Reservoirs and Hosts

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M. yokogawai has a complex lifecycle involving two intermediate hosts (see lifecycle). Pollution of water in which the intermediate hosts live  provides the original sources of infection.

The first intermediate hosts (Semisulcospira libertina, and possibly related species), are freshwater snails. They ingest fully embroynated unhatched M. yokogawai eggs from human fecal matter deposited in the water. The eggs hatch and release miracidia, which undergo a complex lifecycle in the snail, to produce cercariae.

Korean Freshwater Snail - Semisulcospira libertina. Animal Pictures Archive


The cercariae are tailed, and swim around in the water until they encounter a suitable fish, at which point they attack the fish, and encyst under its scales, in the skin, or in the flesh. These fish are the second intermediate host required for the M. yokogawai lifecycle. The Oriental Freshwater Trout Plectoglossus altivelis is the most common of the fish species involved in this cycle. P. altivelis (also known as Sweetfish) is particularly common in waters off the coast of South East Asia. Other freshwater fishes, especially Odontobutis obscurus and Salmo perryi,  are common hosts in this and other endemic foci.

Plectoglossus altivelis altivelis. Ken, The Angling Authority

Ingestion of the raw flesh of these fishes containing M. yokogawai metacariae provides the means of human infection. Heterophyids tend to lack host specificity, and will mature in a wide range of birds and mammals. In addition to humans, fish-eating mammals and pelicans also act as reservoir hosts of the parasite.