By Elina Body, The George Farm Vets
Parasites are an increasing burden upon livestock farming across all of the UK, through increasing anthelmintic resistance plus changing epidemiology attributed to climate change.
Liver fluke infection costs UK agriculture around £300 million per year, not to mention the possible impacts on health and welfare, including accuracy of bovine tuberculosis testing through modulation of the immune system.
Lots of discussion for fluke control is centred around testing and treatment options, but can we help manage disease by changing the way we manage grazing land?
Life cycle
In order to effectively control fluke, it’s important to understand the lifecycle (see figure). It is heavily weather dependent, relying on temperatures >10C to allow lifecycle development but also on water to allow the miracidium to swim and reach snails, and for mud snail survival. The mud snail itself is critical for the fluke lifecycle; a snail infected with just one miracidium amplifies to release 600 cercariae. The final stage involves development on grass, and is resistant to cold winter temperatures.
A key part of control is limiting livestock exposure to possible snail habitat. The mud snail itself feeds on algae on exposed mud surfaces, thriving on bare mud not recently disturbed, with slow to little water movement – typically edges of ponds or watercourses, or hoofprints or tractor ruts in muddy fields. They also prefer open areas (not shaded by trees, hedges or long vegetation) and water needs to be pH neutral with calcium and minerals for shell growth.
Weather and lying water
Despite trends toward higher rainfall and milder winters which favour snail survival, it isn’t all necessarily about the weather. Various models have attempted to attribute the importance of climatic conditions to explain variability in liver fluke incidence across the UK. This figure varies from 45%-79%, thus the remaining 21-55% is influenced by on-farm management and environment, which can be controlled on an individual basis based on each farms risk factors.
A good example of this was shown by a recent study examining the grazing of wetland areas under agri-environmental schemes to benefit wading birds and natterjack toads. This concluded that the cattle were under lower risk of fluke infection when compared to the intensely grazed fields they came from. This was possibly attributed due to snails not surviving well in saltmarshes, the grazed sward heights or low levels of fluke having been introduced by previous livestock grazing. But there was still a need to regularly monitor, as conditions change. Testing and quarantine of purchased stock is also important to avoid introducing fluke infection to a favourable habitat.
One of the common quoted pieces of advice for control of fluke – “avoid grazing flukey pastures and fence off possible mud snail habitats” – can be impractical and economically unfeasible for some. But it can be achieved through risk mapping flukey areas on farm over time and avoiding grazing on heavily contaminated pastures particularly in Autumn. Cattle are less susceptible to disease resulting from fluke than sheep, so in a mixed enterprise then prioritise low-risk grazing for sheep.
It’s possible to reduce boggy, wet parts of the pasture by fixing any leaking water troughs or pipes which create the conditions for mud snails. The type of drinking water supply in some studies has been shown to influence the risk. Hence providing sound troughs, or even better, water supply infrastructure that can be moved frequently, to prevent repeated poaching of the same areas of the pasture could aid in reducing mud snail environment.
Grazing management and infiltration
More long-term reduction of mud snail habitats could be addressed through improved soil health using grazing management. Improving water infiltration not only means the soil will have better ability to absorb water but there would be reduced risk of soil erosion, more water retained within the soil rooting zone as well as reduced risk of subsurface flow and leaching. It’s especially important when considering future predictions of increasing single day rainfall events, which could exacerbate run-off, soil loss and worsening soil structure.
To reduce this effect, ensure there is suitable vegetation cover and living roots, with no bare soils. On grasslands this can be achieved by ensuring there is sufficient sward height and root network to protect the soil from erosion, evaporation and shielding from extreme temperature changes which benefits soil’s biological function. Long rotations of multispecies swards or conservation grazing at low stocking density would be two ways to achieve this.
Another key physical aspect of soil health would be avoiding compaction, this reduces the amount and size of pore spaces within the soil for water infiltration. Compaction due to machinery and livestock is important to consider. Intensive or heavy grazing, in addition to the loss of vegetation cover (as above), has been shown to reduce water infiltration, especially with heavy cattle or on clay/silt soils.
A common culprit of this could be set stocking. Although the apparent stocking rate may be low, spatial patterns of repetitive use result in much higher stocking rates than intended in these preferred areas, leading to degradation that is exacerbated by heavy rain.
Rotational grazing offers a solution to this, with multi paddock grazing and deferred grazing found to benefit water infiltration. This is due to less bare ground, higher soil stability, higher soil organic matter, less sediment loss and a higher fungal/bacterial ratio, allowing for better water holding capacity and nutrient availability and retention.
However, studies have shown the grazing intensity and frequency needs to be considered. By overall reducing grazing pressures and adding complexity to managed grazing system, rather than continuous grazing, water infiltration can be improved.
Nutrient cycling and trees
Considering soil chemistry, organic matter content is also important as it allows binding to stabilize micro-aggregates, helps break up compaction as well as stimulate microbial activity. The effect of increasing organic matter content upon water holding capacity varies depending on physical soil properties. It’s estimated for every 1% increase in soil organic matter in the top six inches this enables stores of an additional 10,800 litres of water.
An alternative method of achieving this could be through silvopastures, these can have a higher soil organic matter content compared to conventional pasture systems. Trees within a pasture could also enable better water infiltration, further support mycorrhizal networks, and provide more biodiversity and deeper rooting systems. It also utilizes the fact that mud snails seem to prefer unshaded environments, although this isn’t a scientifically proven aspect of control it could be worth considering. This also plays into the importance of soil biology, root networks, earthworms and fungal hyphae allowing larger aggregates to be held together and creates networks of macropores for water infiltration and aeration of soils.
Utilising predation and minimizing anthelmintic use
Beyond the soil itself, it’s important to consider the whole ecosystem. There are many possible natural predators which could support regulation of fluke. Lapwings, thrushes and other birds can feed on snails and there’s some evidence that commercial duck/goose systems may even eliminate fluke-hosting snails from habitats. Ground and water beetle larvae, larvae of marsh flies and some fluke-hosting snails can also predate and feed on mud snails.
Being aware of the negative impact of all anthelmintic products on invertebrates should also be a key consideration of livestock keepers.
Excretion of these products through excrements/urine as well as incorrect disposal of these products allows lethal and sub-lethal exposure of all invertebrates to these products which have been proven to have significant effects.
Summary
Liver fluke control in the long term requires a more holistic approach, going beyond diagnostic and appropriate treatment of stock. If current rainfall patterns worsen with climate change and flukicide resistance continues to grow, livestock keepers won’t have a choice but to have to utilize other control measures to tackle liver fluke as an endemic and emerging disease.
Medium-term changes centre around understanding and addressing soil health to improve water infiltration, through better grazing management, supporting wider ecosystem function and considering novel approaches such as silvopasture. In the short term, implementing other simpler solutions such as housing, water infrastructure and fencing off risky areas or pastures will make a positive and sustainable impact.