Biology Reference
In-Depth Information
A well-known and common nematode in Asia is the
Angiostrongylus
sp.
(e.g.,
Angiostrongylus cantonensis
). The adult worm is found in the lungs
of rats and the intermediate hosts are snails, freshwater prawns and
land crabs. The parasite has been shown to cause meningitis in humans
(Brier, 1992).
Cestodes
Only few
cestodes
or tapeworms in man are known to be transmitted
by fi sh. However, the broad fi sh tapeworm
Diphyllobothrium latum
is a
common human parasite reaching up to 10 m or more in length in the
intestinal tract of humans. This parasite has a microcrustacean as fi rst
intermediate host and freshwater fi sh are required as second intermediate
host. The related species (
D. pacifi cum
) is transmitted by marine fi sh and
commonly occurs in the coastal regions of Peru, Chile and Japan where
raw fi sh preparations (ceviche, sushi and others) are common.
The broad fi sh tapeworms,
Diphyllobothrium
sp., reach sexual maturity
in the intestinal tract of mammals. Eggs may pass in the feces and develop
in water into larvae that hatch and swim freely. If consumed by a copepoda
or other suitable crustacean host, the larvae may then become infective for
fi sh that consume the infected crustacean. These larvae then develop into
forms that may infect other fi shes, where they do not develop further, or
mammals, where they may reach sexual maturity (Brier, 1992).
Trematodes
Some of the
Trematodes
or fl ukes are extremely common, particularly
in Asia. Thus it is estimated that the
Clonorchis sinensis
(the liver fl uke)
is infecting more than 20 million people in Asia. In southern China
human clonorchiasis rates can surpass 40% in some regions (Rim, 1982).
Intermediate hosts are snails and freshwater fi sh, while dogs, cats, wild
animals and humans are fi nal hosts where the fl uke live and develop in
the bile ducts in the liver. The predominant problem in transmission is the
contamination of snail-infested waters by egg-laden feces from humans
(e.g., use of “night soil” as fertilizers). Two very small fl ukes (1-2 mm)
Metagonimus yokagawai
and
Heterophyes heterophies
differ from
Clonorchis
by living in the intestines of the fi nal host, causing infl ammation,
symptoms of diarrhea and abdominal pain. Intermediate hosts are snails
and freshwater fi sh. The adult oriental lung fl uke
Paragonimus
sp. is 8-12
mm and encapsulated live in cysts in the lungs of man, cats, dogs and
pigs and many wild carnivore animals. Snails and crustaceans (freshwater
crab) are the intermediate hosts. All parasites of concern are transmitted to
humans by eating raw or uncooked fi sh products.
