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Why does the fluke risk vary?

Two key factors cause the significant issues associated with liver fluke:

  • when the infectious stages (the metacercaria) first appear on the pasture ie when the challenge starts
  • the level of challenge – the number of metacercaria, the larval stage of the fluke, on the pasture

The fluke risk for each season is affected by the conditions over the months before. Different active ingredients are needed to target the different stages of fluke

It is important to assess the fluke risk by monitoring both the fluke forecast and the situation on your farm, and adjust the choice of treatment accordingly.

In most areas of the UK, temperatures during the winter months are low enough (<10C) to kill off a lot of the mud snails (the intermediate host in the life cycle of the liver fluke), leaving just a small number that burrow down into the mud and hibernate to survive the winter. During hibernation, no new infective cysts (metacercariae) are coming out of the snail. The cysts sheep are picking up are ones put on the pasture in the autumn. These will decrease over time so the amount of new infection that sheep pick up is less. Therefore, we tend to get more immature and adult fluke in sheep at this time.

BE AWARE: the fluke levels vary from year to year and in some years we can still have high levels of infective cysts on the pasture early in the year so you must be aware of the fluke forecast, or monitor the situation on your farm.


In late winter and early spring, depending on the weather, use closantel, the active ingredient in Flukiver™ and Supaverm™, or a nitroxinil product, as these actives kill immature and adult fluke.


Warm and/or wet conditions:As the weather warms up in the spring, and soil temperatures rise, the fluke eggs will start to develop and the snail population will start to increase. As it gets warmer this development happens faster, so that most of the fluke eggs that landed on the pasture through the winter and spring will hatch around the end of May or beginning of June.

By this time there is usually a very healthy mud snail population, provided there has been enough rain to maintain the mud conditions they need. The miracidium, the active stage that hatches from the fluke egg, also needs wet conditions to be able to swim from the egg and find a snail to infect. Once inside the snail, the liver fluke will take around 8 weeks before the metacercaria (the stages infectious to cattle and sheep) appear on the pasture. Therefore, in warm and wet weather, the fluke eggs will hatch in May, find a snail and develop quite quickly, so that metacercaria appear on the grass in August. The fluke don’t kill the snails, so the more snails that get infected, and the longer the weather stays above 10C in the autumn, the more metacercaria will accumulate on pasture. The higher the metacercaria numbers, the higher the risk to grazing animals.

Cold and/or dry conditions: Dry or cold conditions slow down snail activity and development of the fluke, so that instead of taking 8 weeks for metacercaria to appear, it could take 3-4 months. A cold/dry spring means eggs might not hatch until the end of June, relatively few will find a snail, and those that do may take 3-4 months before metacercaria are released. Therefore, in these conditions there would be no infectious stages on the grass for livestock to pick up until October.


In the summer months, liver fluke in sheep are likely to be adult. Therefore, treatments should target adult liver fluke only. Not all flukicide products are able to kill all stages of the liver fluke in sheep. The active ingredients of choice would be albendazole, the active ingredient in Rycoben SC™, or oxyclosanide as these will only kill adult liver fluke in sheep. As there are only a limited number of actives available, it is important to use the right active at the right time, for the right stages of liver fluke to:

  • avoid overuse of any one active
  • reduce the risk of developing resistance


Once the autumn/winter temperatures drop below 10C again, most of the snails will die off and the survivors will hibernate in the mud. No more metacercaria will be released, and as those present on grass die off over time, the numbers and the risk to grazing animals will decrease through the late winter and into the spring. As the spring grass starts to grow, and there has not yet been any snail activity, there will be no metacercaria present, and so little or no risk to grazing stock.


In the autumn/early winter, we get the highest number of early immature fluke and so need a product that kills these. The only active to kill early immature fluke - ie fluke that is under 5 weeks old is triclabendazole, the active in Combinex for Sheep™, Fasimec Duo™ and Fasinex 240™.