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.     

Seasonal transmission of Fasciola hepatica in north central Florida (U.S.A.)

To assess seasonal transmission of Fasciola hepatica 37 groups of fluke-free tracer sheep, one group of four sheep each month in succession, grazed an infected pasture in north central Florida (U.S.A.) from June 1984 to June 1987. At the end of each month, tracers were moved to a fluke-free barn for 2 months, then necropsied and flukes counted. Fluke transmission mostly occurred during the first half of the year, with peak transmission from February to April. No transmission occurred during the warm summer months, having ceased in either May or June of each year to begin again in late autumn or early winter (November-January).     

Fasciola hepatica: surfaces involved in movement of miracidia and cercariae

Rapid freezing and substitution with fixative prior to scanning electron microscopy was used to demonstrate the pattern of beat and recovery of the cilia of free swimming miracidia of Fasciola hepatica. There were stages of dexioplectic metachronal co-ordination and the power stroke was approximately 15 degrees anticlockwise from the anterior-posterior axis. Around the circumference of the body of the miracidia there were approximately 12 metachronal waves of power and recovery. Free-swimming cercariae were recorded by time-lapse photography and, after conventional fixation, by scanning electron microscopy. Cercarial tail-beats were to the posterior of the body in the lateral plane at a rate of 8 Hz. The tail has paired lateral ridges positioned to act as leading edges. There is an array of 32 sensory papillae on the mid-ventral surface of the tail. The tegument of the most distal part of the tail is described: it is free of sensory endings and the surface shows a spiral pattern.     

Molecular characterization revealed Fasciola specimens in Ecuador are all Fasciola hepatica,none at all of Fasciola gigantica or parthenogenic Fasciola species

All 225 Fasciola flukes obtained from domestic animals (73 cattle, 7 sheep and 1 pig) of 18 distinct geographic areas in Ecuador-South America, were identified as Fasciola hepatica, based on molecular analyses of nuclear pepck and pold genes, and mitochondrial nad1gene as well as the morphological observation of sperm within the seminal vesicles. Fasciola gigantica and parthenogenic Fasciola forms endemic to Asian countries were not found in this study, although zebu cattle and water buffalos have introduced into South America from Asia; this could be due to the absence of suitable intermediate host snails. The results of pepck analysis using multiplex PCR developed previously showed that 32 of the flukes could not be confirmed as F. hepatica, suggesting that the method is unreliable for the accurate discrimination of F. hepatica, and that pepck gene of the species consists of multiple loci, not a single locus. The results of genetic diversity, phylogenetic, and network analyses based on mitochondrial nad1 sequences suggest that F. hepatica populations in South America, including Ecuador, formed from the ancestral F. hepatica individuals introduced into the continent along with anthropogenic movement of livestock infected with the species.     

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     

Fasciola hepatica: identification of molecular markers for resistant and susceptible Pseudosuccinea columella snail hosts

Protein electrophoresis, RAPD-PCR and nuclear rDNA ITS sequencing were performed to search for genetic differences between Pseudosuccinea columella snails susceptible and resistant to Fasciola hepatica infection. Of the 21 enzymatic loci analyzed in both populations, none of them exhibited neither within- or between-group variation. Such an absence of enzyme polymorphism support the hypothesis of selfing as the "prevalent" mating system for this hermaphroditic species. Conversely, the RAPD profiles displayed clear differences between susceptible and resistant isolates for 17 of the 26 primers tested while no within-group variation was detected. rDNA ITS sequence analysis from snails of each isolates showed only two bases that differed between groups accounting for a 0.17% of variation confirming that susceptible and resistant snails belong to the same species. This is the first time that a genetic variation using RAPD markers is demonstrated between susceptible and resistant lymnaeid snails vis-a-vis of F. hepatica infection in absence of experimental selection.     

Interrelations in the Fasciola hepatica-water snail system

Peculiarities of relationships in the "Fasciola hepatica-Lymnaea truncatula" system were studied by means of hystological and histochemical methods. Trematodes cause deep histopathological and histochemical changes of the injured organs of L. truncatula. They are determined by mechanical destruction of mollusc's tissues under the effect of parasites and by intoxication of animals with metabolites of parasites. Under the effect of parasites the content of glycogen and phospholipids in hosts' tissues decreases while the concentration of neutral fats increases. In most cases, however, the infected hosts remain viable that under normal ecological conditions results in the preservation of the "F. hepatica-L. truncatula" system.     

Characterization of sheep antibodies involved in precipitate formation with surface antigens of Fasciola hepatica in vitro

Sandeman R. M. and Howell M. J. 1982. Characterization of sheep antibodies involved in precipitate formation with surface antigens of Fasciola hepatica in vitro. International Journal for Parasitology12: 467–471. The role of sheep antibodies which precipitate with surface antigens of Fasciola hepatica is unclear. In an attempt to clarify their function these antibodies were characterized as to their immunoglobulin class and ability to affect the survival of fluke in rats. The ability of fluke antigens complexed with sheep antibody to vaccinate rats against infection was also tested. IgM antibodies were involved in precipitate formation on the teguments of fluke 3 weeks after infection but IgG₁ predominated at later stages of infection. The decreased survival of fluke in rats after culture with increasing levels of sheep antibodies suggests that the antibodies exert some deleterious effect on the fluke in vitro. The fluke antigen-sheep antibody complex failed to immunize rats against infection. Since sheep antibodies to F. hepatica can impair the ability of fluke to resist further attack in rats but not sheep, it is suggested that some effector mechanism other than antibody is defective in the latter.     

Mitochondrial respiratory activity of the trematode Fasciola hepatica

Polarographic studies have been made on the respiratory activity of isolated mitochondria of the trematode F. hepatica. Respiratory chain transferring electrons to oxygen and which is sensitive to cyanide was found in the mitochondria. Certain coupling between respiration and phosphorylation was observed. Intact mitochondria of the trematode exhibit the respiratory control although its level is significantly lower than that in the mitochondria from rat liver. The existence of an alternative respiratory chain was demonstrated in which electron transport is not associated with ATP synthesis.     

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.     

Collagenolytic activities of the major secreted cathepsin L peptidases involved in the virulence of the helminth pathogen, Fasciola hepatica

Background

The temporal expression and secretion of distinct members of a family of virulence-associated cathepsin L cysteine peptidases (FhCL) correlates with the entry and migration of the helminth pathogen Fasciola hepatica in the host. Thus, infective larvae traversing the gut wall secrete cathepsin L3 (FhCL3), liver migrating juvenile parasites secrete both FhCL1 and FhCL2 while the mature bile duct parasites, which are obligate blood feeders, secrete predominantly FhCL1 but also FhCL2.

Methodology/Principal Findings

Here we show that FhCL1, FhCL2 and FhCL3 exhibit differences in their kinetic parameters towards a range of peptide substrates. Uniquely, FhCL2 and FhCL3 readily cleave substrates with Pro in the P2 position and peptide substrates mimicking the repeating Gly-Pro-Xaa motifs that occur within the primary sequence of collagen. FhCL1, FhCL2 and FhCL3 hydrolysed native type I and II collagen at neutral pH but while FhCL1 cleaved only non-collagenous (NC, non-Gly-X-Y) domains FhCL2 and FhCL3 exhibited collagenase activity by cleaving at multiple sites within the α1 and α2 triple helix regions (Col domains). Molecular simulations created for FhCL1, FhCL2 and FhCL3 complexed to various seven-residue peptides supports the idea that Trp67 and Tyr67 in the S2 subsite of the active sites of FhCL3 and FhCL2, respectively, are critical to conferring the unique collagenase-like activity to these enzymes by accommodating either Gly or Pro residues at P2 in the substrate. The data also suggests that FhCL3 accommodates hydroxyproline (Hyp)-Gly at P3-P2 better than FhCL2 explaining the observed greater ability of FhCL3 to digest type I and II collagens compared to FhCL2 and why these enzymes cleave at different positions within the Col domains.

Conclusions/Significance

These studies further our understanding of how this helminth parasite regulates peptidase expression to ensure infection, migration and establishment in host tissues.