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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{|cellpadding="10" cellspacing="0" border="1"
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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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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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|-
 
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|You might also be interested in:
 
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|The organism, ''[[Fasciola hepatica]]''
 
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|}
 
  
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==Description==
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A slightly larger trematode,  ''F. gigantica'', causes a similar condition in tropical regions.
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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.jpg|thumb|right|150px|''Fasciola hepatica - Wikimedia Commons]]
 
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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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The adult liver fluke, ''[[Fasciola hepatica]]'',  lives in the bile ducts of a wide range of animals, including sheep,  cattle, rabbits and, less often, horses. It can infect humans causing a  painful abdominal disease. The intermediate host in the UK is a mud  snail, ''Lymnaea truncatula''. It causes significant economic losses in  western parts of the British Isles due to deaths, clinical and  subclinical cattle disease confined mostly to the younger stock.  Fasciolosis is a seasonal disease with more serious outbreaks occurring  in some years than in others. A similar but slightly larger species,  ''F. gigantica'', occurs in wetter tropical regions.
 
  
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==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.
  
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=== Geographical distribution of disease ===
 
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Transmission  is dependent on the snail, and therefore associated with snail  habitats. These are more plentiful in high rainfall areas such as found  on the western side of the British Isles, or alongside drainage canals  in low-lying areas such as the Romney Marsh (or the Netherlands). It is  important to remember that sheep and cattle are often bred in Ireland or  western Britain and transported to the east of the country for  fattening. Infected animals may therefore be found in areas where no  transmission occurs
 
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=== Weather patterns and disease risk ===
 
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'''Wet summer'''
 
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Many fluke and snail eggs hatching and snail habitats  expanded, therefore:
 
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→ snail  population increases rapidly
 
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→ many infected snails
 
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→ many cercariae shed
 
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→ high density of metacercariae on herbage
 
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→ '''increased''' risk of disease
 
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'''Dry summer'''
 
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Few  fluke and snail eggs hatching and snails restricted to permanent  habitats, therefore:
 
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→ snail  populations small and overcrowded
 
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→ few infected snails, high mortality
 
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→ few cercariae shed
 
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→ few metacercariae confined to restricted  areas
 
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→ '''reduced''' risk  of disease
 
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*Large  numbers of metacercariae ingested over a short period of time leads to  acute diseasegnosis==
 
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*Smaller numbers ingested over a longer period of  time leads to chronic disease
 
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*'''Feed intake'''
 
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**Fluke  infections cause reduced food intake
 
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NOTE: chronic fasciolosis  occurs at a time of year when animals are on a low plane of nutrition.  This combined with the reduced food
 
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intake causes a significant  effect on the development and severity of clinical and subclinical  fasciolosis.
 
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*'''Species susceptibility'''
 
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**The proportion  of flukes that reach the bile ducts is determined mainly by
 
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***The  fibroplastic potential of the liver
 
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***The effectiveness of the  protective immune responses (which are ineffective in sheep)
 
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**Therefore,  establishment rate is
 
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'''sheep>cattle>pig'''
 
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==Signalment==
 
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==Diagnosis==
 
 
===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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==== Ovine Fasciolosis ====
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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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'''Acute fasciolosis'''
 
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*Sudden death
 
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*Normally  '''September-November'''
 
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*Enlarged pale, friable,  haemorrhagic liver
 
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*More than 1000 immature flukes in  liver parenchyma
 
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NOTE: Acute disease is rarely seen in  cattle
 
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'''Sub-acute  fasciolosis'''
 
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*Rapid weight-loss over 1-2 weeks
 
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*Occurs '''October-December'''
 
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*Normochromic  anaemia
 
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*Enlarged liver with large haemorrhages
 
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*More than 500 flukes - 50:50 immature and adult
 
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'''Chronic fasciolosis'''
 
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*Progressive  weight-loss over weeks or months
 
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*Occurs  '''January-March'''
 
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*Normochromic anaemia often becoming  hypochromic
 
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*Hypoalbuminaemia leading to oedema
 
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*Small, distorted cirrhotic liver
 
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*Enlarged bile  ducts
 
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*More than 250 adult flukes
 
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NOTE:  Calcification of bile duct in cattle → 'pipe-stem' liver
 
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'''Sub-clinical effects'''
 
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*In  sheep, fleece weight and fibre quality are affected even by small fluke  burdens. There is some evidence that reproductive performance (number of  lambs born and growth-rate of lambs) is inversely influenced, but this  has not been well documented.
 
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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
 
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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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*The prepatent period is  10-12 weeks
 
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====Acute Fascioliasis====
 
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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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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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====Subactute Fascioliasis====
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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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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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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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====Chronic  Fascioliasis====
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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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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.  
 
  
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====Cattle====
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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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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===
 
===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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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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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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===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.
  
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'''Acute  fasciolosis'''
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===Biopsy===
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*Clincal 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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*Post-mortem  examination
 
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'''Chronic fasciolosis'''
 
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*Clinical  history
 
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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===
 +
[[Image:Fasciola  hepatica - bile duct.jpg|thumb|right|150px|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.
  
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==== Pathogenesis of acute fasciolosis ====
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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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*'''Liver pathology'''
 
  
 +
'''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.
  
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**Flukes develop from 0.1mm-1cm within the liver  parenchyma causing trauma, necrotic tracts, and haemorrhages
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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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**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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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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*'''Black disease (Infectious  Necrotic Hepatitis)'''
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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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**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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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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*'''Bile duct damage'''
 
  
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**Adult flukes (2-5cm long) in bile ducts feed on epithelium  and blood
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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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**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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=== Forecasting  Fasciolosis ===
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==Prognosis==
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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.
 
  
 +
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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A drought in late  summer can reverse a potentially dangerous  trend, and so unqualified  forecasts should not be issued prematurely.
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==Links==
  
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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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*''[[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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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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==Test yourself with the Trematodes Flashcards==
  
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[[Trematodes_Flashcards|Trematodes Flashcards]]
  
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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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==Literature Search==
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<br>There are several control objectives
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[[File:CABI logo.jpg|left|90px]]
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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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Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).
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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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<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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'''Alternative  strategies'''
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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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*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==
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#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:Sheep]] [[Category:Sheep Parasites]] [[Category:To_Do_-_Lizzie]]
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{{review}}
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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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