A growing multitude of factors has the potential to greatly increase the risk of liver fluke disease on much of the land area typically farmed with sheep. This warning was delivered by Dr Philip Skuce, principal scientist at the Moredun Research Institute, at the recent Teagasc National Hill Sheep Conference.

Philip outlined that in recent years, four primary factors have aligned to deliver a more favourable environment for liver fluke to prosper and spread.

  • Climate/weather patterns: Changing climatic conditions or weather patterns are giving rise to warmer, wetter summers and milder winters, with a heightened risk of more extreme weather events such as flooding. This is providing greater opportunities and a longer season for the liver fluke parasite to complete its life cycle, while a potentially longer grazing season means that animals may be exposed to the threat of liver fluke infestation for longer. This is especially true for sheep, with a high percentage of flocks outwintered and only housed for a short period, if at all.
  • Drug resistance: The emergence of resistance to trichlabendazole is greatly limiting farmer choices when it comes to selecting products to tackle liver fluke disease. Triclabendazole was the drug of choice for acute liver fluke, especially in sheep and where resistance has been confirmed, which leaves farmers with more limited treatment options. Liver fluke treatment programmes must be developed more strategically, taking into account the disease risk, using technologies to confirm the presence of fluke and then using the most appropriate product.
  • Animal movements: The movement of animals from farm-to-farm without an effective quarantine protocol on arrival greatly increases the risk of importing liver fluke onto your farm and parasites which may have developed resistance to certain flukicides.
  • Agri-environment schemes: Measures contained in agri-environment schemes promoting, for example wetland/ peatland restoration or the creating of new wetland environment, also pose a risk of providing enhanced environmental conditions for liver fluke. Many schemes which aim to enhance the population of endangered bird species require agricultural activity and, ideally, livestock grazing to provide suitable environmental conditions. The associated risk of liver fluke is not considered in such discussions, but must be to promote sustainable grazing systems.

    • Flooding outside Maghera, Co Derry. \ Houston Green

    Perfect snail habitat

    The successful completion of the liver fluke life cycle is dependent on the mud snail. This tiny snail craves wet conditions, but will not operate under water. Bare mud which has not been recently disturbed and which is located in open areas (for example not shaded by hedges, trees, long vegetation, etc) is the optimum environment.

    Typical areas highlighted by Philip include depressions caused by tractor tyre ruts, poaching or landscape features (banks on the side of streams/ponds). Cleared drainage ditches or ditches which are not fenced and trampled by livestock, or soft ground around leaking water taps or pipes, are also high risk areas for harbouring mud snails.

    Diagnostic options

    The diagnostic tools available for identifying the presence of liver fluke are quite limited. The identification via post-mortem is obviously too late for the animal in question while significant damage could also have occurred across the flock, but nevertheless it is still an important tool.

    The health status of livers in slaughtered sheep can also be a useful indicator. Automatic results printed with a farmers’ remittance is where the industry needs to get to, but in the meantime asking your agent to follow up on the health status of livers can be a useful aid.

    Philip explains that blood tests can be a useful indicator in young, first-season grazing animals for both sheep and cattle. They are less useful in older animals as antibodies can persist, even after successful treatments. The advice is to use monthly on small groups of sentinel animals from mid-summer, to indicate when and where they have encountered fluke and to inform treatment, timing and product choice.

    Faecal egg counts offer limited scope in the fight against fluke, as they work on the premise of identifying fluke eggs. This means that animals can be infected for 10 to 12 weeks post-infection before eggs from adult fluke appear. They can be used in determining efficacy of treatment, choice of product or in assessing resistance to flukicides.

    Coproantigen tests can be used to determine the presence of liver fluke six to eight weeks post-infection, ie will identify late immature-adult fluke.

    Flooding river Finn, Donegal.

    Farm management

    In the future, farmers will also need to utilise farm management options to reduce the risk of liver fluke infestation. Fencing is an option to prevent animals from entering high-risk areas at certain times of the year. It is less practical in marginal land areas, where there may be a high risk across the farm.

    Drainage is a similar option, in that it can work in targeted areas to help control the environment for the mud snail to survive. Housing is unfortunately becoming a must on some farms with a very high risk and can be used to take the pressure off animals and widen treatment options.

    Treatment options

    The most obvious farm management option is treatment. Philip highlighted that there is no flukicide available with residual properties. With no new flukicides released for decades and none in the pipeline, he urged producers to be cautious in their approach and to select a flukicide which best targets the stage of the liver fluke parasite present.

    Table 1 was presented at the conference and covers the main flukicides on the market. Nitroxynil is not included as Trodax is no longer on the market. However, farmers in Ireland have options to select nitroxynil flukicides, which have been afforded emergency licensing.

    A paper presented at a previous Teagasc hill sheep conference listed efficacy for nitroxynil of 50% to 90% in immature fluke aged upwards of six weeks and 91% to 100% efficacy for fluke aged 10 to 12 weeks. Philip strongly advised farmers against the use of combination products that include flukicides. He said that such drugs that kill worms tend not to kill fluke and that most flukicides only target a narrow profile of fluke stages.

    Take home messages

    Many animals face a constant risk of reinfection if grazing outside due to no natural immunity and no products with residual activity. The great hope for long-term liver fluke treatment is the development of a vaccine and while work is ongoing in this area, it is proving highly challenging.

    Farmers were asked to remember the five Rs – “the need to use the right product at the right dose on the right animal at the right time and in the right way”.

    Philip also raised the fact that some of the frontline flukicides (and wormers) also have potentially detrimental impact on important dung, soil and aquatic life.

    He said: “If you need to treat, use as little as possible but as much as necessary for successful treatment and dispose of unused chemical and containers carefully.”