Infection of Liver Fluke in Grazing Animals

Liver fluke in Sheep

Introduction

Fasciolosis is the affliction acquired 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, illness can range from seemingly unaffected animals with sub-clinical infection to sudden death in heavy, acute infections. Due to its circuitous life cycle disease is seasonal in nature, with moisture, warm conditions favouring its evolution on pastures. This also makes illness more mutual in wetter parts of the UK. Changing atmospheric condition 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 U.k. previously considered to exist low risk.

Cause

The liver fluke, Fasciola hepatica, infects a number of animals including cattle, sheep, goats and horses. Different other parasites of grazing animals, the liver fluke life bicycle requires an intermediate host, the mud snail (Galba truncatula). These snails are simply nowadays in clammy, muddy areas such as the borders of permanent water bodies, wet flushes (often identifiable through presence of rushes and other water loving institute 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 to a higher place 10^oC, meaning development of liver fluke on pastures in the UK generally occurs from mid- to late-Bound through to early on Autumn.

The development time of liver fluke within the mud snail host can be prolonged, taking six weeks or more than 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 wintertime, pastures can remain infective for most of the year.

One time ingested, the juvenile flukes drift to, and so through the liver causing tissue damage as they go earlier reaching the liver bile ducts, where they mature to adults and brainstorm 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 non develop against this parasite. Due to the biological science of the external life stages, and its mud snail host, fluke hazard is greater when in that location has been sustained warm, wet weather condition. Unsurprisingly, fluke is a bigger problem in parts of the Uk with suitable weather conditions, although it besides appears to be increasingly common in areas previously considered lower take a chance equally a result of changing weather condition patterns.

Additionally, and again dissimilar 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 i: Fluke lifecycle

Welfare and Economics

Fasciolosis can have a serious impact on sheep wellness and welfare. Inadequate command during high risk periods for acute affliction can lead to stock losses of upwardly to x% in an affected flock through internal bleeding and anaemia or secondary complications caused by extensive liver damage. Chronic disease leads to reduced weight-proceeds and loss in body status and is besides associated with poor fertility in ewes. It is estimated that increased bloodshed of ewes and reduced weight-gain in lambs caused past chronic fasciolosis may reduce farm profits considerably. Fluke is too the second highest crusade for butchery condemnations. Overall, economic losses to the Great britain sheep industry caused past fasciolosis were estimated at £xiii-15 million in 2011 making information technology 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 sure times of yr.

Astute disease is caused by the ingestion and migration of big numbers of juvenile flukes through liver tissue. This tends to occur most commonly in the early autumn, but may exist seen before in the season under certain circumstances. Following the emergence of very big numbers of infective "cysts" onto pastures at this fourth dimension 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 iii weeks afterwards ingestion where collectively they cause extensive and severe liver damage and massive blood loss. Animals may simply be found dead as a result of the resulting internal bleeding, or may succumb to secondary infection with clostridial bacteria (Black Disease) which tin can multiply rapidly in the damaged livers of unvaccinated animals. Sudden expiry is therefore often the beginning sign of astute fluke. Other less severely afflicted animals in the group may be lethargic and have reduced grazing action. 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-acutefasciolosis is likewise the upshot 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 big numbers of new "cysts" on pasture. Animals present with a rapid loss of trunk condition and poor fleece quality despite acceptable flock nutrition. Typically, some sheep nowadays with severe depression, inappetence, weakness and may be unable to stand up. The liver damage caused past sub-acute fasciolosis has a huge result on growth rate, end weights and body condition. Mortality rates tin be high, but deaths commonly occur post-obit 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 belatedly winter and early jump, although since fluke infections in sheep can exist long-lived may nowadays at other times of year also. Affected animals have poor body condition and fleece quality together with a general dullness and angst. These sheep may also have fluid accumulation such as the feature "bottle-jaw" resulting from long-term anaemia and impaired liver function. Affected sheep may progressively waste material 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 of import source of pasture contagion with fluke eggs.

Effigy 5: Classic "bottle jaw" fluid accumulation under the mentum

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

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

Serum antibody ELISA. This detect antibodies in the claret raised in response to fluke infection. Antibody levels increase from as early equally 2 weeks following initial infection and are therefore very useful for monitoring of acute illness in leap lambs. However, antibody levels may remain elevated for weeks or months fifty-fifty subsequently infections accept been effectively treated.
Fluke egg counts. These are based on ten-40g of faeces and are useful in detecting developed chronic infection of 12 weeks or more than. Egg counts are of no value in diagnosing astute affliction, since juvenile stage parasites do not produce eggs. Faecal samples can exist pooled to produce a unproblematic screening method for infection in groups of animals and are of detail use in tardily winter/ early spring and for evaluating treatment efficacy.
Faecal antigen ELISA. Similar to egg counts, this ELISA detects fluke fabric 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 illness the liver is enlarged with bleeding below the surface and numerous tracks acquired by immature fluke present in the liver tissue, whilst with chronic fluke the liver may appear stake with thickened enlarged bile ducts containing adult fluke, identifiable equally leaf-shaped parasites measuring i-2cm.
Where available, slaughterhouse inspection information can provide a useful insight into the level of infection within a flock and the success of control programmes in identify.
Claret sampling of suspected cases tin 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 exist identified in a faeces sample. Shedding may exist intermittent however, and then a negative faeces screen does non eliminate the chance of liver fluke infection

Prevention

Liver fluke command 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 muddied areas bordering permanent water bodies, moisture flushes (oft 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 summit risk periods. Drainage of marshes and wetlands on farms may assist to reduce snail habitat and fluke risk on-farm, merely is also often at odds with agri-environment schemes.

Due to the importance of weather on its development, fluke risk varies betwixt years, by region and flavor. 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 flavour can pb to a risk over the summer months too. To this end, NADIS produces both a summer fluke forecast in August, and an fall fluke forecast in Oct to December. These tin be useful in gauging level of risk in an area, simply must be interpreted alongside local, farm-level factors such equally grazing strategies, history of disease, diagnostic testing and the nature of "flukey" pastures, some of which may remain permanently wet fifty-fifty over dry out 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. Claret antibody ELISAs are the about appropriate selection for this in first season lambs. Routine, monthly sampling of six to 10 animals volition let identification of early phase infection and treatment at the right time with an appropriate product effective confronting immature flukes to prevent clinical illness such as triclabendazole.

Drug resistance, particularly against triclabendazole is as well a major business concern in the UK, with cases of handling failure reported in sheep flocks across the United kingdom. To reduce selection for triclabendazole resistance on subcontract, alternative treatments should be considered where possible. Delaying treatment in clinically salubrious animals at winter housing will allow use of a range of culling products constructive against mature adults (run across beneath). 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 simply no current show of liver fluke demand to keep fluke out with effective biosecurity and quarantine measure. Gamble of fluke infection for any stock brought onto farms should be considered, and quarantine measures applied as role of the wider flock wellness plan. Quarantine is also of import on farms where liver fluke is already present to forbid the introduction of drug-resistance. Recommended quarantine measures for fluke and current SCOPS guidelines are based on sequential treatments, diagnostic testing to bank check for treatment failure, and keeping animals on dry out well drained grazing until these measures have been completed. Consequently, treatments with culling products such every bit closantel or nitroxynil should exist considered on arrival with repeat treatments administered six to 7 weeks later depending on the product used. Where acute fasciolosis is of concern, treatment with triclabendazole may be indicated, followed past treatment with an alternative product seven weeks afterward.

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

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

For most farms with endemic fluke, prevention of clinical and subclinical illness volition be based on testing and strategic dosing with flukicides. Products ineffective confronting young fluke tin can exist used for strategic dosing when you are uncertain of fluke age, provided they are repeated at the right interval.

Treatment

When selecting an appropriate treatment for fasciolosis, understanding the age of fluke existence treated is essential. Seek a veterinary diagnosis for suspected clinical cases and take guidance on treatment. It should exist noted that no treatments that are constructive against liver fluke have whatsoever residual activity, making timing of treatments and grazing strategy important considerations. For acute fasciolosis, treatment with triclabendazole is recommended, the but production that is effective against the very early juvenile stages, both in the confront of an outbreak and as a preventative treatment when used in conjunction with antibiotic testing. Where possible, treated sheep should then be moved to safe pastures, otherwise animals may become re-infected - repeating treatment every 3 weeks is not a sustainable strategy. Where handling failure is suspected, veterinarian advice should be sought for advice on alternative treatments and follow-upwards diagnostic testing. Aside from drug resistance, information technology 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 impairment (due to high levels of young fluke) may reduce the animal's ability to metabolise triclabendazole into its agile form.
Inaccurate or underdosing dosing due to poorly maintained and calibrated equipment, inaccurate weight assessment, incorrect administration and utilize 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 bug into consideration. This involves performing egg counts on a pooled faecal sample from 10 chronically infected sheep earlier, and 21 days after handling with triclabendazole.

For chronic fasciolosis, particularly when treating in late winter and bound, 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 data near triclabendazole resistance and how to avoid can be found hither: https://world wide web.scops.org.united kingdom/internal-parasites/liver-fluke/tackling-resistance/

Information technology should be noted that some flukicide products are combination treatments for roundworms and liver fluke. These should only be used where a concurrent worm brunt also needs to exist treated. It should as well exist 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 adventitious worm resistance pressure, and ever 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