When do we need to treat for liver fluke again?

As described in the treatments section, treatments for liver fluke are not “long acting”, they will only remove the target age range of liver fluke that are present at the time of treatment. Knowing when to treat again depends on:

• Current level of fluke risk
• Grazing on farm
• Weather – providing the snail’s habitat

Current level of fluke risk

Assuming that some testing has been done to demonstrate the fluke challenge has started (see ‘Testing for liver fluke’), we know that animals grazing that same pasture will continue to pick up new/more metacercaria. In an average year, using the coproantigen test (dung samples) will let you know when there is an active fluke infection before adult fluke are present, and give a good indication that a treatment is necessary. However, in a very high challenge year, enough new fluke can be picked up to kill an adult sheep within a month or so. The only option in this situation is to move animals to a lower risk pasture (no snail habitat) or house them to avoid re-infection. Even a very small number of fluke can cause enough liver damage to significantly reduce the performance of livestock, so avoiding the fluke challenge as far as possible should always be the first priority.

Post treatment checks

As even a few liver fluke can have a significant impact on the performance of livestock, it is always worth checking that any treatment for fluke has been effective. The easiest way to do this is to use the coproantigen test (10 individual dung samples) and collect samples 2 weeks after treatment. The coproantigen result should be zero on all samples after an effective treatment with triclabendazole or closantel. (Other actives may not remove enough of the immature fluke to give a zero coproantigen result after treatment. For information on how to check treatment efficacy for other actives please seek specialist advice)

Grazing risk and housing

Fields with areas of snail habitat will present a risk of liver fluke infection to stock grazing those areas once the metacercaria start to appear on pasture. This risk varies depending on the weather and the stage of the life cycle so it is important to test for fluke to determine the degree of risk. As the metacercaria survive very well in the environment, this risk does not go away until the following spring. However, metacercaria and mud snails remain only in the areas of mud snail habitat (the wet muddy bits), and are not spread across the drier parts of the field. Temporary fencing to exclude animals from these wet areas, after fluke infection has been detected (blood samples for fluke antibodies), will allow grazing of the remainder of the field. If the whole area is covered in snail habitat, the safest option is not to graze that area until there is new grass growth the following spring. In these circumstances housing animals is a cost effective option.