Natural infections of Omphiscola glabra (Lymnaeidae) with Fasciola hepatica in central France

Towards the comprehension of fasciolosis (re-)emergence: an integrative overview

The increasing distribution and prevalence of fasciolosis in both human and livestock are concerning. Here, we examine the various types of factors influencing fasciolosis transmission and burden and the interrelations that may exist between them. We present the arsenal of molecules, 'adjusting' capabilities and parasitic strategies of Fasciola to infect. Such features define the high adaptability of Fasciola species for parasitism that facilitate their transmission. We discuss current environmental perturbations (increase of livestock and land use, climate change, introduction of alien species and biodiversity loss) in relation to fasciolosis dynamics. As Fasciola infection is directly and ultimately linked to livestock management, living conditions and cultural habits, which are also changing under the pressure of globalization and climate change, the social component of transmission is also discussed. Lastly, we examine the implication of increasing scientific and political awareness in highlighting the current circulation of fasciolosis and boosting epidemiological surveys and novel diagnostic techniques. From a joint perspective, it becomes clear that factors weight differently at each place and moment, depending on the biological, environmental, social and political interrelating contexts. Therefore, the analyses of a disease as complex as fasciolosis should be as integrative as possible to dissect the realities featuring each epidemiological scenario. Such a comprehensive appraisal is presented in this review and constitutes its main asset to serve as a fresh integrative understanding of fasciolosis.

A Universal Approach to Molecular Identification of Rumen Fluke Species Across Hosts, Continents, and Sample Types

Rumen fluke are parasitic trematodes that affect domestic and wild ruminants across a wide range of countries and habitats. There are 6 major genera of rumen fluke and over 70 recognized species. Accurate species identification is important to investigate the epidemiology, pathophysiology and economic impact of rumen fluke species but paramphistomes are morphologically plastic, which has resulted in numerous instances of misclassification. Here, we present a universal approach to molecular identification of rumen fluke species, including different life-cycle stages (eggs, juvenile and mature fluke) and sample preservation methods (fresh, ethanol- or formalin-fixed, and paraffin wax-embedded). Among 387 specimens from 173 animals belonging to 10 host species and originating from 14 countries on 5 continents, 10 rumen fluke species were identified based on ITS-2 intergenic spacer sequencing, including members of the genera Calicophoron, Cotylophoron, Fischeroedius, Gastrothylax, Orthocoelium, and Paramphistomum. Pairwise comparison of ITS-2 sequences from this study and GenBank showed >98.5% homology for 80% of intra-species comparisons and <98.5% homology for 97% of inter-species comparisons, suggesting that some sequence data may have been entered into public repositories with incorrect species attribution based on morphological analysis. We propose that ITS-2 sequencing could be used as a universal tool for rumen fluke identification across host and parasite species from diverse technical and geographical origins and form the basis of an international reference database for accurate species identification.

Genetic diversity and relationships of the liver fluke Fasciola hepatica (Trematoda) with native and introduced definitive and intermediate hosts

Fasciolosis is a worldwide spread parasitosis mainly caused by the trematode Fasciola hepatica. This disease is particularly important for public health in tropical regions, but it can also affect the economies of many developed countries due to large infections in domestic animals. Although several studies have tried to understand the transmission by studying the prevalence of different host species, only a few have used population genetic approaches to understand the links between domestic and wildlife infections. Here, we present the results of such genetic approach combined with classical parasitological data (prevalence and intensity) by studying domestic and wild definitive hosts from Camargue (southern France) where fasciolosis is considered as a problem. We found 60% of domestic hosts (cattle) infected with F. hepatica but lower values in wild hosts (nutria, 19%; wild boars, 4.5%). We explored nine variable microsatellite loci for 1,148 adult flukes recovered from four different populations (non-treated cattle, treated cattle, nutria and wild boars). Populations from the four groups differed, though we found a number of migrants particularly non-treated cattle and nutria. Overall, we detected 729 different multilocus genotypes (from 783 completely genotyped individuals) and only 46 genotypes repeated across samples. Finally, we experimentally infected native and introduced intermediate snail hosts to explore their compatibility with F. hepatica and assess the risks of fasciolosis expansion in the region. The introduced species Galba truncatula and Pseudosuccinea columella attained the higher values of overall compatibility in relation to the European species. However, concerning the origin, sympatric combinations of G. truncatula were more compatible (higher prevalence, intensity and survival) than the allopatric tested. According to our results, we should note that the assessment of epidemiological risks cannot be limited to a single host-parasite system, but should focus on understanding the diversity of hosts in the heterogeneous environment through space and time.

Galba truncatula and Fasciola hepatica: Genetic costructures and interactions with intermediate host dispersal

Antagonistic interactions between hosts and parasites are key structuring forces in natural populations. Demographic factors like extinction, migration and the effective population size shape host-parasite metapopulational dynamics. Therefore, to understand the evolution of host-parasite systems it is necessary to study the distribution of the genetic variation of both entities simultaneously. In this paper, we investigate the population genetics co-structure of parasites and hosts within a metapopulation of the liver fluke, Fasciola hepatica, and two of its intermediate hosts, the main intermediate host in Europe, Galba truncatula, and a new intermediate host, Omphiscola glabra, in Central France. Our results reveal an absence of specificity of flukes as regard to the two alternative hosts though O. glabra shows higher prevalence of F. hepatica. Host and parasites displayed contrasting population genetics structure with very small, highly inbred (selfing) and strongly isolated G. truncatula populations and much bigger, panmictic and more dispersive F. hepatica. This could indicate a local adaptation of the parasite and a local maladaptation of the host. We also unveil a parasite-mediated biased population genetics structure suggesting that infected G. truncatula disperse more; have higher dispersal survival than uninfected snails or, more likely, that immigrant snails are infected more often than local snails (local parasites are less adapted to local hosts). Finally, an absence, or at least an ambiguous signature of isolation by distance was observed in both host and parasite population. A very weak migration rate for G. truncatula provides a reasonable explanation for this ambiguous result. Alternatively, smaller sample sizes combined with modest migration rates might explain the difficulties to unveil the signal in F. hepatica.

Fasciola hepatica demonstrates high levels of genetic diversity, a lack of population structure and high gene flow: possible implications for drug resistance

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Self-fertilisation does occur but is rare in field populations of Fasciola hepatica.

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Some hosts harboured genotypically identical parasites (clones).

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The presence of clones is consistent with clonal expansion and clumped transmission.

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84% of 1579 F. hepatica had unique genotypes, indicating high genetic diversity.

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We found high gene flow, no population structure and low self-fertilisation rate.

Fasciola hepatica, the liver fluke, is a trematode parasite of considerable economic importance to the livestock industry and is a re-emerging zoonosis that poses a risk to human health in F. hepatica-endemic areas worldwide. Drug resistance is a substantial threat to the current and future control of F. hepatica, yet little is known about how the biology of the parasite influences the development and spread of resistance. Given that F. hepatica can self-fertilise and therefore inbreed, there is the potential for greater population differentiation and an increased likelihood of recessive alleles, such as drug resistance genes, coming together. This could be compounded by clonal expansion within the snail intermediate host and aggregation of parasites of the same genotype on pasture. Alternatively, widespread movement of animals that typically occurs in the UK could promote high levels of gene flow and prevent population differentiation. We identified clonal parasites with identical multilocus genotypes in 61% of hosts. Despite this, 84% of 1579 adult parasites had unique multilocus genotypes, which supports high levels of genotypic diversity within F. hepatica populations. Our analyses indicate a selfing rate no greater than 2%, suggesting that this diversity is in part due to the propensity for F. hepatica to cross-fertilise. Finally, although we identified high genetic diversity within a given host, there was little evidence for differentiation between populations from different hosts, indicating a single panmictic population. This implies that, once those emerge, anthelmintic resistance genes have the potential to spread rapidly through liver fluke populations.

Study of surface carbohydrates in Galba truncatula tissues before and after infection with Fasciola hepatica

The presence and distribution of surface carbohydrates in the tissues of Galba truncatula snails uninfected or after infection with Fasciola hepatica as well as on the surface of the snail-pathogenic larval stages of the parasite were studied by lectin labelling assay. This is an attempt to find similarities that indicate possible mimicry, utilised by the parasite as an evasion strategy in this snail-trematode system. Different binding patterns were identified on head-foot-mantle, hepatopancreas, genital glands, renopericardial complex of the host as well as of the snail-pathogenic larval stages of F. hepatica. The infection with F. hepatica leads to changes of labelling with Glycine max in the head-mantle cells and Arachis hypogaea in the tubular epithelium of the hepatopancreas. The lectin binding on the other snail tissues is not changed by the development of the larvae. Our data clearly demonstrated the similarity in labelling of G. truncatula tissues and the surface of the snail-pathogenic larval stages of F. hepatica. The role of glycosylation of the contact surfaces of both organisms in relation to the host-parasite interactions is also discussed.

Confirmation of Galba truncatula as an intermediate host snail for Calicophoron daubneyi in Great Britain, with evidence of alternative snail species hosting Fasciola hepatica

BACKGROUND

Fasciola hepatica is a highly prevalent parasite infecting livestock in Great Britain, while Calicophoron daubneyi is an emerging parasite within the GB livestock industry. Both F. hepatica and C. daubneyi require an intermediate host snail to complete their life-cycles and infect ruminants; however, there has been no confirmation of the intermediate host of C. daubneyi in GB, while there are questions regarding alternative host snails to Galba truncatula for F. hepatica. In this study, PCR was used to identify C. daubneyi hosting snail species on Welsh pastures and to identify any alternative snail species hosting F. hepatica.

FINDINGS

Two hundred and sixty four snails were collected between May-September 2015 from six farms in mid-Wales known to have livestock infected with C. daubneyi and F. hepatica. Fifteen out of 134 G. truncatula were found positive for C. daubneyi, one of which was also positive for F. hepatica. Three snail species were found positive for F. hepatica [18/134 G. truncatula, 13/52 Radix balthica, and 3/78 Potamopyrgus antipodarum (New Zealand mud snail)], but no evidence of C. daubneyi infection in the latter two species was found.

CONCLUSION

This study indicates that G. truncatula is a host for C. daubneyi in GB. Galba truncatula is also an established host of F. hepatica, and interactions between both species at intermediate host level could potentially occur. Radix balthica and P. antipodarum were found positive for F. hepatica but not C. daubneyi. This could indicate a role for alternative snail species other than G. truncatula in infecting pastures with F. hepatica in GB.

Longitudinal study on the temporal and micro-spatial distribution of Galba truncatula in four farms in Belgium as a base for small-scale risk mapping of Fasciola hepatica

Background

The trematode parasite Fasciola hepatica causes important economic losses in ruminants worldwide. Current spatial distribution models do not provide sufficient detail to support farm-specific control strategies. A technology to reliably assess the spatial distribution of intermediate host snail habitats on farms would be a major step forward to this respect. The aim of this study was to conduct a longitudinal field survey in Flanders (Belgium) to (i) characterise suitable small water bodies (SWB) for Galba truncatula and (ii) describe the population dynamics of G. truncatula.

Methods

Four F. hepatica-infected farms from two distinct agricultural regions were examined for the abundance of G. truncatula from the beginning (April 2012) until the end (November 2012) of the grazing season. Per farm, 12 to 18 SWB were selected for monthly examination, using a 10 m transect analysis. Observations on G. truncatula abundance were coupled with meteorological and (micro-)environmental factors and the within-herd prevalence of F. hepatica using simple comparison or negative binomial regression models.

Results

A total of 54 examined SWB were classified as a pond, ditch, trench, furrow or moist area. G. truncatula abundance was significantly associated with SWB-type, region and total monthly precipitation, but not with monthly temperature. The clear differences in G. truncatula abundance between the 2 studied regions did not result in comparable differences in F. hepatica prevalence in the cattle. Exploration of the relationship of G. truncatula abundance with (micro)-environmental variables revealed a positive association with soil and water pH and the occurrence of Ranunculus sp. and a negative association with mowed pastures, water temperature and presence of reed-like plant species.

Conclusions

Farm-level predictions of G. truncatula risk and subsequent risk for F. hepatica occurrence would require a rainfall, soil type (representing the agricultural region) and SWB layer in a geographic information system. While rainfall and soil type information is easily accessible, the recent advances in very high spatial resolution cameras carried on board of satellites, planes or drones should allow the delineation of SWBs in the future.

Lymnaea glabra: progressive increase in susceptibility to Fasciola hepatica through successive generations of experimentally infected snails

Experimental infections of Lymnaea glabra (two populations) with Fasciola hepatica were carried out during seven successive snail generations, to determine if prevalence and intensity of snail infection increased over time through descendants of snails already infected with F. hepatica. Controls were descendants coming from uninfected parents and infected according to the same protocol. No larval forms were found in the bodies of control snails coming from uninfected parents. In contrast, prevalence and intensity of F. hepatica infection in snails originating from infected parents progressively increased from the F2 or F3 to the F6 generation of L. glabra. In another experiment carried out with the F7 generations of L. glabra and a single generation of Galba truncatula (as controls), the prevalence of F. hepatica infection and the total number of cercariae were lower in L. glabra (without significant differences between both populations). If the number of cercariae shed by infected snails was compared to overall cercarial production noted in snails containing cercariae but dying without emission, the percentage was greater in G. truncatula (69% instead of 52-54% in L. glabra). Even if most characteristics of F. hepatica infection were lower in L. glabra, prevalence and intensity of parasite infection increased with snail generation when tested snails came from infected parents. This mode of snail infection with F. hepatica suggests an explanation for cases of fasciolosis occurring in cattle-breeding farms where paramphistomosis is lacking and G. truncatula is absent.

Lymnaea palustris and Lymnaea fuscus are potential but uncommon intermediate hosts of Fasciola hepatica in Sweden

Background

Lymnaea palustris and L. fuscus are members of the European stagnicolines (Gastropoda: Lymnaeidae). The role of stagnicolines in transmission of Fasciola hepatica has been often proposed. To assess the possible relationship between these two stagnicolines and F. hepatica in Sweden, field monitoring in parallel with experimental infections of L. palustris and L. fuscus were conducted.

Methods

Stagnicoline snails were collected and identified on pastures grazed by either sheep or cattle on four farms suffering from fasciolosis in Sweden during 2011–2012. Field-collected L. palustris and L. fuscus were examined for F. hepatica DNA by PCR. In the laboratory, different age groups of L. palustris, L. fuscus and G. truncatula were each exposed to two F. hepatica miracidia and main infection characteristics were obtained.

Results

One field-collected L. palustris (out of n = 668) contained F. hepatica as determined by PCR. On the other hand, stagnicolines artificially exposed to F. hepatica miracidia resulted in successful infection with fully differentiated cercariae, but only in juvenile snails (size, 1–2 mm at exposure) and with a prevalence of 51% and 13% in L. palustris and L. fuscus, respectively. In contrast, 90% of juvenile (size, 1–2 mm) and 92% of preadult G. truncatula (size, ≥ 2-4 mm), respectively, were successfully infected. Delayed, reduced and/or no spontaneous cercarial shedding was observed in the two stagnicolines when compared to G. truncatula. However, at snail dissection most cercariae from L. fuscus and L. palustris were able to encyst similarly to those from G. truncatula.

Conclusion

Both L. fuscus and L. palustris can sustain larval development of F. hepatica but with an apparent level of age resistance. The finding of a single F. hepatica positive specimen of L. palustris, together with infection characteristics from the experimental infection, suggest that L. palustris is a more suitable snail vector of F. hepatica than L. fuscus. The reduced growth observed in both stagnicolines was contrary to the ‘parasitic gigantism’ theory. Overall, it seems that the epidemiological role of L. palustris in transmission of F. hepatica in Sweden is likely to be much lower than for G. truncatula.

Natural infection ofFasciola hepatica(Trematoda: Fasciolidae) inBulinus truncatus(Gastropoda: Planorbidae) in northern Tunisia

Monthly samples ofBulinus truncatuswere collected during a year from a cattle-breeding farm located in the region of Sejnane (North Tunisia) to detect natural infections withFasciola hepaticaand determine seasonal variations of the prevalence throughout a year. Of the 163 adult bulinids, larval forms ofF. hepaticawere found in 39% of snails. Two peaks in prevalence, the first in June and the second in October, were also noted.Bulinus truncatuscan be added to the list of potential intermediate hosts ofF. hepatica.

Evidence of Fasciola hepatica infection in Radix peregra and a mollusc of the family Succineidae in Ireland

Worldwide molluscs of the genera Lymnaea, Pseudosuccinea, Galba and Stagnicola act as intermediate hosts of the common liver fluke, Fasciola hepatica. Galba truncatula is the only recorded intermediate host of F. hepatica within Ireland and is the principle intermediate host within Europe. To investigate the transmission of liver fluke on the Teagasc hill sheep farm, Co. Mayo, Ireland, snail species other than G. truncatula, were collected from the lowland pasture (snail sp. 'A') in May (n=23) and from the hill area (snail sp. 'B') in March, April and May 2008 (n=78, 53 and 36 respectively). Morphological characteristics identified snail sp. 'A' as a Succinea sp. and snail sp. 'B' as Radix peregra. PCR and subsequent sequencing of the 18S gene and internal transcribed spacer 2 (ITS-2) region of the snail rDNA were used for molecular identification. The 18S ribosomal DNA (rDNA) sequences revealed a 99% similarity with Omalonyx matheroni (Succineidae) for snail sp. 'A', and for snail sp. 'B' a 99% and 98% similarity with Radix auricularia and R. peregra, respectively. The ITS-2 sequences of snail sp. 'B' revealed a 100% similarity with R. peregra. Using PCR F. hepatica DNA was identified in 73.9% (n=17) of snail sp. 'A' collected during May and 10.3% (n=8), 60.4% (n=32) and 61.1% (n=22) of snail sp. 'B' collected during March, April and May, respectively. A priori knowledge of habitat preference and environmental tolerances of G. truncatula has failed to explain the prevalence of infection recorded in the hill sheep flock on Teagasc hill sheep farm. These findings may explain the presence of infection in sheep populations grazing hill habitats considered too acidic to maintain G. truncatula populations.

Pseudosuccinea columella (Say 1817) (Gastropoda, Lymnaeidae), snail host of Fasciola hepatica: first record for France in the wild

The lymnaeid snail Pseudosuccinea columella has shown strong invasive capabilities in the last decades, and this species has now a worldwide distribution. So far, the presence of this snail in Europe was restricted to botanical gardens, but the recent discovery of a few specimens along the banks of the Lot River, southwestern France is the first record of this species in the wild. The first generation of this P. columella isolate obtained in the laboratory was used for parasitological tests with a French Fasciola hepatica sample. Experimental infections showed that 100% of snails (n=26) developed infection when exposed to F. hepatica miracidia. Consequences for the epidemiology of F. hepatica transmission in France are discussed.

Prevalence and intensity of infections in the lymnaeid snail Omphiscola glabra experimentally infected with Fasciola hepatica, Fascioloides magna and Paramphistomum daubneyi

Single and double infections of juvenile Omphiscola glabra (Gastropoda: Lymnaeidae) with Paramphistomum daubneyi and/or Fasciola hepatica were carried out to determine the redial burden and cercarial production in snails dissected at day 60 or at day 75 post-exposure (p.e.) in the laboratory at 20 degrees C. The results were compared with those obtained with single-miracidium infections by Fascioloides magna. Compared to F. hepatica, low values were noted at day 75 p.e. for the prevalence of snail infections with P. daubneyi (4.6-8.3% instead of 23.6-25.9%), the total number of free rediae (10.7-17.9 per snail instead of 26.3-34.7), and that of free cercariae (112.8-136.9 per snail instead of 177.8-248.5). Despite a greater number of free rediae at day 75 p.e. (36.2-45.6 per snail), the prevalences of snail infections with F. magna and cercarial production were similar to those noted for F. hepatica. The results concerning F. hepatica and P. daubneyi might partly be explained by a progressive adaptation of O. glabra to sustain the larval development of these digeneans over the years, as this snail is a natural intermediate host of F. hepatica and P. daubneyi in central France since 1995. Compared with the high number of fully-grown rediae of F. magna in O. glabra, cercarial production seemed limited and this might be explained by the presence of high numbers of rediae which reduced the avaibility of nutrients for cercarial differentiation within the snail.

Comparison between shell morphology and genetic diversity in two sympatric lymnaeid snails, vectors of fasciolosis

Species determination of freshwater snails is particularly important in the case of disease vectors. In central France, for example, it has been shown that Galba truncatula (Müller, 1774) and Omphiscola glabra (Müller, 1774), which can be found in sympatry, are intermediate hosts for fasciolosis. Although these two species are distinguishable based on their shell morphology, some snails present in sympatric zones possess shell characteristics that are common to both species, suggesting possible gene flow. To test this hypothesis, we carried out multilocus enzyme electrophoreses on individuals sampled in one zone of sympatry. Our results clearly show that all snails with shell characteristics common to both species are O. glabra. We also note an absence of hybridization between G. truncatula and O. glabra in the study area. Remarkably, we observe a total monomorphism at the six studied loci among individuals of G. truncatula. Similarly, we find a total lack of heterozygotes in the O. glabra sample (with classic or intermediate shell characteristics). These results suggest a predominance of selfing in the reproductive mode of both species. Our results suggest that conchological characteristics can provide information about the taxonomic position of lymnaeids. However, they are insufficient to confidently differentiate species.

Fasciola hepatica: an unusual case of adaptation to a Moroccan population of Galba truncatula

Parent snails, F1, and F2, originating from a Moroccan population of Galba truncatula, were experimentally subjected to sympatric miracidia of Fasciola hepatica at 20 degrees C to determine the mode of adaptation of this parasite to the local intermediate host via the study of redial burdens and by following cercarial shedding. In spite of an increase in the global prevalences of infections, the frequencies recorded for immature or mature infections in dissected snails did not show any significant variations from parents to F2. If the redial burdens in mature infections were mainly composed of first-generation rediae in parent snails, the numbers of rediae belonging to subsequent generations significantly increased in the F1 and F2 generations. In mature infections, the lengths of first-generation rediae and their contents significantly decreased from parent snails to F2. Conversely, in the first redial cohort of the second generation (R2a rediae), only the intraredial contents increased significantly with snail generation. In mature infections, daughter redia production was ensured by all mother rediae of the first generation in parent snails, whereas it was progressively effected by the first mother redia in the F1 and F2 infections. The numbers of cercariae shed by snails increased significantly from parents to F2. If the cercariae produced by first-generation rediae decreased with snail generation, an inverse finding was noted in the case of R2a rediae. We suggest that snails impose a bottleneck on the development of the first mother redia, so that other mother rediae of the first generation react with greater growth (until 3.9 mm in parent snails), an important daughter redia production and a high cercarial production. As the first mother redia remained alive until day 49 post-exposure but without growth after week 2, this mode of development must be considered as a new developmental pattern of redial generations, and it is useful to determine its frequency in snails, as it has never been found in naturally or experimentally infected snails in central France.

The contamination of wild watercress with Fasciola hepatica in central France depends on the ability of several lymnaeid snails to migrate upstream towards the beds

As most natural watercress beds in central France are located upstream of the permanent habitats of two lymnaeid species, Galba truncatula and Omphiscola glabra, field investigations were made from 1999 to 2004 on 67 beds to determine why the contamination of watercress with Fasciola hepatica is irregular over time in these sites, while definitive hosts, especially lagomorphs, are regularly found infected around them. Snails are able to migrate upstream in winter and spring towards the beds, and a 4-year survey demonstrated the existence of annual variation in the colonization of these sites by snails. In the 45 beds irregularly contaminated with F. hepatica over time, 37.7-62.2%, according to the year, were not populated by lymnaeids, in spite of the presence of snail populations downstream. G. truncatula was found alone in 8.8-13.3% of sites and was the first colonizing snail in 24.3-33.3% when the two lymnaeid species successively settled in these waterholes. The colonizing ability of O. glabra was more limited, as it was observed alone in 2.2% of beds and was the first colonizing snail in only 2.2-20% of them. The distances covered by these snails significantly increased with increasing migration time. After migration, a few overwintering snails (3.8% of G. truncatula and 6.8% of O. glabra) are able to colonize the beds, but their numbers decreased when the distance of migration was greater. O. glabra migrated more quickly and reached more watercress beds than G. truncatula. However, this did not influence the natural infections of snails, which were more frequent in the F1 of G. truncatula. The variability observed in the colonization of beds by snails might explain the regular or irregular contamination of wild watercress by metacercariae of F. hepatica. One of the factors which may explain this variability is the occurrence of showers in spring, so that a few snails are able to reach watercress beds after their upstream migration.

Fasciola hepatica: epidemiological surveillance of natural watercress beds in central France

A total of 59 natural watercress beds in the Limousin region (central France) was surveyed over a 15-year period (1990-2004) to detect the contamination of watercress by the metacercariae of Fasciola hepatica and to determine the presence of larval forms in the two species of lymnaeids which live in these waterholes in June and July. The number of beds contaminated with F. hepatica metacercariae varied over the years, and the burden of the larvae on plants was low: a mean of 2.6-6.3 per bed. The same variability was also noted for natural infections of Galba truncatula with F. hepatica, as the annual prevalences ranged from 1.2% to 2.4%. Natural infections of Omphiscola glabra with F. hepatica were only detected from 1996 and the annual prevalences subsequently increased up to 1.4-1.8% between 2001 and 2004. However, for both lymnaeids, the variations in these prevalences with year were insignificant. The contamination of these beds with F. hepatica over the past 15 years was similar to that recorded in the same sites between 1970 and 1986. The main changes were the appearance of another digenea, Paramphistomum daubneyi, in the beds, and the possibility for O. glabra to naturally sustain the larval development of F. hepatica.