Difference between revisions of "Fasciolosis"

Revision as of 23:26, 22 December 2008

Fasciolosis

The adult liver fluke, Fasciola hepatica, lives in the bile ducts of a wide range of animals, including sheep, cattle, rabits 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. Significant economic losses occur in western parts of the British Isles. Deaths, clinical and subclinical disease in cattle is confined to a 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.

Ovine Fasciolosis

Acute fasciolosis: sudden death; normally September-November; enlarged pale, friable, haemorrhagic liver with more than 1000 immature flukes in liver parenchyma. (In cattle, acute disease is rarely seen).

Sub-acute fasciolosis: rapid weight-loss over 1-2weeks; October-December; normochromic anaemia; enlarged liver with large haemorrhages; more than 500 flukes - 50:50 immature and adult.

Chronic fasciolosis: progressive weight-loss over weeks or months; January-March; anaemia normochronic, leads to hypochromic; hypoalbuminaemia, which leads to oedema; small distorted cirrhotic liver; enlarged bile ducts; more than 250 adult flukes. (In cattle, calcification of bile ducts leads to 'pipe-stem' liver).

Sub-clinical effects:

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.
Liver conemnations cause economic losses at slaughter. (Note: in beef cattle - sub-clinical infection leads to longer finishing times to slaugter weight, reduced carcass value).
In dairy cows - reduced milk yield (and quality).

Migration and Pathology

Migration

Metacercariae excyst, so immature flukes are present in the small intestine. They then migrate across the peritoneal cavity (takes approximately 1week) to the liver. They meander through the liver parenchyma for 6-7weeks, becoming more destructive as they grow, and then enter bile ducts. The prepatent period is 10-12weeks.

Pathogenesis of Fasciolosis

Liver pathology - in parenchyma, flukes develop from 0.1mm-1cm, which causes trauma (glutamate dehydrogenase (GDH) released by damaged cells), necrotic tracts, and haemorrhages, leading to acute disease.
Acute damage to liver causes post-necrotic scarring, which in turn causes shrinkage of affected tissues and hypertrophy of normal tissue (this causes the typical appearance of the liver in chronic disease).
Chronic damage to bile ducts leads to peribiliary fibrosis.

(Note: other complex events also occur, including disruption of haemodynamics, monolobular fibrosis, egg-granulomas etc.)

Black disease (Infectious Necrotic Hepatitis) - is caused by a toxin produced by Clostridium novyi type B. It is commonly associated with liver fluke infestation because:
- migrating flukes → liver necrosis → anaerobic conditions → clostridial multiplication → toxin production → disease.

Adults in Bile Ducts

Bile duct damage - adult flukes (2-5cms long) in bile ducts feed on epithelium and blood; chronic inflammatory responses → fibrosis of bile duct wall (and, in cattle, calcification).
Bile duct damage - adult flukes feed on biliary epithelium (gamma glutamyl transpeptidase released by damaged cells) and blood, leading to ulceration and haemorrhage, which causes epithelial hyperplasia and increased mucosal permeability.
Anaemia - 250 flukes can cause up to 50ml of blood loss daily → 10 times increase in rate of erythropoiesis → normochronic anaemia until iron stores are exhausted → hypochromic anaemia.
Hypoalbuminaemia - anaemia (and other plasma proteins) lost into bile duct because of:

a) whole blood loss, and

b) increased epithelial permeability

→ catabolic rate incereased by 2.5times → increased nitrogen loss via urine.

Depending on magnitude of nitrogen loss:

a) no obvious effect (although animal is still in abnormal physiological state)

b) reduced weight-gains and/or wool growth and/or milk production

c) loss of body tissue (i.e. weight-loss); hypoalbuminaemia → reduced plasma oncotic pressure → oedema.

Disease Manifestations

Large numbers of metacercariae ingested over a short period of time leads to acute disease.
Smaller numbers ingested over a longer period of time leads to chronic disease.
Feed intake - fluke infections cause reduced food intake. Note: chronic fasciolosis occurs at a time of year when animals are on a low plane of nutrition. This combined with the reduced food intake causes a significant effect on the development and severity of clinical and subclinical fasciolosis.
Species susceptibility - the proportion of flukes that reach the bile ducts is determined mainly by the fibroplastic potential of the liver and the effectiveness of the protective immune responses (which are ineffective in sheep). Therefore, establishment rate: sheep>cattle>pig.

Snail Biology

Lymnaea truncatula

is 5-10mm long
has a brown-black shell with 5-6spirals
first spiral is greater than half the total length
the shell opens on the right (when held with the opening upwards)
feeds on green slime
can multiply rapidly if food is abundant
most die during the British winter (unless very mild)
survivors lay eggs in spring, which hatch in June.

Habitats

Lymnaea is found in muddy areas (but not on highly acidic soils)
habitats may be permanent (dry summer) or temporary (wet summer).

Epidemiology

In temperate areas, there are two superimposed epidemiological cycles, known as the summer and winter infections of the snail. On mainland Britain, the summer cycle predominates as a high proportion of snails perish during the winter, but very occasionally, weather sequences allow the winter cycle to affect the pattern of disease. On the west coast of Ireland, the winter cycle of events determines the timing of clinical outbreaks.

Summer Infection of the Snail

Fluke eggs that are passed in spring:

→ hatch in June (i.e. coincident with snail hatch)

→ miracidia infect newly hatched snails

→ develop and multiply in snail hepatopancreas during summer

→ cercariae shed from late August onwards

→ metacercariae ingested by sheep

→ immature flukes migrate through liver

→ acute disease September-November; or chronic diesease January onwards.

Winter Infection of the Snail

Eggs dropped in late summer:

→ infect snails

→ development halted when temperature <10°C (i.e. flukes trapped in hibernating snails through the winter)

→ development resumes when temperature >10°C

→ cercariae shed from July

→ disease from August.

Geographical Distribution of Disease

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 th country for fattening. Infected animals may therefore be found in areas where no transmission occurs.

Weather Patterns and Disease Risk

Wet summer: Many fluke and snail eggs hatching and snail habitats expanded, therefore:

→ snail population increases rapidly

→ many infected snails

→ many cercariae shed

→ high density of metacercariae on herbage

→ increased risk of disease.

Dry summer: Few fluke and snail eggs hatching and snails restricted to permanent habitats, therefore:

→ snail populations small and overcrowded

→ few infected snails, high mortality

→ few cercariae shed

→ few metacercariae confined to restricted areas

→ reduced risk of disease.

@@ Line 1: / Line 1: @@
-Also known as: '''''Fascioliasis  — Fasciolasis — Fluke
+{{toplink
+|backcolour =
+|linkpage =Parasites
+|linktext =PARASITES
+|pagetype=Bugs
+|sublink1=Trematodes
+|subtext1=TREMATODES
+}}
+<br>
-==Introduction==
+== Fasciolosis ==
-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.
+The adult liver fluke, ''Fasciola hepatica'', lives in the bile ducts of a wide range of animals, including sheep, cattle, rabits 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''. Significant economic losses occur in western parts of the British Isles. Deaths, clinical and subclinical disease in cattle is confined to a 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.
-[[Image:Fasciola   hepatica.jpg|thumb|right|150px|''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.
+=== Ovine Fasciolosis ===
+'''Acute fasciolosis''': sudden death; normally September-November; enlarged pale, friable, haemorrhagic liver with more than 1000 immature flukes in liver parenchyma. (In cattle, acute disease is rarely seen).
-A slightly larger trematode,  ''F. gigantica'', causes a similar condition in  tropical regions.
+'''Sub-acute fasciolosis''': rapid weight-loss over 1-2weeks; October-December; normochromic anaemia; enlarged liver with large haemorrhages; more than 500 flukes - 50:50 immature and adult.
-==Signalment==
+'''Chronic fasciolosis''': progressive weight-loss over weeks or months; January-March; anaemia normochronic, leads to hypochromic; hypoalbuminaemia, which leads to oedema; small distorted cirrhotic liver; enlarged bile ducts; more than 250 adult flukes. (In cattle, calcification of bile ducts leads to 'pipe-stem' liver).
-Fasciolosis affects both young and adult animals, primarily sheep and cattle.
-==Diagnosis==
+'''Sub-clinical effects''':
-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.
+*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.
+*Liver conemnations cause economic losses at slaughter. (Note: in beef cattle - sub-clinical infection leads to longer finishing times to slaugter weight, reduced carcass value).
+*In dairy cows - reduced milk yield (and quality).
-===Clinical Signs===
-[[Image:Fasciola  hepatica    adult.jpg|thumb|right|150px|''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 [[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.
+=== Migration and Pathology ===
+==== Migration ====
+Metacercariae excyst, so immature flukes are present in the small intestine. They then migrate across the peritoneal cavity (takes approximately 1week) to the liver. They meander through the liver parenchyma for 6-7weeks, becoming more destructive as they grow, and then enter bile ducts. The prepatent period is 10-12weeks.
-'''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.
-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.
+==== Pathogenesis of Fasciolosis ====
+*'''Liver pathology''' - in parenchyma, flukes develop from 0.1mm-1cm, which causes trauma (glutamate dehydrogenase (GDH) released by damaged cells), necrotic tracts, and haemorrhages, leading to acute disease.
+*Acute damage to liver causes post-necrotic scarring, which in turn causes shrinkage of affected tissues and hypertrophy of normal tissue (this causes the typical appearance of the liver in chronic disease).
+*Chronic damage to bile ducts leads to peribiliary fibrosis.
-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.
+(Note: other complex events also occur, including disruption of haemodynamics, monolobular fibrosis, egg-granulomas etc.)
-====Cattle====
+*'''Black disease (Infectious Necrotic Hepatitis)''' - is caused by a toxin produced by ''Clostridium novyi'' type B. It is commonly associated with liver fluke infestation because:
-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.
+**migrating flukes → liver necrosis → anaerobic conditions → clostridial multiplication → toxin production → disease.
-===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 testing|ELISA test]] that identifies anti-fluke antibodies in the serum or in milk.
+=== Adults in Bile Ducts ===
+*'''Bile duct damage''' - adult flukes (2-5cms long) in bile ducts feed on epithelium and blood; chronic inflammatory responses → fibrosis of bile duct wall (and, in cattle, calcification).
+*'''Bile duct damage''' - adult flukes feed on biliary epithelium (gamma glutamyl transpeptidase released by damaged cells) and blood, leading to ulceration and haemorrhage, which causes epithelial hyperplasia and increased mucosal permeability.
+*'''Anaemia''' - 250 flukes can cause up to 50ml of blood loss daily → 10 times increase in rate of erythropoiesis → normochronic anaemia until iron stores are exhausted → hypochromic anaemia.
+*'''Hypoalbuminaemia''' - anaemia (and other plasma proteins) lost into bile duct because of:
-===Diagnostic Imaging===
+a) whole blood loss, and
-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===
+b) increased epithelial permeability
-Biopsy or histopathology may reveal necrotic areas of liver, haemorrhage, hyperplastic bile ducts or indeed the presence of ''[[Fasciola hepatica]]'' within the section.
-===Pathology===
+→ catabolic rate incereased by 2.5times → increased nitrogen loss via urine.
-[[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.
-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.
+*Depending on magnitude of nitrogen loss:
-'''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".
+a) no obvious effect (although animal is still in abnormal physiological state)
-==Treatment==
+b) reduced weight-gains and/or wool growth and/or milk production
-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.
+c) loss of body tissue (i.e. weight-loss); hypoalbuminaemia → reduced plasma oncotic pressure → oedema.
-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.
+=== Disease Manifestations ===
+*Large numbers of metacercariae ingested over a short period of time leads to acute disease.
+*Smaller numbers ingested over a longer period of time leads to chronic disease.
+*'''Feed intake''' - fluke infections cause reduced food intake. Note: chronic fasciolosis occurs at a time of year when animals are on a low plane of nutrition. This combined with the reduced food intake causes a significant effect on the development and severity of clinical and subclinical fasciolosis.
+*'''Species susceptibility''' - the proportion of flukes that reach the bile ducts is determined mainly by the fibroplastic potential of the liver and the effectiveness of the protective immune responses (which are ineffective in sheep). Therefore, establishment rate: '''sheep>cattle>pig'''.
-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 [[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]].
+=== Snail Biology ===
+==== ''Lymnaea truncatula'' ====
+*is 5-10mm long
+*has a brown-black shell with 5-6spirals
+*first spiral is greater than half the total length
+*the shell opens on the right (when held with the opening upwards)
+*feeds on green slime
+*can multiply rapidly if food is abundant
+*most die during the British winter (unless very mild)
+*survivors lay eggs in spring, which hatch in June.
-==Prognosis==
+==== Habitats ====
+*''Lymnaea'' is found in muddy areas (but not on highly acidic soils)
+*habitats may be permanent (dry summer) or temporary (wet summer).
-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==
+=== Epidemiology ===
+In temperate areas, there are two superimposed epidemiological cycles, known as the summer and winter infections of the snail. On mainland Britain, the summer cycle predominates as a high proportion of snails perish during the winter, but very occasionally, weather sequences allow the winter cycle to affect the pattern of disease. On the west coast of Ireland, the winter cycle of events determines the timing of clinical outbreaks.
-*''[[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)]
-==Test yourself with the Trematodes Flashcards==
+=== Summer Infection of the Snail ===
+Fluke eggs that are passed in spring:
-[[Trematodes_Flashcards|Trematodes Flashcards]]
+→ hatch in June (i.e. coincident with snail hatch)
+→ miracidia infect newly hatched snails
-==Literature Search==
+→ develop and multiply in snail hepatopancreas during summer
-[[File:CABI logo.jpg|left|90px]]
+→ cercariae shed from late August onwards
-Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).
+→ metacercariae ingested by sheep
-<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]
-[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]
+→ immature flukes migrate through liver
-==References==
+→ '''acute''' disease '''September-November'''; or '''chronic''' diesease '''January''' onwards.
-#Merck & Co (2008) '''The Merck Veterinary Manual (Eighth Edition)''', ''Merial''.
-#Aitken, I D (2007) '''Diseases of Sheep''', ''Blackwell''.
-#Pugh, D G (2002) '''Sheep and Goat Medicine''', ''Elsevier Health Sciences''.
-#Taylor, M A et al (2007) '''Veterinary Parasitology (Third Edition)''', ''Wiley-Blackwell''.
+=== Winter Infection of the Snail ===
+Eggs dropped in late summer:
-{{review}}
+→ infect snails
-[[Category:Liver Diseases - Cattle]] [[Category:Liver Diseases - Sheep]]  [[Category:Brian Aldridge reviewing]]
+→ development halted when temperature <10°C (i.e. flukes trapped in hibernating snails through the winter)
+→ development resumes when temperature >10°C
+→ cercariae shed from July
+→ disease from August.
+=== Geographical Distribution of Disease ===
+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 th country for fattening. Infected animals may therefore be found in areas where no transmission occurs.
+=== Weather Patterns and Disease Risk ===
+'''Wet summer''':
+Many fluke and snail eggs hatching and snail habitats expanded, therefore:
+→ snail population increases rapidly
+→ many infected snails
+→ many cercariae shed
+→ high density of metacercariae on herbage
+→ '''increased''' risk of disease.
+'''Dry summer''':
+Few fluke and snail eggs hatching and snails restricted to permanent habitats, therefore:
+→ snail populations small and overcrowded
+→ few infected snails, high mortality
+→ few cercariae shed
+→ few metacercariae confined to restricted areas
+→ '''reduced''' risk of disease.