2.10 Flukes of liver, intestine and lungs

Flukes, which are hermaphrodites, live in the body cavities of warmblooded vertebrates and develop within two intermediate hosts (IHs), the first one of which is a snail, with all further IHs being less specific: fishes, freshwater crabs, insects or plants. The members of the last-mentioned group have to be eaten by the definitive hosts (DHs). Inside the IH snail, the miracidium becomes a sporocyst, which creates rediae and cercariae. The first IHs serve for the multiplication of the parasite, whereas the second IHs act as collecting hosts to which the cercariae adhere (plants) or into which they enter and encyst to metacecercariae surviving in diapause for years.

The large liver-fluke (Fasciola hepatica) lives in the bile ducts of cattle and sheep. In the first IH (the limnic snail Lymnaea truncatula), the sporocyst gives rise to rediae provided with a pharynx, gut, protonephridia and birth pore. They generate cercariae, which adhere to grasses and encyst as metacercariae. They remain infestive for 6 months even under dry conditions. When taken up by DHs, they penetrate the gut wall and reach the liver via the peritoneal cavity, the diaphragm and the pleural cavity. They penetrate the fibrous liver capsule and pass via the parenchyma to the bile ducts. Prepatency lasts 7-8 weeks; a single fluke delivers up to 20,000 eggs per day, although this is reduced to one quarter by crowding in the presence of a high worm load. At Sicily / Italy fasciolasis occurs as an anthropozoonosis, whereas in Bolivia, it is present as an anthroponosis.

The lancet fluke (Dicrocoelium dendriticum) of sheep occurs on dry pastures. The first IHs are xerophilous pulmonate snails (Helicella spec., Zebrina spec., Cochlicopa spec.), which ingest the eggs together with detritus. The miracidium hatches inside the gut of the snail. The xiphidocercariae are ejected through the snail’s breathing pore inside slimeballs, which are delivered to plants and eaten by ants (Formica spec.). By means of their spine, the xiphidiocercariae penetrate the gut wall of the ant and one xiphidiocercaria (seldom two) migrates to the suboesophageal ganglion ("brain worm"). The resulting biting cramp of the mandibles prevents the return of the ant to the nest and so it will be eaten by grazing sheep together with the plant. The other cercariae encysted in the ant’s haemocoel become infestive after one month. They hatch inside the gut of the DH and migrate via the ductus choledochus to the bile ducts of the liver.

The Chinese liver fluke (Clonorchis syn. Opisthorchis sinensis) develops in limnic prosobranchiate snails (Parafossolurus syn. Bithynia spec.) as the first IH and in carpiform fish (Cyprinidae) as the second IH. The cercariae encyst under the scales of the fish inside its skin or muscles. In addition to man, wild carnivores, pigs, rats, dogs and cats are natural DHs. Having excysted in the duodenum, the metacercariae reach the bile ducts via the ductus choledochus. Prepatency lasts 2-4 weeks. The flukes may survive in man for up to 10 years and cause icterus, secondary infections and carcinomas.

The lung fluke (Paragonymus westermani) has as its natural DHs not only man, but also felids, rabbits, mongeese, rats, pigs (rarely), and dogs, cats and other fur-bearing animals. The first IHs are limnic prosobranchiate snails (Melania spec., Semisulcospira spec., Oncomelania spec.), whereas the second IHs are decapod crustaceans (Astacus spec., Potamon spec.) in the muscles and gills of which the parasites encyst and survive for at least 200 days. In man as a DH, they penetrate the small intestine and arrive at the lungs via the body cavities. In the lungs, two or three flukes at a time nest in a cyst surrounded by fibrous tissue and survive for several years. After 6 weeks, the eggs are delivered into the bronchial tubes and coughed out or swallowed. The juice of crustacean muscles is used, in East Asia, as a popular remedy for fever and diarrhoea and can therefore cause serious infestations.

The giant intestinal fluke (Fasciolopsis buski) lives in the small intestine of man and pigs and adheres to the gut wall. The first IHs are limmnic pulmonate snails of the family Planorbidae (Polyplis spec., Helicorbis spec.). The metacercariae encyst on submerged parts of freshwater plants such as water caltrop (Trapa natans) and chestnut (Eleocharis tuberosa). Infestation happens on peeling the nuts with the teeth. Prepatency lasts one month.

The body cover (tegument) of adult trematodes and cestodes is of mesodermal origin and hence they are grouped as Neodermata. The tegumental cells form a syncytium, the nuclei of which are submerged in sacklike extensions in the underlying parenchyma. Separate cells with the same construction only exist in the free living Turbellaria. The tegumental system is bounded apically (externally) and basally by typical trilaminate plasma membranes. The tegumental matrix contains mitochondria, ribosomes, endoplasmic reticulum, Golgi complexes, various secretory complexes and sense organs. In addition to its protective role, its functions include the absorption of nutrition, the synthesis and secretion of various materials (e.g. during migration through the tissues of the hosts), the excretion of metabolic products and osmoregulation (fig. 2.72, page 154).

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