Austropeplea ollula (Pulmonata: Lymnaeidae): a new molluscan intermediate host of a human intestinal fluke, Echinostoma cinetorchis (Trematoda: Echinostomatidae) in Korea

Three freshwater snail species of the family Lymnaeidae have been reported from Korea, Radix auricularia coreana, Austropeplea ollula and Fossaria truncatula. Out of 3 lymnaeid snail species, A. ollula was naturally infected with the Echinostoma cinetorchis cercariae (infection rate = 0.7%). In the experiments with the laboratory-bred snails, F. truncatula as well as A. ollula was also susceptible to the E. cinetorchis miracidia with infection rates of 25% and 40%, respectively. All of three lymnaeid snail species exposed to the E. cinetorchis cercariae were infected with the E. cinetorchis metacercariae. It is evident that A. ollula acts as the first molluscan intermediate host of E. cinetorchis in Korea, and F. truncatula may be a possible candidate for the first intermediate host of this intestinal fluke. Also, three lymnaeid snail species targeted were experimentally infected with E. cinetorchis metacercariae.     

Neodiplostomum leei n. sp. (Digenea: Neodiplostomidae) from chicks infected with metacercariae from the grass snake Rhabdophis tigrina

Neodiplostomula from the European grass snake Rhabdophis tigrina, in the Republic of Korea, were thought to represent 2 species: Pharyngostomum cordatum (large-sized neodiplostomula), an intestinal trematode of cats, and Neodiplostomum seoulense (small-sized neodiplostomula), an intestinal trematode of humans and rodents. The present study describes N. leei n. sp. (Digenea: Neodiplostomidae) on the basis of adult flukes recovered from the small intestines of chicks experimentally infected with small-sized neodiplostomula from the grass snake. The new species differs from N. seoulense in terms of the more extensive distribution of vitellaria, the more severe bilobation of 2 testes (each having a dumbbell shape), the absence of a genital cone, and the smaller size of the tribocytic organ and eggs. Neodiplostomula of the new species and N. seoulense are almost indistinguishable. The new species develops into an adult in chicks, whereas N. seoulense matures in mice and rats. Neodiplostomula of the new species migrate to the liver in mice and rats, without maturing. Results show that there are at least 3 species of neodiplostomid trematodes in the grass snake in the Republic of Korea, i.e.. P. cordatum, N. seoulense and N. leei n. sp.     

Apodemus agrarius as a new definitive host for Neodiplostomum seoulense

A total of 1,496 rodents and insectivores were live-trapped at Yeoncheon-gun (n = 351), Paju-shi (804), and Pocheon-gun (343), Gyeonggi-do (Province), and examined for intestinal helminths, including Neodiplostomum seoulense, seasonally from December 2004 to September 2005. Six species of rodents, including Apodemus agrarius (1,366), Mus musculus (32), Micronytus fortis (28), Eothenomys regulus (9), Micronys minutus (6), and Cricetulus triton (3), and 1 species of insectivores Crocidura lasiura (54) were collected. A total of 321 adult N. seoulense were collected from 19 (1.4%) A. agrarius. The worm burden ranged from 1 to 101 per A. agrarius (mean; 16.9). No N. seoulense was observed in other rodent or insectivore species examined. The infection rate during autumn (4.5%) was higher than those during spring (0.8%), summer (0.8%), and winter (0.5%). The average number of N. seoulense in infected A. agrarius was the highest in spring (66.0 specimens), followed by autumn (15.2), winter (4.5), and summer (3.3). This study first confirms that A. agrarius is a natural definitive host for N. seoulense, and demonstrates that the infection rates and intensities vary seasonally and geographically.     

A new neodiplostomid (Digenea) from the intestine of chicks infected with metacercariae from the grass snake, Rhabdophis tigrina

Three species of neodiplostomula are known to inhabit the European grass snake, Rhabdophis tigrina, in the Republic of Korea: Pharyngostomum cordatum (large-sized neodiplostomula), an intestinal trematode of cats; Neodiplostomum seoulense (small-sized neodiplostomula), an intestinal trematode of humans and rodents; and Neodiplostomum leei (small-sized neodiplostomula), which migrates to the livers of rodents and is an intestinal trematode of birds. The present study describes a fourth species, Neodiplostomum (Conodiplostomum) boryongense n. sp. (Digenea: Neodiplostomidae), based on adult flukes recovered from the small intestines of chicks experimentally infected with small-sized neodiplostomula from the grass snake. The new species differs from 13 previously known species. It also differs from N. seoulense in its larger body size, severely bilobed testes, and smaller genital atrium, and from N. leei in its larger body size, smaller ventral sucker, presence of a genital cone, and vitelline follicles distributed chiefly in the forebody. The new species does not migrate to the livers of rodents nor does it develop to adulthood in the rodent intestines. However, the neodiplostomula of the new species are indistinguishable from those of the other 2 species. Results show that at least 4 species of neodiplostomula inhabit the grass snake in the Republic of Korea.     

Antigenic properties of cystatin-binding cysteine proteinases from Neodiplostomum seoulense

The antigenic properties of cysteine proteinases binding to cystatin were analyzed in Neodiplostomum seoulense, an intestinal trematode that infects humans and rodents in the Republic of Korea. Cystatin was found to effectively capture cysteine proteinases present in the crude extract of N. seoulense. The IgG levels against cystatin-binding cysteine proteinases in sera of mice infected with N. seoulense were higher than those in sera of mice immunized with the crude extract of N. seoulense. The production of IgG antibodies against cystatin-binding cysteine proteinases increased according to the length of infection period. In immunoblots of purified cystatin-binding proteinases, 2 molecules, approximately 50 kDa and 60 kDa, reacted with N. seoulense-infected mouse sera. Of the sera from patients infected with various helminths, those of sparganum-infected patients showed the strongest affinities for cystatin-binding cysteine proteinases of N. seoulense. Cystatin-binding cysteine proteinases of N. seoulense are suggested to be putative antigens for serodiagnosis of human N. seoulense infection.     

Cipangopaludina chinensis malleata (Gastropoda: Viviparidae): a new second molluscan intermediate host of a human intestinal fluke Echinostoma cinetorchis (Trematoda: Echinostomatidae) in Korea

Two species of the viviparid snails have been reported in Korea, e.g., Cipangopaludina chinensis malleata and Cipangopaludina japonica. Cipangopaludina chinensis malleata collected at 3 of 12 localities were found to be infected with metacercariae of Echinostoma cinetorchis, one of the snail-borne human intestinal trematodes in Korea. Metacercariae from these snails were fed to rats (S/D strain), and adult worms of E. cinetorchis, characterized by 37-38 collar spines on the head crown, were recovered from the ileocecal regions. However, no C. japonica collected from 2 localities harbored the metacercariae. In experiments with laboratory-bred viviparid snails, all viviparids were not susceptible to miracidia of E. cinetorchis. To confirm the identity of second intermediate hosts of E. cinetorchis experimentally, 2 species of viviparid snails were exposed to the cercariae from Segmentina hemisphaerula that had been infected with miracidia of E. cinetorchis. Both species of snails were susceptible to cercariae of E. cinetorchis. This is the first report of Cipangopaludina spp. serving as the second intermediate host of E. cinetorchis and as a potential source of human infection.     

Potamopyrgus antipodarum(Mollusca:Hydrobiidae) in continental aquatic gastropod communities: impact of salinity and trematode parasitism

The structure of gastropod communities was examined from January to June 1999 in four sites of the streams of Mont Saint-Michel Bay along a gradient of salinity, and the occurrence of larval trematodes infecting snails was studied. Abundance and species richness of gastropods increased from polyhaline (95 snails, 1 species) to oligohaline waters (6672 snails, 6 species). Whatever the salinity, the most abundant species was Potamopyrgus antipodarum, an invasive non-indigenous species that represented 80% of the gastropods. Only one male was found in P. antipodarum populations suggesting a predominantly parthenogenetic mode of reproduction. Among 7218 gastropods collected, 1.2% were infected by larval trematodes: 5 species in Lymnaea peregra (4.4%), 4 species in Planorbis planorbis (12.0%), one echinostome in Physa acuta (0.2%), and a new species of Sanguinicola in P. antipodarum (0.5%). This is the first record of infected P. antipodarum in Europe. No parasites were found in polyhaline waters. The prevalence per host population varied from 0 to 100% depending on time of collection, salinity and host species. In the lowest-salinity site, abundance of gastropods and prevalence of trematodes were negatively correlated. The dominance of P. antipodarum in the gastropod communities is discussed in relation with euryhalinity, parthenogenesis and weak rate of parasitism.     

Fasciola hepatica in Snails Collected from Water-Dropwort Fields using PCR

Fasciola hepatica is a trematode that causes zoonosis mainly in cattle and sheep and occasionally in humans. Fascioliasis has been reported in Korea; however, determining F. hepatica infection in snails has not been done recently. Thus, using PCR, we evaluated the prevalence of F. hepatica infection in snails at 4 large water-dropwort fields. Among 349 examined snails, F. hepatica-specific internal transcribed space 1 (ITS-1) and/or ITS-2 markers were detected in 12 snails and confirmed using sequence analysis. Morphologically, 213 of 349 collected snails were dextral shelled, which is the same aperture as the lymnaeid snail, the vectorial host for F. hepatica. Among the 12 F. hepatica-infected snails, 6 were known first intermediate hosts in Korea (Lymnaea viridis and L. ollula) and the remaining 6 (Lymnaea sp.) were potentially a new first intermediate host in Korea. It has been shown that the overall prevalence of the snails contaminated with F. hepatica in water-dropwort fields was 3.4%; however, the prevalence varied among the fields. This is the first study to estimate the prevalence of F. hepatica infection using the vectorial capacity of the snails in Korea.     

First report of Lymnaea cousini Jousseaume, 1887 naturally infected with Fasciola hepatica (Linnaeus, 1758) (Trematoda: Digenea) in Machachi, Ecuador

We report the first finding of Lymnaea cousini naturally infected with Fasciola hepatica in Ecuador. A sample of 70 snails was collected in April 2005 from a wetland located in a valley at approximately 3000 m a.s.l., near the locality of Machachi, Pichincha Province. The prevalence of natural infection in L. cousini was 31.43%, which is the highest value ever recorded for naturally infected lymnaeid species.     

Different thermal conditions of lakes affect host–parasite systems: A case study of Viviparus contectus (Millet, 1813) and digenean trematodes

1. Thermal disturbance of aquatic ecosystems directly and/or indirectly affects interspecific interactions, including parasitism. Both hosts and parasites respond differently to environmental changes, thus, predicting how host–parasite systems behave under the influence of disturbance remains a challenge. The aim of the study was to check how the differences in thermal conditions of lakes affect life-history traits of hosts and the level of parasitism, using a Viviparus contectus–digenean trematodes model.
2. Overall, we examined 480 individuals of V. contectus collected from a thermally polluted lake (TPL) and a natural lake (NL). Host features, including body size and fecundity, as well as the prevalence and species richness of digenean trematodes in snail populations were investigated.
3. We found that V. contectus from the TPL were significantly larger, heavier, and females were more fertile than snails collected from the NL. A total of 20.4% of the collected snails were infected with digenean larvae. The species richness of parasites was twice as high in the NL compared to the TPL (six and three species, respectively). A significant difference in the percentage of snails infected with parasites was identified between both types of lakes, with a higher prevalence of V. contectus in the NL (31.3%) compared to the TPL (7.3%).
4. These results indicate that host–parasite systems follow the environmental changes in lakes due to thermal pollution by increasing fertility and metabolism rate of viviparid hosts and by decreasing the prevalence and diversity of digenean trematodes.

     

滨鹬马蹄吸虫(复殖目:微茎科)的尾蚴和囊蚴期的季节动态

对科威特湾微茎科滨鹬马蹄吸虫幼虫期的中间宿主双带盾桑椹螺(Clypeomorus bifasciata)及小相手蟹(Nanosesarma minutum)的季节动态进行了研究。调查期超过一年,在检查的1 600只螺和415只蟹中, 11.8 %的螺感染了8种马蹄属线虫中的一种,且以滨鹬马蹄吸虫的感染占优势(9.9 %螺感染) ; 80 %的蟹感染滨鹬马蹄吸虫囊蚴。虽然一年四季两种宿主都会感染,但吸虫的流行和尾蚴(指成熟期感染)在夏季呈现高蜂。从螺体排出的尾蚴具有明显季节性,在此海湾必须要超过最低温度20℃。总的感染率在较大(较老)的螺里有所下降,显示吸虫影响宿主生存并随之影响宿主群体结构。囊蚴的感染丰度与蟹的个体大小有明显相关性;较大的蟹感染较多的囊蚴,显示宿主能耐受更多的吸虫。调查显示,囊蚴的感染率与蟹的大小或性别无相关性。囊蚴体外脱囊以及产卵吸虫的释放证明,成熟虫体终年存在于所有大小和性别不同的蟹里,显示从蟹到鸟的持续感染是可能的。总的来说,滨鹬马蹄吸虫在海湾的传播动态是由这两种无脊椎动物宿主来协调,并似乎是被一系列依赖于温度的活动控制,这些活动影响易感宿主种群及感染性幼虫期尾蚴和囊蚴的存在。     

Spatial and temporal structure of the trematode component community in Valvata macrostoma (Gastropoda, Prosobranchia)

We conducted the first comprehensive study on the spatiotemporal structure of trematode communities in the large-mouthed valve snail, Valvata macrostoma. A total of 1103 snails were examined monthly between May and October 2007 from Lake Konnevesi, Central Finland, from a shallow (1-2 m deep) and an offshore site (5-6 m deep), located ca. 50-70 m apart. Snails were infected by 10 trematode species. The species composition and prevalence were strikingly different between the sites with high species diversity in the shallow site (all 10 species; total prevalence of sporocysts/rediae 12.1%, metacercariae 55.4%) compared to the deeper site (3 species; prevalence 15.0% and 1.9%, respectively). This difference persisted throughout our study and is probably related to the spatial distribution of bird definitive hosts, whereas the seasonal parasite dynamics are likely to be affected by changes in the age-structure of the snail population. The probability of sporocyst infections increased with snail size, but no such trend was observed in redial or metacercarial infections which decreased with host size. Our results show that generally well-described spatiotemporal differences in trematode infection of molluscs can emerge in very narrow spatial and temporal scales, which emphasizes the importance of these factors in community studies.     

Associations between two trematode parasites, an ectosymbiotic annelid, and Thiara (Tarebia) granifera (Gastropoda) in Jamaica

This work describes associations of Thiara (Tarebia) granifera, its larval trematode community, and Chaetogaster limnaei limnaei at a freshwater reservoir in Jamaica. Larvae of 2 trematodes were present, i.e., a notocotylid (15.3%) and Philophthalmus sp. (1.3%), in 3,575 T. granifera examined. The prevalence of both infections increased with snail shell length (H = 56, P < 0.01, H = 23.1, P < 0.01, respectively). Only 3.0% (n = 595) of infected snails possessed reproductive stages, compared with 90.3% (n = 2,980) of uninfected snails (χ(2) = 2,059.8, df = 1, P < 0.001); both trematodes negatively impacted snail reproduction. Chaetogaster l. limnaei occurred within the mantle cavity of T. granifera with a prevalence of 2.3% (n = 3,575); intensity ranged from 1 to 6 annelids. Notocotylid larvae occurred in 32.5% (n = 83) of snails also harboring C. l. limnaei, compared with 14.9% (n = 3,492) of snails lacking the annelid (χ(2) = 18.127; P < 0.001). Chaetogaster l. limnaei appears not to influence the recruitment of egg-transmitted, notocotylid infections to snails. Ingestion of emergent cercariae by the annelid was observed; this may impact transmission of the parasite. The article presents the first report of a notocotylid and C. l. limnaei in T. granifera, and of Philophthalmus sp. in Jamaica.     

Molecular Identification of Haemadipsa rjukjuana (Hirudiniformes: Haemadipsidae) in Gageo Island,Korea

There are 60 species of blood-feeding land leeches, 50 species belonging to the family Haemadipsidae and 10 species belonging to the family Xerobdellidae. Despite recent papers on the land leeches, their taxonomic identification is not fully understood, especially at a species level. In Korea, there have been no historical records of the terrestrial leeches, but recently an unrecorded blood-feeding land leech was discovered at Gageo-do (Island), Korea. Molecular analysis was used to identify the species of 29 leeches collected from Mt. Dock-Sil in Gageo-do. Conventional PCR was conducted using nuclear 18S rRNA and mitochondrial cytochrome c oxidase subunit 1 (CO1) genetic marker. The 18S rRNA sequences revealed that the leeches share 99.9% identity with Haemadipsa rjukjuana (inhabiting Taiwan), and the CO1 sequences revealed that the leeches are very close to H. rjukjuana (inhabiting Taiwan). The CO1 sequences were separated into 2 categories, 1 with 94.6% and the other with 94.3% similarity to the H. rjukjuana L00115A (inhabiting Taiwan). This new finding of the land leech is the first record in Korea. In addition, the north range of the distribution of the blood-feeding leech (Hirudiniformes: Haemadipisidae) should be reconsidered including Korea.     

Prevalence and intensity of Angiostrongylus cantonensis in freshwater snails in relation to some ecological and biological factors

The purpose of the study was to record different intermediate hosts of A. cantonensis and to determine the infection prevalence and intensity of this parasite in freshwater snails in relation to some ecological and biological factors. The study was conducted at Al-Salam irrigation Canal and Al-Abtal village (north Sinai) for one year, from March 2004 to February 2005. Thirteen species of freshwater snails of nine families were examined for A. cantonensis infection. Six species were found infected with A. cantonensis larvae. These species were L. carinatus, C. bulimoides, C. cyclostomoides, B. alexandrina, L. natalensis and M. tuberculta. The infection prevalence of A. cantonensis in the examined snails ranged from 0.63 to 2.24%. L. carinatus snail had the highest prevalence, mean abundance and mean intensity of A. cantonensis infection. Positive correlations were found between both prevalence and mean abundance of A. cantonensis and host size in L. carinatus and M. tuberculata. Negative correlations were detected between salinity and prevalence, mean abundance and mean intensity of larvae of A. cantonensis. The results demonstrated seasonal and spatial variation in the prevalence, mean abundance and mean intensity of infection among examined snails. In this study, A. cantonensis larvae were found in a wide range of freshwater snails and M. tuberculata snail was recorded as a new intermediate host for the first time. In conclusion, further investigations in other areas and controlled laboratory experiments of infection approaches are required to evaluate the possible threat of this parasite on humans.     

Host life history and the effect of parasitic castration on growth: A field study of Cerithidea californica Haldeman (Gastropoda : Prosobranchia) and its trematode parasites

The prevalence of parasitic infection by larval digenetic trematodes in natural populations of the mud snail, Cerithidea californica Haldeman, was found to increase with snail length; all snails ≥ 33 mm were infected. Distributions of infections by the seven most common larval trematodes were heterogeneous due to two species being more common than expected in the smaller size classes of snails, two being more common than expected in the larger size-classes of snails and three species being most prevalent in snails of intermediate length. The relative abundances of trematodes in different size-classes reflected these distributional patterns.A mark-recapture field study of snail growth rates failed to demonstrate that parasitic infection causes gigantism in Cerithidea. Parasitism tended to stunt the growth of juvenile snails and to a lesser degree, that of adult snails. The effects of trematodes on snail growth was shown to be species specific. This finding contrasts with those of earlier studies in which gigantic growth was observed in infected snails. This discrepancy is attributed to differences in the life histories of the host snails. It is predicted that gigantism will occur commonly in short-lived or semelparous species of snails but rarely, if ever, in long-lived iteroparous species which are predominately marine.     

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.     

Trombiculid mites (Acarina: Trombiculidae) from Apodemus agrarius coreae caught at Pochun-gun, Gyeonggi Province and Chinhae-City, Kyongnam Province

During October-December 1987, chigger mites infested on the striped field mice (Apodemus agrarius coreae) collected at Pochun-gun, Gyeonggi Province and Chinhae city, Kyongnam Province were identified. 1. Of 171 back-striped mice collected, chigger mites were found from 58 mice, showing 33.9% of infestation rate. 2. Total 865 chigger mites collected were classified into seven species; Leptotrombidium pallidum (4.3%), L. palpale(23.0%), L. orientalis(20.2), L. zetum(19.0%). Neotrombicula tamiyai(32.9), N. japonica(0.2%) and Euschongastia koreaensis(0.3%). Though the most dominant species in number was N. tamiyai(32.9% of the total), this species was collected at very limited locality and for a short period. Therefore, most common species in Korea seems to be L. palpale which was found at all localities throughout the whole survey period. L. pallidum which is known as the vector species of tsutsugamushi disease in Korea was collected in a very low number(4.3% of the total chiggers collected). 3. The number of chigger mites infested on a host animal showed great variations from one chigger up to 207 chiggers. The present study has shown that there may be other Leptotrombidium species mite(s) for the vector and host of tsutsugamushi disease in Korea.     

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.