Cercaria

This article is about the flatworm life stage. For the genus of flatworms, see Cercaria (genus). For the genus of single-celled organisms, see Euglena.

A cercaria (plural cercariae) is a larval form of the trematode class of parasites. It develops within the germinal cells of the sporocyst or redia.^[1] A cercaria has a tapering head with large penetration glands.^[2] It may or may not have a long swimming "tail", depending on the species.^[1] The motile cercaria finds and settles in a host where it will become either an adult, or a mesocercaria, or a metacercaria, according to species.

Cercarial infection in water environments by non-human schistosome species causes dermatologic burden to nearby swimmers, fishermen and farmers. The cycle as mentioned above, starts with egg distribution whether fecal in route or from the nostril of a duck or goose.^{[3]: 157–159} Miracidia infect snail reservoirs and form successive sporocysts. Released cercariae travel and infect nearby non-human mammals or birds, depending on species, or accidental hosts such as humans. This dead-end infection and host allergic reaction has been referred to as cercarial dermatitis or "swimmer's itch".^[4][5] Dermatitis can also be seen following exposure to the cercariae of schistosome species that cause patent infections in humans (schistosomiasis) in endemic areas.^[6]

Among fish, infection beneath the scale bedding by cercariae of other trematode species can result in black spot disease.^[7][8] This is an example of an encysted form, or metacercaria. For some trematode species, cercarial encystment takes place on aquatic plants (e.g. in the liver fluke Fasciola gigantica^[9] and the intestinal fluke Fasciolopsis buski^[10]).

The term Cercaria is also used as a genus name in descriptions of species when only the larval form is known.^[11]

Rotifers (Rotaria rotatoria) produce a chemical, Schistosome Paralysis Factor, that suppresses cercaria swimming and reduces infections.^[12]