Studies on the prevalence and nature of triclabendazole resistance in liver fluke populations from cattle in Australia

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Agricultural Sciences, Faculty of Science, Technology and Engineering, LaTrobe University, Bundoora.Triclabendazole (TCBZ) is widely used to control liver fluke (Fasciola hepatica) infections in livestock in Australia which has led to the development of liver fluke resistant to this drug. This Thesis reports several studies investigating fluke prevalence and the nature of TCBZ resistance in cattle in South-Eastern Australia. (i) We identified a high prevalence (81%) and intensity of fluke infection in the Maffra region indicating that fasciolosis is causing economic losses in dairy cattle. TCBZ resistance ([less than]90% reduction in fluke burdens) was detected on 2 dairy properties in Maffra and on 4/7 beef properties in NSW and Victoria. Live flukes were recovered at slaughter following TCBZ treatment of cattle from 3 beef properties, confirming the TCBZ resistance status of F. hepatica. This is the first report of widespread resistance to TCBZ in F. hepatica from cattle in Australia. (ii) The genetic diversity of TCBZ-resistant, TCBZ-susceptible and field fluke populations was evaluated using mitochondrial genes. We identified several common mtDNA haplotypes that were shared amongst the populations and many less frequent haplotypes. This study revealed a high level of diversity in field and drug resistant flukes in South-Eastern Australia. (iii) The mechanism of TCBZ resistance is yet to be discovered. P-glycoproteins (Pgp) are thought to be involved with the transport of TCBZ out of the parasite and an over-expression or increased efficiency in these proteins may be linked to resistance. A single nucleotide polymorphism (SNP) in a Pgp gene has been proposed as a molecular marker to identify TCBZ resistance. We tested this hypothesis using well-characterised isolates via PCR and DNA cloning and rejected the hypothesis that the SNP could be used as molecular marker, at least in Australian populations. These studies have greatly improved our understanding of the nature of TCBZ resistance in F. hepatica in Australia.