Life Cycle

The life cycle of M. yokogawai includes two intermediate hosts and one definitive host. Each will be discussed in turn, beginning with the parasite's egg.

Egg

http://www.nih.go.jp/niid/para/atlas/images/yokol.gif

The egg of M. yokogawai is small, and indistinguishable from those of another intestinal fluke, Heterophyes heterophyes. It is found in the stool of the definitive host. The eggs possess prominent opercular shoulders, are brownish yellow, and are 26.5 to 30µm by 15 to 17µm. Each egg contains a fully-developed miracidium, which is the larva that infects the first intermediate host.

 

First Intermediate Host - Snail

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The first intermediate host of M. yokogawai is a gastropod, or snail, most frequently of the genus Semisulcospira. The snail ingests the egg, after which the miracidia emerge and penetrate the snail's intestine. The parasite undergoes several developmental stages in the snail:

The cercaria is the form that is released from the snail into the surrounding water and infects the second intermediate host.

 

Second Intermediate Host - Fish

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The second intermediate host is a fresh-water or brackish-water fish. Most frequently infected are species of Plectoglossus, Tribolodon, Lateolabex, and Salmo. The fish is the only player in M. yokogawai's life cycle that does not ingest the parasite. The cercariae encyst as metacercariae underneath the fish's scales, or in the flesh after penetrating the fish’s skin. They can also encyst in the gills, fins, and tails. The metacercaria is the form that infects the definitive host.

 

Definitive Host - Humans & Other Fish-Eating Mammals/Birds

The definitive host is a fish-eating mammal or bird that ingests metacercariae along with its fish meal. A variety of definitive hosts serve as reservoirs of infection. The metacercariae excyst in the definitive host's small intestine, where they attach to, and at times burrow into, the mucosa. There they mature to become adults, and produce eggs which are released in fecal matter, thus completing the life cycle.

Next: Human Transmission

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