Chronic liver fluke – clinical signs and diagnosis
- What is the impact and clinical signs of chronic fluke?
- Is my farm at risk?
- The impact of climate change
- Diagnosis of chronic liver fluke
Chronic liver fluke is not only a huge challenge for the sheep industry regarding animal welfare but also comes with a significant economic cost to the industry due to potential production losses, mortalities, treatment costs, and losses to processors at the abattoir. Losses per infected sheep are estimated to be £3-£5.1 One significant outbreak in a flock with triclabendazole resistance present was estimated to have cost £8.73 per ewe in the flock.2
What is the impact and clinical signs of chronic fluke?
As discussed in our August blog, there are 3 types of disease seen in sheep in the UK due to liver fluke – acute, subacute and chronic. Acute and subacute disease is discussed in ‘Acute and subacute liver fluke (fascioliasis) in lambs – clinical signs and diagnosis’, here we will look at chronic fluke in more detail.
Chronic fluke mainly affects sheep from January to April. This type of disease may be more of a concern for adults as most lambs have been marketed by the time chronic fluke becomes a problem. Longer term fat lambs and replacement ewe lambs should be monitored, however. This type of liver fluke disease is caused by adult fluke in bile ducts of the liver causing clinical signs which include chronic weight loss, diarrhoea, and ‘bottle jaw’.
Is my farm at risk?
The lifecycle of liver fluke is a relatively complicated lifecycle involving the intermediate host, the mud snail - Galba (lymnaea) truncatula and several free-living stages. Have a look here for more on the lifecycle of liver fluke (Fasciola hepatica).
As the mud snail is essential for the liver fluke lifecycle, identification of any areas of the farm that are suitable for the mud snail to live are important. Such areas include mud around the edge of ponds, streams, rivers and even hoof prints or tractor tyre ruts in muddy fields. They don’t tend to be found in areas that are shady or heavily poached. Slow-moving water with a neutral pH is preferred.
Identification of these areas will allow exposure of livestock to the mud snail to be avoided, if not at least reduced. Permanent fencing of snail habitats can be costly in the short term, impractical and reduce available grazing. Land drainage will reduce the presence of snail habitats; however, this is costly and may not be suitable for the environment. The simplest answer to reduce the risk from fluke is to keep stock off the wettest fields in the autumn and winter when the risk of disease is at its highest. The use of temporary fencing could be considered to help reduce the access sheep have to high-risk areas at high risk times of the year.
The impact of climate change
Over recent years we have seen milder winters and wetter summers which has had an impact upon the occurence of the parasite. Adequate moisture levels and temperatures greater than 10°c are required for the hatching of fluke eggs and multiplication of snails. Such conditions usually occur from May-October in the UK, however due to the change in climatic conditions over recent years timing and level of incidence can vary depending upon the type of summer and/or winter that we experience. In a bid to identify the level of risk posed by liver fluke there is a liver fluke forecast for the UK. This forecast is based on the current climatic conditions in different parts of the UK and how they are likely to influence the snail host and the subsequent fluke infection risk on pasture. Monthly parasite forecasts and alerts are produced by the National Animal Disease Information Service (NADIS) and are based on detailed weather data for individual 40 sq km areas across the UK (NADIS - National Animal Disease Information Service). Contact your veterinary practice to access the latest information to assist the decision-making process of when to treat for liver fluke if treatment is necessary at all.
Your veterinary practice may also have access to the Animal and Plant Health Agency Disease Surveillance Dashboard which gives up to date information regarding current UK fluke prevalence. The SCOPS website is a great go-to for further information on the control and treatment options for Liver Fluke. SCOPS is an industry led group that works in the interest of the UK sheep industry. It recognises that, left unchecked, anthelmintic resistance (including Triclabendazole-resistant Fluke) is one of the biggest challenges to the future health and profitability of the sector.
Diagnosis of chronic liver fluke
Adult liver flukes produce eggs, so fluke egg detection can be useful for diagnosis in late autumn/ winter, when chronic liver fluke is more likely. Liver fluke eggs look very similar to rumen fluke eggs (the adults of which are not thought to cause significant disease) but a lab should be able to distinguish between the two. Your vet will be able to advise whether individual or pooled samples are best for your farm and discuss treatment options.
Some of the diagnostic tests discussed here can also be of use in ewes to diagnose chronic fluke i.e. post mortem and abattoir feedback. Blood samples should be used with caution for chronic fluke infection as antibody levels can remain high in previously exposed older animals or even after a successful treatment, and therefore may not indicate a true current infection.
One other method of detection that may be utilised is the coproantigen ELISA test performed on individual faecal samples. This test can detect fluke infection around two to three weeks earlier than the faecal fluke egg count. However, if fluke burdens are low, there may be no advantage to this test as coproantigen levels can fluctuate from day to day. If the test returns a positive result, then this confirms active infection. Where a negative result is found but fluke is suspected, you may need to use other forms of diagnosis as seen above, investigate other causes of disease and repeat the coproantigen test in 4-8 weeks.
References:
1. NADIS - National Animal Disease Information Service
2. Sargison ND, Scott PR. Diagnosis and economic consequences of triclabendazole resistance in Fasciola hepatica in a sheep flock in south-east Scotland. Vet Rec. 2011 Feb 12;168(6):159. doi: 10.1136/vr.c5332. Epub 2011 Feb 1. PMID: 21493511.
Chronic liver fluke – clinical signs and diagnosis
- What is the impact and clinical signs of chronic fluke?
- Is my farm at risk?
- The impact of climate change
- Diagnosis of chronic liver fluke
Chronic liver fluke is not only a huge challenge for the sheep industry regarding animal welfare but also comes with a significant economic cost to the industry due to potential production losses, mortalities, treatment costs, and losses to processors at the abattoir. Losses per infected sheep are estimated to be £3-£5.1 One significant outbreak in a flock with triclabendazole resistance present was estimated to have cost £8.73 per ewe in the flock.2
What is the impact and clinical signs of chronic fluke?
As discussed in our August blog, there are 3 types of disease seen in sheep in the UK due to liver fluke – acute, subacute and chronic. Acute and subacute disease is discussed in ‘Acute and subacute liver fluke (fascioliasis) in lambs – clinical signs and diagnosis’, here we will look at chronic fluke in more detail.
Chronic fluke mainly affects sheep from January to April. This type of disease may be more of a concern for adults as most lambs have been marketed by the time chronic fluke becomes a problem. Longer term fat lambs and replacement ewe lambs should be monitored, however. This type of liver fluke disease is caused by adult fluke in bile ducts of the liver causing clinical signs which include chronic weight loss, diarrhoea, and ‘bottle jaw’.
Is my farm at risk?
The lifecycle of liver fluke is a relatively complicated lifecycle involving the intermediate host, the mud snail - Galba (lymnaea) truncatula and several free-living stages. Have a look here for more on the lifecycle of liver fluke (Fasciola hepatica).
As the mud snail is essential for the liver fluke lifecycle, identification of any areas of the farm that are suitable for the mud snail to live are important. Such areas include mud around the edge of ponds, streams, rivers and even hoof prints or tractor tyre ruts in muddy fields. They don’t tend to be found in areas that are shady or heavily poached. Slow-moving water with a neutral pH is preferred.
Identification of these areas will allow exposure of livestock to the mud snail to be avoided, if not at least reduced. Permanent fencing of snail habitats can be costly in the short term, impractical and reduce available grazing. Land drainage will reduce the presence of snail habitats; however, this is costly and may not be suitable for the environment. The simplest answer to reduce the risk from fluke is to keep stock off the wettest fields in the autumn and winter when the risk of disease is at its highest. The use of temporary fencing could be considered to help reduce the access sheep have to high-risk areas at high risk times of the year.
The impact of climate change
Over recent years we have seen milder winters and wetter summers which has had an impact upon the occurence of the parasite. Adequate moisture levels and temperatures greater than 10°c are required for the hatching of fluke eggs and multiplication of snails. Such conditions usually occur from May-October in the UK, however due to the change in climatic conditions over recent years timing and level of incidence can vary depending upon the type of summer and/or winter that we experience. In a bid to identify the level of risk posed by liver fluke there is a liver fluke forecast for the UK. This forecast is based on the current climatic conditions in different parts of the UK and how they are likely to influence the snail host and the subsequent fluke infection risk on pasture. Monthly parasite forecasts and alerts are produced by the National Animal Disease Information Service (NADIS) and are based on detailed weather data for individual 40 sq km areas across the UK (NADIS - National Animal Disease Information Service). Contact your veterinary practice to access the latest information to assist the decision-making process of when to treat for liver fluke if treatment is necessary at all.
Your veterinary practice may also have access to the Animal and Plant Health Agency Disease Surveillance Dashboard which gives up to date information regarding current UK fluke prevalence. The SCOPS website is a great go-to for further information on the control and treatment options for Liver Fluke. SCOPS is an industry led group that works in the interest of the UK sheep industry. It recognises that, left unchecked, anthelmintic resistance (including Triclabendazole-resistant Fluke) is one of the biggest challenges to the future health and profitability of the sector.
Diagnosis of chronic liver fluke
Adult liver flukes produce eggs, so fluke egg detection can be useful for diagnosis in late autumn/ winter, when chronic liver fluke is more likely. Liver fluke eggs look very similar to rumen fluke eggs (the adults of which are not thought to cause significant disease) but a lab should be able to distinguish between the two. Your vet will be able to advise whether individual or pooled samples are best for your farm and discuss treatment options.
Some of the diagnostic tests discussed here can also be of use in ewes to diagnose chronic fluke i.e. post mortem and abattoir feedback. Blood samples should be used with caution for chronic fluke infection as antibody levels can remain high in previously exposed older animals or even after a successful treatment, and therefore may not indicate a true current infection.
One other method of detection that may be utilised is the coproantigen ELISA test performed on individual faecal samples. This test can detect fluke infection around two to three weeks earlier than the faecal fluke egg count. However, if fluke burdens are low, there may be no advantage to this test as coproantigen levels can fluctuate from day to day. If the test returns a positive result, then this confirms active infection. Where a negative result is found but fluke is suspected, you may need to use other forms of diagnosis as seen above, investigate other causes of disease and repeat the coproantigen test in 4-8 weeks.
References:
1. NADIS - National Animal Disease Information Service
2. Sargison ND, Scott PR. Diagnosis and economic consequences of triclabendazole resistance in Fasciola hepatica in a sheep flock in south-east Scotland. Vet Rec. 2011 Feb 12;168(6):159. doi: 10.1136/vr.c5332. Epub 2011 Feb 1. PMID: 21493511.