NAME: Fasciola hepatica, Fasciola gigantica
SYNONYM OR CROSS REFERENCE: Fascioliasis, Human Fascioliasis, common liver fluke, trematode, sheep liver fluke disease.
CHARACTERISTICS: F. hepatica is a parasitic flatworm of the Fasciolidae family. Hermaphroditic adults are flat and leaf-like, ranging from 20-30 mm in length and 7-14 mm in width, with oral and ventral suckers that are the organs of attachment Footnote 1, Footnote 2. F. gigantica are about double the size of F. hepatica. The eggs are ovoid, light yellow-brown, operculate and large, and range from 130 to 150 µm in length, and 60-90 µm in width.
PATHOGENICITY/TOXICITY: F. hepatica induces pathology in the liver and biliary tracts Footnote 3, where it can survive up to 10-15 years in the infected host Footnote 1. Infection consists of two stages, the hepatic stage and the biliary stage Footnote 4. The first hepatic stage is acute and invasive while flukes are in the liver parenchyma and can last for 2 to 4 months. This causes destruction of the hepatic tissue and hepatic lesions, resulting in fever, nausea and vomiting, urticaria, right upper quadrant abdonminal pain , hepatomegaly, hypergamma-globulinaemia, anaemia and marked eosinophilia. Other signs include mild hepatitis, severe subcapsular haemorrhage, and frank hepatic necrosis. The second biliary stage (chronic stage) occurs when flukes move into the biliary tract. Eggs appear in the stool after a prepatent period of 3 to 4 months. This can result in intermittent right upper quadrant pain, with or without cholangitis or cholestasis, chronic bile duct inflammation, and ectopic infections that may occur in the intestine, liver, and bile duct Footnote 5. Detection of absolute eosinophilia, fever, and infection of extrahepatic sites such as the heart, brain, intestines, lungs, and skin can also be attributed to F. hepatica infection Footnote 4, Footnote 6. In addition, acute nasopharyngitis may occur in areas of high prevalence Footnote 7.
EPIDEMIOLOGY: Worldwide human infections have been reported in areas of high rainfall, poorly drained areas and irrigated pastures Footnote 8. Regions include South America, Southeast Asia, Africa, the Middle East, Europe, and the Pacific area. They are especially prevalent in areas inhabited by freshwater snails, since these are the intermediate host of this trematode Footnote 9, as well as areas where cattle or sheep are raised. F. gigantica is commonly found in tropical and subtropical areas, most notably in Africa, Asia, and western Pacific regions Footnote 10.
HOST RANGE: Humans are accidental hosts. The snails serve as intermediate hosts. Animal infections have been documented in rats, rabbits, chickens, ducks, cattle, horses, buffalo, goats, sheep, and other herbivore ruminants Footnote 4, Footnote 9, Footnote 11, Footnote 12. Eggs are located in the bile duct of the host, and can be released into the environment along with excretion of faeces Footnote 4.
INFECTIOUS DOSE: Unknown for humans; however, chewing of encysted plants is sufficient to become infected Footnote 8. Rats aged 5 weeks or older showed symptoms of infection after ingesting 5 metacercariae Footnote 13.
MODE OF TRANSMISSION: Consumption of uncooked vegetation (commonly watercress) or raw parasitized liver of cattle or sheep containing metacercariae can lead to F. hepatica infection Footnote 14. It is also possible for the organism to break off of the foliage, so drinking unboiled water from pastures containing encysted plants can also lead to infection Footnote 4, Footnote 12.
INCUBATION PERIOD: Onset of the first hepatic infection stage is 3 – 11 weeks after the metacercariae enters the body Footnote 14, followed by the chronic biliary stage which can occur after months or years following first ingestion Footnote 4. This period may vary depending on the infectious dose Footnote 13.
COMMUNICABILITY: Cannot be transmitted between humans.
RESERVOIR: Eggs are released into the environment by infected hosts. In water with optimal temperatures, miracidia hatch from the eggs and then enter into a snail for developmental stages Footnote 8. The mature cercariae emerge from the snail and encyst onto aquatic plants (watercress are the most common), where they develop into the infectious metacercariae stage in a matter of a few hours – the maturity process may be accelerated by irrigation and washing of the plants Footnote 11. The mature metacercariae can infect cattle, sheep, humans (accidental hosts), and other mammal hosts.
ZOONOSIS: Yes – humans can become infected through ingestion of metacercariae or indirectly through cattle, sheep, and other herbivore ruminants Footnote 8.
VECTORS: F. hepatica develops into its infectious stage (metacercariae) in the freshwater snail (usually lymnaeid) Footnote 8.
DRUG SUSCEPTIBILITY: Triclabendazole, a benzimidazole, is the current drug of choice Footnote 15-Footnote 17. Bithionol is an alternative drug for the treatment of fascioliasis Footnote 4, Footnote 15 .
DRUG RESISTANCE: Immature F. hepatica displays resistance to clorsulon, closantel, nitroxynil, and albendazole fasciolocides. Resistance to triclabendazole is also developing in isolated strains from Australia and Spain Footnote 18.
PHYSICAL INACTIVATION: Sensitive to heat, sterilize by autoclave.
SURVIVAL OUTSIDE HOST: Freshly hatched cercariae from the snail can survive about one hour to encyst on a suitable object Footnote 20. If the water in which the snails live dries up, the snails can still be infected for months and in the presence of water the snails emerge and shed many cercariae. Metacercariae can survive prolonged periods (up to one year) on aquatic plants or even on hay grown in infested meadows, as they are resistant to drying Footnote 21. They can also be found free-floating in shallow, still water Footnote 4, Footnote 22.
SURVEILLANCE: Serological techniques (ELISA, immunoblotting, complement fixation, indirect hemagglutination) can detect antibodies against antigens secreted from adult flukes 2 – 4 weeks after infection Footnote 23, Footnote 24. Evidence of infection can be obtained using radiographic techniques such as computed tomography (CT), ultrasonography, magnetic resonance imaging, and the indirect hemagglutination test Footnote 25. Stool examination for F. hepatica ova can confirm infection after 2 months. Other techniques include percutaneous cholangiography, endoscopic retrograde cholangiography, and liver biopsy, which can be used but are invasive and not essential in most cases Footnote 4.
FIRST AID/TREATMENT: Administer appropriate drug therapy. A single oral dose or two dose 12 hours apart of 10 mg/kg body weight of Triclabenazole can clear infection in a few months for humans (may require a second round of treatment in some cases) Footnote 16. Bithionol can be used with a 50% cure rate in one month Footnote 15.
IMMUNIZATION: None to date.
PROPHYLAXIS: None to date. Avoid oral exposure to uncooked plants growing in moist grounds or in flooded areas, or drinking unboiled or unfiltered surface water Footnote 8.
LABORATORY-ACQUIRED INFECTIONS: None reported to date.
SECONDARY HAZARDS: None.
RISK GROUP CLASSIFICATION: Risk Group 2 Footnote 28.
CONTAINMENT REQUIREMENTS: Containment Level 2 facilities, equipment, and practices are required for work with F. hepatica and F. gigantica. Exposure hazards include ingestion, inoculation, and through mucous membrane. Avoid the production of aerosols or splashes, although it cannot be transmitted while airborne, it may set on bench tops where it can become an ingestion hazard by coming in contact with hands Footnote 29.
PROTECTIVE CLOTHING: Lab coat. Gloves when direct skin contact with infected materials or animals is unavoidable. Eyes protection must be used where there is known or potential risk of exposure to splashes Footnote 29.
OTHER PRECAUTIONS: All procedures that may produce aerosols, or involve high concentrations or large volumes should be conducted in a biological safety cabinet (BSC). The use of needles, syringes, and other sharp objects should be strictly limited. Additional precautions should be considered with work involving animals or large scale activities Footnote 29 .
SPILLS: Allow aerosols to settle and, wearing protective clothing, gently cover spill with paper towels and apply an appropriate disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time before clean up.
DISPOSAL: Decontaminate all wastes that contain or have come in contact with the infectious organism by autoclave, chemical disinfection, gamma irradiation, or incineration before disposing Footnote 19.
STORAGE: The infectious agent should be stored in leak-proof containers that are appropriately labelled.
REGULATORY INFORMATION: The import, transport, and use of pathogens in Canada is regulated under many regulatory bodies, including the Public Health Agency of Canada, Health Canada, Canadian Food Inspection Agency, Environment Canada, and Transport Canada. Users are responsible for ensuring they are compliant with all relevant acts, regulations, guidelines, and standards.
UPDATED: June 2010
PREPARED BY: Pathogen Regulation Directorate, Public Health Agency of Canada
Although the information, opinions and recommendations contained in this Pathogen Data Sheet are compiled from sources believed to be reliable, we accept no responsibility for the accuracy, sufficiency, or reliability or for any loss or injury resulting from the use of the information. Newly discovered hazards are frequent and this information may not be completely up to date.
Public Health Agency of Canada, 2010