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Fascioliasis |
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There are an estimated 2.4 million people infected with the two
larger human liver flukes, Fasciola hepatica and
Fasciola gigantica, often causing serious acute and chronic
morbidity. These parasites
commonly infect domestic ruminants; F. hepatica
(the sheep liver fluke) and F. gigantica (mainly of
cattle). Humans usually become infected by eating aquatic
plants grown in water that is contaminated with faeces from
animals harbouring Fasciola. |
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| Life cycle | ||||||||||||||
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Immature eggs are discharged in the biliary ducts and in the
stool
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| Epidemiology | ||||||||||||||
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Human infection with Fasciola is most common in villages
and larger towns within rural areas, especially
sheep- and cattle-grazing
areas. Levels of infection depend on the frequency of
humans eating plants (mainly watercress in Europe, morning glory
in Asia) from water bodies contaminated with animal faeces. In
most endemic areas, human infection is relatively rare, even
where prevalence among domestic animals is high. Outbreaks of
F. hepatica occur in households and communities, and are
often traced to consumption of wild, rather than cultivated,
watercress. Infection might also occur from contaminated
drinking water or cooking utensils. |
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Pathogenesis and Pathology |
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The parasites cause considerable mortality in sheep and cattle, and human morbidity which is dependent on the number of worms and stage of infection. The acute phase occurs during migration of the immature flukes through the liver. Severe pathology results from parasite ingestion and destruction of parenchymal tissue, haemorrhage, parasite death and inflammatory responses largely mediated by eosinophils. Repair mechanisms can lead to extensive periportal fibrosis. The chronic phase, during which parasites are present in the bile ducts, tends to be less severe. Tissue change, including bile duct proliferation, dilatation and fibrosis, is largely caused by mechanical obstruction of the ducts, inflammatory responses and the activity of proline, which the fluke excretes in large quantities. Anaemia may result from blood loss through bile duct lesions. Death is uncommon, but is usually caused by haemorrhaging in the bile duct. |
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| Clinical features | ||||||||||||||
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Fascioliasis can be symptomatic or asymptomatic. Symptomatic
cases
may begin approximately 2 months following ingestion of
metacercariae. During the acute phase (caused by the migration
of the immature fluke through the hepatic parenchyma),
manifestations include abdominal pain, hepatomegaly, fever,
vomiting, diarrhea, urticaria and eosinophilia, and can last for
months. In the chronic phase (caused by the adult fluke within
the bile ducts), the symptoms are more discrete and reflect
intermittent biliary obstruction and inflammation. Occasionally,
ectopic locations of infection (such as intestinal wall, lungs,
subcutaneous tissue, and pharyngeal mucosa) can occur. |
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| Diagnosis | ||||||||||||||
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Fascioliasis has been diagnosed by observation of eggs during faecal examination, by parasite-specific antibody detection in a variety of immunodiagnostic assays, by radiological methods and by laparotomy. Dietary history is helpful for differential diagnosis and in investigating outbreaks. Microscopic identification of eggs is useful in the chronic (adult) stage. Eggs can be recovered in the stools or in material obtained by duodenal or biliary drainage. They are morphologically indistinguishable from those of Fasciolopsis buski. False fascioliasis (pseudofascioliasis) refers to the presence of eggs in the stool resulting not from an actual infection but from recent ingestion of infected livers containing eggs. This situation (with its potential for misdiagnosis) can be avoided by having the patient follow a liver-free diet several days before a repeat stool examination. Antibody detection tests are useful especially in the early invasive stages, when the eggs are not yet apparent in the stools, or in ectopic fascioliasis. |
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| Treatment | ||||||||||||||
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The treatment of fascioliasis remains highly problematic. Unlike
infections with other flukes, Fasciola hepatica
infections may not respond to praziquantel. The drug of choice
is triclabendazole with bithionol as an alternative.
Triclabendazole at a dose of 10 or 20mg/kg
has been shown to be effective against Fasciola infection
and without serious side effects. |
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| Prevention and control | ||||||||||||||
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Ultimate control of Fasciola must focus on strategic
treatment or immunization of livestock and other herbivorous
animals that maintain the life cycle. Advances in the
development of veterninary vaccines are very encouraging.
Widespread livestock immunization is being considered by some
countries to reduce human infection and economic loss to the
parasite. Control of the snail vectors using molluscicides is
not considered practical in most situations. Health education to
discourage human consumption of raw wild watercress and other
edible water plants may be effective in areas where the disease
is prevalent. Increased awareness by clinicians of the problem
and its diagnostic difficulties, plus data from community-based
studies assessing seroprevalence, will help quantify the extent
to which fascioliasis affects human health. |
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Bibliography |
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1. Sithithaworn
P, Sripa B, Kaewkes S, Haswell MR. Foodborne trematodes. In
Manson’s Tropical Diseases 22 th Ed. (in press) |
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.
Eggs become embryonated in water
,
eggs release miracidia
,
which invade a suitable snail intermediate host
,
including many species of the genus Lymnae. In the snail
the parasites undergo several developmental stages (sporocysts
,
rediae
,
and cercariae
). The
cercariae are released from the snail
and encyst as metacercariae on aquatic vegetation or other
surfaces. Mammals acquire the infection by eating vegetation
containing metacercariae. Humans can become infected by
ingesting metacercariae-containing freshwater plants, especially
watercress
. After
ingestion, the metacercariae excyst in the duodenum
and migrate through the intestinal wall, the peritoneal cavity,
and the liver parenchyma into the biliary ducts, where they
develop into adults
. In
humans, maturation from metacercariae into adult flukes takes
approximately 3 to 4 months. The adult flukes (Fasciola
hepatica: up to 30 mm by 13 mm; F. gigantica: up to
75 mm) reside in the large biliary ducts of the mammalian host.
Fasciola hepatica infect various animal species, mostly
herbivores. (from
