FASCIOLIASIS is the disease caused by liver fluke (fasciola hepatica) and the fluke is common on farms on the western side of Britain due to the increased rainfall, but it also becoming increasingly established in the east.

This is due to the increased movement of infected animals (particularly to and from farms with poor biosecurity measures) and changing weather patterns (increased rainfall and milder winters).

It is estimated that fluke costs the UK agricultural industry £300m annually. This cost accounts for veterinary treatment/investigation, poor performance, flukicides, condemnations and the cost of preventative measures (such as fencing off wet areas).

By far the biggest cost is the sub-optimal performance of affected animals showing no obvious signs of fluke. These animals cannot efficiently utilise their feed intake due to liver damage and will perform poorly. Infestation can reduce the market value of a finished animal by around 10%.

By taking action to control fluke, liver condemnations can be avoided, performance improved and welfare maintained.

Disease can result in:

- Reduced weight gain

- Lower milk production

- Lower feed conversion ratio (FCR)

- Lack of thrive in younger and growing stock

- Reduced fertility

- Death of stock

- Poor welfare

CHRONIC DISEASE:

Chronic disease occurs most commonly in cattle and occurs when small numbers of encysted larvae are gradually consumed.

This results in a prolonged period of migration of a smaller number of metacercariae through the liver parenchyma. Liver damage is less acute and so the hosts tend to survive allowing the fluke to develop in to adults in the bile duct.

The adult fluke shed eggs and these chronically infected animals are the main source of infection when the animals return to pasture.

ACUTE DISEASE:

Acute disease strikes when large numbers of immature fluke migrate though the liver parenchyma causing severe liver damage and blood loss. Sheep are more often affected by acute infection which occurs when large numbers of metacercariae are consumed during their mass emergence in the late summer/early autumn.

Sheep often have positive fluke burdens, even in the presence of solid faecal pellets so sheep owners must be vigilant and should talk to their vet or SQP about taking remedial action in the Spring by using a flukicide targeting older fluke to prevent build-up of fluke in the autumn.

CLINICAL SIGNS:

CATTLE:

- Weight loss

- Anaemia

- Oedema

- Diarrhoea

- Infertility

- Milk production drop

- Increased susceptibility to other diseases eg salmonella diarrhoea and blacks disease

- Poor weight gain

SHEEP:

- Sudden death

- Haemorrhage

- Ill-thrift

- Diarrhoea

- Anaemia

- Pale mucous membranes

- Weakness

- Icterus (jaundice)

- Peripheral oedema

LIFE CYCLE:

The completion of the liver fluke life cycle is dependent on environmental conditions - mainly temperature and humidity because they affect the survival rate of the miracidium; the rate of development of the cercaria; availability of the intermediate host lymnaea truncatula (mud snail); and distribution of the intermediate host.

Below 10oC the eggs cannot develop on the pasture. Any eggs which are shed on to the pasture over winter and during spring tend to develop around April with their development accelerating as the temperature increases.

By late spring, large numbers of miracidium will have hatched and be in search of mud snails for the next stage of their life cycle. The snails also tend to awaken from hibernation around 10oC which makes the liver fluke such an effective parasite.

Once the miracidium find a snail host they burrow in to them through the muscular foot. One infecting one snail can produce hundreds of cercariae which emerge in the summer and autumn.

These form infective cysts known as metacercariae on the pasture. After ingestion by the ruminant the juvenile flukes hatch from the cysts in the intestines and migrate through the gut wall and locate the liver.

They then begin to burrow through the hepatic parenchyma towards the bile ducts. As they feed on the hosts tissues and blood they grow and develop rapidly causing increasing damage and haemorrhage as they increase in size.

Animals with a lower burden of metacercariae usually survive the acute phase of infection, during which the immature flukes migrate through the liver parenchyma. Once flukes become established in the major bile ducts they mature and begin to produce eggs 8-10 weeks after the initial infection. In the event of no subsequent invasion by immature fluke the liver parenchyma is able to regenerate and regain functionality albeit with severe fibrous scarring.

DIAGNOSIS:

The gold standard diagnostic tool is visualising fluke within the liver tissues, which be an unplanned post-mortem following the sudden death of animals which have succumbed to acute disease (normally sheep) or can be carried out routinely in the abattoir on request.

Faecal worm egg count (FWEC) can be useful in chronic infection as the adult fluke shed eggs via the bile duct into the gastrointestinal tract. It's worth noting that they shed a relatively small number of eggs sporadically so it is possible that chronic infection may be missed.

Blood testing is a useful tool. We can measure the liver enzymes which will detect if any liver damage has occurred and also check for fluke antibodies. This is a useful monitoring test which gives a long term indication of the fluke control strategy.

The antibodies can persist for up to nine months, so a positive result indicates that the animal has been exposed to fluke within that time period - not that it is necessarily currently infected.

Bulk milk testing for dairy herds is helpful for detecting any challenge. A sample of milk from the bulk tank can be analysed for antibodies to fluke. As mentioned these can persist but if monitored regularly can alert you to a new challenge.

CONTROL:

- Vets are integral to ensuring a sound fluke control and prevention strategy. They should integrate fluke control into herd and flock health plans based on the individual risk and history of disease.

NADIS and SCOPs provide parasite forecasts so use these to your advantage to check the risk in your region. Word of mouth from Vets, SQPs and neighbouring farms is invaluable for local knowledge.

- In high risk situations (farms with uncontrolled parasite burden, acute outbreak) it may be necessary to dose surviving animals as frequently as every four weeks until winter housing with a flukicidal drug active against immature stages.

- In moderate risk situations dosing can be as infrequent as 10-weekly during risk periods.

- Additional treatment(s) should be administered over winter to destroy any adult fluke in the bile ducts as these chronically infected animals act as a source of infection in the coming grazing season.

- Ensure you treat using the most appropriate active ingredient which is suitable for the risk at that time of year. All animals must be dosed accurately as under-dosing predisposes to anthelmintic resistance. Remember no flukicidal treatment is persistent so animals are immediately susceptible to infection again.

- Any areas which are water logged should be drained or fenced off.

- Ask the abattoir for feedback on the condition of the liver following slaughter.

- Any sudden loss of sheep on farm should be investigated as it may be the first indication of a fluke problem.

- All incoming stock should be quarantined. Treat any bought-in livestock with a product which can kill immature fluke and where possible keep treated animals on drier pastures or housed for 3-4 weeks.

FLUKICIDES:

When deciding which flukicide to use you must consider which stage of parasite you are trying to target. To prevent and control acute infestation in the late summer/autumn you should use a product that is active against the earliest stages.

Triclabendazole is the only active that can kill the early immature stage. Treatment of chronic fluke infection in sheep and cattle during winter and early spring requires product that targets the adult stages.

When using any anthelmintic for the purpose of controlling fluke ensure you are using a product with an active ingredient which is appropriate for the life stage you are trying to control. If you are in any doubt contact your SQP or vet for advice.

* Rachel Mallet is a veterinary surgeon and territory manager for animal health specialist, Bimeda. She is passionate about animal health and promoting best practice amongst farmers and animal owners.