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.     

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.     

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.     

Small estuarine fishes feed on large trematode cercariae: lab and field investigations

In aquatic ecosystems, dense populations of snails can shed millions of digenean trematode cercariae every day. These short-lived, free-living larvae are rich in energy and present a potential resource for consumers. We investigated whether estuarine fishes eat cercariae shed by trematodes of the estuarine snail Cerithidea californica. In aquaria we presented cercariae from 10 native trematode species to 6 species of native estuarine fishes. Many of these fishes readily engorged on cercariae. To determine if fishes ate cercariae in the field, we collected the most common fish species, Fundulus parvipinnis (California killifish), from shallow water on rising tides when snails shed cercariae. Of 61 killifish, 3 had recognizable cercariae in their gut. Because cercariae are common in this estuary, they could be frequent sources of energy for small fishes. In turn, predation on cercariae by fishes (and other predators) could also reduce the transmission success of trematodes.     

Effect of osmolality and selected ions on retraction of the distome body into the cercaria tail chamber of Proterometra macrostoma (Trematoda: Azygiidae)

The furcocystocercous cercariae of the digenetic trematode, Proterometra macrostoma , possess a tail chamber into which their distome body withdraws prior to emergence from their snail intermediate host. The process of distome retraction and the conditions that trigger it in this species are not clear. The objectives of the present study were (1) to describe the retraction process in P. macrostoma; (2) to assess whether osmolality affects cercarial retraction; (3) to evaluate the effect of selected ions on retraction; and (4) to compare the swimming effectiveness of naturally (?= in vivo) retracted versus in vitro retracted cercariae. Retraction of the cercaria body into its tail chamber required only 2 min or less once initiated. The process began with the development of a chamber within the anterior end of the worm's tail. The chamber's lip advanced in a pulsating motion over the stationary distome. Retraction was completed with the constriction and fusion of the chamber lip once it passed over the anterior end of the distome, sealing the latter within the tail chamber. There was a significant difference in the proportions of cercariae with bodies retracted into tails, bodies not retracted, and bodies separated from tails in artificial pond water (APW) versus artificial snail water (ASW). A greater number of cercariae withdrew into their tail chambers in ASW (59/124; 47.6%) than in APW (21/124; 16.9%). In APW, more bodies separated from their tails (24/124; 19.4%) than in ASW (3/124; 2.4%). In both solutions (APW: 63.7% = 79/124; ASW: 50% = 62/124), a majority of cercariae never retracted. In APW, 76.2% of distomes retracting into their tails did so within the first 5 min compared to only 30.5% in ASW. There was no significant difference in the proportions of cercariae with bodies retracted into tails, bodies not retracted, and bodies separated from tails based on isosmotic replacement of individual ions, i.e., Na(+), K(+), Ca(++), or Mg(++), in ASW with Li(+). There was also no significant difference in the vertical swimming burst distance in cercariae whose bodies were initially retracted into their tails in vitro versus in vivo.     

Prevalence of amphistomiasis and its association with Fasciola gigantica infections in Zambian cattle from communal grazing areas

To estimate prevalence of amphistomiasis and its association with bovine Fasciola gigantica infections in Zambia, 709 faeces of cattle presented for slaughter from three major cattle rearing areas of Central, Southern and Western provinces were analysed. The prevalence rate of amphistomes was 51.6%. Egg counts per gram (EPG) of faeces ranged from 0 to 385 with a mean (+/- SEM) of 11.96 +/- 1.07. The origin of the cattle had a significant influence (P < 0.001) on the prevalence rate. Fasciola gigantica infections accounted for 46.7% of the cattle examined. The mean EPG count (+/- SEM) was 6.3 +/- 0.66 with a range of 0 to 223. A total of 34.6% were mixed infections while single Fasciola and amphistome infections represented 12.1% and 17.1%, respectively. Significantly more cattle (63.8%) were infected with either single or both trematode infections (P < 0.0001) than not. Mixed trematode infections were highest in the Southern province (80.0%) while the lowest were recorded in Central province (16.3%). A positive correlation (r(2) = 0.0428) was present in mixed infections. In order to minimize losses, epidemiology and cross-resistance of amphistomiasis and fascioliasis should be studied in different ecological regions of Zambia to formulate efficient control programmes.     

Cercarial shedding of Echinostoma cinetorchis and experimental infection of the cercariae to several kinds of snails

The development of Echinostoma cinetorchis in several snail species reared in laboratory aquaria was observed. The eggs from adult flukes collected from the intestine of rats were cultivated to miracidia, and exposed to Hippeutis sp. snails. Observations were made for cercarial shedding from the exposed snails. The cercariae shed from the snails were again exposed to several species of fresh water snails in order to observe metacercarial formation in the snails and their infectivity to final hosts. The results obtained in this study were as follows: 1. Twenty miracidia were exposed to each snail of Hippeutis sp. About 58.3% of the above snails (7 out of 12) were dead before shedding the cercariae, and the remainder shed the cercariae for a period of 7 to 9 days before death. 2. Cercarial shedding from the infected snails started from the 25th day after the exposure to miracidia, and the total number of cercariae shed per snail was 684 in average (range; 482-904). 3. The size of rediae developed in the infected Hippeutis sp. snails was 1,242 x 214 microns in average, and the number of rediae per snail was 350 in average (range; 120-510). 4. About 40 to 50 cercariae shed from the Hippeutis sp. snails were each exposed to several species of snails reared in the laboratory. The metacercarial formation was confirmed by dissecting the infected snails, 12 to 16 days after the infection.(ABSTRACT TRUNCATED AT 250 WORDS)     

Development of Fasciola hepatica in Lymnaea columella infected with miracidia derived from cattle and marmoset infections

The development of Fasciola hepatica from two species of definitive hosts, i.e. cattle (Bos taurus) and a marmoset (Callithrix penicillata) in the snail Lymnaea columella was determined based on the production of rediae and cercariae and snail survival rate. More rediae and cercariae at 60-74 days post-infection were produced by snails infected by cattle-derived miracidia (cattle group) than by those infected by marmoset-derived miracidia (marmoset group). Among the L. columella parasitized by the marmoset group, the survival rate and the percentage of positive snails were higher than among those parasitized by the cattle group. Eggs of F. hepatica released in cattle faeces were significantly bigger than those released in marmoset faeces. Miracidia originating from parasites that completed their development in cattle were more efficient in infecting the intermediate host. These results suggest that vertebrate-host origin influences the eggs produced by the parasite and the infection rates in the snail host L. columella.     

Clonal diversity of the marine trematode Maritrema novaezealandensis within intermediate hosts: the molecular ecology of parasite life cycles

We quantified the clonal diversity of the New Zealand marine trematode Maritrema novaezealandensis (n = 1250) within Zeacumantus subcarinatus snail (n = 25) and Macrophthalmus hirtipes crab (n = 25) intermediate hosts using four to six microsatellite loci, and investigated the potential biological and physical factors responsible for the observed genetic patterns. Individual snails harboured one to five trematode genotypes and 48% of snails were infected by multiple parasite genotypes. Overall, the number of parasite genotypes did not increase with snail size, but was highest in intermediate-sized snails. Significantly larger numbers of parasite genotypes were detected in crabs (relative to snails; P < 0.001), with 16-25 genotypes recovered from individual crabs. Although crabs are typically infected by small numbers of cercariae sourced from many snails, they are occasionally infected by large numbers of cercariae sourced from single snails. The latter cases explain the significant genetic differentiation of trematode populations detected among their crab hosts (F(ST) = 0.009, P < 0.001). Our results suggest that the timing of infection and/or intraspecific competition among parasite clones within snails determine(s) the diversity of parasite clones that snails harbour. The presence of a large number of infected snails and tidal mixing of cercariae prior to infection results in crabs potentially harbouring hundreds of parasite genotypes despite the crabs' territorial behaviour.     

Long-term analysis of Charlie's Pond: fecundity and trematode communities of Helisoma anceps

Charlie's Pond (North Carolina) harbors a diverse community of trematodes that infect the planorbid snail Helisoma anceps. Research at the Pond began in 1984 and serves as a foundation on which to investigate long-term changes in trematode communities. In 2002, 2005, and 2006 average size and fecundity of H. anceps were calculated each month, and seasonal trends analyzed with randomization tests. Concomitantly, trematode infections were recorded, and the community composition compared to those from previous studies. Helisoma anceps in 2002, 2005, and 2006 were smaller and less fecund than snails in 1984. The trematode community was consistently diverse, with 11 species recovered in 2006 versus 7 in 1984. However, the prevalence of Halipegus occidualis was much lower than previously observed (60% in 1984) and never exceeded 20% during the latter years. The decline of emergent vegetation is likely contributing to these changes. Aquatic macrophytes increase the surface area for growth of periphyton, the food source of these snails. Limited food supplies result in lower snail growth rates and fecundity. Similarly, emergent vegetation creates foci of transmission for H. occidualis between the frog definitive host and the snail intermediate host. When these areas are lost from the Pond, probability of transmission is reduced, and prevalence in the snail declines.     

Radix natalensis (Gastropoda: Lymnaeidae), a potential intermediate host of Fasciola hepatica in Egypt

Experimental infections of Egyptian Radix natalensis with French miracidia of Fasciola hepatica were carried out to determine if this snail might act as an intermediate host in the life cycle of this digenean in Egypt. Single exposures of R. natalensis to miracidia (2/snail) and two successive exposures (a total of 4 miracidia/ snail) were performed using lymnaeids measuring 1 to 6 mm in height. Live larval forms of F. hepatica were noted in single- and double-exposed snails. In double exposures, a significant increase of snail survival on day 28 post-exposure (at 24 degrees C) and an decrease in prevalence were noted when the height of snails at exposure was increasing. Cercariae of F. hepatica were shed by these snails (90.7/snail) during a mean patent period of 24.3 days. All snails have released these cercariae during 2-13 waves of shedding. According to these results, R. natalensis can be considered a potential intermediate host of F. hepatica in Egypt.     

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。     

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.     

Parasite-induced changes in nitrogen isotope signatures of host tissues

To estimate isotopic changes caused by trematode parasites within a host, we investigated changes in the carbon and nitrogen isotope ratios of the freshwater snail Lymnaea stagnalis infected by trematode larvae. We measured carbon and nitrogen stable isotopes within the foot, gonad, and hepatopancreas of both infected and uninfected snails. There was no significant difference in the delta13C and delta15N values of foot and gonad between infected and uninfected snails; thus, trematode parasite infections may not cause changes in snail diets. However, in the hepatopancreas, delta15N values were significantly higher in infected than in uninfected snails. The 15N enrichment in the hepatopancreas of infected snails is caused by the higher 15N ratio in parasite tissues. Using an isotope-mixing model, we roughly estimated that the parasites in the hepatopancreas represented from 0.8 to 3.4% of the total snail biomass, including the shell.     

Population dynamics of two snail species,Planaxis sulcatus and Cerithium moniliferum,and their trematode species at Heron Island,Great Barrier Reef

Summary The population dynamics of the prosobranch snail Planaxis sulcatus and its trematode parasites on 600 m of beachrock on the southern side of Heron Island, Great Barrier Reef, were studied. Populations of Planaxis show little mixing, due to lack of long-distance movements. From March 1973 to June 1975, an increase in the numbers of small and medium-sized snails occurred, but the biomass (dry tissue weight) of the snail population did not change. Snails were infected with one species of Aspidogastrea and six species of cerariae. Infections with cercariae did not significantly affect the relative weight of the snails. Multiple infections were random, i.e. neither negative nor positive interactions between trematode species could be demonstrated. Proportions of uninfected snails and snails infected with various trematode species remained more or less constant from July 1973 to February 1975.The population dynamics of the prosobranch snail Cerithium moniliferum and its 11 species of cercariae and one species of Aspidogastrea in a small area at Heron Island were studied. Numbers of large snails decreased and numbers of small snails increased from August 1973 to June 1975. The biomass of the Cerithium population increased only slightly. Prevalence of infection changed significantly only in large snails.Infections of Cerithium moniliferum and Peristernia australiensis with an aspidogastrid species decreased strongly from January 1971 to March/April 1972 and had not recovered by mid 1975.Seasonal fluctuations could not be demonstrated for any of the snails or parasites.The data for Planaxis suggest equilibrium conditions and saturation of the habitat.     

Larval development of Fasciola hepatica in experimental infections: variations with populations of Lymnaea truncatula

A retrospective study was undertaken on 70 French populations of Lymnaea truncatula experimentally infected with Fasciola hepatica to determine whether or not susceptibility of snails to infection influenced redial and cercarial production. Results were compared with those obtained from two control populations, known for prevalences higher than 60% when experimentally infected with F. hepatica. In the 70 other populations examined, the prevalences ranged from 2 to 75%. In 55 of these populations, where the prevalence was more than 20%, a high proportion (50.1-56.8%) of snails died after cercarial shedding, whereas in the other groups (non-shedding snails with the most differentiated larvae being free cercariae, rediae containing cercariae, immature rediae, or sporocysts, respectively), snail death was significantly less. In 11 populations, where the prevalence values were 5-19%, only 14% of snails died after cercarial shedding, whereas snails with free cercariae, rediae with cercariae, or immature rediae showed significant increases in snail mortality. In the remaining four snail populations, with prevalences of less than 5%, the most differentiated larval forms were only immature rediae and/or sporocysts. Overall, the number of rediae containing cercariae significantly decreased with decreasing prevalence values. The low prevalence of experimental infection in several populations of snails might be explained by the occurrence of natural infections with miracidia originating from a mammalian host other than cattle, and/or by genetic variability in the susceptibility of snails to infection.     

Fasciola hepatica: cercarial productivity of redial generations in long-surviving Galba truncatula

Bimiracidial infections of Galba truncatula with Fasciola hepatica were carried out to determine the effect of food quality on the frequency of 1- and 2-sporocyst infections, to analyse its impact on the developmental patterns (normal, or abnormal) of redial generations, and to verify its consequences on cercarial production. These investigations were performed in snails reared at 20 degrees C and provided with cos lettuce and commercial fish food (Tetraphyll) as a food source until their death. Double-sporocyst infections with normal development of redial generations were recorded in 43.9% of infected snails (out of 296). Single-sporocyst infections were noted in the other snails, with normal development of generations in 53.7% and abnormal development (the first mother redia early degenerated) in 2.4%. Four successive redial generations were found in long-surviving snails (more than 90 days). In both 1- and 2-sporocyst infections, showing normal development of generations, the daughter rediae, which exited from the first mother redia (R2a rediae), constituted the greater group of free rediae and produced the highest percentages of cercariae (46.2-48.2%). However, the development of these rediae inside the snail body was slower in 2-sporocyst infections than in 1-sporocyst infections. The numbers of rediae noted in subsequent generations (R2b/R3a and R3b/R4a rediae) were similar, whatever the number of full-grown sporocysts. The number of shed cercariae recorded in the 1- and 2-sporocyst infections did not significantly differ. When long-surviving snails died, 19.8-20.7% of cercariae produced by free rediae (essentially by R2b/R3a and R3b/R4a rediae) were still present in their bodies. The increased frequency of 2-sporocyst infections demonstrated that food quality had a significant effect on the redial burden of F. hepatica developing inside G. truncatula.     

Plagiorchis elegans (Digenea: Plagiorchiidae) infections in Stagnicola elodes (Pulmonata: Lymnaeidae): host susceptibility, growth, reproduction, mortality, and cercarial production.

Eggs of Plagiorchis elegans were readily ingested by Stagnicola elodes of all ages, but were more infective to immature than mature snails. Infection enhanced the growth of the host in a dose-dependent manner. The number of cercariae released by immature snails increased with the age of the snail host; mature snails yielded fewer cercariae. Heavily infected snails tended to die prematurely, thereby reducing their total production of cercariae to levels below those of more lightly infected individuals. Even light infections castrated the snail host. Snails that acquired the infection as juveniles never produced eggs. Actively reproducing snails ceased egg laying within days of infection and never recovered. All parasite effects on the growth and reproduction of the snail host first manifested themselves during the early stages of infection, long before the development of daughter sporocysts and cercariae, and are therefore attributable to the effects of mother sporocysts. The study provides insight into how this natural enemy of mosquito larvae may be established in natural snail populations by means of strategically timed introductions of parasite eggs, with a goal of maximizing cercarial production for the biological control of sympatric mosquito larvae.     

Does behavioural fever occur in snails parasitised with trematode larvae?

The objective of this study was to determine whether individuals of Lymnaea stagnalis naturally parasitised with trematode larvae had different thermal preferences from non-parasitised ones, and what was the influence of constant temperature on biology of the host and the parasite.

The snails under study were placed in and oblong thermal gradient and their preferences were recorded during 24 h. Non-parasitised individuals chose a rather high temperature (25 °C). In the group of parasitised snails, only individuals which released cercariae showed statistically significant differences in thermal preference than the non-larvae releasing ones. Parasitised snails kept in constant temperatures lived shorter than non-parasitised ones. At high temperatures, they released more cercariae per day than at low temperatures. Temperature had a great influence on the longevity and viability of cercariae.

The thermal behaviour of snails parasitised with trematode larvae is not a symptom of behavioural fever but probably can play a therapeutic role in the host.     

The presence of predators modifies the larval development of Fasciola hepatica in surviving Lymnaea truncatula

Experimental infections of Lymnaea truncatula with Fasciola hepatica were performed to study the consequences of the presence of predators (sciomyzid larvae or zonitid snails) on the characteristics of larval F. hepatica development in surviving snails. Controls consisted of infected snails that were not subjected to predators. Compared to controls, the survival rate at day 30 post-exposure, the duration of cercarial shedding, and the number of cercariae shed by surviving snails were significantly lower when predators were present in snail breeding boxes, whatever the type of predator used. In contrast, the prevalences of Fasciola infections in snails, and the length of time between exposure and the onset of cercarial shedding showed no significant variation. The progressive development of a stress reaction in surviving snails against predators during the first 30 days of experimental exposure to F. hepatica would influence snail survival during the cercarial shedding period and, consequently, the number of cercariae shed by the snails.