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Liver Fluke Control in Sheep

Liver fluke infestation (fasciolosis) has always been a problem during the late autumn and winter in the wetter western areas of the UK. More recently, liver fluke has been reported increasingly in eastern areas of the country due to the introduction of infested sheep. Slaughterhouse liver condemnations indicate that fasciolosis is now widespread and increasing in frequency.

Fig 1: Store lambs grazing wet pastures - at risk from fluke?

Most recent update

The most recent fluke forecast can alwasy be found in the NADIS parasite forecast.

While the numbers of infective metacercariae decline throughout the winter, allowing sheep access to pastures not grazed during autumn/early winter exposes sheep to further challenge and often presents disease problems of subacute fluke during January/February. In this situation infective fluke stages emerging from snails are not ingested immediately as there are no sheep on those pastures but infestation occurs when the sheep are returned to graze those pastures during the winter months. Although the numbers of infective metacercariae decline throughout the winter, many pastures were heavily contaminated during the wet summer of 2008 and challenge may still be high in January and February 2009. In some areas of the country however, a prolonged freeze during much of December and early January may well have caused a sharp decline in infectivity but another drench with triclabendazole during February would probably be well advised. For more specific information concerning the likely risk of liver fluke in your area contact your veterinary practice.

Fig 2: Sheep with acute fluke die suddenly from haemorrhage and liver damage with the first evidence of a problem being sudden death.

Fig 3: Sudden death caused by acute liver fluke.

The causal parasite

Fasciola hepatica infects the liver in both cattle and sheep. For part of its life cycle it inhabits the snail, Limnea truncatula. This important stage of the parasite life cycle necessitates wet and warm conditions (above 7 - 10°C) during the summer months. 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.

Fig 4: Sub acute fluke afected liver (left) compared to normal liver (right). Such damage has a huge effect on growth rate and body condition

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 (Figs 2-3). 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. Sudden deaths may affect up to 10 per cent of sheep at-risk causing grave financial loss.

Fig 5: Sheep affected by sub acute fluke.  The ewe lamb is severely depressed, inappetant and weak.

Fig 6: Liver damage caused by chronic fluke infestation

Fig 7: Emaciation caused by chronic liver fluke infestation (not all affected sheep have "bottle-jaw")

Other causes of sudden death your vet will consider include:

  • Clostridial disease; pulpy kidney, blackleg, Black disease, braxy in unvaccinated stock
  • Tick diseases as habitats often similar for both snails and ticks:
  • Pasteurellosis or other septicaemic diseases
  • Louping ill

Subacute fasciolosis

The major presenting clinical findings are rapid loss of body condition and poor fleece quality despite adequate flock nutrition. (Figs 4-5) Typically, some sheep present with severe depression, inappetance, weakness, and may be unable to stand. Losses typically occur from December onwards but may be much earlier (October) 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 (Figs 6-8) ). Affected sheep may die in an emaciated state especially when infestation is compounded by the metabolic demands of advanced pregnancy/early lactation. Loss of the ewe and her lamb(s) can severely affect farm profits

  • 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.

Diagnosis

Acute/subacute fasciolosis

Diagnosis of acute/subacute fasciolosis is based upon the epidemiological data (high risk year such as 2008), 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.

Fig 8: Classical 'Bottle-jaw" may not always be the result of fluke infestation.

Treatment

Triclabendazole is highly effective at killing all stages of flukes responsible for acute fasciolosis. Drenched sheep should be moved to clean pasture or re-treated every three weeks for the next three months at least as prescribed by your veterinary surgeon.

Nitroxynil and oxyclosanide 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.

Improved nutrition is essential.

Fig 9: Unacceptable losses caused by chronic liver fluke.  Five dead ewes within two days is a considerable financial loss to the farmer.

Management/Prevention/Control measures

Fluke infestations are controlled by strategic drenching based upon advice written in the veterinary flock health plan. During low risk years 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 such as 2008, 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. In certain areas wildlife hosts may play an important role in the disease dynamics.

Fencing off snail habitats is rarely practicable and in most situations is cost prohibitive as these are often extensive sheep enterprises. Drainage is cost prohibitive and many properties are subject to environmental controls.

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.

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