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.

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 use of tetraphyll as food for snails increases the intensity of cercarial shedding in Galba truncatula infected with Fasciola hepatica.

Experimental infections of Galba truncatula with Fasciola hepatica were carried out under laboratory conditions to determine why the use of Tetraphyll, when provided every 3 days as food for snails, stimulated the intensity of cercarial shedding. In snails raised on cos lettuce and Tetraphyll, the number of metacercariae was significantly higher than that recorded in controls on lettuce only. In the former group, numerous metacercariae were counted on day 1 of every 3-day period (when snails fed on Tetraphyll), while the numbers of these larvae strongly decreased on day 2 and were very low on day 3. The repetition of this experiment using microalgae or modified Boray's diet every 3 days as food for snails gave the same results. However, the number of metacercariae recorded on day 1 of every 3-day period was significantly higher in snails raised on lettuce and Tetraphyll than in those reared on lettuce and algae (or Boray's diet). According to the authors, the first contact of an infected snail with Tetraphyll induces a faster differentiation of cercariae within the rediae and the accumulation of many free cercariae in the snail's hemocoel, so that a second contact of the snail with this food 3 days after allows free cercariae to complete their glycogen and fat reserves and to rapidly exit from the snail. However, a temporary immobilization of the infected snail by Tetraphyll, or rather its degradation products, and a massed exit of cercariae cannot be excluded.

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.

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.

Fasciola hepatica: the effect of food quality on the development of redial generations in Galba truncatula infected with allopatric miracidia.

Individual bimiracidial infections of French Galba truncatula with an allopatric (Moroccan) isolate of Fasciola hepatica were carried out under laboratory conditions to count free rediae and intraredial daughter rediae in snails just after their death, and to determine whether feeding the snails on lettuce only or lettuce plus Tetraphyll had an effect on the development of redial generations. Controls consisted of snails infected with sympatric (French) miracidia and reared with lettuce only. In the allopatric group, reared with lettuce only as food, a higher mortality rate of snails in the first 4 weeks of the experiment was noted, and most G. truncatula showed 1-sporocyst infections, with the usual development of redial generations. Compared to the sympatric group, the growth of the first mother redia (R1a redia) in each snail of the allopatric group was significantly slower during the first weeks of infection, whereas a significantly increased formation of intraredial daughter rediae in the other mother rediae (R1b rediae) was noted over the same period. In the allopatric group, reared with cos lettuce and commercial fish food, 1-sporocyst infections (45.5% of snails) and 2-sporocyst infections (53.9%), with the usual development of redial generations, were noted. Compared to the sympatric group, a significantly slower development of the first mother redia during the first weeks of the experiment was noted in the 1-sporocyst infections, whereas the values in the 2-sporocyst infections did not significantly differ from those noted in the sympatric group. Similar results were noted for the production of intraredial daughter rediae in the R1b rediae during the first weeks of the experiment. In all infections derived from the growth of a single sporocyst per snail, the differentiation of numerous daughter rediae inside R1b rediae would be induced by the slow growth of the R1a redia (from which the most important group of R2a rediae originated) during the first weeks of infection and might represent a mechanism to replace free rediae absent from the inside of the snail.