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Liver Fluke
Confirmation of triclabendazole resistance in liver fluke in the UK
  1. Danielle Gordon1,
  2. Ruth Zadoks1,
  3. Philip Skuce1 and
  4. Neil Sargison2
  1. 1Moredun Research Institute, Pentlands Science Park, Bush Loan, Penicuik, Midlothian EH26 0PZ
  2. 2Royal (Dick) School of Veterinary Studies, University of Edinburgh, Easter Bush Veterinary Centre, Roslin, Midlothian EH25 9RG
  1. e-mail: ruth.zadoks{at}moredun.ac.uk

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WE would like to report confirmation of triclabendazole resistance (TCBZ-R) in liver fluke, Fasciola hepatica, from the UK, based on a dose and slaughter trial. Recently, there has been considerable debate regarding the usefulness of reports of triclabendazole treatment failure in liver fluke infected sheep and cattle as an indicator of TCBZ-R (Fairweather 2011, Sargison and Scott 2011). It has been suggested that the diagnosis of TCBZ-R in the field should be based on a combination of diagnostic tests (Fairweather 2011). We used a combination of faecal egg count reduction testing (FECRT) and coproantigen reduction testing (CRT) to demonstrate TCBZ-R in liver fluke in sheep from Scotland (Gordon and others 2012). We have now confirmed this diagnosis based on a dose and slaughter trial, the only method that was put forward by a panel considering new World Association for the Advancement of Veterinary Parasitology (WAAVP) guidelines on the detection of anthelmintic resistance (Coles and others 2006).

F hepatica eggs were obtained at postmortem examination from the gall bladder of two naturally infected ewes from Dumfries and Galloway, Scotland. Large numbers of live adult fluke were present in the liver of both animals (46 and 75, respectively) despite three triclabendazole treatments. Fasciola eggs were propagated through Galba truncatula snails and metacercariae were harvested for reinfection of sheep. Ethical approval for the study was obtained through the Moredun Research Institute's Experiments and Ethical Review Committee.

Six animals (one-year-old males) were challenged with an estimated 150 cysts each. Infection status was monitored for 17 weeks by means of a serum antibody ELISA, a coproantigen ELISA and faecal egg count (first positive at three, three and nine weeks after challenge, respectively). At 12 weeks after challenge, infection was patent in all animals. The animals were then treated with triclabendazole. Calibrated scales were used to measure bodyweight and calculate adequate dosage. Treatment failure was confirmed based on FECRT and CRT, as described by Gordon and others (2012). Three weeks later, animals entered a dose and slaughter trial, that is, triclabendazole treatment was repeated and the numbers of fluke present were counted at postmortem examination 14 days later (Fig1). Between 19 and 70 live adult fluke were retrieved from the liver and bile ducts of each animal, confirming the TCBZ-R status of the fluke isolate.

FIG 1:

Sheep liver in situ showing damage to the liver tissue and thickening of the bile ducts 17 weeks after challenge with Fasciola hepatica

The dose and slaughter trial-based confirmation of TCBZ-R in liver fluke in the UK, which was suspected on the basis of FECRT and CRT, should be seen as a further incentive to apply the best possible management strategies for control of liver fluke on sheep and cattle farms. This should include use of preventive measures where possible, treatment where necessary and the evaluation of treatment efficacy. Before declarations of TCBZ-R are made in the field, problems due to inadequate dosing, poor product storage or inferior quality products need to be ruled out, as triclabendazole is still the drug of choice for migrating juvenile fluke, particularly in sheep.

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