We’re into the problem period for Fluke in sheep – late Autumn to Winter, and due to its financial implications including body condition deterioration and death, it’s a disease most sheep producers are tuned in to. However, it’s now commonly suspected that resistance is building against the most effective treatment. Alun gives us more…

Triclabendazole can kill liver fluke from just two weeks of age, making it the most popular choice for farmers. However, inappropriate use of this treatment is leading to the development of resistance in the fluke population in the UK. Triclabendazole has its place in the management of fluke in sheep, but must be used appropriately to preserve it for future use.

Several factors need to be taken into consideration when selecting the correct treatment for fluke infestation. The time of year and thus the current stage of the flukes life cycle is vital to the choice. Different flukicides target different stages in the life cycle, and using different treatments over the year prevents repeat exposure to the same drug, and reduces the rate of resistance development (please see table 1).

Fluke Life cycle and different treatments

The most common indicator that resistance is developing is the apparent failure of the product to kill the immature fluke, resulting in re-infestation appearing sooner than anticipated. If no action was to be taken at this point then the resistance would continue to develop and ultimately adult fluke will develop resistance. In any situation where resistance is suspected a different flukicide should be used taking into account the spectrum of activity.

There are several easy steps which can be taken to prevent the development of resistance to flukicides. The strategic use and rotation of nitroxynil, closantel and triclabendazole should be considered, although additional treatments may be needed in the years when triclabendazole isn’t used. As well as ensuring that all dosing equipment is calibrated correctly and that the correct dose for the sheep’s weight is being administered.

Efficacy of Flukicides

The incorrect selection of an adulticide treatment during autumn when there is the highest burden of immature fluke will result in a large number of untreated fluke remaining in the liver. These will then continue to progress through their life cycle, causing disease and giving the appearance of a failed treatment and the early stages of resistance developing. Another scenario in which it may appear that resistance is developing is, the continual grazing of heavily contaminated pastures, resulting in rapid reinfection despite the correct treatment.

If you are concerned that resistance may be developing on your farm we can perform a fluke egg count reduction test to determine if this is the case. Faecal samples from 10 individual animals need to be collected on the day of treatment, and sent away for a fluke egg count. Three weeks post treatment faecal samples from the same 10 animals need collecting, in cases where there is no resistance you would expect to see a decrease of greater than 90% in the numbers of eggs seen. Please do not hesitate contacting a member of the team if you need any additional help.