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

Clonal amplification of Fasciola hepatica in Galba truncatula: within and between isolate variation of triclabendazole-susceptible and -resistant clones

Background

Fasciola hepatica is of worldwide significance, impacting on the health, welfare and productivity of livestock and regarded by WHO as a re-emerging zoonosis. Triclabendazole (TCBZ), the drug of choice for controlling acute fasciolosis in livestock, is also the drug used to treat human infections. However TCBZ-resistance is now considered a major threat to the effective control of F. hepatica. It has yet to be demonstrated whether F. hepatica undergoes a genetic clonal expansion in the snail intermediate host, Galba truncatula, and to what extent amplification of genotypes within the snail facilitates accumulation of drug resistant parasites. Little is known about genotypic and phenotypic variation within and between F. hepatica isolates.

Results

Six clonal isolates of F. hepatica (3× triclabendazole-resistant, TCBZ-R and 3× triclabendazole-susceptible, TCBZ-S) were generated. Snails infected with one miracidium started to shed cercariae 42–56 days post-infection and shed repeatedly up to a maximum of 11 times. A maximum of 884 cercariae were shed by one clonally-infected snail (FhLivS1) at a single time point, with > 3000 clonal metacercariae shed over its lifetime. Following experimental infection all 12 sheep were FEC positive at the time of TCBZ treatment. Sheep infected with one of three putative TCBZ-S clones and treated with TCBZ had no parasites in the liver at post-mortem, whilst sheep each infected with putative TCBZ-R isolates had 35–165 adult fluke at post-mortem, despite TCBZ treatment. All six untreated control animals had between 15–127 parasites. A single multi-locus genotype was reported for every fluke from each of the six clonal isolates. Adult F. hepatica showed considerable variation in weight, ranging from 20–280 mg, with variation in weight evident within and amongst clonal isolates.

Conclusions

A genetic clonal expansion occurs within G. truncatula, highlighting the potential for amplification of drug resistant genotypes of F. hepatica. Variation in the weight of parasites within and between clonal isolates and when comparing isolates that are either susceptible or resistant to TCBZ represent inherent variation in liver fluke and cannot be attributed to their resistance or susceptibility traits.

Fasciola and fasciolosis in ruminants in Europe: Identifying research needs

Fasciola hepatica is a trematode parasite with a global distribution, which is responsible for considerable disease and production losses in a range of food producing species. It is also identified by WHO as a re-emerging neglected tropical disease associated with endemic and epidemic outbreaks of disease in human populations. In Europe, F. hepatica is mostly associated with disease in sheep, cattle and goats. This study reviews the most recent advances in our understanding of the transmission, diagnosis, epidemiology and the economic impact of fasciolosis. We also focus on the impact of the spread of resistance to anthelmintics used to control F. hepatica and consider how vaccines might be developed and applied in the context of the immune-modulation driven by the parasite. Several major research gaps are identified which, when addressed, will contribute to providing focussed and where possible, bespoke, advice for farmers on how to integrate stock management and diagnosis with vaccination and/or targeted treatment to more effectively control the parasite in the face of increasing the prevalence of infection and spread of anthelmintic resistance that are likely to be exacerbated by climate change.

A retrospective study on the metacercarial production of Fasciola hepatica from experimentally infected Galba truncatula in central France

A retrospective study of the experiments performed during the past 15 years on infections of Galba truncatula with Fasciola hepatica was carried out to determine what susceptible populations of snails might be used for the commercial production of metacercariae, and to examine this metacercarial production in relation to the characteristics of snail infections. Of the four groups of snail populations studied, the ablest snails to sustain a complete larval development of F. hepatica originated from populations living on siliceous soils at 600 m and more in altitude. In contrast, snail populations living along river banks on siliceous soils were inappropriate due to the poor characteristics of snail infections (high snail mortality, low prevalence of snail infections, and low number of cercariae produced). Except for these last populations, 86-87% of cercaria-shedding (CS) snails in the other populations shed less than 300 cercariae, even if a maximum of 1,772 cercariae were obtained from a single snail. The date of the first cercarial shedding at 20 degrees C began during week 7 or 8 PE for 80.1-83.5% of CS snails. Most metacercariae (82.0-85.9% of the total production) were recorded during the first 10 days of the patent period. In these conditions, the authors collected metacercarial production up to the beginning of week 10 PE (20 degrees C) and did not use snails that shed their cercariae during the following weeks due to too low numbers of parasites. This method allows to have a continuous production of metacercariae over time by using successive groups of infected snails, each being separated from the other by a fortnight's time.