Difference between revisions of "Fasciolosis"

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Also known as: '''''Fascioliasis  — Fasciolasis — Fluke
  
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==Introduction==
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Fasciolosis is a condition of ruminants which causes subclinical and clinical disease leading to ill thrift and deaths. The causative organism is the trematode ''[[Fasciola hepatica]]'' which primarily parasitises the bile ducts of sheep and cattle but may occasionally be found in the horse. ''Lymnaea truncatula'', a mud snail, is the intermediate host of ''[[Fasciola hepatica]]'', and transmission of disease is dependent on the presence of appropriate snail habitats. These habitats are more plentiful in areas of high rainfall, such as the western British Isles. However, infected animals may be found outwith these areas due to the transportation of livestock, or unusual weather patterns. The association of fasciolosis with wetter habitats lends a [[Fasciola hepatica#Epidemiology|seasonal nature to disease outbreaks]], and can help predict the severity of these.
  
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[[Image:Fasciola   hepatica.jpg|thumb|right|150px|''Fasciola hepatica''. Source: Wikimedia Commons; Author:Adam Cuerden(2007)]]
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|Also known as:
 
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|'''Fascioliasis''' <BR> '''Fasciolasis''' <BR> '''Fluke'''
 
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|}
 
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==Description==
 
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[[Image:Fasciola hepatica ruminant.jpg|thumb|right|150px|''Fasciola hepatica'' - Joaquim Castellà Veterinary Parasitology Universitat  Autònoma de Barcelona]]
 
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[[Image:Fasciola  hepatica  adult.jpg|thumb|right|150px|''Fasciola hepatica'' adults from a  horse -  Castellà Veterinary Parasitology Universitat Autònoma de  Barcelona]]
 
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Fasciolosis is a condition of ruminants which causes subclinical and clinical disease leading to ill thrift and deaths. The causative organism is the trematode ''[[Fasciola hepatica]]'' which primarily parasitises the bile ducts of sheep and cattle but may occasionally be found in the horse. ''Lymnaea truncatula'', a mud snail, is the intermediate host of ''[[Fasciola hepatica]]'', and transmission of disease is dependent on the presence of appropriate snail habitats. These habitats are more plentiful in areas of high rainfall, such as the western British Isles. However, infected animals may be found outwith these areas due to the transportation of livestock, or unusual weather patterns. The association of fasciolosis with wetter habitats lends a seasonal nature to disease outbreaks, and can help predict the severity of these.
 
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For instance, wet summers increase both the number of snail habitats and the hatching of fluke eggs, leading to many infected snails. These in turn shed many cercariae, which form a high density of metacercariae on herbage to increase the risk of fasciolosis. Conversely, in dry summers, fewer fluke eggs hatch and snails are restricted to their permanent habitats. Fewer snails become infected and cercariae and metacercariae numbers are low and confined to the areas where snails can survive. The risk of fasciolosis is therefore reduced.
 
  
 
In sheep, "acute" disease caused by fluke larvae is the most common presentation, and generally occurs in the wetter Autumn and early Winter months in both lambs and ewes. Fasciolosis in cattle can occur at any time of year and tends to involve adult fluke, causing "chronic" disease.
 
In sheep, "acute" disease caused by fluke larvae is the most common presentation, and generally occurs in the wetter Autumn and early Winter months in both lambs and ewes. Fasciolosis in cattle can occur at any time of year and tends to involve adult fluke, causing "chronic" disease.
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==Signalment==
 
==Signalment==
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[[Image:Fasciola  hepatica.jpg|thumb|right|150px|''Fasciola hepatica -  Wikimedia Commons]]
 
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Fasciolosis affects both young and adult animals, primarily sheep and cattle.
 
Fasciolosis affects both young and adult animals, primarily sheep and cattle.
  
 
==Diagnosis==
 
==Diagnosis==
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A diagnosis can usually be made using clinical findings and the seasonal occurrence of disease, and may be supported by a history of fasciolosis on the farm and post-mortem findings. Certain adjunctive tests may also prove useful.
 +
 
===Clinical Signs===
 
===Clinical Signs===
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[[Image:Fasciola  hepatica    adult.jpg|thumb|right|150px|''Fasciola hepatica'' adults from a  horse -    Castellà Veterinary Parasitology Universitat Autònoma de  Barcelona]]
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====Sheep====
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In sheep, fasciolosis may present as acute, chronic, or infrequently sub-acute manifestations.
  
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In sheep, fasciolosis may present as acute, chronic, or infrequently sub-acute manifestations. Acute fasciolosis usually occurs between September and December and is caused by large numbers of immature ''[[Fasciola hepatica]]'' migrating through the liver parenchyma and causing massive damage. If sheep are not exposed to at-risk pasture until later in the year, acute fasciolosis may occur as late as the following Feburary. Hepatic damage caused by migration of fluke larvae gives clinical signs including lethargy, pallor, dyspnoea and death in both young and adult animals. Handling of sheep may cause liver rupture and sudden death, and sudden death may also occur due to Black's disease (''Clostridium novyi'' type B) or bacillary haemoglobinuria (''Clostridium novyi'' type D) in unvaccinated sheep. This is a result of larval migrationg facilitiating the activation of clostridial spores.
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'''Acute''' fasciolosis usually occurs between September and December and is caused by large numbers of immature ''[[Fasciola hepatica]]'' migrating through the liver parenchyma and causing massive damage. It arises within around two to six weeks of ingestion of metacercariae. If sheep are not exposed to at-risk pasture until later in the year, acute fasciolosis may occur as late as the following Feburary. Hepatic damage caused by migration of fluke larvae gives clinical signs including lethargy, pallor, dyspnoea and death in both young and adult animals. Handling of sheep may cause liver rupture and sudden death, and sudden death may also occur due to [[Infectious Necrotic Hepatitis|Black's disease]] (''[[Clostridium novyi]]'' type B) or [[Bacillary Haemoglobinuria|bacillary haemoglobinuria]] (''[[Clostridium haemolyticum|Clostridium novyi'' type D]]) in unvaccinated sheep. This is a result of necrosis caused by larval migration within the liver: anaerobic conditions are created, enabling multiplication of clostridial organisms and thus toxin production.  
  
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Chronic fasciolosis in sheep is caused by adult flukes in the bile ducts and is usually seen in February and March. However, cases may present in early summer if snails become infected during the winter. Progressive weight loss over weeks to months results in poor body condition, and anorexia is often seen. Other features include anaemia, and ascites and/or submandibular oedema due to hypoalbuminaemia.  
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'''Chronic''' fasciolosis in sheep is caused by adult flukes in the bile ducts and is usually seen in February and March, 4-5 months after ingestion of metacercariae. However, cases may present in early summer if snails become infected during the winter. Progressive weight loss over weeks to months results in poor body condition, and anorexia is often seen. As adult flukes feed on blood and are capable of consuming 0.5ml each per day, [[Regenerative and Non-Regenerative Anaemias|anaemia]] and pallor frequently occur in chronic fasciolosis. Initially, this regenerative anaemia is normochromic, but becomes hypochromic as iron reserves are depleted. Hypoalbuminaemia may also result from whole blood loss, and from reduced hepatic production. This gives a reduced plasma oncotic pressure, leading to ascites and/or submandibular oedema in advanced cases.
  
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In some cases, sub-acute fasciolosis may occur if infections has occured over a prolonged period. In these instances, disease is caused by both adult flukes and larvae and ill thrift, lethargy, dyspnoea is seen from around December to March.
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In some cases, sub-acute fasciolosis may occur if infection has occurred over a prolonged period. In these instances, disease is caused by both adult flukes and larvae and ill thrift, lethargy, dyspnoea is seen from around December to March.
  
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In addition to these presentations, ''[[Fasciola heptatica]]'' has subclinical effects on sheep. Fleece weight and fibre quality are affected by even small fluke burdens, and there is some evidence that lambing percentage and lamb growth rates may be negatively influenced. Condemnation of affected livers at slaughter also causes economic losses.
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In addition to these presentations, ''[[Fasciola hepatica]]'' has subclinical effects on sheep. Fleece weight and fibre quality are affected by even small fluke burdens, and there is some evidence that lambing percentage and lamb growth rates may be negatively influenced. Condemnation of affected livers at slaughter also causes economic losses.
  
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====Cattle====
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In cattle, fasciolosis is usually a disease of calves occuring between winter and spring, but may affect any animal at any time of year. Disease is usually chronic (caused by adult flukes) and signs tend to be less severe than in sheep. Poor nutrition and gastrointestinal parasitism do however exacerbate disease. As in sheep, subclinical losses include liver condemnation following slaughter and extended finishing times. Carcass values and milk yield may be reduced in beef and dairy cattle respectively. Milk quality may also be poorer in ''[[Fasciola hepatica]]'' infection.
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*Liver condemnations cause  economic losses at slaughter  
 
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*Longer finishing times to  slaughter weight and reduced carcass value in beef cattle
 
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*Reduced  milk yield and quality in dairy cows
 
  
 
===Laboratory Tests===
 
===Laboratory Tests===
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This is performed primarily on the clinical findings, seasonal  occurance, as well as a previous history of fasciolosis on the farm.  These along with post-mortem form the basis for diagnosis of Fasciola hepatica. In practice, diagnosis of ovine fasciolosis is often more  straightforward than bovine fasciolosis. 
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A simple and effective test for ''[[Fasciola hepatica]]'' infection is examination of the faeces for fluke eggs. These are large, ovoid and golden brown, but may be few in number and thus difficult to find. It must also be remembered that eggs are produced only by adult flukes, and so will not be detectable in acute disease.
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Examination  of faeces for liver fluke eggs is also useful, and may be complemented  by laboratory tests.  Firstly the measurement of Glutamate dehydrogenase  (GLDH) is used.  This is an enzyme released by damaged hepatic cells,  and levels become elevated within the first few weeks of infection.  Another test commonly used is measuring the glutamyl transpeptidase  (GGT), and this indicates damage to the epithelial cells of the bile  ducts, and is seen later in the cycle, and maintained for a longer  duration.
 
  
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An [[ELISA]] can also be used in the  detection process. It identifies antibodies against flukes in the serum and milk samples, and is deemd to be extremely reliable in this case.
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Measurement of serum glutamate dehydrogenase  (GLDH) and/or glutamyl transpeptidase (GGT) levels may prove useful. GLDH is an enzyme released by damaged hepatic cells and becomes elevated within the first few weeks of infection. GGT indicates damage to the epithelial cells lining the bile ducts and rises later in disease, although high levels are maintained for longer. Fluke serology is possible by means of a reliable [[ELISA testing|ELISA test]] that identifies anti-fluke antibodies in the serum or in milk.
  
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'''Acute  fasciolosis'''
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===Diagnostic Imaging===
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*Clincal history
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Although not commonly performed in the diagnosis of fasciolosis, ultrasonography can show hepatic changes that may be suggestive of fluke. The probe should be applied immediately caudal to the costal arch, approximately two-thirds of the way down the right hand side. This positioning may reveal an excess of peritoneal exudate and fibrinous adhesions between the liver and the small intestine or the body wall. The liver is seen to be enlarged, and the capsule appears hyperechoic due to fibrin deposition. The liver parenchyma also exhibits changes, appearing diffusely granular, rather than uniformly hypoechoic.
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**Season
 
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**Grazing
 
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*Post-mortem  examination
 
  
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'''Chronic fasciolosis'''
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===Biopsy===
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*Clinical  history
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Biopsy or histopathology may reveal necrotic areas of liver, haemorrhage, hyperplastic bile ducts or indeed the presence of ''[[Fasciola hepatica]]'' within the section.
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**Season
 
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**Grazing
 
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**Lethargy
 
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**Weight  loss
 
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**Anaemia (pallor)
 
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**Hypoalbuminaemia (bottle-jaw)
 
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**Gamma-glutamyl  transpeptidase increase (i.e. increase in hepatic enzymes)
 
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**Fluke  eggs in faeces
 
  
 
===Pathology===
 
===Pathology===
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====Acute Fascioliasis====
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[[Image:Fasciola   hepatica - bile duct.jpg|thumb|right|150px|Bile duct fibrosis due to chronic ''Fasciola hepatica'' infestation. Source: Wikimedia Commons; Author: Flukeman]]
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The  acute disease  is a less common type of Fasciola hepatica, and generally  occurs 2-6  weeks after large ingestion of metacercariae. The young  liver flukes  migrate through the liver parenchyma causing severe  haemorrhaging, due to the damage to the [[Liver - Anatomy &  Physiology|liver]] vasculature.  
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The pathology of fasciolosis is similar in both sheep and cattle, although the species have a different incidence of acute and chronic disease, as described above.
  
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In '''acute''' fasciolosis, immature flukes grow and migrate within the liver parenchyma, causing necrotic tracts and haemorrhage. On post-mortem, the liver is enlarged and a fibrinous peritonitis may be be present. Fibrin tags are often particularly apparent on the ventral lobe. Histologically, tracts are seen as areas of degenerate hepatocytes and haemorrhage, which may later become infiltrated with eosinophils and lymphocytes. In the long term, these areas will become fibrosed. In sub-acute fasciolosis, the liver again is enlarged on post-mortem, and haemorrhagic tracts can be seen.
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This occurs in late autumn  and winter, mainly  between the months of August to October. Outbreaks  of acute  fascioliasis usually present as sudden deaths.  On examination  infected  animals are weak, with pale mucous membranes.  They may also  have  enlarged livers, and the liver surface may be cover with a fibrinous peritonitis, particularly evident on the ventral lobe. Tracts  become  filled with blood and degenerate hepatocytes later infiltrated with [[Eosinophils|eosinophils]], [[Lymphocytes|lymphocytes]] and replaced  by fibrosis.
 
  
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[[Image:Fasciola  hepatica - bile duct.jpg|300px|thumb|right|'''Fasciola hepatica  (Copyright Flukeman, Wikimedia Commons) ''']]
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'''Chronic''' fasciolosis is associated with damage to the bile ducts by adult flukes. On post-mortem, the liver is distorted by areas of fibrosis caused by the migration of the original immature flukes. The bile ducts are dilated, and flukes may be expressed from within. The gall bladder may also be enlarged. The walls of the bile ducts may be ulcerated and haemorrhagic with areas of epithelial hyperplasia. The walls eventually become fibrosed and may calcify in cattle. Calcified bile ducts can be seen protruding from the liver surface - this is known as "pipe stem liver".
  
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====Subactute Fascioliasis====
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==Treatment==
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This  is caused by  ingestion of metacercariae over a longer period of time.  Some may have  migrated to the bile ducts, causing [[cholangitis]],  whilst other  metacercariae are migrating through the liver causing  lesions similar to those present in acute fascioliasis. The infected  host may present  with severe haemorrhagic anaemia, with  [[hypoalbuminaemia]], rapid loss  of body condition, reduced appetite,  pale mucous membranes, and submandibular oedema may also be present. On  post-mortem, an enlarged  liver is common and haemorrhagic tracts are  usually visible on the liver  surface. If left untreated, it is often  fatal. This form of  fascioliasis occurs around 6-10 weeks after  ingestion of the  metacercariae by the host, and like acute fascioliasis  occurs in late  autumn and winter.
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Due to the reliance of disease transmission on appropriate snail habitats and therefore weather, it has been possible to develop models to predict the occurrence of fasciolosis to help its control within flocks and herds. These models evaluate the soil moisture content from May to October by considering rainfall patterns and evapo-transpiration, weighted for season. Although June is a particularly influential month in these models, a drought in late summer can reverse predictions of potentially high snail density, and so forecasts should not be issued prematurely. A complicating factor in the prediction of fasciolosis is the fact that snail density is insufficient for disease in the absence of infection (i.e. deposited fluke eggs), and so forecasts generated must be interpreted in the context of local biology.
  
 +
Anthelmintic drugs are used in the control of fasciolosis. Not all flukicides are effective against each parasitic developmental stage, and so some may not be suitable for use in an outbreak of acute disease. Triclabendazole, a benzimidazole, is the flukicide with the broadest spectrum of activity against both immature and adult ''[[Fasciola hepatica]]'' and is therefore used to control acute disease. However, triclabendazole-resistant fluke populations are beginning to emerge. Albendazole, closantel, clorsulon and nitroxanyl all have a narrower spectrum of activity, primarily against adult fluke.
  
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====Chronic  Fascioliasis====
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There are two objectives to anthelmintic control of fluke in sheep and cattle. The first is to limit shedding of fluke eggs onto snail habitats, which is achieved by the use of any adulticidal drug in late winter/early spring. The second aim is to protect animals grazing metacercariae-contaminated pasture against fluke infection, and the approach to this is not so simplistic. Here, the choice of drug, the timing of treatment and dosing interval is dependent on: a) whether acute or chronic disease is the (anticipated) problem; b) the likely intensity of challenge, based on local knowledge or fluke forecasting and c) the persistence of the selected drug. When treating for chronic fasciolosis it is important to provide good quality nutrition following anthelmintic dosing, and response and need for further treatment can be monitored using faecal egg counts.
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This is usually seen in late winter,  early spring and is currently the most common fascoloiasis seen.  It  occurs around 4-5  months after ingestion of the metacercariae.  Hypochromic and macrocytic  anaemia and hypoalbuminaemia are common, as  the adult flukes are  capable of sucking up to 0.5ml of blood each day.  In heavy infections, this can prove to be a severe loss.  
 
  
 +
Previously, molluscicides have been employed to control fasciolosis and were successful. However, these are now infrequently used. This is in part due to the need for application before the likely impact of fluke can be predicted, potentially creating an unnecessary cost. Molluscides can also be difficult to use effectively since careful application is needed to avoid rapid recolonisation of treated land from any habitat that has been missed.
  
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Infected  animals may present with progressive loss of body  condtion, reduced  appeptite, which along with hypoalbuminaemia can  result in an gaunt  animal. Other common signs include pale mucous  membranes, and  submandibular oedema, more commonly known as 'bottle  jaw.'  On biopsy  the liver will have an irregular shape, distorted shape  with areas of  fibrous tissue replacing the cells damaged by the  migrating flukes. The  bile ducts appear dilated, and dark, and it is  often possible to express numerous numbers of adult flukes from within  the ducts. Pathology is similar in both sheep and cattle, expect in  cattle you may see calcification of the bile ducts, and enlargement of  the gall  bladder. The calcified bile ducts are often seen protruding  from the  liver surface, which is known as 'pipe stem liver.'
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The nature of fluke transmission means that is it possible to use environmental strategies to help control. Recognition of potential snail habitats allows areas to be fenced off or drained to limit snail numbers. Alternatively, animals may simply be grazed away from wet or boggy areas during high-risk periods.
  
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Metacercariae  excyst → immature  flukes present in the  small intestine → migrate  across the peritoneal  cavity (about 1 week) →  to the liver → migrate  through the liver  parenchyma for 6-7 weeks  becoming more destructive as  they grow →  enter bile ducts
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Finally, a [[Vaccines|vaccine]] against fluke in cattle is currently in development. This is a recombinant vaccine which is thought to provide around 70% protection by stimulating a range of immune responses not normally seen in chronically infected cattle, for example the [[T cell differentiation#TH1 Cells|Th1 response]].
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*The  prepatent  period is  10-12 weeks
 
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==== Pathogenesis of acute fasciolosis ====
 
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*'''Liver pathology'''
 
  
 +
==Prognosis==
  
 +
With effective treatment, the prognosis for fasciolosis is reasonable, but this is dependent upon the severity of disease at diagnosis, for example the prognosis for chronic infections is better than that for acute fasciolosis. Clearly strategies aimed at prevention provide the optimum prognosis for herd/flock health.
  
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**Flukes develop from 0.1mm-1cm within the liver  parenchyma causing trauma, necrotic tracts, and haemorrhages
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==Links==
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**Glutamate dehydrogenase (GDH) is released by damaged cells
 
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*Acute damage to liver causes post-necrotic scarring →  shrinkage of affected tissues and hypertrophy of normal tissue → the  typical appearance of the liver in chronic disease
 
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*Chronic  damage to bile ducts → peribiliary fibrosis
 
  
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(Note: other complex events also occur, including disruption  of haemodynamics, monolobular fibrosis, egg-granulomas etc.)
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*''[[Fasciola hepatica]]''.
 +
*[http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/22702.htm The Merck Veterinary Manual - ''Fasciola hepatica'']
 +
*[http://www.who.int/neglected_diseases/diseases/fascioliasis/en/index.html World Health Organisation - Fascioliasis]
 +
*[http://www.sac.ac.uk/mainrep/pdfs/tn557liverfluke.pdf Treatment and Control of Liver Fluke (Scottish Agricultural College)]
  
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*'''Black disease (Infectious  Necrotic Hepatitis)'''
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==Test yourself with the Trematodes Flashcards==
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**caused by a toxin produced by  ''Clostridium novyi'' type B.
 
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**It is commonly  associated with liver fluke infestation because migrating flukes → liver  necrosis → anaerobic conditions → clostridial multiplication → toxin  production → disease
 
  
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==== Pathogenesis  of chronic fasciolosis ====
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[[Trematodes_Flashcards|Trematodes Flashcards]]
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*'''Bile duct damage'''
 
  
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**Adult flukes (2-5cm long) in bile ducts feed on epithelium  and blood
 
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**Chronic inflammatory responses → fibrosis of  bile duct wall (and, in cattle, calcification)
 
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**Gamma  glutamyl transpeptidase released by damaged cells
 
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**Ulceration  and haemorrhage of bile duct → epithelial hyperplasia and increased  mucosal permeability
 
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*'''Anaemia'''
 
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**250  flukes → up to 50ml of blood loss daily → 10 times increase in rate of  erythropoiesis → normochronic anaemia until iron stores are exhausted →  hypochromic anaemia.
 
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*'''Hypoalbuminaemia'''
 
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**Albumin (and other plasma proteins) lost into bile duct  because of
 
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#Whole blood loss
 
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#Increased  epithelial permeability
 
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** → Increased catabolic rate by  2.5x → increased nitrogen loss via urine
 
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**The effects  seen depend on the magnitude of nitrogen loss
 
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#There is  no obvious effect (although animal is still in abnormal physiological  state)
 
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#Reduced weight-gain and/or wool growth and/or  milk production
 
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#Loss of body tissue (i.e. weight-loss);  hypoalbuminaemia → reduced plasma oncotic pressure → oedema
 
  
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==Treatment==
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==Literature Search==
 +
[[File:CABI logo.jpg|left|90px]]
  
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=== Forecasting  Fasciolosis ===
 
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Several prediction  models have been  developed. They evaluate the wetness of the soil from  '''May''' to  '''October''' by taking account of rainfall patterns and  evapo-transpiration. Seasonal weighting factors are applied. '''June'''  is a particularly influential month.
 
  
 +
Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).
 +
<br><br><br>
 +
[http://www.cabdirect.org/search.html?rowId=1&options1=AND&q1=%28title%3A%28%22Fasciolosis%22%29+OR+title%3A%28Fascioliasis%29+OR+title%3A%28Fasciolasis+%29+OR+title%3A%28%22liver+fluke%22%29+OR+title%3A%28%22fasciola+hepatica%22%29%29+AND+od%3A%28cattle%29&occuring1=freetext&rowId=2&options2=AND&q2=&occuring2=freetext&rowId=3&options3=AND&q3=&occuring3=freetext&publishedstart=2000&publishedend=yyyy&calendarInput=yyyy-mm-dd&la=any&it=any&show=all&x=56&y=10 Fasciolosis in cattle publications since 2000]
  
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A drought in late  summer can reverse a potentially dangerous  trend, and so unqualified  forecasts should not be issued prematurely.
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[http://www.cabdirect.org/search.html?rowId=1&options1=AND&q1=%28%28title%3A%28%22Fasciolosis%22%29+OR+title%3A%28Fascioliasis%29+OR+title%3A%28Fasciolasis+%29+OR+title%3A%28%22liver+fluke%22%29+OR+title%3A%28%22fasciola+hepatica%22%29%29+AND+od%3A%28sheep%29%29&occuring1=freetext&rowId=2&options2=AND&q2=&occuring2=freetext&rowId=3&options3=AND&q3=&occuring3=freetext&publishedstart=2000&publishedend=yyyy&calendarInput=yyyy-mm-dd&la=any&it=any&show=all&x=68&y=9 Fasciolosis in sheep publiations since 2000]
  
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A high snail density will only  lead to disease if infection is present (i.e. if fluke eggs have been  deposited onto the habitat by farm or wild animals). Local biological  interpretation of computer generated predictions is therefore required.
 
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These forecasting  models can  provide valuable information for the farming community. MAFF  (before  DEFRA) used to provide an annual forecast, but this is now  discontinued.
 
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=== Control ===
 
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'''Anthelmintics'''
 
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*Few flukicides kill all parasitic developmental stages
 
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*Not all products, therefore, are suitable for controlling  acute outbreaks
 
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*The anthelmintic with the broadest  spectrum of activity against immature and adult ''F. hepatica'' is  '''Triclabendazole'''
 
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*Triclabendazole-resistant ''F.  hepatica'' populations are beginning to emerge
 
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*The more  potent products tend to be the most expensive
 
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*Many  flukicides bind to plasma protein and have long plasma half-lives
 
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'''Chemoprophylaxis'''
 
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<br>There are several control objectives
 
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*To  prevent fluke eggs being dropped onto snail habitats
 
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**This  is done by treating sheep/cattle with an adulticidal drug in the  '''late winter/early spring'''
 
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*To protect animals  grazing pasture known to be contaminated with metacercariae. The choice  of drug, time of treatment and dosing interval will depend on
 
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#Whether you are trying to prevent acute or chronic disease
 
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#The likely intensity of challenge (local knowledge/fluke  forecast)
 
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#The persistent effect of the drug (i.e. the  period after dosing, during which the animal is protected from  reinfection)
 
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'''Vaccination''' 
 
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*A recombinant vaccine providing approximately 70%  protection for cattle is being developed
 
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*It exerts its  effect by stimulating a range of immune responses not normally seen in  chronically infected animals (including TH1-type responses)
 
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'''Molluscicides'''
 
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*These have been employed with success in the past, but are no  longer used. This is because they have to be applied before any fluke  forecast can be issued. (Farmers are unwilling to invest in control  measures before they are known to be necessary). Also, they have to be  applied very carefully as snails can rapidly recolonise sprayed land if  any habitat has been missed
 
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'''Alternative  strategies'''
 
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*An ability to recognise and define the  extent of snail habitats allows alternative cost-effective control  options such as fencing and drainage
 
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===Treatment===
 
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If the fuke is present  treat with triclabendazole, which is effective against all stages of  Fasciola hepatica.  Treatment should be applied in September/October and  again in January, if faecal egg count is still postitive.  One may also  treat against adult only stages in May/June, preventing any future  pasture contamination.  However, do not use the same treatment in  September/October as used in May/June, as resistance to drugs is  becoming a real problem within the UK due to overuse.  If it has been a  particularly wet season, it may be necessary to treat again, as Fasciola  hepatica becomes more prevalent under such conditions.
 
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Isolation and treatment of all new animals  entering from another farm has also be shown to be effective.  Other  control measures include fencing off wet areas, and increasing soil  drainage.
 
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==Prognosis==
 
 
==References==
 
==References==
  
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==Links==
+
#Merck & Co (2008) '''The Merck Veterinary Manual (Eighth Edition)''', ''Merial''.
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* The causative organism, ''[[Fasciola hepatica]]''.
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#Aitken, I D (2007) '''Diseases of Sheep''', ''Blackwell''.
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#Pugh, D G (2002) '''Sheep and Goat Medicine''', ''Elsevier Health Sciences''.
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#Taylor, M A et al (2007) '''Veterinary Parasitology (Third Edition)''', ''Wiley-Blackwell''.
  
  
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[[Category:Liver Diseases - Cattle]] [[Category:Liver Diseases - Sheep]] [[Category:Brian Aldridge reviewing]]

Latest revision as of 14:24, 6 May 2011

Also known as: Fascioliasis — Fasciolasis — Fluke

Introduction

Fasciolosis is a condition of ruminants which causes subclinical and clinical disease leading to ill thrift and deaths. The causative organism is the trematode Fasciola hepatica which primarily parasitises the bile ducts of sheep and cattle but may occasionally be found in the horse. Lymnaea truncatula, a mud snail, is the intermediate host of Fasciola hepatica, and transmission of disease is dependent on the presence of appropriate snail habitats. These habitats are more plentiful in areas of high rainfall, such as the western British Isles. However, infected animals may be found outwith these areas due to the transportation of livestock, or unusual weather patterns. The association of fasciolosis with wetter habitats lends a seasonal nature to disease outbreaks, and can help predict the severity of these.

Fasciola hepatica. Source: Wikimedia Commons; Author:Adam Cuerden(2007)

In sheep, "acute" disease caused by fluke larvae is the most common presentation, and generally occurs in the wetter Autumn and early Winter months in both lambs and ewes. Fasciolosis in cattle can occur at any time of year and tends to involve adult fluke, causing "chronic" disease.

A slightly larger trematode, F. gigantica, causes a similar condition in tropical regions.

Signalment

Fasciolosis affects both young and adult animals, primarily sheep and cattle.

Diagnosis

A diagnosis can usually be made using clinical findings and the seasonal occurrence of disease, and may be supported by a history of fasciolosis on the farm and post-mortem findings. Certain adjunctive tests may also prove useful.

Clinical Signs

Fasciola hepatica adults from a horse - Castellà Veterinary Parasitology Universitat Autònoma de Barcelona

Sheep

In sheep, fasciolosis may present as acute, chronic, or infrequently sub-acute manifestations.

Acute fasciolosis usually occurs between September and December and is caused by large numbers of immature Fasciola hepatica migrating through the liver parenchyma and causing massive damage. It arises within around two to six weeks of ingestion of metacercariae. If sheep are not exposed to at-risk pasture until later in the year, acute fasciolosis may occur as late as the following Feburary. Hepatic damage caused by migration of fluke larvae gives clinical signs including lethargy, pallor, dyspnoea and death in both young and adult animals. Handling of sheep may cause liver rupture and sudden death, and sudden death may also occur due to Black's disease (Clostridium novyi type B) or bacillary haemoglobinuria (Clostridium novyi type D) in unvaccinated sheep. This is a result of necrosis caused by larval migration within the liver: anaerobic conditions are created, enabling multiplication of clostridial organisms and thus toxin production.

Chronic fasciolosis in sheep is caused by adult flukes in the bile ducts and is usually seen in February and March, 4-5 months after ingestion of metacercariae. However, cases may present in early summer if snails become infected during the winter. Progressive weight loss over weeks to months results in poor body condition, and anorexia is often seen. As adult flukes feed on blood and are capable of consuming 0.5ml each per day, anaemia and pallor frequently occur in chronic fasciolosis. Initially, this regenerative anaemia is normochromic, but becomes hypochromic as iron reserves are depleted. Hypoalbuminaemia may also result from whole blood loss, and from reduced hepatic production. This gives a reduced plasma oncotic pressure, leading to ascites and/or submandibular oedema in advanced cases.

In some cases, sub-acute fasciolosis may occur if infection has occurred over a prolonged period. In these instances, disease is caused by both adult flukes and larvae and ill thrift, lethargy, dyspnoea is seen from around December to March.

In addition to these presentations, Fasciola hepatica has subclinical effects on sheep. Fleece weight and fibre quality are affected by even small fluke burdens, and there is some evidence that lambing percentage and lamb growth rates may be negatively influenced. Condemnation of affected livers at slaughter also causes economic losses.

Cattle

In cattle, fasciolosis is usually a disease of calves occuring between winter and spring, but may affect any animal at any time of year. Disease is usually chronic (caused by adult flukes) and signs tend to be less severe than in sheep. Poor nutrition and gastrointestinal parasitism do however exacerbate disease. As in sheep, subclinical losses include liver condemnation following slaughter and extended finishing times. Carcass values and milk yield may be reduced in beef and dairy cattle respectively. Milk quality may also be poorer in Fasciola hepatica infection.

Laboratory Tests

A simple and effective test for Fasciola hepatica infection is examination of the faeces for fluke eggs. These are large, ovoid and golden brown, but may be few in number and thus difficult to find. It must also be remembered that eggs are produced only by adult flukes, and so will not be detectable in acute disease.

Measurement of serum glutamate dehydrogenase (GLDH) and/or glutamyl transpeptidase (GGT) levels may prove useful. GLDH is an enzyme released by damaged hepatic cells and becomes elevated within the first few weeks of infection. GGT indicates damage to the epithelial cells lining the bile ducts and rises later in disease, although high levels are maintained for longer. Fluke serology is possible by means of a reliable ELISA test that identifies anti-fluke antibodies in the serum or in milk.

Diagnostic Imaging

Although not commonly performed in the diagnosis of fasciolosis, ultrasonography can show hepatic changes that may be suggestive of fluke. The probe should be applied immediately caudal to the costal arch, approximately two-thirds of the way down the right hand side. This positioning may reveal an excess of peritoneal exudate and fibrinous adhesions between the liver and the small intestine or the body wall. The liver is seen to be enlarged, and the capsule appears hyperechoic due to fibrin deposition. The liver parenchyma also exhibits changes, appearing diffusely granular, rather than uniformly hypoechoic.

Biopsy

Biopsy or histopathology may reveal necrotic areas of liver, haemorrhage, hyperplastic bile ducts or indeed the presence of Fasciola hepatica within the section.

Pathology

Bile duct fibrosis due to chronic Fasciola hepatica infestation. Source: Wikimedia Commons; Author: Flukeman

The pathology of fasciolosis is similar in both sheep and cattle, although the species have a different incidence of acute and chronic disease, as described above.

In acute fasciolosis, immature flukes grow and migrate within the liver parenchyma, causing necrotic tracts and haemorrhage. On post-mortem, the liver is enlarged and a fibrinous peritonitis may be be present. Fibrin tags are often particularly apparent on the ventral lobe. Histologically, tracts are seen as areas of degenerate hepatocytes and haemorrhage, which may later become infiltrated with eosinophils and lymphocytes. In the long term, these areas will become fibrosed. In sub-acute fasciolosis, the liver again is enlarged on post-mortem, and haemorrhagic tracts can be seen.

Chronic fasciolosis is associated with damage to the bile ducts by adult flukes. On post-mortem, the liver is distorted by areas of fibrosis caused by the migration of the original immature flukes. The bile ducts are dilated, and flukes may be expressed from within. The gall bladder may also be enlarged. The walls of the bile ducts may be ulcerated and haemorrhagic with areas of epithelial hyperplasia. The walls eventually become fibrosed and may calcify in cattle. Calcified bile ducts can be seen protruding from the liver surface - this is known as "pipe stem liver".

Treatment

Due to the reliance of disease transmission on appropriate snail habitats and therefore weather, it has been possible to develop models to predict the occurrence of fasciolosis to help its control within flocks and herds. These models evaluate the soil moisture content from May to October by considering rainfall patterns and evapo-transpiration, weighted for season. Although June is a particularly influential month in these models, a drought in late summer can reverse predictions of potentially high snail density, and so forecasts should not be issued prematurely. A complicating factor in the prediction of fasciolosis is the fact that snail density is insufficient for disease in the absence of infection (i.e. deposited fluke eggs), and so forecasts generated must be interpreted in the context of local biology.

Anthelmintic drugs are used in the control of fasciolosis. Not all flukicides are effective against each parasitic developmental stage, and so some may not be suitable for use in an outbreak of acute disease. Triclabendazole, a benzimidazole, is the flukicide with the broadest spectrum of activity against both immature and adult Fasciola hepatica and is therefore used to control acute disease. However, triclabendazole-resistant fluke populations are beginning to emerge. Albendazole, closantel, clorsulon and nitroxanyl all have a narrower spectrum of activity, primarily against adult fluke.

There are two objectives to anthelmintic control of fluke in sheep and cattle. The first is to limit shedding of fluke eggs onto snail habitats, which is achieved by the use of any adulticidal drug in late winter/early spring. The second aim is to protect animals grazing metacercariae-contaminated pasture against fluke infection, and the approach to this is not so simplistic. Here, the choice of drug, the timing of treatment and dosing interval is dependent on: a) whether acute or chronic disease is the (anticipated) problem; b) the likely intensity of challenge, based on local knowledge or fluke forecasting and c) the persistence of the selected drug. When treating for chronic fasciolosis it is important to provide good quality nutrition following anthelmintic dosing, and response and need for further treatment can be monitored using faecal egg counts.

Previously, molluscicides have been employed to control fasciolosis and were successful. However, these are now infrequently used. This is in part due to the need for application before the likely impact of fluke can be predicted, potentially creating an unnecessary cost. Molluscides can also be difficult to use effectively since careful application is needed to avoid rapid recolonisation of treated land from any habitat that has been missed.

The nature of fluke transmission means that is it possible to use environmental strategies to help control. Recognition of potential snail habitats allows areas to be fenced off or drained to limit snail numbers. Alternatively, animals may simply be grazed away from wet or boggy areas during high-risk periods.

Finally, a vaccine against fluke in cattle is currently in development. This is a recombinant vaccine which is thought to provide around 70% protection by stimulating a range of immune responses not normally seen in chronically infected cattle, for example the Th1 response.

Prognosis

With effective treatment, the prognosis for fasciolosis is reasonable, but this is dependent upon the severity of disease at diagnosis, for example the prognosis for chronic infections is better than that for acute fasciolosis. Clearly strategies aimed at prevention provide the optimum prognosis for herd/flock health.

Links

Test yourself with the Trematodes Flashcards

Trematodes Flashcards


Literature Search

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Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).


Fasciolosis in cattle publications since 2000

Fasciolosis in sheep publiations since 2000

References

  1. Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition), Merial.
  2. Aitken, I D (2007) Diseases of Sheep, Blackwell.
  3. Pugh, D G (2002) Sheep and Goat Medicine, Elsevier Health Sciences.
  4. Taylor, M A et al (2007) Veterinary Parasitology (Third Edition), Wiley-Blackwell.


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