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.

Variations in local adaptation of allopatric Fasciola hepatica to French Galba truncatula in relation to parasite origin.

Two French populations of Galba truncatula were subjected to experimental infections with Egyptian and French isolates of Fasciola sp. miracidia, originating from cattle and sheep, to compare characteristics of snail infections in allopatric and sympatric groups. All sampled Egyptian isolates were identified as Fasciola hepatica using microsatellite markers. Compared to snails infected with French miracidia, snail survival at day 30 post-exposure was significantly greater in the Egyptian groups, while prevalence of infection was significantly lower (in an Egyptian group infected with cattle-derived miracidia) or did not show any significant differences in the other three cases. The total number of metacercariae was significantly higher in the four Egyptian groups. However, snail population and the mammalian origin of F. hepatica had also a significant effect on this parameter. The dissection of snail cadavers showed a significantly higher number of free rediae in the Egyptian groups, even if snail population also had a significant effect on the redial burden. Both Egyptian isolates of F. hepatica could easily develop in French snails, causing a low mortality in snails and inducing a metacercarial production higher than that noted in sympatric infections. However, the mammalian origin of F. hepatica eggs and the quality of snail populations as intermediate hosts had to be taken into account for studying local adaptation in reason of their effects on this process.

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.

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.

Significant population genetic structure of the Cameroonian fresh water snail, Bulinus globosus, (Gastropoda: Planorbidae) revealed by nuclear microsatellite loci analysis.

In order to characterize the demographic traits and spatial structure of Cameroonians Bulinus globosus, intermediate host of Schistosoma haematobium, genetic structure of seven different populations, collected from the tropical zone, was studied using six polymorphic microsatellites. Intrapopulation genetic diversity ranged from 0.37 to 0.55. Interpopulation genetic diversity variation clearly illustrated their significant isolation due to distance with gene flow substantially limited to neighbouring populations. The effective population sizes (Ne) were relatively low (from 3.0 to 18.6), which supposes a high rate from which populations would lose their genetic diversity by drift. Analysis of genetic temporal variability indicated fluctuations of allelic frequencies (35 of 42 locus-population combinations, P<0.05) characteristic of stochastic demography, and this is reinforced by events of bottlenecks detected in all populations. These findings demonstrated that Cameroonian B. globosus were mixed-maters with some populations showing clear preference for outcrossing. These data also suggest that genetic drift and gene flow are the main factors shaping the genetic structure of studied populations.

Strong genetic structure in Cameroonian populations of Bulinus truncatus (Gastropoda: Planorbidae), intermediate host of Schistosoma haematobium.

In order to evaluate the snail host contribution on the variability of mollusk/schistosome compatibility, the genetic structure of seven Cameroonian populations of the schistosome vector, Bulinus truncatus, was studied using four variable microsatellite loci. A substantial polymorphism mainly distributed among populations was observed. No heterozygous genotype was scored, confirming the high level of selfing rate occurring in B. truncatus populations. Contemporaneous samples were highly and significantly differentiated with a marginally significant correlation with geographical distances (P-value=0.069). The different sites sampled seemed to rarely exchange migrants with very small Nm (∼0.22 or below). The data also suggest that B. truncatus subpopulations might be composed of very small and isolated units at much smaller surfaces than what was investigated. Even if more data (in particular more loci) will be needed to confirm these issues, they suggest that restricted gene flow plays an important role in maintaining differentiation among snail populations in the transmission foci, potentially leading to specific adaptation between each B. truncatus population and its local Schistosoma haematobium population.