THE VARIED ROLES OF SNAILS - National Universities Commission

Clonorchiasis is caused by Clonorchis sinesis.
The species was earlier known as Opisthorchis
sinensis. Infection due to this parasite is largely
confined to Vietnam to Japan (Including Korea,
Taiwan, China). It affects about 10 million
persons.
Humans are the principal definitive hosts, but
dogs and other fish eating canines act as
reservoir hosts. Two intermediate hosts are
required to complete its life cycle, the first being
snails and the second is fish.
The eggs passed in faeces contain the ciliated
miracidia. They do not hatch in water, but only
when ingested by suitable species of operculate
snails such as Parafoassarulu, Bulimus,
Alocinma. The cercariae escape from these snails
and swim about in water, waiting to get attached
to the second intermediate host, suitable
freshwater fish of the carp family. They then
shed their tails and encyst under the scales or in
the flesh of the fish to become in about 3 weeks
the meta cercariae which are the infective stage.
Human infection occurs when such a fish is
eaten raw or improperly processed. Frozen, dried
or pickled fish may act as source of infection.
Infection may also occur through fingers or
cooking utensils contaminated with the meta
cercariae during preparation of fish for cooking
(f) Other Trematodes of Medical Importance
(i) Dicrocoeliasis: Disease caused by the
trematode parasite Dicrocoelium dendriticum
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