Fasciola hepatica  By Jessica Sand

Sheep or common liver fluke Fasciola hepatica is a parasitic fluke or trematode that is found in the liver and bile duct. It is known as the sheep or common liver fluke and causes a disease called fascioliasis.

Interesting Facts Adult liver flukes reach a length of 3 cm and a width of 1 cm. Fasciola hepatica is one of the largest flukes in the world. The adult worm has a very characteristic leaf shape with the anterior end being broader than the posterior end.

Interesting Facts Cont. The fluke has a cone on the anterior end of its body. It has a powerful oral sucker at the anterior end of the cone and a ventral sucker at the base of the cone which allow it to attach to the lining of the biliary ducts. 

Interesting Facts Cont. Each worm possesses ovaries and testes, making it hermaphroditic. These reproductive organs are highly branched and allow for individual flukes to produce eggs independently. Eggs of Fasciola hepatica have been found in mummies, showing that human infection was occurring at least as early as the age of the Pharaohs in Egypt.

Audience Participation Draw your own liver fluke! 

Geographic Range Fasciola hepatica is found on every continent with nearly 180 million people at risk and an estimated 2.4 million people already infected worldwide. Prevalence of this parasite is highest in areas where extensive sheep and cattle raising occurs and where dietary practices include the consumption of raw aquatic vegetables. Fascioliasis is one of the most economically important parasitic diseases of livestock, causing disease in sheep and other domestic animals in Latin America, Africa, Europe, and China.

Definitive Host Its hosts include herbivorous mammals, especially sheep and cattle, and it is found in 46 species of domestic and wild animals as well as humans.

Intermediate Host The intermediate host is the Lymnaea genus of snail which lives in marshy areas and standing water.

Life Cycle Immature eggs are discharged in the biliary ducts and in the stool. The eggs become embryonated in water and then release miracidia, which invade a suitable snail intermediate host. In the snail, the parasites develop from sporocyst to rediae, and finally into cercariae, which are released from the snail and encyst as metacercariae on aquatic vegetation or other surfaces. Mammals become infected by eating contaminated vegetation. Humans become infected by ingesting contaminated 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. Human infection by consumption of raw liver from infected sheep, goats, and cows has also been reported.

Life cycle

Pathogenesis / Clinical Signs Fasciola hepatica causes a disease called fascioliasis. Up to 50% of Fasciola hepatica infections are asymptomatic and disease may appear anywhere from a few days to several years after infection. There are four different types of fascioliasis. They include a chronic phase, an acute phase, a condition called halzoun, and an ectopic infection.

Chronic Phase The symptoms of the chronic phase of fascioliasis include gallstones, abdominal pain, tender and enlarged liver, and jaundice. In children, severe anemia is a common result of the infection and is the greatest source of disability from infection in this age group.

Acute Phase The acute phase of infection is rarely seen in humans and occurs only when a large number of metacercariae are ingested at once. Fever, tender, enlarged liver, and abdominal pain are the most frequent symptoms of this stage of infection although vomiting, diarrhea, hives, and anemia may all be present. The acute phase continues for 6-8 weeks until the larvae mature and settle in the bile ducts.

Halzoun The condition commonly known as halzoun is a type of Fasciola hepatica infection in which the worm settles in the pharynx. This occurs when an individual consumes infected raw liver. The young adult worms then attach themselves to the pharyngeal mucosa which causes considerable pain, edema, and bleeding that can interfere with respiration.

Ectopic Infection Ectopic infections through normal transmission are infrequent but can occur in the peritoneal cavity, intestinal wall, lungs, subcutaneous tissue, and very rarely in other locations.

Diagnosis Fasciola hepatica is identified from eggs in a stool sample, duodenal aspirate or biliary aspirate. Early stages of the infection can be diagnosed from a blood sample, if antibodies are found. Where eggs are not present in the stool, serological tests can be used, the FAST-ELISA ( Falcon Assay Screening Test-Enzyme-Linked Immunosorbent) being the most popular. Ultrasound can be used to visualize the adult flukes in the bile ducts and a CT scan may reveal the burrow tracts made by the worms and dilation of the bile ducts.

Treatment Triclabendazole (Fastinex) or bithionol are the drugs of choice but are not in the British National Formulary and may need to be prescribed on a named patient basis. Praziquantel is recommended only if bithionol or triclabendazole is unavailable. Nitazoxanide is also used as an alternative.

Control Methods Keeping domestic animal herds separate from the growing sites of aquatic vegetation limits the risk of contaminating the vegetation and thus decreases both human infection and the animal reservoir. Washing of aquatic vegetables can destroy the encysted metacercariae. This approach has proven more acceptable to communities than past attempts to entirely halt the consumption of raw vegetables.

Control Methods Cont. Training physicians and health workers in recognizing the signs and symptoms of Fasciola hepatica infection would increase timely identification and treatment of the disease, decreasing both the individual disease burden and transmission by way of the human reservoir. The most frequently used public health intervention is the application of molluscicides to decrease the population of Lymnaea snails, the intermediate hosts of Fasciola hepatica. Molluscicides have been particularly popular because they also decrease transmission of many other trematodes that also use snails as intermediate hosts.

References http://www.parasitesinhumans.org/fasciola-hepatica-liver-fluke.html http://www.stanford.edu/class/humbio103/ParaSites2001/fascioliasis/Fasciola.htm http://www.patient.co.uk/doctor/Fasciola-Hepatica.htm http://www.tpub.com/content/armymedical/MD0589/MD05890060.htm http://www.dpd.cdc.gov/dpdx/HTML/Fascioliasis.htm