Liver Fluke


Liver fluke infection (fasciolosis) is caused by a trematode (flat leaf-like parasite) known as Fasciola hepatica. It can infect all grazing animals but mainly affects sheep and cattle. Disease can result from the migration of large numbers of immature flukes through the liver, or from the presence of adult flukes in the bile ducts or both. Liver fluke can have severe effects on herd and flock performance and therefore needs to be dealt with correctly. Over the past few years the disease has become much more common with a greater geographical distribution with slaughterhouse liver condemnations indicate that fasciolosis is now widespread and increasing in frequency

Sheep are particularly susceptible to fluke and this can leave them seriously ill or cause sudden death. Fluke in cattle, on the other hand, is often over-looked because the signs are subtle but can be serious, for example: a low grade infection can reduce milk yield by 400l/cow/lactation; it also reduces butterfat %, calf birth weights from infected cows can be up to 10% lower; youngstock have poorer growth rates and lowered immunity which can leave the animal susceptible to other infections  such as Black disease or salmonella.

The causal parasite

Fasciola hepatica infects the liver in both cattle and sheep. For part of its life cycle it inhabits the snail, Lymnea truncatula. Eggs from adult female fluke pass in the dung to contaminate pastures. When conditions are suitable i.e. damp and above above 7 - 10°C, the eggs hatch to form mobile larvae that seek out the mud snails to complete their life cycle. Therefore the presence of these mud snails determines the distribution of the fluke - highest risk grazing areas are wet with rutted pools of water. The larvae multiply in the snails and develop further until they emerge and attach to the grass as cysts. Once ingested by a grazing animal an immature fluke breaks out the cyst and makes its way to the liver. Immature fluke tunnel through the liver causing serious damage and adults live in the bile ducts where they feed on blood potentially causing anaemia. Liver fluke causes three types of disease, acute, sub-acute and chronic. Essentially, late spring early/summer infestation of snails result in the autumn fluke challenge to sheep with immediate acute disease, sub-acute disease over the following weeks, or chronic disease apparent three months later depending upon the level of challenge.

liver fluke life cycle

Life-cycle of Fasciola Hepatica


Clinical presentation

Acute fasciolosis

Affected sheep die suddenly from haemorrhage and liver damage with the first evidence of a problem being sudden deaths in previously healthy sheep from August to October. Inspection of others in the group reveals lethargy and reduced grazing activity. Gathering may prove difficult because sheep are reluctant to run caused by pain.

Subacute fasciolosis

The major presenting clinical findings are rapid loss of body condition and poor fleece quality despite adequate nutrition. Others may be depressed, inappetant and weak. Usually seen from December onwards but may be earlier with severe challenge. 

Chronic fasciolosis

The major presenting clinical findings are very poor body condition score and poor fleece quality and in many sheep, bottle jaw.  Death can occur following severe emaciation and liver failure.

Poor condition affecting many of your sheep may also result from:

  • Inadequate flock nutrition
  • Chronic parasitism including anthelmintic-resistant strains
  • Virulent footrot
  • Johne's disease
  • Poor dentition especially cheek teeth,
  • Chronic severe lameness.


Acute/subacute fasciolosis

Diagnosis of acute/subacute fasciolosis is based upon the epidemiological data and veterinary investigation of blood samples which reveals raised liver enzymes. Immature flukes are demonstrated in the bile ducts and gall bladder at necropsy.

Chronic fasciolosis

Chronic fasciolosis is diagnosed by demonstration of fluke eggs in faecal samples. Mature flukes are demonstrated in the bile ducts and gall bladder at necropsy.


Triclabendazole (Fasinex) is an oral drench effective at killing all stages of fluke resposible for acute fasciolosis. Following treatment animals should be moved to clean pasture or re-treated every three weeks for the next three months. Unfortunately resistance problems with this drug have been found on a minority of farms. If you are concerned you may have a problem with resistance please contact County Vets for advise.

Nitroxynil (Trodax) and Closantel (Closamectin) are less effective against immature flukes and should be used only in the treatment of subacute and chronic fasciolosis. Once again, treated sheep must be moved to clean pastures.

Management/Prevention/Control measures

Fluke infestations are controlled by strategic drenching based upon veterinary advice. During low risk years (long spells of dry hot weather) triclabendazole is administered in advance of the predicted challenge during October and January, with another flukacide drug administered in May. In years when epidemiological data indicate a high risk of fasciolosis (long spells of warm, wet weather) additional triclabendazole treatments are given in November and February. While it may be possible to eradicate fluke from a property, there are considerable risks from not drenching as the appearance of clinical disease in a few sheep represents serious losses in the whole flock.

Isolation and treatment of all bought-in stock is essential to avoid introducing fluke if your farm or parts of your farm are free of it at present.

Economic importance

Fasciolosis can have a serious financial impact on a sheep farm with immediate losses up to 10 per cent caused by acute/subacute disease. Chronic disease could half profits by reducing lamb crop and increasing ewe mortality. Infestations are controlled by strategic drenching based upon veterinary advice. The role of health planning by the farmer's veterinary surgeon is essential to maintain profitability and prevent losses.