January - March
The risk posed by liver fluke to grazing stock depends entirely on the number of metacercaria (infectious stages) present on the grass/herbage they are eating. Weather conditions have a major impact on this: warm wet weather from May/June through the summer allowing a large mud snail population to develop, and also allowing fluke eggs to develop quickly, hatch and infect the snails. Six to eight weeks after infection, the snails will start to produce metacercaria, and continue to do so until environmental temperatures drop below 10C. Although metacercaria will survive for months on pasture and can survive freezing, their numbers will gradually decline once snail activity has stopped, and will cease to be a risk when “old season” grass dies back and “new season” grass begins to grow.
The fluke risk after Christmas will therefore depend on how many snails are infected (late spring/summer weather conditions) and how long these snails stay active (autumn/winter weather conditions).
April - August
In late Spring and early Summer the risk of serious disease in sheep from liver fluke is small but it is still just as important to control liver fluke on your farm. Any liver fluke in sheep and cattle in the Spring are most likely to be egg laying adults. The warm, damp weather at this time of year provides the ideal climate for the liver fluke larvae and the mud snails resulting in increasing levels of fluke contamination on the pasture as we head towards late Summer. Wet weather and muddy areas in fields will significantly increase the liver fluke risk as there will be more habitat suitable for mud snails.
Using a flukicide that targets adult liver fluke in the late Spring / early Summer period kills the egg laying adult liver fluke, reducing the early season contamination of the pasture. This will help to control the build up of liver fluke on the pasture as we go through the warm Summer months.
September - December
Autumn is the time of year when livestock are most likely to ingest large numbers of liver fluke cysts on the pasture. Once ingested these liver fluke larvae migrate through the liver causing damage to the liver tissue and blood loss. The level of damage caused is directly related to the number of liver fluke ingested. Large numbers of liver fluke ingested in the Autumn can cause serious disease and death.
The biggest factor affecting the amount of liver fluke present in the Autumn is the weather. Wet and warm weather increases the liver fluke risk by:
The weather will be an important factor in determining when animals need to be treated in the Autumn. A warm, wet Spring may bring the risk period forward to late August / early September whereas long, hot dry spells may move the risk period back to October. As the weather changes every year the timing and severity of the liver fluke challenge will also vary every year.
The type of ground is also an important factor. Wet, boggy lower lying fields will be a much higher risk than dry, higher ground due to their suitability to host the mud snails. Avoid grazing high risk areas in Autumn and Winter (particularly in a wet year). Fence off high risk areas to prevent access by livestock.
Managing the pasture to minimise snail habitat, reduce snail numbers and in turn reduce the infective stage on the pasture should be considered. This includes fixing leaky troughs, avoiding poached ground and maintaining effective drainage.
Moving animals onto Winter fodder crops is another option to reduce the risk. In Autumn crops such as brassicas and root crops are low risk options compared to grass.
Housing animals will also negate the risk of liver fluke if an effective liver fluke dose is given at housing.
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