Liver Fluke Control in Sheep

Introduction

Faciolosis is the disease caused by liver fluke infestation. In the sheep sector slaughterhouse condemnations and laboratory surveillance show it is increasing and being seen further north and east in areas previously considered to be fluke free.

Climate change is reducing the predictability of the geographical range and season of fluke and the movement of infected sheep into previously free areas allows the parasite to establish

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Fasciolosis can have a serious financial impact on a sheep farm with immediate losses of up to 10 per cent caused by the acute or subacute form of disease. Chronic disease could halve profits by reducing the lamb crop and increasing ewe mortality. Losses are estimated at £3 to £5 per infected sheep. Fluke is the second highest cause for abattoir condemnations.

The liver fluke parasite, Fasciola hepatica, infects the liver of both cattle and sheep. Adult fluke are 2 to 3cm in size and live in the bile ducts laying eggs which enter the animals intestinal tract and end up on pasture. The eggs hatch and enter a specific species of snail, Galba truncatula. This stage requires wet and warm conditions above 7 to 10°C occurring from late spring to mid-autumn. The fluke develops in the snail,hatches out onto pasture and forms a cyst on the grass which is eaten by sheep. The immature fluke migrate through the liver structure causing damage as they develop into adults. It takes 10 to 12 weeks after ingestion for the fluke to reach maturity and start to lay eggs.

Figure 1. The fluke lifecycle.

Clinical Signs

Liver fluke causes three types of disease - acute, sub-acute and chronic depending upon the level of challenge and the animals’ resilience. The timing of disease depends on the timing of infestation of the snails but acute is usually seen from August to October, sub-acute from October onwards and chronic from December onwards.

Acute cases occur from three weeks after high numbers of immature fluke are eaten by the sheep. It causes severe disease due to massive blood loss in the liver or from secondary infection of the damaged liver by clostridial bacteria (Black Disease). Sudden deaths are the usual first indication of acute fluke. Others in the group may be lethargic and have reduced grazing activity. Gathering may be difficult because sheep have abdominal pain and are reluctant to run. Sudden deaths may affect up to 10 percent of sheep at risk causing grave financial loss.

Figure 2. Sheep with acute fluke die suddenly from severe bleeding and liver damage.

Figure 3. Sudden death caused by acute liver fluke.

Sub-acute fluke presents with rapid loss of body condition and poor fleece quality despite adequate flock nutrition. Typically, some sheep present with severe depression, inappetance, weakness and may be unable to stand. Losses usually occur from December onwards but may be seen much earlier (October) with a severe challenge. The damage from sub-acute fluke has a huge effect on growth rate, milk yield, finish weights and body condition. Mortality rates can be high but deaths are usually only seen after a period of clinical signs.

 Figure 4. Sub-acute fluke in a ewe lamb causing severe depression, weakness and inappetance.

The major signs of chronic fluke are very poor body condition, poor fleece quality and in many sheep, bottle-jaw. Affected sheep may die in an emaciated state especially during the high metabolic demands of advanced pregnancy or early lactation. Loss of the ewe and her lamb crop can severely affect farm profits. Production losses can also be seen from subclinical, chronic fluke.

Figure 5. Classical 'Bottle-jaw" is a common sign of chronic fluke but can be caused by other diseases

Figure 6. Emaciation caused by chronic liver fluke infestation (not all affected sheep have "bottle-jaw")

Figure 7. Unacceptable loss of five ewes in two days caused by chronic liver fluke - a considerable financial loss to the farmer.

Diagnosis

Suspicion of acute and sub-acute fluke as a cause of death or severe disease can be based on the farm history and on epidemiology data suggesting high fluke risk. A confirmed diagnosis can be made from post mortem findings where the liver is enlarged with bleeding below the surface and immature fluke are present in the liver tissue. Blood samples from suspected cases can be also be tested for elevated liver enzymes and low blood protein levels. Fluke egg counts will not detect acute and sub-acute cases - fluke eggs are not being produced yet.

Figure 8. Sub-acute fluke affected liver (left) compared to normal liver (right). 

Chronic fluke can be diagnosed by demonstration of fluke eggs in faecal samples, although the secretion of eggs can be intermittent. Fluke egg counts from individual infected sheep and pooled samples can test negative. Faecal samples can also be tested using the coproantigen ELISA test which can detect adult fluke infection 2 to 3 weeks earlier than a conventional fluke egg count. At post mortem, chronic liver damage is seen and mature fluke are found in the bile ducts and gallbladder. Blood samples can be used to check for evidence of exposure to fluke but they do not indicate current infection - antibody levels to fluke remain high for many months after effective treatment.

Figure 9. Liver damage caused by chronic fluke.

A diagnosis should be sought for sudden death cases which may also be a result of clostridial disease in unvaccinated stock (pulpy kidney, blackleg, Black disease or braxy), pasteurellosis, septicaemia and tick-borne illnesses including Louping Ill.

A veterinary diagnosis for poor condition affecting many sheep should also be sought. Other causes include: inadequate energy, protein or mineral nutrition; chronic parasitism including resistant worms; severe lameness; Johne's disease; Maedi-Visna and poor teeth.

Treatment

Understanding the age of fluke being treated is essential. Seek a veterinary diagnosis for suspected clinical cases and take guidance on treatment. For clinical outbreaks of acute and sub-acute fluke the only effective treatment is triclabendazole. Drenched sheep should be moved to clean pasture - repeating treatment every three weeks is not sustainable.

For chronic fluke cases and for strategic dosing always use an alternative to triclabendazole wherever possible. The extent of triclabendazole resistance is unknown and poses a real risk to our ability to treat immature fluke.

Some listed products are combinations of wormers and flukicides and should only be used where a concurrent worm burden also needs to be treated – seek advice and use worm egg counts to guide choices. Wherever possible use a narrow spectrum flukicide to avoid accidental worm resistance pressure.

Figure 10. Flukicides - Adapted from SCOPS (Detection and treatment options for liver fluke) and NOAH Compendium online July 2019.

Prevention and Control

Farms with a suitable snail habitat but no evidence of liver fluke need to keep fluke out with effective biosecurity. When buying in sheep, liver fluke quarantine treatment strategies should be considered based on the risk posed by the incoming sheep and the risk status of the farm. Details of these strategies can be found at: https://www.scops.org.uk/internal-parasites/liver-fluke/fluke-quarantine/

Farms with no fluke history must continue to monitor for the disease based on abattoir reports or testing. 

For farms with fluke, avoiding the highest risk pastures might be possible in some cases. Fencing off snail habitats is rarely practicable and in most situations it is cost prohibitive.  Drainage to remove snail habitats is also costly and many grazing areas are subject to environmental controls.

For most farms with endemic fluke, prevention of clinical and subclinical disease will be based on testing and strategic dosing with flukicides. Timing and treatment choices should be based on forecasts from weather data specific to the area (e.g. the NADIS parasite forecasts) in conjunction with a specific testing and treatment regime devised for your flock and recorded in the flock health plan. Products ineffective against immature fluke can be used for strategic dosing when you are uncertain of fluke age, provided they are repeated at the correct interval (6-7 weeks). 

Figure 10. Store lambs grazing wet pastures are at risk from fluke

An example treatment plan may suggest during low risk years to drench with triclabendazole in advance of the predicted challenge during October, with an alternative flukicide (possibly closantel) administered in January. Additional treatments required in spring should be effective against adults only. In years when epidemiological data indicate a high risk of fasciolosis, an earlier triclabendazole treatment may be necessary in September.