongtinghuensis sp.nov(新种)及其全程生活史,其第一中间宿主为湖北钉螺Oncomelania hupensis;第二中间宿主为鲤鱼、鲫鱼和金鱼;终宿主为鲶鱼Parasilurus asotus。作者对各期宿主作了人工感染试验和现场自然感染调查。对其发育过程作了观察比较。     

Larval trematode infections in freshwater snails from the highveld and lowveld areas of Zimbabwe

Between November 1998 and October 2000, freshwater snails were collected monthly from the highveld and lowveld areas of Zimbabwe to determine the occurrence of larval trematodes. A total of 13,789 snails, representing ten species, were collected from 21 sites and 916 (6.6%) harboured patent trematode infections. Eight morphologically distinguishable types of cercariae were identified. Bulinus tropicus had the highest overall prevalence of infection (13.1%). The echinostome was the most common type of cercaria recovered, contributing 38.2% of all infections. Schistosoma cercariae were recovered mainly from the highveld and comprised 8.0% of all infections. Amphistome cercariae contributed 37.6% of all infections and were recorded from both the highveld and lowveld areas with a peak prevalence occurring during the post-rainy period (March-May). The main intermediate host for amphistomes was B. tropicus. Infections in B. globosus, B. forskalii and Biomphalaria pfeifferi with amphistome cercariae are new records for Zimbabwe.     

Use of rDNA polymorphism for identification of Heterophyidae infecting freshwater fishes

Infections by trematodes are among the most common fish-borne zoonoses. Metacercariae of the Family Heterophyidae in marine and freshwater fishes are nonfastidious in their choice of definitive hosts, and therefore, cause infections in human and domestic animals. In the present study, species-specific polymerase chain reaction (PCR) assays were developed for identifying and differentiating the various species examined. Sequencing and aligning the 18S (SSU) rDNA revealed interspecific variation for which species-specific DNA oligonucleotides were designed and used for the identification of 6 heterophyid species recovered from piscivorous birds. The oligonucleotides were further used to evaluate the various stages (cercariae recovered from snails, metacercariae recovered from fish and adult trematodes) of the digeneans. By applying this method we elucidated for the first time the life cycle of Pygidiopsis genata. The phylogenetic interrelationship among the newly sequenced species of Heterophyidae is outlined.     

Relationship between Snail Population Density and Infection Status of Snails and Fish with Zoonotic Trematodes in Vietnamese Carp Nurseries

Background

Fish-borne zoonotic trematodes (FZT) are a food safety and health concern in Vietnam. Humans and other final hosts acquire these parasites from eating raw or under-cooked fish with FZT metacercariae. Fish raised in ponds are exposed to cercariae shed by snail hosts that are common in fish farm ponds. Previous risk assessment on FZT transmission in the Red River Delta of Vietnam identified carp nursery ponds as major sites of transmission. In this study, we analyzed the association between snail population density and heterophyid trematode infection in snails with the rate of FZT transmission to juvenile fish raised in carp nurseries.

Methodology/Principal Findings

Snail population density and prevalence of trematode (Heterophyidae) infections were determined in 48 carp nurseries producing Rohu juveniles, (Labeo rohita) in the Red River Delta area. Fish samples were examined at 3, 6 and 9 weeks after the juvenile fish were introduced into the ponds. There was a significant positive correlation between prevalence of FZT metacercariae in juvenile fish and density of infected snails. Thus, the odds of infection in juvenile fish were 4.36 and 11.32 times higher for ponds with medium and high density of snails, respectively, compared to ponds where no infected snails were found. Further, the intensity of fish FZT infections increased with the density of infected snails. Interestingly, however, some ponds with no or few infected snails were collected also had high prevalence and intensity of FZT in juvenile fish. This may be due to immigration of cercariae into the pond from external water sources.

Conclusions/Significance

The total number and density of potential host snails and density of host snails infected with heterophyid trematodes in the aquaculture pond is a useful predictor for infections in juvenile fish, although infection levels in juvenile fish can occur despite low density or absence infected snails. This suggests that intervention programs to control FZT infection of fish should include not only intra-pond snail control, but also include water sources of allochthonous cercariae, i.e. canals supplying water to ponds as well as snail habitats outside the pond such as rice fields and surrounding ponds.     

Larval trematodes (Digenea) of planorbid snails (Gastropoda: Pulmonata) in Central Europe: a survey of species and key to their identification

A survey of the larval stages (cercariae and metacercariae) of trematodes (Digenea) found in planorbid snails in Central Europe (Austria, Czech Republic, south-east Germany, Hungary, Poland and the Slovak Republic) is presented based on a study of 7,628 snails of 12 species examined between 1998-2006. A total of 34 trematode larval stages, comprising cercariae of 28 species and metacercariae of seven species (one species occurred both as cercaria and metacercaria) of nine families were found in 898 (11.5%) snails of eight species. The dominant cercariae were those belonging to the Rubenstrema exasperatum (Rudolphi, 1819)/Neoglyphe locellus (Kossack, 1910) species complex, Tylodelphys excavata (Rudolphi, 1803) and Echinostoma spiniferum (La Valette, 1855) sensu Nasincová (1992), all from Planorbarius corneus (Linnaeus). Almost the same spectrum of cercariae of the families Echinostomatidae, Plagiorchiidae and Omphalometridae was found in the present study as in previous reports; however, a considerably lower spectrum of cercariae of the families Diplostomidae and Strigeidae was recorded. The most frequent metacercariae were those of Echinoparyphium aconiatum Dietz, 1909, Neoglyphe locellus and Moliniella anceps (Molin, 1859), all occurring mainly in P. corneus. The most heavily infected snail species was P. corneus, followed by Planorbis planorbis (Linnaeus) and Segmentina nitida (Müller). The widest spectrum of trematode species was found in P. planorbis and P. corneus. Forty-two cercariae identified to the species level belonging to 15 families, plus an additional 43 taxa recorded under generic or provisional names, were reported from 11 species of planorbids in previous studies carried out in Central Europe. However, the actual number of trematode species occurring in the planorbid snails is probably much lower, because many, if not most, larval stages reported under provisional names or unidentified to the species level may be conspecific with identified adult forms. A key to the cercariae and metacercariae recorded from planorbids in Central Europe, together with illustrations of those species encountered most frequently in the field, is provided to facilitate identification.     

Can parasites be indicators of free-living diversity? Relationships between species richness and the abundance of larval trematodes and of local benthos and fishes

Measuring biodiversity is difficult. This has led to efforts to seek taxa whose species richness correlates with the species richness of other taxa. Such indicator taxa could then reduce the time and cost of assessing the biodiversity of the more extensive community. The search for species richness correlations has yielded mixed results, however. This may be primarily because of the lack of functional relationships between the taxa studied. Trematode parasites are highly promising bioindicators. Diverse assemblages of larval trematode parasites are easily sampled in intermediate host snails. Through their life cycles these parasites are functionally coupled with the surrounding free-living diversity of vertebrate and invertebrate animals. It has been shown that larval trematodes in snails correlate positively with bird diversity and abundance. Here, we explore whether trematodes also correlate with standard measures of fishes, and large and small benthos, for 32 sites in three wetlands. We found associations between trematodes and benthic communities that were not consistent across wetlands. The associations were, however, consistently positive for large benthic species richness and density. Some of the contrasting associations between trematode and benthos may be explained by negative associations between large and small benthos. We found no associations with fish communities (probably because of the inadequacy of standard “snapshot” sampling methods for highly mobile fishes). The results support further exploration of trematodes as bioindicators of diversity and abundance of animal communities. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Ecosystem consequences of fish parasites*

In most aquatic ecosystems, fishes are hosts to parasites and, sometimes, these parasites can affect fish biology. Some of the most dramatic cases occur when fishes are intermediate hosts for larval parasites. For example, fishes in southern California estuaries are host to many parasites. The most common of these parasites, Euhaplorchis californiensis, infects the brain of the killifish Fundulus parvipinnis and alters its behaviour, making the fish 10–30 times more susceptible to predation by the birds that serve as its definitive host. Parasites like E. californiensis are embedded in food webs because they require trophic transmission. In the Carpinteria Salt Marsh estuarine food web, parasites dominate the links and comprise substantial amount of biomass. Adding parasites to food webs alters important network statistics such as connectance and nestedness. Furthermore, some free‐living stages of parasites are food items for free‐living species. For instance, fishes feed on trematode cercariae. Being embedded in food webs makes parasites sensitive to changes in the environment. In particular, fishing and environmental disturbance, by reducing fish populations, may reduce parasite populations. Indirect evidence suggests a decrease in parasites in commercially fished species over the past three decades. In addition, environmental degradation can affect fish parasites. For these reasons, parasites in fishes may serve as indicators of environmental impacts.     

Trematode communities in snails can indicate impact and recovery from hurricanes in a tropical coastal lagoon

In September 2002, Hurricane Isidore devastated the Yucatán Peninsula, Mexico. To understand its effects on the parasites of aquatic organisms, we analyzed long-term monthly population data of the horn snail Cerithidea pliculosa and its trematode communities in Celestún, Yucatán, Mexico before and after the hurricane (February 2001 to December 2009). Five trematode species occurred in the snail population: Mesostephanus appendiculatoides, Euhaplorchis californiensis, two species of the genus Renicola and one Heterophyidae gen. sp. Because these parasites use snails as first intermediate hosts, fishes as second intermediate hosts and birds as final hosts, their presence in snails depends on food webs. No snails were present at the sampled sites for 6 months after the hurricane. After snails recolonised the site, no trematodes were found in snails until 14 months after the hurricane. It took several years for snail and trematode populations to recover. Our results suggest that the increase in the occurrence of hurricanes predicted due to climate change can impact upon parasites with complex life cycles. However, both the snail populations and their parasite communities eventually reached numbers of individuals and species similar to those before the hurricane. Thus, the trematode parasites of snails can be useful indicators of coastal lagoon ecosystem degradation and recovery.     

Effects of the herbicide atrazine's metabolites on host snail mortality and production of trematode cercariae

Environmental stressors have the potential to greatly impact the transmission of parasites with complex, multi-host life cycles such as those of trematodes. The commonly used herbicide atrazine has been shown to affect the susceptibility of second intermediate hosts (such as larval amphibians) to trematode infection, as well as the longevity and infectivity of the free-swimming cercariae, but not eggs or the free-swimming miracidia that infect the gastropod first intermediate hosts. However, we do not know if this pesticide influences the survival of infected snails or whether it affects cercariae production within, or emergence from, these hosts. In addition, previous studies of host-parasite dynamics have only examined the parent atrazine compound, not any of the long-lasting metabolites commonly present in water bodies. Here, we report that a concentration of 0.33 μg/L of an atrazine metabolite, desethyl atrazine, increased the mortality of freshwater gastropods ( Stagnicola elodes ) infected with a gymnocephalus type of cercaria but not that of uninfected snails or those harboring a mature or dormant infection of Echinoparyphium sp. In contrast, 2 wk of exposure to desethyl atrazine did not affect the emergence of gymnocephalus cercariae from snails, although a trend for a decrease in the emergence of Echinoparyphium sp. cercariae was observed. We suggest that simultaneous trematode infection and exposure to contaminants may represent a significant combined stress to gastropods, but this is likely parasite species-specific as well as dependent on whether cercariae are being actively produced.     

Distribution of trematodes of the family Prosthogonimidae in river and lake ecological systems in the south of the Western Siberia

The results of long-term investigations (1994-2003) of an infection rate of trematodes of the family Prosthogonimidae in the first intermediate hosts (snails of the family Bithyniidae) and in the final hosts (birds) from a basin of the Chany Lake (Western Siberia) are discussed. A total of 1824 specimens of Bithynia tentaculata (L., 1758) and 7166 specimens of Opisthorchophorus troscheli (Paasch, 1842) have been collected from eight rivers and two lakes in the south of the Western Siberia. Data of incomplete helminthological dissection of 225 young water-fowl of 18 species were analyzed. Birds were obtained in July, August and September, 1996-2003 from the Chany lake basin. Maritas of the trematode family Prosthogonimidae were revealed in bursa Fabricii (26.12%) of seven bird species of two orders (Anseriformes and Gruiformes): Grey Lag-Goose Anser anser (L.) (Infection rate-28.6% +/- 12.1), Mallard Anas platyrhynchos L. (27.5% +/- 6.2), Shoveler--A. clypeata L. (22.2% +/- 13.9), Pochard Aythya ferina (L.) (51.7% +/- 9.3), Coot Fulica atra L. (28.6% +/- 5.6), Tufted Duck A. fuligula (L.) (1 case), Wigeon A. penelope L. (1 case). Average intensity of invasion is 5.8 marita specimens per infected bird. The maximal number of parasites (37 maritas), and also the highest average intensity of invasion are recorded for the Coot. The 342 maritas of the family Prosthogonimidae belong to three species: Prosthogonimus ovatus (Rud., 1803) (49.7%), P. cuneatus (Rud., 1809) (32.8%) and Schistogonimus rarus (Braun, 1901) (14.6%); these trematode species have been recorded in 35, 12 and 8 bird species, respectively. Grey Lag-Goose, Shoveler, Pochard and Tufted Duck are new host records for P. cuneatus in for Western Siberia. Parthenitae of trematodes (Prosthogonimidae) were revealed from eight populations of bithyniid snails. The infection rate of the trematodes (parthenitae and cercariae) in population of bithyniid snails from lake ecological system varied 6.55-54.4%, and in river ecological systems in was 0.6-4.2 %. The infection rate of trematodes (parthenitae and cercariae) of the first intermediate hosts was 2.64% that is significantly lower than that of the final hosts from the Chany Lake basin. Both O. troscheli and B. tentaculata were noted as the first intermediate hosts of prosthogonimid trematodes of three species: P. ovatus, P. cuneatus and S. rarus. All three species of trematode were found in three water reservoirs: Ob River (B. tentaculata), Kargat River (O. troscheli) and Chany Lake (O. troscheli). Thus it is found out that 39 bird species can be the final hosts of prosthogonimid trematodes. About a quarter of nestlings is infected with these trematodes. The infection rate of parthenitae in snails from the lake ecosystems is significantly higher than in the river ecosystems.     

Crepidula fornicata is not a first intermediate host for trematodes: who is?

Trematode larvae must generally invade a molluscan intermediate host, usually a gastropod, before they can reach reproductive maturity in another definitive host. The research literature to date has focused almost exclusively on the documented specificity between particular trematode species and particular molluscan hosts; little attention has been paid to gastropod species that do not appear to serve as hosts. We sampled Rhode Island and Massachusetts populations of the marine gastropod Crepidula fornicata to determine whether this widespread species serves as a first intermediate host for trematodes. We also sampled from the same habitat populations of Littorina littorea and Ilyanassa obsoleta, gastropods known to serve as first intermediate hosts for several trematode species. All individuals were examined by dissection for the presence of sporocysts, rediae, or developing cercariae. Although 4-28% of L. littorea (N=112) and I. obsoleta (N=84) were infected by larvae of at least one trematode species, no individuals of C. fornicata sampled from the same locations were so infected (N=136). A survey of the Biological Abstracts computer database indicates that snails in only about 10% of marine gastropod families are known to serve as first intermediate hosts for trematodes. We suggest that more attention be paid to marine gastropods that appear not to be infected by trematode miracidia. Such species may productively serve as new models for understanding trematode host specificity and gastropod resistance to infection.     

Diversity of trematode larvae in gastropod molluscs in water bodies of Belarus

The analysis of literary data and own investigations of gastropod molluscs on existence of cercariae of trematodes in Belarus was carried out. 13 856 specimens of 11 species and 2 species complexes of freshwater gastropod molluscs were examined on infection with cercariae of trematodes in 2010 by the authors of this article. Total infestation was 13.1% and infection of some molluscs species varied from 0.4% (Dnieper River, Gomel region) to 66.7% (Lukomskoe reservoir, Vitebsk region). The cercariae of 90 trematode species were found. The majority (51 species) at the adult stage parasitize birds. The cercariae of 14 trematode species have medical significance.     

Distribution Pattern of Serotoninergic Neurons in Trematode Cercariae

The review summarizes data on the number, size and localization of serotoninergic neurons in cercariae of 16 trematode species, referring to 11 families. Presented diagrams show distribution patterns of serotoninergic neurons in cercariae of trematodes from different taxonomic, biological and morphological groups. Data analysis indicates that the number, disposition and size of these neurons vary in cercariae of different species. The data are discussed in relation with a functional significance of serotonin as a neurotransmitter in free-living larvae—trematode cercariae.     

The role of spatial and temporal heterogeneity and competition in structuring trematode communities in the great pond snail, Lymnaea stagnalis (L.)

We assessed how spatial and temporal heterogeneity and competition structure larval trematode communities in the pulmonate snail Lymnaea stagnalis . To postulate a dominance hierarchy, mark-release-recapture was used to monitor replacements of trematode species within snails over time. In addition, we sampled the trematode community in snails in different ponds in 3 consecutive years. A total of 7,623 snails (10,382 capture events) was sampled in 7 fishponds in the Jind?ich?v Hradec and T?eboň areas in South Bohemia (Czech Republic) from August 2006 to October 2008. Overall, 39% of snails were infected by a community of 14 trematode species; 7% of snails were infected with more than 1 trematode species (constituting 16 double- and 4 triple-species combinations). Results of the null-model analyses suggested that spatial heterogeneity in recruitment among ponds isolated trematode species from each other, whereas seasonal pulses in recruitment increased species interactions in some ponds. Competitive exclusion among trematodes led to a rarity of multiple infections compared to null-model expectations. Competitive relationships among trematode species were hypothesized as a dominance hierarchy based on direct evidence of replacement and invasion and on indirect evidence. Seven top dominant species with putatively similar competitive abilities (6 rediae and 1 sporocyst species) reduced the prevalence of the other trematode species developing in sporocysts only.     

Structure and temporal variation of trematode and gastropod communities in a freshwater ecosystem.

The infro- and component community dynamics of digenetic trematodes in a freshwater gastropod community were examined over a 33-month period. The gastropod and trematode communities were composed of 17 and 10 species respectively. A total of 9,831 snails was collected; among them, 192 belonging to 14 species were infected by larval trematodes. The size of infected snails was significantly greater than that of healthy ones, and the increase of prevalence with size/age was interpreted as related to the increased probability of ultimately becoming parasitized. The trematode community was rich in allogenic species, but the most frequent trematode (cercariaeum) was autogenic and generalist (a range of 12 snail host species). There was a significantly positive relationship between the frequency of trematode species in the community and the number of first intermediate host species. A great temporal heterogeneity occurred in the prevalence of the snails, mainly attributed to the great temporal fluctuations of snail host populations and the variability of freshwater ecological conditions. The data on the occurrence of larval trematodes in 14 host species over the 33-month study allowed indicate a significant negative correlation between the abundance of gastropods and the prevalence of trematodes.     

Effects of trematode double infection on the shell size and distribution of snail hosts

Infection with larval trematodes sometimes alters the phenotypes of their snail hosts. While some trematode species have distinct effects on host phenotypes, it is still unclear how snail phenotypes are altered when they are parasitized with multiple trematode species. Here, we report that double infection with trematode species averages the effects of parasitic alteration on host phenotype. We found that snail hosts Batillaria attramentaria (Batillariidae) infected with Cercaria batillariae (Heterophyidae) have abnormally large shells and distribute in lower areas of the intertidal zone. Snails with another dominant trematode species, the renicolid cercaria I (Renicolidae), have slightly larger shells and distribute in upper areas of the intertidal zone. A number of double infections with both trematodes was observed in this study. Snails infected with both trematode species exhibited an intermediate size and inhabited a depth between those of snails solely infected with either trematode species, suggesting that the two trematodes simultaneously affected the snail phenotypes. Because altered host phenotypes are frequently beneficial to parasites, two trematode species may compete for successful transmission through alteration of host phenotypes.     

Synchronous attack is advantageous: mixed genotype infections lead to higher infection success in trematode parasites

Co-infecting parasite genotypes typically compete for host resources limiting their fitness. The intensity of such competition depends on whether parasites are reproducing in a host, or using it primarily as a transmission vehicle while not multiplying in host tissues (referred to as 'competition hypothesis'). Alternatively, simultaneous attack and co-infection by several parasite genotypes might facilitate parasite infection because such a diverse attack could present an additional challenge to host immune defence (referred to as 'facilitation hypothesis'). We tested the competition hypothesis by comparing the production of transmission stages (cercariae) from snails infected with one or two genotypes of the trematode Diplostomum pseudospathaceum. We found that cercarial production did not differ between the two groups of snails, suggesting lower per genotype production in double infections, and competition for host resources. Second, we tested the facilitation hypothesis by comparing parasite infection success on fishes (proportion of parasites establishing in the host) using cercariae originating from single-infected snails, double-infected snails and artificial mixtures of the single genotypes. In both cases, we found higher infection success when fishes were challenged with two parasite genotypes instead of one, supporting the facilitation hypothesis. Our results suggest that constraints defining the success of multiple genotype infections in parasites with multiple host life cycles include both between-genotype resource competition in the host and performance of host immune defences against a diverse parasite challenge.     

Environmental factors influencing community composition of gastropods and their trematode parasites in southern Ontario

Agricultural activity and landscape features have previously been associated with diversity and prevalence of trematode species in amphibian second intermediate hosts. In this study, the density, diversity, and size of snail first intermediate hosts, and the diversity and prevalence of their trematode species, were assessed in 2 types of ponds, i.e., those adjacent to cornfields and those from the same region in southwestern Ontario that were adjacent to nonagricultural settings. Species of trematodes included, but were not restricted to, those that are known parasites of larval and adult frogs. We also assessed landscape factors likely to influence use by definitive hosts. Presence of the herbicide atrazine in ponds was measured to check that ponds adjacent to agriculture had potential to be affected by agricultural runoff. Both snail size and the proportion of snails releasing cercariae were greater in nonagricultural ponds, contrasting with a previous finding of lower trematode infection in tadpoles from nonagricultural ponds. Percentage of forest cover was associated with prevalence of certain trematode species, but not with estimates of combined prevalence. Absence of relations of trematode prevalence to measures of road density also contrasted with previous studies. We interpret our results in light of how agricultural activity might influence trematode viability, snail growth, and use by wildlife definitive hosts, independently of landscape factors.