<Emphasis Type="Italic">Fasciola hepatica</Emphasis> and<Emphasis Type="Italic"> Paramphistomum daubneyi</Emphasis>: vertical distribution of metacercariae on plants under natural conditions

Four experiments on the metacercariae of Fasciola hepatica and Paramphistomum daubneyi were carried out under natural conditions in order to study their vertical location on submerged plants and to determine whether simultaneous cercarial shedding of both digenea causes changes in the distributions of the metacercariae. These experiments were performed in experimental boxes, each containing six tufts of rushes. Most metacercariae (73.0%) of F. hepatica were found along the walls of boxes, while 81.5% of P. daubneyi metacercariae were found on rush stems. In the case of snails infected by either of the two digenea, 80.1% of F. hepatica cercariae encysted on submerged parts of rushes and of the box walls near the water surface (to a depth of 1 cm), whereas 73.0% of P. daubneyi metacercariae were found in the lower sections (from –4 to –7 cm). If snails dually infected with P. daubneyi and F. hepatica were used, the vertical distributions of the metacercariae were significantly different from those found for snails infected by either of the two digenea. If snails having 42-day old infections with F. hepatica and other snails with 70-day old infections with P. daubneyi were simultaneously introduced into the boxes, the frequency of F. hepatica cysts was significantly lower in the section located under the water surface (29% only), while the frequencies of metacercariae in the lower sections (from –1 to –5 cm) were increased. Some significant changes were also observed for the metacercariae of P. daubneyi. The disturbance noted in the vertical distributions of F. hepatica and P. daubneyi metacercariae suggest that the encystment of F. hepatica cercariae can be disturbed by the simultaneous encystment of P. daubneyi cercariae, or conversely.     

Cercarial shedding from <Emphasis Type="Italic">Galba</Emphasis><Emphasis Type="Italic">truncatula</Emphasis> infected with <Emphasis Type="Italic">Fasciola</Emphasis><Emphasis Type="Italic">gigantica</Emphasis> of distinct geographic origins

Galba truncatula snails were experimentally infected with either of two different isolates of Fasciola gigantica, originating from Egypt or China, to determine the influence of these isolates on the characteristics of snail infections. The survival rates of G. truncatula on day 30 post-exposure were 90.0% and 60.2% in the Egyptian and Chinese groups, respectively. The frequency of cercaria-shedding snails within the Egyptian group was 79.8%, whereas in the Chinese group it was 22.4%. The parasite origin had a significant effect on the durations of the prepatent and patent periods. The mean number of cercariae shed from the Egyptian group was significantly greater than that shed from the Chinese group (a mean of 275.5 per cercaria-shedding snail compared with 29.0). These results could be explained by the fact that G. truncatula might be a natural intermediate host for F. gigantica in Egypt, and the greater adaptability of the Egyptian miracidia of F. gigantica to unusual snail hosts. These results demonstrate the influence of the geographic origin of the parasite on the success of trematodes infecting snails.     

<Emphasis Type="Italic">Fasciola hepatica</Emphasis>: the developmental patterns of redial generations in naturally infected<Emphasis Type="Italic"> Galba truncatula</Emphasis>

A retrospective study on 1,211 snails naturally infected with Fasciola hepatica in central France was carried out to determine the numbers of full-grown sporocysts from which infections derived, to specify the developmental patterns of redial generations (normal or abnormal) and to count live and free rediae. In the department of Haute Vienne (siliceous subsoils), most snails showed single-sporocyst infections, with normal (46.3%) or abnormal (33.9%) development of redial generations. Two-sporocyst infections were scarcer (a total of 10.6%), while snail co-infections with F. hepatica and Paramphistomum daubneyi (8.0%) were found since 1996. In the department of Indre (calcareous subsoils), single-sporocyst infections showing normal development of rediae were the most numerous (58.0%), while the frequency of single-sporocyst infections with abnormal development was strongly decreased (4.3%). Two-sporocyst infections (with normal development of generations) and co-infections with F. hepatica and P. daubneyi slightly increased in frequency. The redial burdens found in snails collected from the department of Indre were significantly higher than those noted in snails originating from the department of Haute Vienne, whatever the type of snail infection; and these increases in numbers especially concerned the rediae of the second and subsequent generations. The results might be explained by the lower calcium ion content (7–23 mg/l) present in waters from the department of Haute Vienne which would induce a slower growth of infected snails (the shell height of adults scarcely reached 8 mm) and, consequently, would create barely favourable conditions for the development of the first rediae of the first generation within snails. However, the quality of the diet provided as food for snails and its influence on the development of redial generations cannot be excluded.     

Effects of <Emphasis Type="Italic">Schistosoma mansoni</Emphasis> and <Emphasis Type="Italic">Schistosoma haematobium</Emphasis> infections on calcium content in their intermediate hosts

Flame atomic absorption spectrometry was performed to determine the alteration of calcium concentration in the soft parts and shells of Biomphalaria alexandrina and Bulinus truncatus due to the infection with Schistosoma mansoni and S. haematobium, respectively. The results showed significant lowering in the calcium content of the shells of cercariae shedding B. alexandrina and B. truncatus relative to the calcium content in the shells of uninfected ones. In contrast, the calcium content in the soft parts of cercariae shedding snails was higher than in the soft parts of uninfected snails; the differences were statistically significant. Generally, calcium content was significantly higher in the shells than in the soft parts of the snails, regardless infected or uninfected. The results obtained and the hypothesis of hypercalcification in shells of infected snails were discussed.     

<Emphasis Type="Italic">Paramphistomum daubneyi</Emphasis>: the number of sporocysts developing in experimentally and naturally infected <Emphasis Type="Italic">Galba truncatula</Emphasis>

Experimental infections of Galba truncatula with Paramphistomum daubneyi were carried out to determine at day 50 (at 24 degrees C) the numbers of sporocysts, which grew in infected snails via the count of first- and second-generation rediae. In snails individually exposed to one, two, three, four, or five miracidia, the numbers of first-generation rediae increased from the one-miracidium group to the five-miracidium snails (from a mean of 6.7 to 26.1), while second-generation rediae decreased in number (from 6.2 to 0.9, respectively). This scale of redial numbers was used to determine the number of sporocysts, which grew in naturally infected snails collected from sedimentary or acid soils between 1993 and 2006. In cercariae-containing snails, natural infections resulting from the development of one to five sporocysts were found in both samples of G. truncatula examined. The numbers of 3-, 4-, and 5-sporocyst infections were increasing over time since 1997, 2000, and 2003, respectively. The utility of such multiple-sporocyst infections is open to question, as the differentiation of second-generation rediae and that of procercariae were delayed and always limited. They might be interpreted as a consequence of a zoonosis, which has been spreading since 1990 in ruminants of central France.     

The use of Tetraphyll as food for snails increases the intensity of cercarial shedding in<Emphasis Type="Italic"> Galba truncatula</Emphasis> infected with<Emphasis Type="Italic"> Fasciola hepatica</Emphasis>

Experimental infections of Galba truncatula with Fasciola hepatica were carried out under laboratory conditions to determine why the use of Tetraphyll, when provided every 3 days as food for snails, stimulated the intensity of cercarial shedding. In snails raised on cos lettuce and Tetraphyll, the number of metacercariae was significantly higher than that recorded in controls on lettuce only. In the former group, numerous metacercariae were counted on day 1 of every 3-day period (when snails fed on Tetraphyll), while the numbers of these larvae strongly decreased on day 2 and were very low on day 3. The repetition of this experiment using microalgae or modified Borays diet every 3 days as food for snails gave the same results. However, the number of metacercariae recorded on day 1 of every 3-day period was significantly higher in snails raised on lettuce and Tetraphyll than in those reared on lettuce and algae (or Borays diet). According to the authors, the first contact of an infected snail with Tetraphyll induces a faster differentiation of cercariae within the rediae and the accumulation of many free cercariae in the snails hemocoel, so that a second contact of the snail with this food 3 days after allows free cercariae to complete their glycogen and fat reserves and to rapidly exit from the snail. However, a temporary immobilization of the infected snail by Tetraphyll, or rather its degradation products, and a massed exit of cercariae cannot be excluded.     

High performance column liquid chromatographic analysis of selected carboxylic acids in <Emphasis Type="Italic">Biomphalaria glabrata</Emphasis> patently infected with <Emphasis Type="Italic">Schistosoma mansoni</Emphasis>

High-performance liquid chromatography (HPLC) was used to determine the effects of a patent Schistosoma mansoni infection on certain carboxylic acids in the digestive gland gonad complex (DGG) and hemolymph of Biomphalaria glabrata. An analysis of DGG samples was done using extraction with 50% Locke’s solution, cleanup of the extract by anion exchange solid phase extraction (SPE), and ion exclusion HPLC with ultraviolet detection. Hemolymph was applied directly to the SPE column. Acetic, fumaric, malic, and pyruvic acids were detected, confirmed, and quantified at concentrations ranging from 12 to 280 ppm in the DGG and less than 124 to 8,000 μg/dl in the hemolymph. Infection with S. mansoni caused a significant reduction (Student’s t test, P < 0.05) in the concentrations of acetic, fumaric, malic, and pyruvic acids in the DGG but not the hemolymph of B. glabrata compared to uninfected cohort snails. The significant reduction of certain carboxylic acids in the DGG of B. glabrata patently infected with S. mansoni suggests that these acids are utilized by the sporocysts and cercariae in the snail tissue, or that the infection stimulates reduced production or increased utilization by the snail tissue.     

Prevalence of<Emphasis Type="Italic"> Sarcocystis</Emphasis> species sporocysts in Northern Virginia opossums (<Emphasis Type="Italic">Didelphis virginiana</Emphasis>)

A total of 206 Virginia opossums (Didelphis virginiana) collected from the mid-Michigan region, United States, during a period extending from 1996 to 2002 were sampled for the presence of Sarcocystis spp sporocysts. All isolates were phenotypically identified as Sarcocystis spp and genotyped to the species level by PCR-based techniques. The overall prevalence of Sarcocystis spp in opossums was 18% (37/206). The prevalence of Sarcocystis spp differed significantly with age (P<0.001) and adult opossums were more commonly infected (14.6%; 30/206) than juveniles (3.4%; 7/206). No significant difference in the prevalence of Sarcocystis spp infection was observed between male and female (P<0.15). The highest prevalence was recorded during summer (9.2%; 19/206). PCR-RFLP analyses demonstrated the majority of Sarcocystis isolates to be S. neurona, with some animals co-infected with sporocysts of S. falcatula. Out of the 37 Sarcocystis-infected opossums, 23 (62%) had sporocysts of S. neurona only, four (11%) had sporocysts of S. falcatula only, and eight (22%) had a mixture of S. neurona and S. falcatula sporocysts. These findings indicate that mixed Sarcocystis infections in opossums are common. The propensity for Sarcocystis spp to co-exist in the opossum gut enhances dissemination and environmental contamination with these coccidia. Additionally, this increases the chance for sexual recombination between Sarcocystis spp, given the proclivity of these species to reproduce sexually at high numbers in the intestinal cells of their definitive host.     

Factors affecting the molluscicidal activity of <Emphasis Type="Italic">Asparagus densiflorus</Emphasis> and <Emphasis Type="Italic">Oreopanax guatemalensis</Emphasis> plants and Difenoconazole fungicide on <Emphasis Type="Italic">Biomphalaria alexandrina</Emphasis> snails

Many factors may enhance or suppress the molluscicidal activity regardless of their origin. The present study aims to determine the molluscicidal activity of two plants, namely Asparagus densiflorus and Oreopanax guatemalensis besides, Difenoconazole fungicide against Biomphalaria alexandrina snails, miracidia and cercariae. The most effective agent was Difenoconazole followed by A. densiflorus and O. guatemalensis against B. alexandrina snails, where the LC₉₀ values after 24 h of exposure reached 2.5, 102, and 188 ppm, respectively. Toxicity of tested agents was more pronounced at high temperatures. A. densiflorus and Difenoconazole showed no changes in their activities after their exposure to sunlight for 6 h. Also, neutral and acidic media were favorable for maintaining their toxic action. The presence of aquatic plants, mud particles, and storage period had negative effects on the activities of the tested agents. Both A. densiflorus and Difenoconazole had a fast effect against miracidia and cercariae of Schistosoma mansoni after 3 h of the exposure. Moreover, the tested agents recorded adverse impact on the total reproductive rate of exposed snails. Such effects were supported by histopathological examinations of the hermaphrodite gland. It showed an abnormal necrosis in male and female gametogenic cells, besides a decrease in the number of ova and other gametogenic stages.     

Genetic differentiation of cercariae infrapopulations of the avian schistosome <Emphasis Type="Italic">Trichobilharzia szidati</Emphasis> based on RAPD markers and mitochondrial <Emphasis Type="Italic">cox1</Emphasis> gene

Avian schistosome Trichobilharzia szidati is a member of the largest genus within the family Schistosomatidae (Trematoda). Population genetic structure of Trichobilharzia spp. schistosomes, causative agents of cercarial dermatitis in humans, has not been studied yet. The knowledge of the genetic structure of trichobilharzian populations is essential for understanding the host–parasite coevolutionary dynamics and epidemiology strategies. Here we examined genetic diversity in three geographically isolated local populations of T. szidati cercariae inhabiting Russia based on nuclear (randomly amplified polymorphic DNA, RAPD) and mt (cox1) markers. We analyzed T. szidati cercariae shed from seven naturally infected snails of Lymnaea stagnalis. Using three random primers, we demonstrated genetic variation among populations, thus posing genetic structure across geographic sites. Moreover, T. szidati cercariae have been genetically structured among hosts (infrapopulations). Molecular variance analysis was performed to test the significance of genetic differentiation within and between local populations. Of total parasitic diversity, 18.8% was partitioned between populations, whereas the higher contribution (48.9%) corresponds to the differences among individual cercariae within infrapopulations. In contrast to RAPD markers, a 1,125-bp fragment of cox1 mt gene failed to provide any significant within-species structure. The lack of geographic structuring was detected using unique haplotypes which were determined in the current work for Moscow and Western Siberian local populations as well as obtained previously for European isolates (Czech Republic and Germany). All T. szidati/Trichobilharzia ocellata haplotypes were found to be mixed across their geographical origin.     

The life cycle of <Emphasis Type="Italic">Stephanoprora aylacostoma</Emphasis> n.sp. (Digenea: Echinostomatidae), parasite of the threatened snail <Emphasis Type="Italic">Aylacostoma chloroticum</Emphasis> (Prosobranchia,Thiaridae), in Argentina

A new species, Stephanoprora aylacostoma is described and its life cycle was resolved experimentally. The prosobranch snail Aylacostoma chloroticum Hylton Scott, collected in the Yacyretá dam, Province of Misiones, Argentina, was found naturally infected with large-tailed cercariae possessing a prepharyngeal body and corpuscles in the excretory system but lacking collar spines. Metacercariae, which encysted on the gills of experimentally infected fishes Cnesterodon decemmaculatus (Jenyns) and Poecilia reticulata (Peters) (Poecilidae), developed collar spines after 10 days. Tetragonopterid fishes Moenckhausia dichroura (Kner), Astyanax erythropterus (Holmberg) and Hyphesobrycon serpae (Durbin in Eigenmann) were found infected naturally. Sexually mature adults were recovered from domestic chicks at day 7 post-exposure. Eggs shed in chick faeces developed to miracidia within 13–15 days; sporocysts were found on the gills of snails. The new species differs from other species of the genus in its larger eggs, in the smaller, slender body and smaller collar spines of the adult and in the morphological and biological features of the larval stages.     

The effects of various chemical and physical factors on encystment and excystment of <Emphasis Type="Italic">Zygocotyle lunata</Emphasis>

The effects of various chemical and physical treatments on the cercariae and metacercariae of Zygocotyle lunata were studied. Cercariae that actively emerged from Helisoma trivolvis snails were examined in 4 ml of test solution in a 3-cm diameter Stender dish. Observations were usually made on encystment and excystment from 2 to 24 h after cercariae were placed in the Stender dishes. Preferences for certain substrates for encystment were noted, such as smooth glass rather than rough glass, the macrophyte Eleocharis acicularis, and plastic Petri dishes. Many of the solutions tested allowed for encystment, i.e., artificial spring water (ASW), 10% artificial ocean water (AOW), and saline (0.85% NaCl). Certain hypertonic solutions allowed for both in vitro encystment and subsequent excystment, i.e., Locke's solution, 25% AOW, and Locke's 1:1. Serial dilutions of ammonium hydroxide, neutral buffered formalin, potassium permanganate, hydrochloric acid, and acetic acid killed cercariae at all concentrations. Ethanol was effective in killing cercariae at all concentrations except at 3.1%, which allowed for encystment. Cercariae were able to encyst at a low temperature of 16 degrees C and a high temperature of 41 degrees C. Morphometric analysis was performed on both live and fixed cercariae of Z. lunata as well as metacercarial cysts formed in Locke's, Locke's 1:1, and ASW. Live cercariae had larger body dimensions than cercariae fixed in hot 5% neutral buffered formalin and mounted in glycerin jelly. Cysts formed in ASW had very similar length and width measurements. Cyst dimensions became less similar to one another as the tonicity of the medium increased. The phenomenon of spontaneous excystation in Z. lunata after in vitro encystment in a hypertonic medium appears to be reported for the first time in digeneans.     

Natural infections of<Emphasis Type="Italic"> Omphiscola glabra</Emphasis> (Lymnaeidae) with<Emphasis Type="Italic"> Fasciola hepatica</Emphasis> in central France

As larval forms of Fasciola hepatica have periodically been detected in Omphiscola glabra after their collection from watercress beds or from meadows since 1995, field investigations in 37 populations of O. glabra were carried from 1996 to 2002. This was done in order to determine the changes in prevalences and intensities of these natural infections with F. hepatica in relation to the type of snail habitat and the year of snail collection. Snails infected with F. hepatica were found in all samples made in swampy meadows and roadside ditches in all years. In fenced pools and walled gardens, snail infections were only found from 1998 and 1999 onwards, respectively. In the four types of habitats, the prevalences of F. hepatica infections increased slightly over time (0.8–2.1% for snails sampled in swampy meadows, for example) but this increase varied with the habitat. The mean shell heights of infected snails (6.2–7.8 mm) were similar whatever the type of habitat. The numbers of cercariae-containing rediae counted in snails sampled in swampy meadows, roadside ditches, and fenced pools significantly increased over time. Significant numerical variation between these redial burdens was also observed in relation to snail habitat. As the larval development of F. hepatica is facilitated by the presence of another trematode larval form (Paramphistomum daubneyi), the finding of some naturally infected O. glabra in watering places known to have no contact with domestic or wild large mammals might be explained by the development of P. daubneyi in small mammals such as lagomorphs. However, a progressive adaptation of F. hepatica miracidia to O. glabra over time, which would permit the infection of snails at sizes larger than 2 mm, could not be excluded.     

<Emphasis Type="Italic">Fasciola hepatica</Emphasis> miracidia: Lectin binding and stimulation of <Emphasis Type="Italic">in vitro</Emphasis> miracidium-to-sporocyst transformation

The lectin binding properties of Fasciola hepatica miracidia were studied by a panel of fluorescein- and gold-conjugated lectins (ConA, LCA, WGA, LEA, SBA, HPA and UEA-I). The presence of mannose and/or glucose residues was demonstrated with ConA and LCA as weak diffuse fluorescence of the miracidial surface, which was more intense at the anterior part of the larva. The N-acetylglucosamine-binding lectins WGA and LEA reacted intensely with the whole miracidial surface. No labelling with N-acetylgalactosamine and/or galactose-specific (SBA and HPA) and fucose-specific UEA-I lectins was observed. The possibility that the specific recognition of the miracidial surface carbohydrates by lectins may initiate the process of transformation of the miracidia into sporocysts was examined in vitro in physiological saline for Galba truncatula. Incubation in the presence of ConA and WGA resulted in facilitation of the transformation process. Facilitation was absent in the presence of inhibitor sugars. Incubation in the presence of SBA or UEA-I had no effect. The results suggested a possible impact of carbohydrate-lectin interactions in transformation of miracidia of F. hepatica to sporocysts in vivo.     

Experimental infections with <Emphasis Type="Italic">Fasciola</Emphasis> in snails,mice and rabbits

Experimental infection trails of Lymnaea (cailliaudi) natalensis snails with miracidia of Fasciola hepatica revealed neither cercariae nor larval stages shed. Infection of white mice with metacercariae from field-collected snails proved to be negative for Fasciola eggs and immature juveniles or adults after 84 days post infection. The infection of eight rabbits (Oryctolagus cuniculus) has succeeded; two rabbits were infected, with a very low infection rate. Faeces of rabbits were negative for eggs. The worm burden was one and three worms from 40 fed metacercariae. The obtained fluke measures 23 mm in length by 4 mm in width. The tegument is covered with sharp-ending spines. The uterus contains few eggs. The intrauterine eggs measured 158 μm × 80 μm. According to the morphological characters of these flukes, they belong to F. gigantica.     

Decreased humoral and pathologic responses in undernourished mice infected with<Emphasis Type="Italic"> Schistosoma mansoni</Emphasis>

Studies of selected species of Lymnaeidae and Planorbidae showed that infection with bird schistosomes occurred in 14 of the 26 Polish lakes studied. In Planorbarius corneus larvae of Bilharziella polonica were found, and in Lymnaea stagnalis and Radix auricularia larvae of Trichobilharzia ocellata. In spite of their comparatively low prevalence, cercariae, potentially pathogenic to humans, may have caused dermatitis in patients registered in one of the regions under study. The low prevalence of infection among the host snails may have been compensated for by the enormous number of cercariae released by them, which, moreover, showed a long-lasting viability, particularly at low ambient temperature.     

Sensitive and species-specific detection of <Emphasis Type="Italic">Clonorchis sinensis</Emphasis> by PCR in infected snails and fishes

In the present study, PCR procedures have been established for a rapid and easy preparation of DNA of parasite stages from the intermediate hosts, i.e. sporocysts and rediae in snails and metacercariae in fishes. Primers have been developed, which enable a highly sensitive and species-specific detection of Clonorchis sinensis.     

<Emphasis Type="Italic">Angiostrongylus vasorum</Emphasis> (Baillet, 1866) Kamensky, 1905: emergence of third-stage larvae from infected<Emphasis Type="Italic"> Biomphalaria glabrata</Emphasis> snails

Biomphalaria glabrata snails were experimentally infected with Angiostrongylus vasorum first-stage larvae and divided into four groups of 30 snails. To assess the shedding of third-stage larvae (L3), the snails were maintained under different stimuli: group 1 60 W light bulb for 24 h, group 2 37 °C water bath for 24 h, group 3 room temperature (23–25 °C) for 24 h, Group 4 room temperature (23–25 °C) for up to 15 days. After 24 h, a total of 512 A. vasorum L3, alive and active, were released by snails from group 1, while 2,446 L3 were released from group 2 and five L3 from group 3. After 15 days, snails from group 4 released a total of 44 L3. To evaluate the infectivity of A. vasorum L3, two mongrel dogs were successfully infected with L3 released by snails from groups 1 and 2, confirming that the infection of dogs with A. vasorum L3 was possible, independently of ingestion of the mollusk intermediate host. The results shown in these experiments suggest that angiostrongylosis could be directly transmitted to the definitive hosts, with implications for the parasites life cycle.     

Malo-ethanolic fermentation in<Emphasis Type="Italic"> Saccharomyces</Emphasis> and<Emphasis Type="Italic"> Schizosaccharomyces</Emphasis>

Yeast species are divided into the K(+) or K(–) groups, based on their ability or inability to metabolise tricarboxylic acid (TCA) cycle intermediates as sole carbon or energy source. The K(–) group of yeasts includes strains of Saccharomyces, Schizosaccharomyces pombe and Zygosaccharomyces bailii, which is capable of utilising TCA cycle intermediates only in the presence of glucose or other assimilable carbon sources. Although grouped together, these yeasts have significant differences in their abilities to degrade malic acid. Typically, strains of Saccharomyces are regarded as inefficient metabolisers of extracellular malic acid, whereas strains of Sch. pombe and Z. bailii can effectively degrade high concentrations of malic acid. The ability of a yeast strain to degrade extracellular malic acid is dependent on both the efficient transport of the dicarboxylic acid and the efficacy of the intracellular malic enzyme. The malic enzyme converts malic acid into pyruvic acid, which is further metabolised to ethanol and carbon dioxide under fermentative conditions via the so-called malo-ethanolic (ME) pathway. This review focuses on the enzymes involved in the ME pathway in Sch. pombe and Saccharomyces species, with specific emphasis on the malate transporter and the intracellular malic enzyme.Communicated by S. Hohmann     

A first infection of<Emphasis Type="Italic"> Galba truncatula</Emphasis> with<Emphasis Type="Italic"> Fasciola hepatica</Emphasis> modifies the prevalence of a subsequent infection and cercarial production in the F1 generation

Snails from two populations highly susceptible to Fasciola hepatica and their F1 generations were subjected to individual bimiracidial exposures to determine if changes noted in infection parameters were due to an effect imposed on the snail by the parasite, or to some other effect such as the food used for the snails. Apart from the higher survival of unexposed parents at day 30 post exposure (p.e.) and their higher shell heights at day 45 p.e., the differences between the survival rates of exposed parents, prevalences of infections, and shell sizes were not significant. In the F1 snails born to previously infected parents, the prevalences of F. hepatica infection and cercarial production were significantly lower than those noted for the F1 born to unexposed parents. The survival of these snails and their shell growth did not show any significant variation. The F1 snails born to previously exposed snails would have developed a partial resistance against F. hepatica and this process would probably be maximal in the first 2 weeks of larval development inside the snail.