[Metagonimus yokogawai]

Causal Agents

Metagonimus yokogawai, a minute intestinal fluke (and the smallest human fluke).

Life Cycle


Adults release fully embryonated eggs each with a fully-developed miracidium, and eggs are passed in the host’s feces The number 1. After ingestion by a suitable snail (first intermediate host), the eggs hatch and release miracidia which penetrate the snail’s intestine The number 2. Snails of the genus Semisulcospira are the most frequent intermediate host for Metagonimus yokogawai. The miracidia undergo several developmental stages in the snail, i.e. sporocysts The number 2a, rediae The number 2b, and cercariae The number 2c. Many cercariae are produced from each redia. The cercariae are released from the snail The number 3 and encyst as metacercariae in the tissues of a suitable fresh/brackish water fish (second intermediate host) The number 4. The definitive host becomes infected by ingesting undercooked or salted fish containing metacercariae The number 5. After ingestion, the metacercariae excyst, attach to the mucosa of the small intestine The number 6 and mature into adults (measuring 1.0 mm to 2.5 mm by 0.4 mm to 0.75 mm) The number 7. In addition to humans, fish-eating mammals (e.g., cats and dogs) and birds can also be infected by M. yokogawai The number 8.

Geographic Distribution

Mostly the Far East, as well as Siberia, Manchuria, the Balkan states, Israel, and Spain.

Clinical Presentation

The main symptoms are diarrhea and colicky abdominal pain. Migration of the eggs to extraintestinal sites (heart, brain) can occur, with resulting symptoms.

Metagonimus yokogawai, adult fluke.


Metagonimus yokogawai adult flukes are minute intestinal flukes (1-2.5 mm in length) that resemble Heterophyes heterophyes. An important distinctive feature is the position of the ventral sucker (genitoacetabulum), which is to the side of the midline and closely associated with the genital pore. Adults reside in the small intestine of the definitive host.
Figure A: Adult M. yokogawai, stained with carmine. In this figure, the following structures are labeled: oral sucker (OS), pharynx (PH), intestine (IN), genitoacetabulum (GA), ovary (OV), the large, paired testes (TE), and eggs within the uterus (EG).
Snail intermediate hosts of M. yokogawai.


Like all trematodes, Metagonimus requires a snail as an intermediate host. Snails in the genus Semisulcospira are the most common intermediate hosts for M. yokogawai.
Figure A: Snail in the genus, Semisulcospira. Image courtesy of Conchology, Inc, Mactan Island, Philippines.
Figure B: Snail in the genus, Semisulcospira. Image courtesy of Conchology, Inc, Mactan Island, Philippines.

Laboratory Diagnosis

The diagnosis is based on the microscopic identification of eggs in the stool. However, the eggs are indistinguishable from those of Heterophyes heterophyes and resemble those of Clonorchis and Opisthorchis. Specific diagnosis is based on identification of the adult fluke evacuated after antihelminthic therapy, or found at autopsy.

Treatment Information

For information about treatment please contact CDC-INFO.

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