(Trematoda) Metagonimus yokogawai
The agent responsible for metagonimiasis is the heterophyid intestinal fluke Metagonimus yokogawai. The fluke is a member of the trematode family, and is one of the smallest human flukes. M. yokogawai bears a strong resemblance to the parasite Heterophyes heterophyes. This similarity has implications for its management and diagnosis.
The Adult M. yokogawai is 1.0mm to 2.5mm in length, and 0.4mm to 0.75mm in width. Like most of the trematodes adults, M. yokogawai is roughly leaf-shaped. Its outer cuticle is spiny. The fluke has two suckers (attachment organs) - an anterior one, or oral sucker surrounding the mouth; and a more posterior one, the ventral sucker on the ventral surface.
The oral cavity leads to an esophagus, which branches into two intestinal caeca, running parallel to each other down the posterior of the worm. M. yokogawai is hermaphroditic; most of the rest of the fluke is taken up with two testes and an ovary. The uterus is the largest organ, winding up around the fluke, toward the genital pore (from which the eggs are released). Adults normally live up to several months in their host organism.
The Adult M. yokogawai. Georgia Division of Public Health
The eggs of M. yokogawai are fairly impossible to distinguish from those of other trematodes, particularly Heterophyes heterophyes. They also closely resemble the eggs of worms belonging to the genera Clonorchis and Opisthorchis. The eggs have a smooth hard shell, and are about 26.5 to 30μm by 15 to 17μm in size. They are brownish yellow in colour, and have a very distinct opercular shoulders (through which the miracidium emerges when the egg "hatches").
Like most trematodes, M. yokogawai has a complex lifecycle that is dependent on two intermediate hosts. Adult flukes live in the small intestine of the human reservoir, attached to the mucosa. They release fully embryonated eggs that are passed in the feces. After ingestion by a suitable snail (1st intermediate host), the eggs hatch and release miracidia which undergo several developmental stages in the snail. This process produces cercariae, which are released from the snail and encyst as metacercariae in the tissues of a suitable freshwater fish (second intermediate host). The definitive host becomes infected by ingesting undercooked fish containing metacercariae. After ingestion, the metacercariae excyst, attach to the intestinal mucosa, and mature into adults. In addition to humans, fish-eating mammals and birds can also be infected.
The lifecycle of M. yokogawai.