Fun With Microbiology (What's Buggin' You?): Fasciola hepatica

Fasciola hepatica (Trematode – Parasite)

Geographic Distribution & Pathogenicity

Fasciola hepatica is commonly known as the sheep liver fluke and is a common parasite in herbivores. With cosmopolitan distribution, human infections have been reported in many parts of the world. Fasciola hepatica is most frequently found in countries where sheep raising is common, such as China, Taiwan, India, Indonesia and other parts of Asia.

Fasciola hepatica is responsible for the disease fascioliasis, also known as fasciolopsiasis or simply, sheep liver fluke infection. The infection may have first been recognized as early as 1379 when the effects were noticed between certain water plants and the sheep that had eaten them. Fascioliasis is considered a zoonotic disease (passed from animals to man).

Symptoms: The infection may produce symptoms of biliary obstruction and cholangitis. Symptoms may include upper right quadrant pain, fever, chills and jaundice. Symptoms may depend on the worm burden and light infections may be asymptomatic.

Life Cycle & Morphology:

Worms: The Fasciola fluke is quite large and may measure as large as 3 cm by 1.5 cm in size. The anterior end of the worm (fluke) has a distinctive cone shaped projection. The interior organs of the worm appear extensively branched. The adult worms live in the bile ducts of the liver and the gallbladder.

Eggs: The eggs (ova) are large (80-150 µm by about 60 -90 µm) and broadly elliptical in appearance. They are operuclated but the operculum is rather small in relation to the egg and rather inconspicuous. The eggs are unembryonated when passed in the feces. When passed into water, they undergo embryonation and subsequently miracidia are hatched (usually in 1 – 2 weeks). Fasciola hepatica requires an intermediate host for development, which in this case is a freshwater snail (Lymnaea sp). The miracidia within the snail mature and emerge as cercariae which then attach to aquatic vegetation (eg. watercress) where they undergo encystation. Humans are infected by the ingestion of uncooked aquatic vegetation on which the metacercariae are encysted. The metacercariae excyst (hatch) in the duodenum and migrate through the intestinal wall into the peritoneal cavity. The larvae penetrate the liver and wander through the parenchyma for up to 9 weeks. The larvae finally enter the bile ducts where they mature and in about three to four months and begin to produce eggs, which are ultimately passed out in the feces. The adult worms may live for up to a year.

Diagnosis:

Diagnosis is made by the detection of the characteristic eggs in the patient’s faeces. One problem in identification is that the species Fasciolopsis buski produces eggs which are almost indistinguishable from those produced by Fasciola hepatica. Life cycles of these two trematodes are very similar. In some areas of the orient where these two species overlap, the clinical evaluation of symptoms aids the diagnosis of these faciolid eggs. The size of the operculum opening may also assist in diagnosis where the Fasciola hepatica’s operculum is larger than that of Fasciolopsis buski (measurements to follow). Putting pressure on the coverslip of a concentrated faecal specimen with the eraser end of a pencil may be sufficient to cause the operculum to pop open and better reveal itself. Molecular methods may provide a definitive identification.