Fasciola hepatica: characteristics of infection in Lymnaea truncatula in relation to the number of miracidia at exposure.

Experimental infections of Lymnaea truncatula by Fasciola hepatica were carried out in three snail populations to determine whether the number of miracidia used for each snail at exposure (1, 2, 5, 10, or 20 per snail) had any influence on the characteristics of Fasciola infection and metacercarial production. The number of miracidia had a significant influence on snail survival at day 30 postexposure and the frequency of infected L. truncatula that died without shedding (NCS snails). The frequency of NCS snails, the growth of cercaria-shedding snails throughout the experiment, the time between exposure and the first cercarial shedding, the duration of shedding, and the number of metacercariae were independent of the number of miracidia used for each snail. The highest metacercaria productivity for each miracidium was found in single-miracidium infections. Single-miracidium infections were the most effective, as the mean number of cercariae was the same as in other groups, whereas their survival rate was much higher.     

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

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

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.     

The stress of Lymnaea truncatula just before miracidial exposure with Fasciola hepatica increased the prevalence of infection.

Single-miracidium infections of Lymnaea truncatula with Fasciola hepatica were carried out under laboratory conditions to determine whether the stress of snails just before miracidial exposure had any influence on the prevalence of Fasciola infection, redial burden, and cercarial shedding. Three methods, i.e., the fasting of L. truncatula for 3 days in water filtered through a Millipore membrane, the effect of 6-8 degrees C water for 15 min, or the immersion of L. truncatula in a detergent solution at low concentration for 15 min, were used to stress snails. Enhanced susceptibility of snails to F. hepatica infection was noted in stressed groups (93-96% vs 48-50% in controls). The number of free rediae did not show any variation in controls as well as in stressed groups, except for fasted snails in which free rediae were significantly fewer. No differences in cercarial production between controls and the cold group were noted. Fasting, cold shock, or detergent exposure prior to exposure to F. hepatica miracidia might have weakened the snails so that they were not as efficient in avoiding miracidial penetration, thus leading to higher infection rates.     

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.     

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

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

The experimental production of Fasciola hepatica metacercariae from three aquatic populations of Galba truncatula

Laboratory investigations on three aquatic populations of Galba truncatula, originating from the Peruvian Altiplano and French Massif Central, were carried out during three successive snail generations to determine if these populations might be successfully used for the metacercarial production of Fasciola hepatica under experimental conditions. High numbers of surviving snails at day 30 post-exposure (>70%), high prevalences of F. hepatica infections (>60%), and prolonged productions of cercariae for a mean period of 35 to 47 days were observed in the three populations, whatever the snail generation. In the Peruvian population, metacercariae of F. hepatica significantly decreased in numbers from a mean of 251 in the parent snails to 124 per snail in the F2 generation, whereas no significant variation was observed in the two French populations. As these aquatic snails rarely emerged out of water, the use of these populations for the commercial production of F. hepatica metacercariae was of great interest, because the daily time spent watching the breeding boxes of snails was clearly shorter, thereby reducing the cost of producing metacercariae compared with using amphibious snails reared with romaine lettuce.     

The susceptibility of Lymnaea fuscus to experimental infection with Fasciola hepatica

Three experiments on the infection of Lymnaea fuscus with Fasciola hepatica were carried out to determine if successful infections and maturation of the parasite were dependent on the size of snails at miracidial exposure. The first experiment was performed using 1-4-mm-high snails from 2 populations of L. fuscus and 1 population of Lymnaea palustris. In these snails each subjected to a single bimiracidial exposure, the prevalence of F. hepatica infection at day 35 postexposure ranged from 20.3% to 46.2% in snails measuring 1 mm in height at exposure; it was lower in the 2-mm snails and was 0 in higher size classes. The second experiment was performed by subjecting 1- and 4-mm L. fuscus to 1, 2, and 3 bimiracidial exposures. The prevalence of F. hepatica infection at day 35 postexposure was maximum in the 1-mm snails exposed once to miracidia and decreased with increasing number of exposures. The results were negative in 4-mm snails. Cercarial shedding of F. hepatica was studied in the third experiment using 1- and 2-mm L. fuscus each subjected to a single bimiracidial exposure. The total number of cercariae released from these snails was less than 50. From these results, it can be concluded that L. fuscus showed a partial resistance to F. hepatica infection due to snail age.     

Cercarial productivity of redial generations in single-miracidium infections of Lymnaea truncatula with Paramphistomum daubneyi or Fasciola hepatica

Single-miracidium infections of Lymnaea truncatula with Paramphistomum daubneyi or with Fasciola hepatica were carried out under laboratory conditions to count free rediae, their germinal embryos, and to determine the cercarial productivity of each redial generation. In snails infected by P. daubneyi, the cercariae were produced by the first (8.7 cercariae per redia) and second (8.9 per redia) generations. At day 63 post-exposure, they corresponded, respectively, to 53.9% and 46.1% of cercariae produced by all rediae. In snails infected by F. hepatica, the majority of cercariae were produced by the R2a group (18.2 cercariae per redia) and corresponded to 66.0% of cercariae produced all rediae. The cercariae produced by the other redial groups were more limited in number: 17.5 per redia in the R1b group (28.7%) and 2.0 per redia in the R2b/R3a group (5.3%). Cercarial productivity of P. daubneyi until day 63 post-exposure was more limited in number than that of F. hepatica: a total of 145 cercariae per snail versus 427 per snail.     

Unusual snail species involved in the transmission of Fasciola hepatica in watercress beds in central France

Four freshwater pulmonate species (Lymnaea ovata, L. stagnalis, Physa acuta, Planorbis leucostoma) were living in several watercress beds known for their relationships with human cases of fasciolosis, whereas L. truncatula was never found. The aims of these studies were to determine the prevalence of natural infections with Fasciola hepatica in snails and to verify if these species might ensure the full larval development of this trematode (with cercarial shedding) when they were experimentally subjected to F. hepatica only, or to co-infections with an other trematode species. Investigations were so carried out in six snail populations living in watercress beds (including three for P. acuta) and in four others originating from three brooks or a pond (as controls). Snails naturally infected with F. hepatica were found in two watercress beds inhabited by L. ovata (prevalence of infection: 1.4%) and P. leucostoma (0.1%), respectively. The L. ovata from the watercress bed could be infected at a higher size than those from the control population and the prevalence of this infection was greater in the bed population. Similar findings were noted for L. stagnalis. Despite single or dual infections, the results obtained with the four populations of P. acuta were unsuccessful. In contrast, the co-infections of young P. leucostoma with Paramphistomum daubneyi and F. hepatica resulted in the shedding of some F. hepatica cercariae. According to the authors, the occurrence of fasciolosis in these watercress beds would be the consequence of frequent natural encounters between parasite and snails (L. ovata, L. stagnalis), or of co-infections with P. daubneyi and F. hepatica (P. leucostoma). In watercress beds only colonized by P. acuta, a lymnaeid species would have ensured the larval development of F. hepatica but it would have been eliminated by P. acuta, as this last species was known to be invasive and could colonize open drainage ditches on siliceous soil.     

An experimental study of the effects of Nosema eurytremae (Microsporida: Nosematidae) on the liver fluke Fasciola hepatica

A microsporidian parasite, Nosema eurytremae, was fed to Lymnaea trunculata infected with Fasciola hepatica. Microsporidian infection of the snail was always light, though spores were present in all tissues but the rediae became heavily infected in the parenchyma. The proportion of infected rediae increased with time and was especially high in ageing infections. Although rediae were only lightly infected when examined before cercarial release began, the infections became progressively heavier, so that towards the end of the life of the snails a high proportion of rediae were totally destroyed and contained no cercariae. Cercarial output was significantly depressed over a long period of active shedding by snails which had been hyperinfected with 1 x 106 spores and some of the liberated cercariae were themselves infected.     

Larval productivity of Fasciola gigantica in two lymnaeid snails

Two groups of Galba truncatula and two groups of Lymnaea natalensis were experimentally infected with Fasciola gigantica to determine if snail species had an influence on the redial burden and cercarial shedding of this trematode when snails of both species were infected with the same isolate of miracidia. In the two groups used for the study of redial burden, the total number of free rediae was significantly higher at day 49 post-exposure in L. natalensis than in G. truncatula. In the groups used for cercarial shedding, the life-span of cercaria-shedding snails and those of infected snails which died without cercarial emission, and the duration of the prepatent period were significantly longer in L. natalensis than those noted in G. truncatula. However, the mean numbers of shed cercariae did not significantly differ and showed no differences in their daily distribution throughout the shedding period. These results demonstrate that G. truncatula might be the principal intermediate host of F. gigantica in Egypt, at least in the areas where this lymnaeid species lives.     

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

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

Redial growth and cercarial productivity of Fasciola hepatica in three species of young lymnaeid snails

Experimental infections of 1-mm high snails using three populations of Lymnaea (L. glabra, L. ovata and L. truncatula) and a cattle strain of Fasciola hepatica miracidia were carried out under laboratory conditions to determine if the snail species had an effect on the number of free rediae, their growth, and cercarial productivity in relation to each redial category (R1a, R1b, R2a, or R2b/R3a). The total number of rediae ranged from 6.4 to 7.5 per snail. The mean body length of rediae varied from 1-1.2 mm (R1a) to 0.3-0.4 mm (R2b/R3a). The width of the intrapharyngeal lumen also varied from 26.0-38.8 microm to 3.0-4.2 microm, respectively. The redial category had a significant effect on both measurements, whereas snail species only had a significant influence on body length. The mean number of cercariae produced by all living rediae at day 49 post-exposure ranged from 63.0 in L. glabra to 87.2 in L. truncatula. In L. ovata and L. truncatula, 55.8% and 58.6% of cercariae, respectively, were produced by R2a rediae, whereas 53.9% of cercariae in L. glabra were formed by the R1b rediae. When young snails were infected with F. hepatica, the species of snail had an effect on the number of living rediae, their length and their cercarial productivity.     

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

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

Effect of concurrent infection with Muellerius capillaris on the development of redial generations of Fasciola hepatica in Lymnaea truncatula.

The rediae of Fasciola hepatica were counted according to generation in adult and juvenile Lymnaea truncatula following single infection with Fasciola hepatica, double infection with F. hepatica and then Muellerius capillaris, or double infection with M. capillaris and F. hepatica. The rediae found in double infections were essentially first generation and an early cohort from the second generation. The following differences were observed in adult snails which underwent double infection when compared to corresponding single infections: i) dependent rediae were almost completely absent; ii) degenerating independent rediae were found in identical or decreased numbers; iii) living independent rediae of the first generation were decreased in number, while those of the second generation had variable decreased numbers. The results were similar in juvenile snails with double infections, except that the numbers of degenerating independent rediae were higher than those found in corresponding single infections and the numbers of rediae of the second generation were increased. The order of exposure in double infections had no influence on the number and maturation of fasciolid rediae.     

Influence of shell size of Lymnaea columella on infectivity and development of Fasciola hepatica

Experimental infections of Lymnaea columella with Fasciola hepatica were carried out to determine the influence of shell size on the infection rate and on the outcome of rediae and cercariae. Snails were divided into seven groups according to shell size: 2-4 mm, 5-6 mm, 7-8 mm, 9-10 mm, 11-12 mm, 13-14 mm and 15 mm or more. One hundred snails in each group were infected by using four miracidia for each snail. Snails with larger shell size showed a lower infection rate, the groups presenting the highest (79%) and lowest (2%) proportions of positives being those of 5-6 mm and 15 mm or more, respectively. Cercariae were present in 21% of them at 31 days post-infection, and cercarial shedding was observed 61 days post-infection. It was concluded that there is a non-linear negative association between shell size and infection rate.     

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

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

Haplometra cylindracea (Zeder, 1800) (Trematoda:Plagiorchiidae): variation in the dates of cercarial shedding for overwintering Galba truncatula

Natural infections of Galba truncatula with Haplometra cylindracea were followed from 2001 to 2009 to determine if their characteristics were similar when snails came from water collections frequented by Bufo bufo or by frogs and newts for their egg-laying. Snail samples were collected from both types of sites to count shed cercariae for three days and also free cercariae when snails were dissected. In sites only frequented by B. bufo, cercarial shedding occurred earlier than in those colonized by frogs and newts (March instead of April-May). In contrast, the number of cercariae shed during three successive days was significantly higher in May. This variation in the dates of cercarial shedding might be due, either to a synchronism between cercaria-releasing snails and the presence of the definitive host (tadpoles) in water collections, or to an earlier infection of overwintering snails in autumn by H. cylindracea miracidia in the case of toad-frequented sites.     

Schistosoma mansoni: influence of Biomphalaria glabrata size on susceptibility to infection and resultant cercarial production

Biomphalaria glabrata snails of the same age, but different sizes, were used to determine size-related susceptibility to Schistosoma mansoni miracidial infection and the influence of snail size on total cercarial production. Snails with shell diameters from less than 5 to greater than 17 mm were individually exposed to one or several miracidia, depending on the experiment. In snails exposed to multiple numbers of miracidia, the percentage of snails which developed patent infections was lower in snails with larger shell sizes. This was also reflected by fewer primary sporocysts per infected snail found in tissues of the larger snails. Upon determining cercarial production in these groups over a 1-month period there were no statistical differences between any groups in the numbers of cercariae produced per snail. However, upon determining the number of successful primary sporocysts found in cohort snails of each size group, cercarial production increased as a function of the number of successful primary sporocysts. This was verified by examining cercarial production in various size snails with known monomiracidial infections. Our data therefore confirm and extend earlier work using snails infected with unknown numbers of miracidia and clearly show that total S. mansoni cercarial development and decreased susceptibility of snails is a direct reflection of snail size and not necessarily age of the snail.