Geographic distribution. Korea
Infection rate. Human cases were found only from Korea. Some inhabitants revealed 72% of prevalence in an endemic area.
Life cycle. The first intermediate host is unknown, but the second intermediate host was confirmed as the oyster Crassostrea gigas. Man and the palearctic oystercatcher Haematopus ostralegus have been shown to be natural definitive hosts.
Morphology. The adult parasite is very small (the smallest of all human intestinal trematodes reported in the Republic of Korea), 0.4-0.5 by 0.2-0.3 mm in size. This is characterized by a large oral sucker, a small ventral sucker, short caeca, two compact masses of vitellaria, and a unique ventral pit.
Pathology and clinical symptoms. Infected people complain of abdominal discomfort and indigestion. General symptoms such as fever, anorexia, weight loss, easy fatiguability, and weakness may be accompanied.
Diagnosis. Detection of eggs from the feces, but careful differentiation is required to identify the eggs. The eggs are very small, only 20-25 by 11-15 mm in size, smaller than those of C. sinensis, M. yokogawai, or other heterophyids, except for those of P. summa, and have a very thin and transparent shell. The eggs are not readily detected in routine fecal examinations performed by formalin-ether sedimentation or cellophane thick smear techniques. They may be overlooked or misdiagnosed as an air bubble or other artifacts, especially in thick smears.
Prevention. The best way to prevent G. seoi infection is the avoidance of consuming raw or undercooked oysters. In particular, consumption of oysters collected from the endemic areas should be avoided.
Treatment. The treatment can be successfully performed by giving a single oral dose of 10 mg/ kg praziquantel (Distocide?). Albendazole may also be effective against G. seoi infection.
Mesocercariae of Gymnophalloides seoi in an oyster.
Mesocercaria of G. seoi isolated from an oyster.
Mesocercaria of G. seoi isolated from an oyster. Semichon's acetocarmine stained.
Gymnophalloides seoi recovered from a mouse, which was experimentally infected with metacercariae.
G. seoi recovered from a mouse, which was experimentally infected with metacercariae. Semichon's acetocarmine stained.
Crassostrea gigas, oyster.
Haematopus ostralegus (oystercatcher), a final host of Gymnophalloides seoi.
Section of the small intestine of an oystercatcher infected with Gymnophalloides seoi. A worm is in the intervillous space of the mucosa. Adjacent villi are markedly atrophic with epithelial flattening and stromal inflammation and degeneration. Crypts show a moderate degree of hyperplasia. H-E stain, X 100
Yong Suk Ryang
Section of the intestine of Kentish plover experimental infected with Gymnophalloides seoi on 14 days. A showing a fluke worm in the intervillous space. H-E stain, X 400
Yong Suk Ryang