Optimization of metacercarial production for three digenean species by the use of petri dishes for raising lettuce-fed Galba truncatula

A new method for laboratory rearing of Galba truncatula, the intermediate host of Fasciola hepatica

In this study a relatively large and open top aquarium was designed, constructed and introduced as a suitable habitat for nutrition, growth and development as well as for egg laying and breeding of Galba truncatula under laboratory conditions. The soil and water used in the aquarium were collected from the locality in which the snails were collected. The aquarium was placed in a laboratory with temperature of 18-32 ºC and relative humidity of 22-37% respectively, according to the season. The artificial light was controlled by a light timer, giving 12 h of light and 12 h of darkness. The snails were fed with dried lettuce leaves, Cyperus alternifolius (aquatic plant), Spirulina (algae), Orthotrichum rupestre (moss) and cuttlebone (a supplementary source of calcium). Approximately five weeks after the start of study, there was evidence of reproduction and success in rearing of G. truncatula by the appearance of eggs and small snails (0.1-0.5 mm) in the aquarium. In conclusion, large scale laboratory rearing of G. truncatula is a feasible task. The method may be improved by balancing the temperature and relative humidity as well as by optimizing the soil type, the water quality and the type of food.

Transmission of Calicophoron daubneyi and Fasciola hepatica in Galicia (Spain): Temporal follow-up in the intermediate and definitive hosts

Background

Paramphistomosis caused by Calicophoron daubneyi and fasciolosis caused by Fasciola hepatica are common parasitic diseases of livestock animals. Transmission of the diseases depends on the presence of intermediate hosts, i.e. freshwater gastropods such as lymnaeids. We carried out a 2-year-long study of the dynamics of the snail population acting as the intermediate host for these parasites, considering the population structure in terms of size/age and infection status. In addition, we determined the kinetics of trematode egg excretion in grazing cows. Generalized Additive Models (GAMs) were used to analyze the associations between different response variables and snail size, sampling month and weather-related variables.

Results

Of the molluscan species examined, Galba truncatula, Radix peregra, Anisus (Anisus) leucostoma and Pisidium casertanum (n = 2802), only G. truncatula was infected with C. daubneyi or F. hepatica, at prevalence rates of 8.2% and 4.4% respectively. The probability of infection with C. daubneyi or F. hepatica was linearly related to snail size, although in different ways (negative for C. daubneyi and positive for F. hepatica). The total snail population increased in winter, when specimens of all size classes were found. Infected snails were more abundant during spring-autumn. Mature cercariae of both parasites were found in most seasons. In the statistical models, the sampling month accounted for a high percentage (71.9–78.2%) of the observed variability in snail abundance. The inclusion of climatic variables in the models moderately increased the percentage of deviance explained (77.7–91.9%). Excretion of C. daubneyi eggs in cow faeces was always higher than that of F. hepatica eggs.

Conclusions

Particular care should be taken to prevent pastures and the surrounding environment being contaminated with parasite eggs during winter-spring, when the number of snails susceptible to miracidial infections is maximal. This is therefore the optimal time for treating grazing animals. Nevertheless, control of trematodosis based only on chemotherapy is difficult in an area such as the study area, where environmental factors favour the regular appearance of snail populations harbouring mature cercariae.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1892-8) contains supplementary material, which is available to authorized users.

Effect of Fascioloides magna (Digenea) on fecundity, shell height, and survival rate of Pseudosuccinea columella (Lymnaeidae)

Infection with Fascioloides magna (Digenea) causes serious damage to liver tissue in definitive hosts represented by ruminants, especially cervids. The distribution of F. magna includes the indigenous areas in North America, and the areas to which F. magna was introduced-Central Europe, Southeast Europe, and Italy. The North American intermediate host of F. magna, the freshwater snail Pseudosuccinea columella (Lymnaeidae), is an invasive species recorded in South America, the Caribbean, Africa, Australia, and west and Southeast Europe. In Europe, Galba truncatula is the snail serving for transmission, but P. columella has potential to become here a new intermediate host of F. magna. Little is known about interactions between F. magna and P. columella. In this study, the susceptibility of P. columella (Oregon, USA) to the infection by a single miracidium of the Czech strain of F. magna and the influence of F. magna on snail fecundity, shell height, and survival were evaluated. The data show that the Oregon strain of P. columella is a highly suitable host for the Czech strain of F. magna, with the infection rate of 74 %. In addition, a negative effect on survival rate of infected snails was recorded only in the late phase of infection. The infection was accompanied by a major reduction in egg mass production and by a decrease in the number of eggs per egg mass. The shell height of infected snails did not significantly differ from that in unexposed controls.

Fasciola hepatica: comparative metacercarial productions in experimentally-infected Galba truncatula and Pseudosuccinea columella

As large numbers of metacercariae of Fasciola hepatica are necessary for research, experimental infections of Galba truncatula and Pseudosuccinea columella with this digenean were carried out to determine the better intermediate host for metacercarial production and, consequently, the most profitable snail for decreasing the cost price of these larvae. Pre-adult snails (4 mm in shell height) originating from two populations per lymnaeid species were individually exposed to two or five miracidia, raised at 23 °C and followed for cercarial shedding up to their death. Compared to values noted in G. truncatula, the survival of P. columella on day 30 post-exposure was significantly greater, while the prevalence of F. hepatica infection was significantly lower. In the four P. columella groups, metacercarial production was significantly greater than that noted in the four groups of G. truncatula (347-453 per cercariae-shedding snail versus 163-275, respectively). Apart from one population of G. truncatula, the use of five miracidia per snail at exposure significantly increased the prevalence of F. hepatica in P. columella and the other population of G. truncatula, whereas it did not have any clear effect on the mean number of metacercariae. The use of P. columella for experimental infections with F. hepatica resulted in significantly higher metacercarial production than that noted with G. truncatula, in spite of a lower prevalence for the former lymnaeid. This finding allows for a significant decrease in the cost price of these larvae for commercial production.

Life history and biology of Fascioloides magna (Trematoda) and its native and exotic hosts

Host-parasite interactions are model systems in a wide range of ecological and evolutionary fields and may be utilized for testing numerous theories and hypotheses in terms of both applied and fundamental research. For instance, they are important in terms of studying coevolutionary arms races, species invasions, and in economic terms the health of livestock and humans. Here, I present a comprehensive description of the life history, biogeography, and biology of the giant liver fluke, Fascioloides magna, and both its intermediate and definitive hosts. F. magna is native to North America where it uses several species of freshwater snails (Lymnaeidae) as intermediate hosts and four main species of ungulates as definitive hosts. The fluke has also been introduced into parts of Europe where it is now established in two lymnaeid snail species and three ungulate species. This study gives a comprehensive description of different developmental stages of the fluke in its two host classes, as well as detailed notes on historical and present distributions of F. magna in North America and Europe as well as in its snail and deer hosts (with range maps provided). Aberrant and dead-end hosts are also discussed in detail, and descriptive phylogenies are provided for all of the organisms. I briefly discuss how F. magna represents a model example of multiple-level ecological fitting, a phenomenon not yet described in the empirical literature. Lastly, I explore possible future scenarios for fluke invasion in Europe, where it is currently expanding its range.

Differences in the compatibility of infection between the liver flukes Fascioloides magna and Fasciola hepatica in a Colombian population of the snail Galba sp

Experimental infections of Galba sp. (origin, Colombia) with allopatric isolates of Fasciola hepatica from France or Fascioloides magna from the Czech Republic were carried out during five successive snail generations to determine if this lymnaeid might sustain complete larval development of either parasite. In snails exposed to F. hepatica, 7 of 400 snails harboured several rediae and only two snails contained a small number of free cercariae on day 50 post-exposure. In contrast, the intensity of F. magna infection in Galba sp. progressively increased from the F1 to F5 generations. Spontaneous cercarial shedding of F. magna occurred in 7 of 100 Galba sp. belonging to the F5 generation and the number of shed cercariae did not differ significantly from that noted in control Galba truncatula of French origin. Galba sp. from Colombia can be added to the list of potential intermediate hosts for F. magna.

Adaptation of Lymnaea fuscus and Radix balthica to Fasciola hepatica through the experimental infection of several successive snail generations

BACKGROUND

High prevalence of Fasciola hepatica infection (>70%) was noted during several outbreaks before the 2000s in several French farms where Galba truncatula is lacking. Other lymnaeids such as Lymnaea fuscus, L. glabra and/or Radix balthica are living in meadows around these farms but only juvenile snails can sustain complete larval development of F. hepatica while older snails were resistant. The low prevalence of infection (<20%) and limited cercarial production (<50 cercariae per infected snail) noted with these juveniles could not explain the high values noted in these cattle herds. As paramphistomosis due to Calicophoron daubneyi was not still noted in these farms, the existence of another mode of infection was hypothesized. Experimental infection of several successive generations of L. glabra, originating from eggs laid by their parents already infected with this parasite resulted in a progressive increase in prevalence of snail infection and the number of shed cercariae. The aim of this paper was to determine if this mode of snail infection was specific to L. glabra, or it might occur in other lymnaeid species such as L. fuscus and R. balthica.

METHODS

Five successive generations of L. fuscus and R. balthica were subjected to individual bimiracidial infections in the laboratory. Resulting rediae and cercariae in the first four generations were counted after snail dissection at day 50 p.e. (20°C), while the dynamics of cercarial shedding was followed in the F5 generation.

RESULTS

In the first experiment, prevalence and intensity of F. hepatica infection in snails progressively increased from the F1 (R. balthica) or F2 (L. fuscus) generation. In the second experiment, the prevalence of F. hepatica infection and the number of shed cercariae were significantly lower in L. fuscus and R. balthica (without significant differences between both lymnaeids) than in G. truncatula.

CONCLUSION

The F. hepatica infection of several successive snail generations, coming from parents infected with this parasite, resulted in a progressive increase in prevalence and intensity of snail infection. This may explain high prevalence of fasciolosis noted in several cattle-breeding farms when the common snail host of this digenean, G. truncatula, is lacking.

Fasciola hepatica: development of redial generations in experimental infections of Pseudosuccinea columella

Experimental infections of Egyptian Pseudosuccinea columella with one or two miracidia of Fasciola hepatica per snail were carried out to determine the developmental pattern (normal or abnormal) of redial generations and specify the number of free rediae developing in snails according to their generation. Controls were constituted by a French population of Galba truncatula infected according to the same protocol. Most infected P. columella showed a normal development of redial generations (96.2-98.1 vs 75.5-85.7% for G. truncatula). In each redial category, free rediae were more numerous in P. columella than in G. truncatula, and their number were also greater in the two-miracidia groups than in single-miracidium infections for each lymnaeid considered separately. This increase in redial production was mainly due to the number of first mother (R1a) rediae producing daughter rediae only: 2 per P. columella (vs one redia in G. truncatula) in single-miracidium groups and 3.1 (vs 1.9) in the two-miracidia groups. In P. columella, the mean total number of free rediae developing in single-miracidium and bimiracidial infections was 77.2 and 117.6, respectively (instead of 33.5 and 52.1 rediae in G. truncatula). The number of F. hepatica rediae present in P. columella was related to the number of fully grown sporocysts and the quantity of R1a rediae which developed into the snail body.

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.

Fasciola hepatica: effect of the natural light level on cercarial emergence from temperature-challenged Galba truncatula

As abrupt changes in water temperature (thermal shock) triggered a significantly greater cercarial emergence of Fasciola hepatica from experimentally infected Galba truncatula, laboratory investigations were carried out to study the influence of light on cercarial emergence in snails subjected to a thermal shock every week (a mean of 12 °C for 3 h) during the patent period. Thermal shock for these temperature-challenged (TC) snails was carried out outdoors under artificial or natural light, or indoors under constant artificial light. Compared with the infected control snails always reared indoors at 20 °C, the number of cercariae in TC snails subjected to a thermal shock and natural light outdoors was significantly greater. The repetition of this experiment by subjecting TC snails to the same thermal shock indoors under an artificial light level ranging from 600 to 3000 lux did not show any significant difference among the numbers of cercariae in the different subgroups. A detailed analysis of the results noted in the TC snails subjected to natural light during the thermal shock demonstrated that the number of cercariae-releasing snails was significantly higher between 601 and 1200 lux and for the highest nebulosity values (7–8 octas, which corresponds to a sufficiently or completely overcast sky). Contrary to the intensity of artificial light, which did not influence cercarial emergence, the natural light level had a significant effect on this process when F. hepatica-infected snails were subjected to a regular thermal shock during the patent period.

Fasciola hepatica: variations in redial development and cercarial production in relation to the geographic origin of the parasite

Two hundred Galba truncatula, originating from a French population, were subjected to single-miracidium infections with an allopatric isolate (origin, Argentina) of Fasciola hepatica. The control group was constituted by 100 snails coming from the same population and exposed to sympatric miracidia of F. hepatica according to the same protocol. Snail samples were collected bimonthly from both groups between day 14 and day 112 p.e. (at 20 °C) and snail dissections were performed to count free rediae, intraredial morulae and free cercariae. Third and fourth generation rediae were significantly more numerous in the allopatric group, while the number of first generation rediae was significantly lower. In the sympatric group, the decrease in the number of intraredial morulae in the first, second and third redial generations was significantly faster. Free cercariae within the snail body were significantly more numerous in the sympatric than in the allopatric groups, whatever the date of snail dissection. The changes in redial development and cercarial production noted in the Argentinean group might be due to the evolution of South American flukes in a divergent way after the introduction of foreign infected ruminants in this continent from the 15th century.

Growth rate of the intermediate snail host Galba truncatula influences redial development of the trematode Fascioloides magna

A total of 850 pre-adult Galba truncatula (shell height, 4 mm), originating from four French snail populations differing in shell height at the adult stage (from 6.5 to 12 mm), were individually subjected at 20°C to single-miracidium infections with Fascioloides magna. At day 75 post-exposure, the surviving snails were dissected, and rediae and cercariae were counted. Snail groups differed in shell growth during the experiment: from 1.8 ± 0.4 mm in group A up to 4.0 ± 1.1 mm in group D. The prevalence of F. magna infection, the numbers of free rediae and cercariae significantly increased together with increasing growth of infected snails during the experiment. Group A produced 1–6 first-generation rediae per snail and the mean daughter redia production ranged from 7.5 second-generation rediae (when a single first generation per snail developed) to 2.3 (6 first-generation rediae per snail). In contrast, up to ten first-generation rediae were noted in group D, and each mother redia gave daughter rediae with averages ranging from 1.5 (ten first-generation rediae per snail) to 13 (a single first generation per snail). In conclusion, the development of F. magna in G. truncatula exhibited both inter- and intrapopulation variability, where the development of rediae and cercariae was positively correlated with snail growth.

Consequence of temperature changes on cercarial shedding from Galba truncatula infected with Fasciola hepatica or Paramphistomum daubneyi

Experimental infections of Galba truncatula (two populations) with Fasciola hepatica or Paramphistomum daubneyi were carried out to study the effect of water temperature changes (3 h at a mean of 12 °C every week) on cercarial shedding during the patent period. The results were compared with those of control snails infected according to the same protocol and always maintained at 20 °C. Compared to controls, a significant increase in the number of cercariae-shedding snails, a significantly longer patent period and significantly greater cercarial production were noted in temperature-challenged snails, regardless of the type of digenean infection. In contrast, the number of incompletely formed metacercariae was significantly higher in temperature-challenged snails than in controls. Incompletely formed metacercariae of F. hepatica consisted of cysts whose colour remained whitish after shedding (25.4% for temperature-challenged snails) or whose dome was flattened after encystment (74.6%). Those of P. daubneyi were totally dark brown or blackish after formation. These incomplete metacercariae might originate from young differentiating cercariae within the snail body (F. hepatica) or from cercariae which died just after encystment (P. daubneyi). The use of regular temperature changes for snails infected with F. hepatica or P. daubneyi must be monitored carefully during collection of metacercariae to select completely formed cysts for infecting definitive hosts.

Comparative strategies and success of sympatric and allopatric Fasciola hepatica infecting Galba truncatula of different susceptibilities

Allopatric infections of French Galba truncatula with an Argentinean isolate of Fasciola hepatica were carried out to determine the infectivity of foreign miracidia in three snail populations differing by their susceptibility to French miracidia (two highly and one poorly susceptible populations). Sympatric infections of G. truncatula with French miracidia were used as controls. Compared to sympatric infections of G. truncatula, snail survival at day 30 post-infection in allopatric groups was significantly lower in a highly susceptible population and significantly greater in the other two. Prevalence in snails infected with the allopatric isolate was significantly lower (16.4-34.5 % instead of 58.6-72.1 %), whereas their patent period was significantly longer (a mean of 69.9-85.9 days instead of 6.4-20.7 days). The mean number of metacercariae was also higher in allopatric groups (236.5-897.3 per cercariae-shedding snail instead of 70.7-222.1). Owing to longer patent periods, the Argentinean isolate of F. hepatica was less pathogenic for these snails. The lower prevalence of infection, the longer patent period and the higher number of metacercariae noted in allopatric groups might be the consequence of an adaptive mechanism used by this digenean introduced to the New World to infect new populations of unusual intermediate hosts.

Local adaptation of the trematode Fasciola hepatica to the snail Galba truncatula

Experimental infections of six riverbank populations of Galba truncatula with Fasciola hepatica were carried out to determine if the poor susceptibility of these populations to this digenean might be due to the scarcity or the absence of natural encounters between these snails and the parasite. The first three populations originated from banks frequented by cattle in the past (riverbank group) whereas the three others were living on islet banks without any known contact with local ruminants (islet group). After their exposure, all snails were placed in their natural habitats from the end of October up to their collection at the beginning of April. Compared to the riverbank group, snails, which died without cercarial shedding clearly predominated in the islet group, while the other infected snails were few in number. Most of these last snails released their cercariae during a single shedding wave. In islet snails dissected after their death, the redial and cercarial burdens were significantly lower than those noted in riverbank G. truncatula. Snails living on these islet banks are thus able to sustain larval development of F. hepatica. The modifications noted in the characteristics of snail infection suggest the existence of an incomplete adaptation between these G. truncatula and the parasite, probably due to the absence of natural contact between host and parasite.

Paramphistomum daubneyi: characteristics of infection in three lymnaeid species

Experimental infections of two South American lymnaeids (Lymnaea neotropica and L. viatrix var. ventricosa) with Paramphistomum daubneyi were carried out to determine if these snail species could sustain larval development of this digenean and, if so, to specify their potential for cercarial production. A French population of Galba truncatula infected and raised according to the same protocol served as controls. In both experiments, prevalence of P. daubneyi infections in snails did not significantly differ from each other. In snail groups evaluated for cercarial shedding (first experiment), a significantly lower number of shed cercariae was noted for L. neotropica, while those from G. truncatula and L. v. ventricosa did not differ significantly from each other. Dissection of infected snails at day 65 post-exposure at 20 °C (second experiment) found significantly lower burdens of P. daubneyi rediae and cercariae in the bodies of L. neotropica than in those of G. truncatula and L. v. ventricosa. Compared to total cercarial production observed in dissected snails, the percentage of cercariae which exited from snails was 75.6% for G. truncatula, 21.6% for L. neotropica, and 91.4% for L. v. ventricosa. This last species seems to be a good candidate for metacercarial production of P. daubneyi.

The production of mammalian trematode infective stages by the snail Galba truncatula

Several experiments on the breeding of trematode-infected Galba truncatula for obtaining and packaging Fasciola hepatica and Paramphistomum daubneyi metacercariae were carried out to determine the more convenient methods to use for commercial production of these infective stages. Compared to the breeding of infected snails in aquaterraria, the use of 14-cm Petri dishes allowed a greater prevalence of snail infection and a higher number of metacercariae. The production of these larvae was still 2.3–3.4 times greater if infected snails were dissected during the patent period. The aspiration of these metacercariae at the extremity of a Pasteur pipette significantly shortens the time necessary for their transfer from Petri dishes to Eppendorf tubes. Using 14-cm Petri dishes, snail dissection and metacercarial aspiration for their transfer strongly reduce the cost price for metacercarial production of the trematodes Fasciola hepatica and Paramphistomum daubneyi.

Lymnaea neotropica and Lymnaea viatrix, potential intermediate hosts for Fascioloides magna

Experimental infections of two South American lymnaeid populations with Fascioloides magna were carried out to determine whether these snails may sustain larval development of this digenean and, if so, to quantify their potential for cercarial production. The reference group was a French population of Galba truncatula infected and raised according to the same protocol. According to the internal transcribed sequence (ITS)-1 segment of their genomic rDNA, these South American populations were identified as Lymnaea neotropica (origin, Argentina) and Lymnaea viatrix var. ventricosa (origin, Uruguay). In the snail groups followed for cercarial shedding, longer prepatent periods and lower numbers of shed cercariae were noted in South American lymnaeids. In other snails dissected at day 65 post-exposure, the redial and cercarial burdens of F. magna found in the bodies of L. neotropica and L. v. ventricosa were significantly lower than those noted in G. truncatula. Compared to the total cercarial production noted in the dissected snails, the percentage of cercariae that exited from snails was 51.3% for G. truncatula, 32.2% for L. neotropica and 46.8% for L. v. ventricosa. The two South American species of snails can thus be considered as potential intermediate hosts of F. magna.

Lymnaea fuscus (Pfeiffer, 1821) as a potential intermediate host of Fascioloides magna in Europe

Experimental infections of two different populations of Lymnaea fuscus in France and Sweden, with a Czech isolate of Fascioloides magna were carried out to determine if this lymnaeid species enables parasite larval development. Species identification of both snail populations was performed using the morphology of the copulatory organ, and also confirmed by sequencing of the internal transcribed spacer 2 (ITS2) region of the snail genomic rDNA. Only juvenile snails measuring less than 3mm (1-3 weeks of age) were successfully infected (the viable cercariae were recorded) and infection prevalence decreased with age, as documented by increased shell height. In both French and Swedish L. fuscus populations, prevalence ranged between 1.1% and 58.8%. The mean number of metacercariae obtained from cercariae-shedding snails was 13.7 (±11.4), while the total cercarial production noted in snails dissected at day 85 post-exposure was 147.5 (±56.6). Compared to uninfected control snails, we observed reduced growth of infected snails. Despite age-related resistance of snail to the parasite, and limited cercarial production in these experimentally infected snails, F. magna was still able to complete larval development in L. fuscus.

Intermediate snail hosts of French Fasciola hepatica: Lymnaea neotropica and Lymnaea viatrix are better hosts than local Galba truncatula

Allopatric and sympatric infections of Lymnaea neotropica and Lymnaea viatrix var. ventricosa with Argentinean and French isolates of Fasciola hepatica were carried out to determine the capacity of these snails to produce metacercariae and to verify if this capacity changed with snail generation. The same process was also made with a French population of Galba truncatula known to be highly susceptible to French isolates of the parasite. In each lymnaeid species separately considered, the survival rate at day 30 post-exposure and prevalence of F. hepatica infection in the group infected with Argentinean miracidia were significantly greater than those recorded in the corresponding French one. Compared to infected G. truncatula, both South American lymnaeids had longer patent periods and produced a higher number of metacercariae. The highest infections were noted with L. v. ventricosa. In the three snail species, metacercarial production was more important with the Argentinean isolate of miracidia than with the French one. If three successive generations of L. v. ventricosa are exposed to the same French isolate of miracidia, cercarial production significantly increased from parents to the F2 generation, while the other characteristics of infection only showed insignificant variations. L. neotropica and L. v. ventricosa are better intermediate hosts for French F. hepatica than local G. truncatula. The numerical increase of shed cercariae in the F1 and F2 generations of L. v. ventricosa demonstrates a rapid adaptation of this species to the French isolate of the parasite.

Recovery of Fascioloides magna (Digenea) population in spite of treatment programme? Screening of Galba truncatula (Gastropoda, Lymnaeidae) from Lower Austria

During the past decade, Fascioloides magna, the large American liver fluke, has spread within free-living deer in wetlands of the Danube in Lower Austria. The aim of this study was to determine the current infection rates with F. magna and other digenean parasites in the intermediate host snail Galba truncatula from risk areas in Lower Austria. A total of 3444 G. truncatula were collected and examined microscopically for the presence of digenean trematodes. A set of randomly selected snails and isolated trematode stages were also investigated molecular biologically by PCR and sequencing. Digenean parasites were detected with a prevalence of 2.41% (1.83% Paramphistomoidea; 0.46% Echinostomatoidea; 0.09% Strigeida; 0.06% Plagiorchiida). F. magna was found with an overall prevalence of 0.23%, which may indicate a recovery of the parasite population in spite of an ongoing triclabendazole treatment programme. Moreover, high risk areas and a seasonality of infections were observed.

The development of rediae ofFasciola hepaticainRadix natalensissubjected twice to bimiracidial exposures

Experimental infections of EgyptianRadix natalensiswith a French isolate ofFasciola hepatica(each snail was subjected twice to a bimiracidial exposure) were carried out to determine how many sporocysts grew in these snails and to study the developmental patterns of redial generations. Single-sporocyst infections were found in 69.3% (34/49) of infected snails, with equivalent numbers of normal and abnormal patterns. Snails with two- and three-sporocyst infections were 24.4% and 6.1%, respectively. In single- and two-sporocyst infections at days 42 and 56 post-exposure, the total redial burden was significantly higher in snails with a normal redial development. In two- and three-sporocyst infections, the overall maturity of rediae was delayed at days 42 and 56. The high frequency of abnormal patterns inR. natalensis(53.1% of all infected snails showed degeneration of a first mother redia) might be due to incomplete adaptation between the snail population and the parasite. The delayed redial maturity in two- and three-sporocyst infections can mainly be explained by the volume of the snail body, which would be insufficient to allow the simultaneous differentiation of most rediae over time.

Characteristics of Fasciola hepatica infections in Galba truncatula originating from riverbank populations

Four geographical strains of Galba truncatula living on riverbanks (the first on a sedimentary soil and the other three on an acid soil) were subjected to bimiracidial exposures with Fasciola hepatica to study their aptitude for cercarial shedding and to count metacercariae in snails dissected at day 42 post-exposure. All snails were reared in 14-cm Petri dishes at 24 °C, with the same spring water (60-73 mg/l of Ca²+) and the same diet (grass and lettuce leaves). Metacercariae of F. hepatica were noted in the four populations after a cercarial shedding or after snail dissection. However, in spite of the breeding method used, the characteristics of snail infections varied with the origin of each geographical strain. For example, the shell heights of infected snails at day 42 were close to those found for the corresponding adults in the field (6.8-8.0 mm for the population living on the sedimentary soil, but only 4.6-5.5 mm for another strain originating from the acid soil). This variability may be explained by assuming that the diet of these riverbank G. truncatula would be different from that of snails living in swampy meadows. However, another hypothesis based on the influence of snail habitat on the characteristics of snail life cannot be excluded.

Total cercarial output in two populations of Galba truncatula experimentally infected with Fasciola hepatica

Experimental infections of pre-adult Galba truncatula (two populations) using one, two, three or five miracidia of Fasciola hepatica per snail were carried out to determine the total number of cercariae produced by snails when they were sacrificed at day 70 post-exposure (at 24 degrees C). When the number of miracidia used for each snail increased at exposure, significant numerical augmentations of live rediae and of intraredial differentiating cercariae were noted. In contrast, only insignificant differences between mean numbers of free cercariae were found. The number of metacercariae produced by each snail (after shedding and/or after dissection) increased in infections with up to three miracidia per snail in each population and strongly decreased in infections with five miracidia per snail. Compared to the other types of infections, the delay of cercarial differentiation noted in the five-miracidia groups was probably due to the volume of the snail host, which would be too small to assure the complete development of five sporocysts.

Paramphistomum daubneyi: production dynamics and infectivity of metacercariae originating from snails dissected at regular intervals

Experimental infections of Galba truncatula with Paramphistomum daubneyi were carried out at 24 degrees C to study the dynamics of larval development in snails dissected at regular intervals and to determine if metacercarial production might be improved. When the shell height of snails (4, 5, 6 or 7 mm) at exposure increased (experiment A), the total number of metacercariae was significantly higher in the 6- and 7-mm snails than in the other two groups, and the differentiation period was shortened (the first cercariae encysted at day 35 post-exposure (p.e.) instead of day 40 in the 4- and 5-mm groups). When the number of miracidia (two, three or five) for each 6-mm high G. truncatula increased (experiment B), a significant decrease of snail survival at day 30 p.e., a significant augmentation of prevalence, and a significant increase of metacercarial production were noted. In the two- and three-miracidium groups, the number of metacercariae was close to that found in the 6-mm snails from experiment 1, whereas they showed slower growth from day 45 to day 65 in the five-miracidium group. In the two groups of lambs infected with metacercariae encysted at days 45 or 60 p.e., no difference in the numbers of adult worms was noted. In contrast, in the case of 35-day encysted larvae, the number of adult worms was clearly lower. Snail dissection allowed higher metacercarial production, a saving of 12-15 days at 24 degrees C to obtain these larvae, and a substantial decrease of their cost price for commercial production.

Fasciola hepatica: the infectivity of cattle-origin miracidia had increased over the past years in central France

A retrospective study on experimental infections of Galba truncatula, originating from four populations, was carried out to determine the infectivity of Fasciola hepatica miracidia in snails either subjected to single-miracidium or bimiracidial exposures in 2006 and to compare it with results found before 1990 in other experiments using the same protocol. In single-miracidium infections, the prevalence of snail infection was significantly greater in 2006 than in experiments performed before 1980, while snail survival at day 30 postexposure showed insignificant differences. In bimiracidial infections, snail survival in two snail groups was significantly lower in 2006 than in experiments performed from 1981 to 1990, while insignificant differences were found for the other two populations of G. truncatula. Compared to results found between 1981 and 1990, the prevalence of snail infection did not significantly differ in 2006. In each population considered alone, the mean number of cercariae shed by infected snails did not significantly vary, whatever the date of experiment and the number of miracidia used for each exposure. The greater infectivity of F. hepatica miracidia towards snails might be the consequence of specific molecules such as triclabendazole used since 1990 to treat fasciolosis in cattle from central France.

The populations of Galba truncatula, known for their natural infections with Haplometra cylindracea (Digenea, Plagiorchioidea), are better intermediate hosts for metacercarial production of Fasciola hepatica

Laboratory investigations on Galba truncatula were carried out to determine if snails coming from four populations known to be natural intermediate hosts of Haplometra cylindracea, a digenean species of frogs, would not be better hosts for experimental infections with Fasciola hepatica than those originating from two communities in which H. cylindracea was never found in the past years. Uninfected G. truncatula were used to constitute six groups of snails (one per population) before being subjected to individual monomiracidial exposures with F. hepatica. Insignificant differences between mean values were noted for snail survival at day 30 post-exposure, prevalence of snail infection with F. hepatica, and prepatent period. In contrast, the duration of cercarial shedding and the number of F. hepatica cercariae in the four groups known for natural infections with H. cylindracea were significantly greater. The use of these last lymnaeid populations for experimental infections with F. hepatica allows to have high numbers of cercariae because of long patent periods and, as a consequence, strongly reduces the cost price of these larvae for scientific purposes and/or commercial production.