Discussion

The liver fluke, Fasciola hepatica is an economically important trematode parasite pathogen of livestock that frequently impacts on the health and welfare of cattle and sheep worldwide and is regarded by the WHO as a re-emerging zoonosis. With predictions for further increases in the prevalence of infection due to a changing climate, increased animal movement and changes in land management F. hepatica infection is likely to have a substantial impact on livestock production and human health in future. Control of F. hepatica relies heavily on drug treatment, in particular the drug triclabendazole (TCBZ), which targets the highly pathogenic juvenile fluke migrating through the liver. However, as with many helminth parasites drug resistance has emerged and is now considered a substantial threat to sustainable liver fluke control. By studying naturally infected sheep and cattle in the UK we have identified high levels of genetic diversity in populations of liver fluke; genetic variation that poses a challenge to identifying the underlying genetic and molecular basis of drug resistance. Similarly the fact that liver fluke are hermaphrodites capable of self fertilization and that a clonal expansion occurs in the snail intermediate host, raises questions about how parasite biology influences the spread of drug resistance genes. This paper will report a series of studies we have taken to better understand the genetic basis of TCBZ resistance in F. hepatica. We have used single miracidial:snail infections of F. hepatica to generate and characterise clonal parental lines of TCBZ-resistant (TCBZ-R) and TCBZ-susceptible (TCBZ-S) liver fluke. In order to identify areas of the genome with signatures of drug selection we have taken an experimental approach by crossing TCBZ-S and TCBZ-R clones and whole-genome mapping of TCBZ-R genes through subsequent F1 and F2 populations. These parental clones have been used to derive F1 progeny using a panel of neutral microsatellite markers to track those parasites that had undergone cross fertilization between -R and -S parasites and to allow subsequent identification of F2 recombinants. By comparing the frequency of SNP alleles derived from the resistant parental clone and linked to the TCBZ resistance loci in pooled, phenotyped F2 recombinants we have localised regions of the genome under selection. Our concurrent population genetic studies have provided insights into how, once drug resistance is present, it may disperse through parasite populations infecting sheep and cattle in the UK.