Climatic variations and Fasciola: a review of impacts across the parasite life cycle.

Fasciolosis, caused by the liver fluke Fasciola spp., is a significant parasitic disease of livestock and humans worldwide. Fasciola transmission and life cycle are highly dependent on climatic conditions, especially temperature and humidity. This dependency has gained significance in the context of ongoing climate change. This literature review examined evidence on the effects of temperature variability on the developmental stages of Fasciola spp. and the snail intermediate hosts. We reviewed free larval stages of Fasciola spp. development, as well as snail intermediate hosts, while investigating the climate-related factors influencing each stage. We found that Fasciola spp. egg hatching and development were inhibited below 10 °C and optimal between 20 and 30 °C, miracidia hatching time decreased with higher temperatures and cercarial shedding by snail hosts accelerated around 27 °C. Further, metacercarial viability declined at higher temperatures but was prolonged by higher humidity. Snail intermediate host growth rates peaked at 25 °C, and their susceptibility to Fasciola infection depends on temperature, underscoring its importance in transmission dynamics. Overall, the Fasciola life cycle and snail host development exhibit stage-specific temperature thresholds, indicating a complex relationship between temperature fluctuations and parasite transmission potential. This research highlights the key role of temperature and humidity on Fasciola spp. and snail development, shedding light on the potential consequences of climate change on their survival, development, and disease transmission. Data limitations, primarily from the scarcity of high-resolution climate-related experiments, should drive future research to enhance predictive models and deepen our understanding of the impact of climate change on this parasitic disease.

Extracellular vesicles from Fasciola gigantica induce cellular response to stress of host cells.

Extracellular vesicles (EVs) from parasitic helminths play an important role in immunomodulation. However, EVs are little studied in the important parasite Fasciola gigantica. Here the ability of EVs from F. gigantica to induce cellular response to stress (reactive oxygen species generation, autophage and DNA damage response) in human intrahepatic biliary epithelial cells (HIBEC) was investigated. F. gigantica-derived EVs were isolated by ultracentrifugation, and identified with transmission electron microscopy, nanoparticle size analysis and parasite-derived EV markers. Internalization of EVs by HIBEC was determined by confocal immunofluorescence microscopy and flow cytometry. ROS levels in HIBEC were detected by molecular probing. EVs-induced autophagy and DNA-damaging effects were determined by evaluating expression levels of light chain 3B protein (LC3B), phosphor- H2A.X and phosphor-Chk1, respectively. Results revealed that EVs with sizes predominately ranging from 39 to 110 nm in diameter were abundant in adult F. gigantica and contained the parasite-derived marker proteins enolase and 14-3-3, and EVs were internalized by HIBEC. Further, uptake of EVs into HIBEC was associated with increased levels of reactive oxygen species, LC3Ⅱ, phosphor-H2A.X and phosphor-Chk1, suggesting EVs are likely to induce autophagy and DNA damage & repair processes. These results indicate F. gigantica EVs are associated with modulations of host cell responses and have a potential important role in the host-parasite interactions.

Genetic diversity and population structure analyses based on microsatellite DNA of parthenogenetic Fasciola flukes obtained from cattle and sika deer in Japan.

Understanding the population structure of Fasciola flukes in domestic and wild animals is important for determining the extent of cross-infection between them. Although the parthenogenetic Fasciola flukes in Japan have been shown to comprise five genetic types based on the ribosomal internal transcribed spacer 1 (ITS1) and mitochondrial NADH dehydrogenase subunit 1 (nad1) regions, these genetic regions are not suitable for analyzing their population structure. In the present study, the genetic diversity and population structure of the parthenogenetic Fasciola flukes in Japan were studied using microsatellite DNA, ITS1, and nad1 regions. A total of 144 parthenogenetic Fasciola flukes, obtained from cattle and sika deer in 16 localities, were individually analyzed using PCR-RFLP for ITS1, PCR-direct sequence analysis for nad1, and post-labeling PCR and capillary electrophoresis for microsatellite DNA regions. The flukes showed higher genetic diversity in the microsatellite DNA regions than ITS1 and nad1. The population structures of parthenogenetic Fasciola flukes were unclear, however, it was suggested that the flukes are more diverse populations. We hypothesized that their distribution throughout Japan is closely related to livestock movement dependent on human activity. Moreover, it is considered that cross-infection of the flukes between cattle and sika deer possibly has occurred in the past.

Newly excysted juveniles (NEJs) of Fasciola gigantica induce mice liver fibrosis and M2 macrophage-like phenotype in vivo.

Liver flukes of animals are parasitic flatworms of major socioeconomic importance in many countries. Particularly, Fasciola gigantica is a leading cause of production losses to the livestock (mainly sheep and cattle) and meat industries due to clinical disease, reduced weight gain and milk production, and deaths. Immune responses induced by helminth have been extensively studied, but there is limited information on this aspect by F. gigantica, especially on macrophages induced with this parasite. Studies have shown that host immune responses induced by parasitic infection is greatly correlated with the macrophage polarization axis. In the present study, we used the murine model of F. gigantica to explore the interaction of host and F. gigantica. We found F. gigantica NEJs promoted pathology and fibrosis of mice liver, and the enlargement of mice spleen. We also showed that macrophages were recruited to mice peritoneal cavity at 5 days post infection. By evaluating the expression of genetic markers of M2 macrophages such as Arg-1, Ym1 and RELMɑ, and genetic marker of M1 macrophages iNOS, we showed that M2 macrophages were induced by F. gigantica. M2 macrophages are central to the immune response during helminth infection, and our findings in this study provided insight into the immune interaction between F. gigantica and host.

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

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

Fasciola species and their vertebrate and snail intermediate hosts in East and Southern Africa: a review.

A systematic review was conducted focusing on the distribution of Fasciola species and their snail intermediate hosts (IHs) in East and Southern Africa. The reviewed literature showed that both Fasciola hepatica and F. gigantica are present in East and Southern Africa, and infect a wide range of domestic and wild ruminants. Fasciola gigantica was reported in six East African and five Southern African countries, where Radix natalensis (found in low altitudes) was reported to be the main IH. Fasciola hepatica was reported in Tanzania and Ethiopia (East Africa), and in South Africa and Zimbabwe (Southern Africa), where Galba truncatula (found in high altitudes) was documented as the IH in all countries except in Zimbabwe. Both Fasciola species were documented in Tanzania, Ethiopia, Zimbabwe and South Africa. An overlap of the two was observed in areas with an intermediate altitude in Ethiopia and South Africa, where Pseudosuccinea columella was widespread and assumed to transmit both species. Pseudosuccinea columella has been reported in South Africa and Namibia, and proven to transmit F. gigantica in South Africa; its role in Namibia in the transmission of Fasciola species has not been reported. Other lymnaeid species such as R. rubiginosa were reported in South Africa, and R. auricularia in South Africa and Botswana; their role in the transmission of Fasciola species has not been proven. Future studies should aim to determine the role of P. columella in the geographical spread of the two species in East and Southern African countries.

Monoclonal antibody against recombinant Fasciola gigantica cathepsin L1H could detect juvenile and adult cathepsin Ls of Fasciola gigantica.

Cathepsin Ls (CatLs), the major cysteine protease secreted by Fasciola spp., are important for parasite digestion and tissue invasion. Fasciola gigantica cathepsin L1H (FgCatL1H) is the isotype expressed in the early stages for migration and invasion. In the present study, a monoclonal antibody (MoAb) against recombinant F. gigantica cathepsin L1H (rFgCatL1H) was produced by hybridoma technique using spleen cells from BALB/c mice immunized with recombinant proFgCatL1H (rproFgCatL1H). This MoAb is an immunoglobulin (Ig)G1 with κ light chain isotype. The MoAb reacted specifically with rproFgCatL1H, the native FgCatL1H at a molecular weight (MW) 38 to 48 kDa in the extract of whole body (WB) of metacercariae and newly excysted juvenile (NEJ) and cross-reacted with rFgCatL1 and native FgCatLs at MW 25 to 28 kDa in WB of 2- and 4-week-old juveniles, adult, and adult excretory-secretory (ES) fractions by immunoblotting and indirect ELISA. It did not cross-react with antigens in WB fractions from other parasites, including Gigantocotyle explanatum, Paramphistomum cervi, Gastrothylax crumenifer, Eurytrema pancreaticum, Setaria labiato-papillosa, and Fischoederius cobboldi. By immunolocalization, MoAb against rFgCatL1H reacted with the native protein in the gut of metacercariae and NEJ and also cross-reacted with CatL1 in 2- and 4-week-old juveniles and adult F. gigantica. Therefore, FgCatL1H and its MoAb may be used for immunodiagnosis of both early and late fasciolosis in ruminants and humans.

Experimental life history and biological characteristics of Fasciola gigantica (Digenea: Fasciolidae).

This study was conducted to investigate the life history, morphology, and maturation of larval stages and adult worms of Fasciola gigantica in experimental mice. Lymnaea auricularia rubiginosa was used as the intermediate host, and Oryza sativa was used for encystment of the metacercariae, while Mus musculus was used as the definitive host for maturation study. Fresh eggs from the gall bladder of water buffaloes fully developed into embryonated ones and hatched out at days 11-12 after incubation at about 29ºC. Free-swimming miracidia rapidly penetrated into the snail host, and gradually developed into the next larval stages; sporocyst, redia, and daughter redia with cercariae. Fully-developed cercariae were separated from the redia and shed from the snails on day 39 post-infection (PI). Free-swimming cercariae were immediately allowed to adhere to rice plants, and capsules were constructed to protect metacercariae on rice plants. Juvenile worms were detected in intestines of mice at days 3 and 6 PI, but they were found in the bile duct from day 9 PI. Juvenile and adult flukes were recovered from 16 mice experimentally infected with metacercariae, with the average recovery rate of 35.8%. Sexually mature adult flukes were recovered from day 42 PI. It could be confirmed that experimentally encysted metacercariae could infect and develop to maturity in the experimental host. The present study reports for the first time the complete life history of F. gigantica by an experimental study in Thailand. The obtained information can be used as a guide for prevention, elimination, and treatment of F. gigantica at environment and in other hosts.

The transmission of Fasciola spp. to cattle and contamination of grazing areas with Fasciola eggs in the Red River Delta region of Vietnam.

At four times during November 2010, cattle with infections of Fasciola spp., in two communes of northern Vietnam, were allocated to two equivalent groups. Cattle in one group were treated with triclabendazole. Faecal samples collected monthly from both groups were tested for Fasciola copro-antigens and the presence of Fasciola eggs. Re-infection of treated cattle occurred from early March to late November, coinciding with high weekly totals of rainfall. Contamination of grazing areas by untreated cattle was high and relatively constant throughout the year. However, contamination was reduced to undetectable amounts for 8 to 12 weeks after treatment and even at 20 weeks was only 50% or less of the pre-treatment amounts. Therefore, treatments given in mid-September and again in early April, at the start of the wet season, may be sufficient to prevent contamination of grazing areas and reduce the prevalence and severity of Fasciola infections in cattle.

Fasciola gigantica: Ultrastructural localisation of neoblast recruitment in somatic tissues during growth and development in the hepatic parenchyma of experimentally infected mice.

Application of 'omics' technology, and advances in in vitro methods for studying the growth of Fasciola hepatica, have highlighted the central role of migrating neoblasts in driving forward development and differentiation towards the adult-like form. Neoblast populations present molecular heterogeneity, morphological variation and changes associated with recruitment of these stem cells into their final tissue locations. However, terminal differentiation towards function, has received much less attention than has been the case for the free-living Platyhelminths. An actively replicating neoblast population, comprising cells with heterochromatic nuclei consistent with regulation of gene expression, has been identified in the parenchyma of juvenile Fasciola gigantica migrating in the liver of experimentally infected mice. In some of these cells, early cytoplasmic differentiation towards myocyte function was noted. Neoblasts have also been identified close to, and incorporated in, the subtegumental zone, the gastrodermis and the excretory ducts. In these locations, progressive morphological differentiation towards terminal function has been described. This includes the appearance of specific progenitors of type-1, type-2 and type-3 tegumental cells, the latter possibly contributing to tegumental spine development. 'Cryptic' surface molecular differentiation is postulated to account for recognition and 'docking' of migrating neoblasts with their final site for terminal differentiation.

Fasciola gigantica: the in vitro effects of artesunate as compared to triclabendazole on the 3-weeks-old juvenile.

The in vitro effect of artesunate (ATS) on the 3-week-old juveniles of Fasciola gigantica was compared with triclabendazole (TCZ) by incubating the parasites in M-199 medium containing the drugs at concentrations of 20, 40, and 80 µg/ml for 1, 3, 6, 12, and 24h. The anthelmintic activities of these drugs were evaluated based on the relative motility value (RM) and the alterations of the tegument as observed by scanning (SEM) and transmission (TEM) electron microscopy. The RM values of TCZ-treated flukes decreased significantly from 6 to 24h for all dosages. For ATS-treated flukes, RM value decreased markedly from 12 to 24h, but the rates of decline were less than TCZ at the same doses. When observed by SEM, the tegument showed similar sequence of morphological changes after treatments with both drugs, comprising of swelling of tegumental ridges, followed by blebbing and later rupturing of the blebs, leading to erosion and lesion, and disruption of the tegument. When examined by TEM, ultrastructural changes in the tegument and associated structures after treatments with TCZ and ATS were similar which comprised of swelling, blebbing of the tegument, dilation of basal infoldings, and depolymerization of the microtrabecular network. After a longer incubation time, the tegument was completely sloughed off and the tegument cell bodies became necrotic. Additionally, in ATS-treated flukes, mitochondria showed severe swelling, rupturing of outer membrane, and their interior filled with flocculent materials.

Prevalence of Fasciola in cattle and of its intermediate host Lymnaea snails in central Vietnam.

The aims of this study were to investigate the prevalence of natural Fasciola infections in both the definitive hosts (cattle) and the intermediate hosts (Lymnaea snails) in central Vietnam. A total of 1,075 fecal samples, randomly collected from cattle in Binh Dinh, Khanh Hoa, and Phu Yen provinces, were examined for Fasciola eggs by a sedimentation method. The overall prevalence of Fasciola was 45.3 %. A subset of the animals (235) was also screened for antibodies against Fasciola by an enzyme-linked immunosorbent assay. Overall, 46.3 % of these animals were shedding Fasciola eggs while 87.2 % were Fasciola seropositive. A lower prevalence of Fasciola was observed in calves ≤ 2 years of age (37.6 %) compared to that in cattle >2 years of age (53.7 %) (p < 0.05). The prevalence in the rainy season (50.8 %) was significantly different to that in the dry season (38.1 %) (p < 0.05). Of the 3.269 Lymnaea viridis and 1.128 Lymnaea swinhoei examined, 31 (0.95 %) and seven (0.62 %), respectively, were found to be infected with Fasciola. This appears to be the first epidemiological survey of the prevalence of Fasciola in cattle and snails in these three provinces in central Vietnam.

A case of biliary Fascioliasis by Fasciola gigantica in Turkey.

A case of Fasciola gigantica-induced biliary obstruction and cholestasis is reported in Turkey. The patient was a 37- year-old woman, and suffered from icterus, ascites, and pain in her right upper abdominal region. A total of 7 living adult flukes were recovered during endoscopic retrograde cholangiopancreatography (ERCP). A single dose of triclabendazole was administered to treat possible remaining worms. She was living in a village of southeast of Anatolia region and had sheeps and cows. She had the history of eating lettuce, mallow, dill, and parsley without washing. This is the first case of fascioliasis which was treated via endoscopic biliary extraction during ERCP in Turkey.

High-quality reference genome of Fasciola gigantica: Insights into the genomic signatures of transposon-mediated evolution and specific parasitic adaption in tropical regions.

Fasciola gigantica and Fasciola hepatica are causative pathogens of fascioliasis, with the widest latitudinal, longitudinal, and altitudinal distribution; however, among parasites, they have the largest sequenced genomes, hindering genomic research. In the present study, we used various sequencing and assembly technologies to generate a new high-quality Fasciola gigantica reference genome. We improved the integration of gene structure prediction, and identified two independent transposable element expansion events contributing to (1) the speciation between Fasciola and Fasciolopsis during the Cretaceous-Paleogene boundary mass extinction, and (2) the habitat switch to the liver during the Paleocene-Eocene Thermal Maximum, accompanied by gene length increment. Long interspersed element (LINE) duplication contributed to the second transposon-mediated alteration, showing an obvious trend of insertion into gene regions, regardless of strong purifying effect. Gene ontology analysis of genes with long LINE insertions identified membrane-associated and vesicle secretion process proteins, further implicating the functional alteration of the gene network. We identified 852 predicted excretory/secretory proteins and 3300 protein-protein interactions between Fasciola gigantica and its host. Among them, copper/zinc superoxide dismutase genes, with specific gene copy number variations, might play a central role in the phase I detoxification process. Analysis of 559 single-copy orthologs suggested that Fasciola gigantica and Fasciola hepatica diverged at 11.8 Ma near the Middle and Late Miocene Epoch boundary. We identified 98 rapidly evolving gene families, including actin and aquaporin, which might explain the large body size and the parasitic adaptive character resulting in these liver flukes becoming epidemic in tropical and subtropical regions.

Oxidative status and changes in the adenosine deaminase activity in experimental host infected with tropical liver fluke, Fasciola gigantica.

Fine tuning of the metabolic, physiological and immunological cues along with interplay between the biomolecules of the host and the parasite could be responsible for the successful establishment of parasitic infections. The present investigation was aimed at evaluating the oxidative status and the level of adenosine deaminase (ADA) in the serum and liver of rabbits experimentally infected with Fasciola gigantica. A significant increase in level of ROS, MDA and 4-HNE along with a decline in the SOD, CAT, GR and GST activity was evident in rabbits experimentally infected with Fasciola gigantica. However, there was an increase in the GPX activity in the sera of infected rabbits. The increased GPX activity and decreased GR activity would have resulted in the depletion of GSH, a key non-enzymatic antioxidant, in the infected animals. The level of GSSG was also found to be higher in the sera and liver tissues of the infected rabbits along with a decline in the GSH/GSSG ratio, indicating a high level of oxidative stress in the infected animals, which also showed a significant increase in the activity of the marker enzymes of liver pathology, AST and ALT. Further, a significant inhibition of the adenosine deaminase (ADA) activity in the infected rabbits was accompanied with the reduction in the level of pro-inflammatory cytokine, IL-6 while the anti-inflammatory cytokine, IL-4 level was significantly elevated. In conclusion, the F. gigantica induced significant oxidative stress as evident from the increased levels of ROS and lipid peroxidation along with the disruption of antioxidant and detoxification cascade ultimately lead to pathogenic and inflammatory responses in the experimental host. Whereas, the altered ADA activity could modulate the host's immune responses toward Th-2 type and would facilitate the successful establishment of flukes within their host, thus indicating that ADA could be exploited as a target for the development of novel anthelmintic drugs against fasciolosis.

Genome-wide identification and characterization of eukaryotic protein kinases.

The tropical liver fluke, Fasciola gigantica is a food-borne parasite responsible for the hepatobiliary disease fascioliasis. The recent completion of F. gigantica genome sequencing by our group has provided a platform for the systematic analysis of the parasite genome. Eukaryotic protein kinases (ePKs) are regulators of cellular phosphorylation. In the present study, we used various computational and bioinformatics tools to extensively analyse the ePKs in F. gigantica (FgePKs) genome. A total of 455 ePKs were identified that represent ~2% of the parasite genome. Out of these, 214 ePKs are typical kinases (Ser/Thr- and Tyr-specific ePKs), and 241 were other kinases. Several FgePKs were found to possess unusual domain architectures, which suggests the diverse nature of the proteins that can be exploited for designing novel inhibitors. 115 kinases showed <35% query coverage when compared to human ePKs highlighting significant divergences in their respective kinomes, further providing a platform for novel structure-based drug designing. This study provides a platform that may open new avenues into our understanding of helminth biochemistry and drug discovery.

Inhibitory effect of essential oils of Allium sativum and Piper longum on spontaneous muscular activity of liver fluke, Fasciola gigantica.

Effects of essential oil of Allium sativum (garlic) and Piper longum (Indian long pepper) were evaluated on muscular activity of whole Fasciola gigantica and its strip preparation. The whole flukes and longitudinal strip preparations of the flukes were isometrically mounted to record the spontaneous muscular activity (SMA) and to evaluate effects of cumulative doses (0.1, 0.3, 1.0 and 3.0mg/ml) of the plant essential oils. Whole flukes and the strip preparations exhibited continuous SMA without any significant difference in its baseline tension, frequency and amplitude for 2h. Essential oil of A. sativum produced significant reduction in the frequency and the amplitude of the SMA of whole fluke at 1 and 3mg/ml concentrations. It caused complete paralysis of the fluke after 15 min of administration of 3mg/ml concentration. Similar to whole fluke, essential oil of A. sativum (3mg/ml) also produced flaccid paralysis in the strip preparations of the flukes. Essential oil of P. longum firstly induced marked excitatory effect and then there was flaccid paralysis of the whole fluke following 15 min exposure at 3mg/ml concentration. Complete flaccid paralysis of the strip preparation was also ensued after 15 min of administration of 3mg/ml concentration of P. longum. In both the essential oils, the whole fluke and strip preparations did not recover from paralysis following 2-3 washes. In conclusion, the observations demonstrated irreversible paralytic effect of essential oils of A. sativum and P. longum on F. giganticain vitro which might possibly help to developing herbal-based anthelmintic.

Fasciola gigantica: anthelmintic effect of the aqueous extract of Artocarpus lakoocha.

The effect of the crude extract of Artocarpus lakoocha (70% composition is 2,4,3',5'- tetrahydroxystilbene -THS) on adult Fasciola gigantica was evaluated after incubating the parasites in M-199 medium containing 250, 500, 750 and 1000 microg/ml of the crude extract, or triclabendazole (TCZ) at the concentrations of 80 and 175 microg/ml as the positive control, for 3, 6, 12 and 24h, using relative motility (RM) assay and observation by scanning electron microscope (SEM). Decreased contraction and motility were first observed after 3h incubation with TCZ at the concentration 80 and 175 microg/ml. TCZ markedly reduced the parasite's motility at the concentration of 175 microg/ml at 6h, and killed the worms after 12h exposure. The crude extract of A. lakoocha at all concentrations reduced the parasite's motility similar to TCZ at 3h incubation. In 250 and 500 microg/ml of the crude extract, the values were decreased from 3 to 12h, then they were stable between 12 and 24h and reduced to the level approximately 30-40% of the control. At 750 and 1000 microg/ml concentrations the crude extract rapidly reduced the RM values from the start to 12h and killed the parasites between 12 and 24h incubation. The crude extract also inhibited the larval migration by 75% and 100% at the concentrations of 250-500 and 750-1000 microg/ml, respectively. TCZ and the crude extract caused sequentially changes in the tegument including swelling, followed by blebbings that later ruptured, leading to the erosion and desquamation of the tegument syncytium. As the result, lesion was formed which exposed the basal lamina. The damage appeared more severe on the dorsal than the ventral surface, and earlier on the anterior part and lateral margins when compared to the posterior part. The severity and rapidity of the damages were enhanced with increasing concentration of the crude extract. Hence, the crude extract of A. lakoocha, may exert its fasciolicidal effect against adult F. gigantica by initially causing the tegumental damage.

Fasciola gigantica: cercarial shedding pattern from Lymnaea natalensis after long-term exposure to cadmium at different temperatures.

Little is known about the response of Fasciola spp., inside its snail host to pollutant toxicity. The effect of long-term exposure of Lymnaea natalensis to 0.1 microg/l, 10 microg/l, or 100 microg/l cadmium (Cd) on the infection rates with F. gigantica, percentage of cercariae-shedding snails and cercariae shedding pattern was investigated. The snails were exposed to Cd for 7 days either pre-infection with single Fasciola miracidium or during the late pre-patent period of the infection. The possible interaction between metal exposure and acclimatization temperature was also studied in three ranges; 16-18 degrees C, 23-25 degrees C and 30-32 degrees C. Results clearly showed that host exposure to 100 microg/l Cd significantly affected the pattern of Fasciola development inside its snail host, and that the acclimatization temperature was a key factor affecting the role played by Cd. Pre-infection exposure to Cd caused a significant reduction in the infection rates where the effect was temperature-dependent. Post-infection exposure to Cd significantly increased the percentage of cercariae-shedding snails; the effect was temperature-independent. Interestingly, post-infection exposure significantly altered the differential cercarial output pattern, while no clear impact on the total was found. The alteration in the differential cercarial output was represented as a significantly higher proportion of floating metacercaria as compared to fixed ones and also higher proportion of dead cercariae which may directly reduce the transmission of Fasciola to the final host. The mechanisms of cadmium impact are briefly discussed.