Fasciola hepatica - Life Cycle, Pathogenesis, Pathology, Host Immunity

Life Cycle of Fasciola hepatica

The life cycle of Fasciola hepatica occurs in the following hosts while aquatic vegetations are also mandatory.

Definitive host: Sheep is the primary definite host while other definitive hosts include humans, goats, cattle, and other herbivores.

Intermediate host:

First intermediate host: Amphibian snails of the family Lymnaeidae while the genera of the host snails depend on the geographical region.

Secondary Intermediate host: watercress (Nasturtium officinale) and aquatic vegetations harbour the infective stage (metacercariae) of Fasciola hepatica.

• the definitive host acquires infection after consumption of the secondary intermediate host - watercress (Nasturtium officinale) and aquatic vegetations harbour – harbouring the metacercariae of Fasciola hepatica

• once metacercariae reach the duodenum, it excysts and penetrates the intestinal wall, and peritoneal cavity to invade the liver capsule

• these larvae reach the hepatic and common bile duct after migrating through the liver parenchyma

• in these ducts, the metacercariae mature into adult Fasciola hepatica

  * in some cases, the larvae may travel to ectopic sites where they mature and live for years while feeding on hepatocytes and duct epithelium

• after mating, they began laying unembryonated eggs

• the time between infection by metacercariae and the production of eggs i.e. the pre-patent period is around 4 months

• these eggs cross the sphincter of Oddi and the intestine from where they are passed along with the host faeces

• once in the water, further development of Fasciola hepatica eggs takes place

• within 2 to 3 weeks, the egg hatches to release one miracidium larva per egg

• the miracidium larvae of the liver fluke can live for 8 hours during which it must find and invade the amphibian snails of the family Lymnaeidae – otherwise, it does

• once inside its primary intermediate host, the miracidium larvae underdo asexual reproduction by a single generation of sporocysts, two generations of rediae (mother rediae and daughter rediae)

• within 4 to 7 weeks, a single miracidium larva gives rise to a large number of cercariae

• the Fasciola hepatica cercariae then escape from the snail into the water and adhere to the secondary intermediate host - watercress (Nasturtium officinale), water lettuce, mint, khat and aquatic vegetations

• after a few hours on the second intermediate host, the cercariae encyst to form metacercariae

• if the second intermediate host harbouring metacercariae is consumed, it causes liver fluke infection in the definitive host including the man – thus the life cycle of Fasciola hepatica is continued

Pathogenesis, Pathology in Fasciola hepatica infection

Both the adult form and the migrating larvae of Fasciola hepatica cause pathogenic lesions/pathology in humans.

Pathogenesis of migrating larvae

• when the developing liver fluke larvae migrate in the hepatic parenchyma, traumatic or necrotic lesions are induced

• characteristics of acute infection

• clinical syndromes caused by the migrating larvae include liver damage, inflammation of the bile duct, extensive haemorrhage, haematomas

• multiple subcapsular cavities (measuring 5 to 10 mm) surrounded by inflammatory infiltrates containing a large number of eosinophils can be found

Pathogenesis of adult Fasciola hepatica

• biliary obstruction symptoms are presented when the adult forms of Fasciola hepatica lodge in the gall bladder or bile duct lumen

• associated with chronic infections

• pathological changes include intermittent obstruction, inflammation, dilation of the biliary tract with thickening of the wall

• in the gall bladder and bile duct, the liver fluke causes hyperplasia and fibrosis

• in chronic cases, the bile duct has epithelial hyperplasia with minimal pericholangitis and proliferation of tissues

• in sheep, liver rot occurs which is a serious illness

Host immunity in Fasciola hepatica infection

Although circulating antibodies alone do not give host immunity against Fasciola hepatica infection, they can combine with cell-mediated immunity to destroy the flukes in the liver and the bile duct.