Liver fluke in sheep - a growing threat

Infecting the sheep: The infectious stage of the liver fluke life cycle is the tough cyst that forms on pasture (metacercariae). When the sheep ingests these cysts, they hatch out in the small intestine and migrate and travel to the liver. The early immature fluke chews its way through the liver substance, becoming the immature fluke when it reaches the small bile duct where it starts feeding on blood and continues to travel down to the main bile duct and gall bladder, where it becomes an adult after 10-12 weeks in the liver.

How the pasture is contaminated with liver fluke: The adult fluke produce thousands of eggs in the host sheep each day; these eggs are then passed out in dung, onto the pasture. When temperatures are above 10°C, the fluke eggs will develop and hatch into larvae, or miracidium; these then swim through water and wet mud to the mud snail, where they burrow in and infect the snail. Once inside the snail, the liver fluke larvae goes through more development stages, multiplying in number many hundreds of times, until they reach the cercariae stage when they leave the snail and swim through the water film onto pasture where they form the tough cysts (metacercariae). These are then eaten by the sheep - and the cycle starts again.

Liver fluke life cycle:

The level of liver fluke risk is dependent on the number of metacercariae cysts on the pasture: the greater the number of cysts, the higher the risk. Weather conditions and mud conditions determine the mud snail population which then has an impact on the number of cysts on the pasture:

Warm, wet and muddy conditions provide a beneficial habitat for the miracidium to swim from the egg to the mud snail, and for the mud snail population to increase. In ideal conditions it takes about 8 weeks for the fluke to develop from the miracidium to the cercariae in the snail. The snails will continue to produce these cercariae until environmental temperatures drop below 10°C. Therefore, the more snails that get infected, and the longer the weather stays above 10°C, the more metacercariae will accumulate on the pasture - meaning the higher the risk to grazing animals.

Cold and/or dry conditions slow down snail activity and development of the fluke so that instead of taking around 8 weeks for metacercariae to appear on the pasture, it could take 3-4 months. Once temperatures drop below 10°C, snail activity decreases and they burrow down into the mud to hibernate, with only a few surviving over winter. Although metacercariae will survive for months on pasture and can survive freezing, their numbers will gradually decline once snail activity has stopped.

While warm, wet weather conditions favour the liver fluke life cycle and increase the infection risk in the autumn and winter, hot dry weather can slow or stop snail and fluke life cycles, which makes the timing and level of challenge very difficult to predict. The challenge will therefore vary considerably from year to year and farm to farm.

The metacercariae will only be present on pasture where the snails are present, so doing a risk assessment of snail habitats is a good first step. Even if snails are present, these areas can be safe to graze before the metacercariae appear.

Testing the sheep will tell us when the fluke risk starts in any field. There are a number of diagnostic tests available to use when looking for liver fluke. Unfortunately, none of them on their own will give us all the information we need at all times of year, so we need to use a combination of different tests and information from other sources to fully understand where and when the fluke challenge is likely to happen on any farm.

The clinical signs of liver fluke in sheep can vary due to the level of infestation and damage to the host’s liver. The severity of disease caused by liver fluke is entirely dependent on the number of infectious metacercariae ingested by the animal.

Effective management of liver fluke in sheep is based on a ‘4 Point Plan’ that target the critical points in the fluke life cycle.

The fluke risk varies significantly from year to year and farm to farm, so monitoring the situation on your farm (see above) will allow the right product to be used at the right time.

The rise of resistance: Resistance to triclabendazole has been identified on a number of farms. With this in mind, it is recommended to check that treatments have been effective.

We cannot rely solely on animal treatments to achieve effective liver fluke control. It is therefore essential to tackle all aspects of the 4 Point Plan.

Rumen fluke (paramphistome) is a different parasite to liver fluke. Generally, mature rumen fluke do not cause clinical disease but there have been reports of disease caused by large numbers of immature rumen fluke in the small intestine, typically in youngstock. In some cases, this has resulted in ill thrift and profuse scour, while in severe cases it has been fatal in both sheep and cattle. 

The clinical importance of rumen fluke is under debate. Diagnosis of rumen fluke is through faecal egg counts in dung samples or through post mortems. It is important to differentiate between liver fluke and rumen fluke eggs as only a small number of flukicides kill rumen fluke.