G. Dreyfuss

A field study of natural infections in three freshwater snails with Fasciola hepatica and/or Paramphistomum daubneyi in central France

Natural infections of three freshwater snails with Fasciola hepatica and/or Paramphistomum daubneyi were studied during two periods in 1996 and 1997 (June-July and September-October) on 18 farms located in the departments of Vienne and Haute Vienne (central France), and known for low prevalences of F. hepatica infections in ruminants. A total of 1573 Lymnaea glabra and 1421 L. truncatula 6 mm high or more were collected in the meadows of 13 farms and dissected under laboratory conditions. Snails with single or concurrent infections of F. hepatica and/or

Paramphistomum daubneyi and Fasciola hepatica: the redial burden and cercarial shedding in Lymnaea truncatula subjected to successive unimiracidial cross-exposures

Abstractu2002The development of redial burden and cercarial shedding were studied in two groups of Lymnaea truncatula subjected to successive cross-exposures to one miracidium of Paramphistomum daubneyi and one of Fasciola hepatica per snail, or vice versa. The results were compared with those obtained in controls subjected to two unimiracidial exposures to the same trematode species. The infection rate was 61% in the group cross-exposed to P. daubneyi/F. hepatica and 37% in that cross-exposed to F. hepatica/P. daubneyi; it was 37% in the control group exposed to F. hepatica and 21% in that exposed to P. daubneyi. Snails harboring larval forms of both trematodes were few in number in cross-exposed groups and the redial burden was low, with one trematode dominating over the other. Free cercariae of F. hepatica and those of P. daubneyi were significantly more numerous at day 35 in the group cross-exposed to P. daubneyi/F. hepatica than in the controls or the other cross-exposed group. Mixed cercarial sheddings were obtained from 40% of snails with emission in the group cross-exposed to P. daubneyi/F. hepatica and from 21% of those in the F. hepatica/P. daubneyi group. The numbers of P. daubneyi metacercariae were significantly greater in the group cross-exposed to P. daubneyi/F. hepatica than in the other cross-exposed group, whereas no significant difference in mean numbers was noted for the F. hepatica cysts. Repartition of metacercariae over the patent period was clearly more irregular for P. daubneyi than for the other trematode.

Human paragonimiasis in Africa

An up-to-date review on human paragonimiasis in Africa was carried out to determine the current geographical distribution of human cases and analyze the animal reservoir, snails and crustaceans which intervene in the local life cycle of Paragonimus species. Two countries, i.e., Cameroon and Nigeria, were mainly affected by this disease, while the distribution of human cases in the other eight states of the intertropical zone was scattered. Infected patients were currently few in number and two Poragonimus species: P. africanus and P. uterobilateralis, were found. The animal reservoir is mainly constituted by crab-eating mammals. The identity of the host snail remains doubtful and was either a prosobranch, or a land snail. Seven crab species belonging to Callinectes, Liberonautes and Sudononautes genera are able to harbour paragonimid metacercariae. Due to the current low prevalence of human paragonimiasis recorded in Africa and the high cost of wide-scale screenings for this disease, training of technicians in anti-tuberculosis centers would be the most realistic attitude to detect mycobacteria and/or Paragonimus eggs during the same sputum examination.

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 15xa0years 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.

Redial generations of Fasciola hepatica: a review.

An update on the redial generations of Fasciola hepatica was carried out to highlight the different developmental patterns of rediae, the effects of some factors on these generations, and the consequences of such developmental patterns on cercarial productivity. The development of generations is dependent on the behaviour of the first mother redia of the first generation. If this redia remains alive throughout snail infection, it produces most second-generation rediae. In contrast, if it dies during the first weeks, daughter redia formation is ensured by a substitute redia (the second mother redia of the first generation, or the first redia of the second generation). Environmental and biotic factors do not modify the succession of redial generations, but most act by limiting the numbers of rediae, either in all generations, or in the second and/or third generations. An abnormal development of rediae reduces the number of cercariae and most are formed by the second cohort of the first generation. By contrast, most cercariae are produced by the first cohort of the second generation when redial development is normal. The mother rediae described by previous authors might correspond to the first generation and the second cohort of the second generation, while daughter rediae would be the second cohort of the second generation and the first cohort of the third generation. Under certain circumstances, daughter redia formation is ensured by the first two mother rediae or all first-generation rediae, thus demonstrating that the first mother redia is not the only larva to ensure daughter redia formation.

Variability in the prevalence of infection and cercarial production in Galba truncatula raised on a high-quality diet

Two experiments using seven populations of Galba truncatula were carried out to analyse the effect of food quality (cos lettuce only, or cos lettuce+Tetraphyll) on the characteristics of infections: (1) in a single population of G. truncatula infected by one of three digenea (first experiment), and (2) in seven populations of G. truncatula differing in their susceptibility to Fasciola hepatica miracidia (second experiment). In most groups, food quality did not have a significant effect on the survival of snails. The prevalence of infection in five populations was significantly higher in snails raised on lettuce+Tetraphyll (first and second experiments), whereas it was close to those noted in lettuce only-reared groups in the last two populations (second experiment). Despite the higher growth of cercariae-shedding snails when raised on the mixed diet, no significant differences were noted. Significant effects of parasite species (first experiment) and of snail population (second experiment) on the life-spans of cercariae-shedding snails were noted, whereas food quality did not influence this parameter. Except for a single snail population, cercarial production in groups raised on lettuce+Tetraphyll was significantly higher than that in groups on lettuce. The variability noted in the prevalence of snail infections and in the intensity of cercarial shedding might be explained by differences in the susceptibility of snail populations to F. hepatica infections, and/or by the fact that Tetraphyll would not have the same appetency for all populations of G. truncatula.

Paramphistomum daubneyi and Fasciola hepatica: influence of temperature changes on the shedding of cercariae from dually infected Lymnaea truncatula

Abstract Dual infections of Lymnaea truncatula with Paramphistomum daubneyi and Fasciola hepatica were performed to determine whether temperature changes in snails (daily water change with spring water at 6°–8u2009°C, which subsequently increased to room temperature at 20u2009°C) would influence snail infection and the production of cercariae by both trematodes. At day 30 postexposure the surviving snails were individually placed in petri dishes to constitute two groups. Snails from the first group were maintained at a temperature of 20u2009°C, and the water in the petri dishes was changed daily. The protocol was identical for the second group of snails except that the water temperature was 6°–8u2009°C when changed. The interval between exposure and the first shedding of cercariae in snails immersed in cold water for a short period was longer (67–69 days instead of 48–50 days in the 20u2009°C group). In both groups, snails infected only with F.u2009hepatica or P.u2009daubneyi or with both trematodes were detected. In snails infected only with F.u2009hepatica the frequency of cercaria-shedding snails and the total number of metacercariae were significantly greater in the 20u2009°C group. Inversely, in snails infected only with P.u2009daubneyi the frequency of cercaria-shedding snails and the number of metacercariae were significantly greater in the 6°–8u2009°C group. In snails harboring both trematode larval forms, no significant difference in the frequencies of cercaria-shedding snails between the two groups was noted. Metacercariae of both trematodes were obtained from these snails. In the 20u2009°C group, F.u2009hepatica metacercariae were more numerous, whereas in the 6°–8u2009°C group the number of P.u2009daubneyi metacercariae was greater. From these results it appears that greater activity of P.u2009daubneyi cercariae occurs in snails subjected to daily temperature changes (from 6° to 20u2009°C).

More efficient allopatric combinations of Fasciola hepatica and Lymnaea truncatula due to modification of redial development

Abstract. Experimental infections of two susceptible French populations of Lymnaeatruncatula (Courcelles and Saint Ours) with an allopatric (Fès, Morocco) or a semi-sympatric (Limoges, France) isolate of Fasciolahepatica miracidia were performed to determine the effect of allopatric miracidia on redial and cercarial production. In both populations, cercarial release was significantly greater in allopatric than in semi-sympatric snails. Compared to semi-sympatric snails, the examination of allopatric snails killed from dayxa014 to dayxa035 post-exposure demonstrated (1) a significant decrease after dayxa028 in the number of daughter rediae (R2a group) exiting from the first-appearing mother redia (R1a redia), and (2) the differentiation of numerous daughter rediae (R2b group) in the body of second-appearing mother rediae (R1b group). These experiments demonstrated that the exposure of L.truncatula to an allopatric isolate of miracidia disturbed the usual developmental pattern of redial generations and caused the formation of numerous R2b rediae. The authors hypothesized that the increase in cercarial release noted in allopatric snails would be assured by the rediae from the R2b group after their emergence from the body of R1b mother rediae.

Fasciola hepatica: an unusual development of redial generations in an isolate of Lymnaea truncatula

Single-miracidium infections of Lymnaea truncatula by Fasciola hepatica were experimentally carried out to identify the redial generations of this trematode when the larval development was unusual (when the first-appearing mother redia, or R1a redia, died after its exit from the sporocyst). Four parameters were measured in the body and pharyngeal region at weekly intervals. At day 49 post-exposure at 20°C, the body of the second mother rediae (R1b) was significantly longer than that of the subsequent generations, R2a and R2b/R3a (a mean of 3.0 mm instead of 1.0 and 0.9 mm, respectively). The body was significantly wider in the R1b and R2a groups than in the R2b/R3a rediae. The pharyngeal lumen was significantly wider in the R1b group than in the R2a and R2b/R3a rediae (a mean of 48.6 μm instead of 10.8 and 3.3 μm at day 49). The thickness of the pharyngeal wall did not differ in the R1b and R2b/R3a groups, but was significantly lower in the R2a group (19.5 μm instead of 23.0-23.6 at day 49). There was better development of R1b and R2b/R3a rediae in the snails when the R1a redia died, compared with normal larval development (with a living R1a redia).

Paramphistomum daubneyi and Fasciola hepatica: the prevalence of natural or experimental infections in four species of freshwater snails in eastern France

Parasitological investigations were performed in July and September–October 1997 in six farms located in the department of Saone et Loire (eastern France) to determine the prevalence of natural infections with Paramphistomum daubneyi and Fasciola hepatica in four species of freshwater snails. Cercaria-containing rediae of P. daubneyi and/or F. hepatica were found in Lymnaea palustris (one snail only) and Lymnaea truncatula . Some living sporocysts and immature rediae were noted in Lymnaea ovata ( P. daubneyi or F. hepatica ) and in Physa acuta ( P. daubneyi only). The prevalence of each trematode infection was often less than 10%. Experimental infections of juvenile and preadult snails (1 and 4 mm in height, respectively) were also performed to test the susceptibility of these four snail species to P. daubneyi , either singly or in combination with F. hepatica . Both 1 and 4 mm high L. truncatula could sustain the full development of P. daubneyi , whether in single or double infections. In L. palustris dually exposed to both trematodes, cercaria-containing rediae of P. daubneyi were found in one juvenile and one preadult snails, while immature infections were noted in ten juvenile and two preadult snails. The overall prevalence of P. daubneyi infection in L. palustris was 11.1% in juvenile snails and 2.1% in preadults. Larval forms of P. daubneyi and F. hepatica were only noted in dually-exposed juvenile L. ovata and P. acuta . In L. ovata , mature and immature rediae of F. hepatica were detected in 17.6% of snails, while immature rediae of P. daubneyi were noted in 4.4% of snails. In P. acuta , only immature infections were detected (5.1% of snails with P. daubneyi , and 1.2% with F. hepatica ). These results demonstrated that Lymnaea species other than L. truncatula could sustain the full development of P. daubneyi and that immature larvae of this trematode might be found in naturally- or experimentally-infected L. ovata and P. acuta .