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Liver fluke in Sheep

Introduction

Fasciolosis is the disease caused by liver fluke (Fasciola hepatica) infection. This parasite is capable of infecting a range of animal species including cattle, sheep, goats and horses. In sheep, disease can range from seemingly unaffected animals with sub-clinical infection to sudden death in heavy, acute infections. Due to its complex life cycle disease is seasonal in nature, with wet, warm conditions favouring its development on pastures. This also makes disease more common in wetter parts of the UK. Changing weather patterns appear to favour development of this parasite. This is reflected in an increasing number of cases identified through laboratory surveillance and abattoir inspection in sheep and its emergence in parts of the UK previously considered to be low risk.

 

Cause

The liver fluke, Fasciola hepatica, infects a number of animals including cattle, sheep, goats and horses. Unlike other parasites of grazing animals, the liver fluke life cycle requires an intermediate host, the mud snail (Galba truncatula). These snails are only present in damp, muddy areas such as the borders of permanent water bodies, wet flushes (often identifiable through presence of rushes and other water loving plant species), ditches, boggy areas etc. often referred to as “flukey” pastures. Mud snails and the external life stages of liver fluke are only active when temperatures rise above 10oC, meaning development of liver fluke on pastures in the UK generally occurs from mid- to late-Spring through to early Autumn.

The development time of liver fluke within the mud snail host can be prolonged, taking six weeks or more depending on weather conditions. Consequently, pastures are usually at their most infectious from August to October, although due to the long-lived nature of the infective “cyst” stages, and survival of infected mud snails over winter, pastures can remain infective for most of the year. 

Once ingested, the juvenile flukes migrate to, and then through the liver causing tissue damage as they go before reaching the liver bile ducts, where they mature to adults and begin producing eggs at around 10-12 weeks post-infection.

Unlike roundworms, sheep of all ages are susceptible to infection with liver fluke, as protective immunity does not develop against this parasite. Due to the biology of the external life stages, and its mud snail host, fluke risk is greater when there has been sustained warm, wet weather. Unsurprisingly, fluke is a bigger problem in parts of the UK with suitable weather conditions, although it also appears to be increasingly common in areas previously considered lower risk as a result of changing weather patterns.

Additionally, and again unlike roundworms, both sheep and cattle are susceptible to infection with liver fluke meaning pastures grazed by one species can be a source of infection for the other, including drug resistant strains of the parasite.

 

 

Figure 1: Fluke lifecycle

Welfare and Economics

Fasciolosis can have a serious impact on sheep health and welfare. Inadequate control during high risk periods for acute disease can lead to stock losses of up to 10% in an affected flock through internal bleeding and anaemia or secondary complications caused by extensive liver damage. Chronic disease leads to reduced weight-gain and loss in body condition and is also associated with poor fertility in ewes. It is estimated that increased mortality of ewes and reduced weight-gain in lambs caused by chronic fasciolosis may reduce farm profits considerably. Fluke is also the second highest cause for abattoir condemnations. Overall, economic losses to the UK sheep industry caused by fasciolosis were estimated at £13-15 million in 2011 making it comparable to sheep scab in terms of economic impact.

It has also been projected that emergence of drug resistant liver fluke may lead to further welfare and economic losses through ineffective treatments.

Figure 2: Both acute and chronic fluke can cause significant losses

Clinical signs and diagnosis

 

Liver fluke can present in different ways depending upon the severity and stage of infection. Due to the seasonal nature of its development, different forms of the disease are typically seen at certain times of year.

 

Acute disease is caused by the ingestion and migration of large numbers of juvenile flukes through liver tissue. This tends to occur most commonly in the early autumn, but may be seen earlier in the season under certain circumstances. Following the emergence of very large numbers of infective “cysts” onto pastures at this time sheep may become infected with large numbers (potentially several thousand) parasites in a very short space of time. These juvenile stage fluke reach the liver from around three weeks after ingestion where collectively they cause extensive and severe liver damage and massive blood loss. Animals may simply be found dead as a consequence of the resulting internal bleeding, or may succumb to secondary infection with clostridial bacteria (Black Disease) which can multiply rapidly in the damaged livers of unvaccinated animals. Sudden death is therefore often the first sign of acute fluke. Other less severely affected animals in the group may be lethargic and have reduced grazing activity. Gathering may be difficult because of their abdominal pain, general dullness and reluctance to run.

 

 

Figure 3: Sheep with acute fluke are often found dead

Sub-acute fasciolosis is also the result of migration of juvenile fluke through the liver tissue, and is consequently also typically seen in animals in the autumn months onwards following emergence of large numbers of new “cysts” on pasture. Animals present with a rapid loss of body condition and poor fleece quality despite adequate flock nutrition. Typically, some sheep present with severe depression, inappetence, weakness and may be unable to stand. The liver damage caused by sub-acute fasciolosis has a huge effect on growth rate, finish weights and body condition. Mortality rates can be high, but deaths usually occur following these clinical signs.

 

Figure 4: Sub-acute infection results in depression, lethargy and inappetance

Chronic fasciolosis is caused by adult flukes residing in the bile ducts of the liver. This is commonly seen in late winter and early spring, although since fluke infections in sheep can be long-lived may present at other times of year also. Affected animals have poor body condition and fleece quality together with a general dullness and malaise. These sheep may also have fluid accumulation such as the characteristic “bottle-jaw” resulting from long-term anaemia and impaired liver function. Affected sheep may progressively waste and die in an emaciated state especially during advanced pregnancy or early lactation due to the increased energy demands at this time. Chronically infected animals may show no obvious outwards signs of infection but still may incur production losses. These animals are also an important source of pasture contamination with fluke eggs.

 

Figure 5: Classic “bottle jaw” fluid accumulation under the chin

 

Figure 6: Severe emaciation associated with chronic liver fluke infection.

Diagnostic testing for fasciolosis can be performed by a number of different methods. These are:

  • Serum antibody ELISA. This detect antibodies in the blood raised in response to fluke infection. Antibody levels increase from as early as 2 weeks following initial infection and are therefore very useful for monitoring of acute disease in spring lambs. However, antibody levels may remain elevated for weeks or months even after infections have been effectively treated.
  • Fluke egg counts. These are based on 10-40g of faeces and are useful in detecting adult chronic infection of 12 weeks or more. Egg counts are of no value in diagnosing acute disease, since juvenile stage parasites do not produce eggs. Faecal samples can be pooled to produce a simple screening method for infection in groups of animals and are of particular use in late winter/ early spring and for evaluating treatment efficacy.
  • Faecal antigen ELISA. Similar to egg counts, this ELISA detects fluke material in faecal samples. This test is capable of detecting infection at an earlier stage than fluke egg counts at between 4 and 8 weeks post-infection.
  • Post-mortem of lost stock allows identification of active fluke infection in both acute and chronic cases. With acute disease the liver is enlarged with bleeding below the surface and numerous tracks caused by immature fluke present in the liver tissue, whilst with chronic fluke the liver may appear pale with thickened enlarged bile ducts containing adult fluke, identifiable as leaf-shaped parasites measuring 1-2cm.
  • Where available, abattoir inspection data can provide a useful insight into the level of infection within a flock and the success of control programmes in place.
  • Blood sampling of suspected cases can also be tested for elevated liver enzymes, low blood protein and elevated white blood cells to support an early diagnosis.

 

Figure 7: Severely damaged liver (left) compared to a normal liver (right)

 

Figure 8: Mature liver fluke lay eggs which can be identified in a faeces sample. Shedding may be intermittent however, so a negative faeces screen does not eliminate the chance of liver fluke infection

Prevention

Liver fluke control needs to take into consideration the farm history, topography, geographical location and prevailing weather. It is worthwhile taking the time to identify “flukey” pastures on farms. These include muddy areas bordering permanent water bodies, wet flushes (often identifiable through presence of rushes and other water loving plant species), ditches, boggy areas etc. and, where possible, either fencing these areas off, or avoid grazing them during peak risk periods. Drainage of marshes and wetlands on farms may help to reduce snail habitat and fluke risk on-farm, but is also often at odds with agri-environment schemes.

Due to the importance of weather on its development, fluke risk varies between years, by region and season. Summers characterised by high levels of rainfall and warm temperatures pose the greatest risk for acute fasciolosis in autumn, although survival of large numbers of infected snails over winter from the previous season can lead to a risk over the summer months too. To this end, NADIS produces both a summer fluke forecast in August, and an autumn fluke forecast in October to December. These can be useful in gauging level of risk in an area, but must be interpreted alongside local, farm-level factors such as grazing strategies, history of disease, diagnostic testing and the nature of “flukey” pastures, some of which may remain permanently wet even over dry summers. 

Due to the potentially serious consequences of outbreaks of acute fasciolosis, lack of clinical signs prior to death and the emergence of drug resistance, increasing emphasis is being placed on the value of diagnostic testing in sheep for acute disease. Blood antibody ELISAs are the most appropriate choice for this in first season lambs. Routine, monthly sampling of 6 to 10 animals will allow identification of early stage infection and treatment at the correct time with an appropriate product effective against immature flukes to prevent clinical disease such as triclabendazole.

Drug resistance, particularly against triclabendazole is also a major concern in the UK, with cases of treatment failure reported in sheep flocks across the UK. To reduce selection for triclabendazole resistance on farm, alternative treatments should be considered where possible. Delaying treatment in clinically healthy animals at winter housing will allow use of a range of alternative products effective against mature adults (see below). Similarly, fluke infections identified in adult sheep by worm egg counts indicate chronic infection and presence of adult flukes.

Farms with suitable habitat for mud snails but no current evidence of liver fluke need to keep fluke out with effective biosecurity and quarantine measure. Risk of fluke infection for any stock brought onto farms should be considered, and quarantine measures applied as part of the wider flock health plan. Quarantine is also important on farms where liver fluke is already present to prevent the introduction of drug-resistance. Recommended quarantine measures for fluke and current SCOPS guidelines are based on sequential treatments, diagnostic testing to check for treatment failure, and keeping animals on dry well drained grazing until these measures have been completed. Consequently, treatments with alternative products such as closantel or nitroxynil should be considered on arrival with repeat treatments administered 6 to 7 weeks later depending on the product used. Where acute fasciolosis is of concern, treatment with triclabendazole may be indicated, followed by treatment with an alternative product 7 weeks later.

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

 

Figure 9: Mixed grazing on marshy land means that liver fluke infection is inevitable, and must be managed appropriately. Image credit: Phoebe McCarter

For most farms with endemic fluke, prevention of clinical and subclinical disease will be based on testing and strategic dosing with flukicides. 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. 

Treatment

When selecting an appropriate treatment for fasciolosis, understanding the age of fluke being treated is essential. Seek a veterinary diagnosis for suspected clinical cases and take guidance on treatment. It should be noted that no treatments that are effective against liver fluke have any residual activity, making timing of treatments and grazing strategy important considerations. For acute fasciolosis, treatment with triclabendazole is recommended, the only product that is effective against the very early juvenile stages, both in the face of an outbreak and as a preventative treatment when used in conjunction with antibody testing. Where possible, treated sheep should then be moved to safe pastures, otherwise animals may become re-infected - repeating treatment every three weeks is not a sustainable strategy. Where treatment failure is suspected, veterinary advice should be sought for advice on alternative treatments and follow-up diagnostic testing. Aside from drug resistance, it is important to consider other reasons for treatment failure on farm. These include: 

  • Grazing of highly infective pastures and immediate re-infection following treatment.
  • Existing liver damage (due to high levels of immature fluke) may reduce the animal’s ability to metabolise triclabendazole into its active form.
  • Inaccurate or underdosing dosing due to poorly maintained and calibrated equipment, inaccurate weight assessment, incorrect administration and use of old, out-of-date product.

To further investigate the cause of treatment failure, a faecal egg count reduction test can be performed on farm taking the above issues into consideration. This involves performing egg counts on a pooled faecal sample from 10 chronically infected sheep before, and 21 days after treatment with triclabendazole.

For chronic fasciolosis, particularly when treating in late winter and spring, and for strategic and quarantine dosing, alternative products should be used where possible to preserve the efficacy of triclabendazole for when it is most needed. Several alternative products are available, with ranging efficacies against the earlier juvenile stages.

More information about triclabendazole resistance and how to avoid can be found here: https://www.scops.org.uk/internal-parasites/liver-fluke/tackling-resistance/

It should be noted that some flukicide products are combination treatments for roundworms and liver fluke. These should only be used where a concurrent worm burden also needs to be treated. It should also be noted that for use as a flukicide albendazole must be administered at a higher dose than is used to treat roundworms. Where in doubt, seek veterinary advice and use diagnostic testing to guide treatment choices. Wherever possible use a narrow spectrum flukicide to avoid accidental worm resistance pressure, and always check the datasheets to determine correct dosing and administration for the product and observe withdrawal periods.

 

 

Figure 10: Table of flukicides adapted from SCOPs and NOAH Compendium online