Digenea: Encyclopedia II - Digenea - Life cycles

Digenea - Life cycles

Digenean fluke eggs leave the vertebrate host in faeces and use various strategies to infect the first intermediate host, in which sexual reproduction does not occur. Digenes may infect the first intermediate host (usually a snail) by either passive or active means. The eggs of some digenes, for example, are (passively) eaten by snails (or, rarely, by an annelid worm) in which they proceed to hatch. Alternatively, in many digenes, eggs hatch in water to release an actively swimming, ciliated larva, the miracidium, which must locate and penetrate the body wall of the snail host.

After post-ingestion hatching or penetration of the snail, the fluke metamorphoses into a simple, sac-like sporocyst. The sporocyst undergoes asexual reproduction, ultimately yielding large numbers of a second free-living stage, the cercaria. This change involves germinal cells within the sporocyst developing into rediae (sing. redia), which emerge from a birth pore or are released by rupture of the sporocyst. Each germ cell in the redia develops into a cercaria (pl. cercariae).

Free-swimming cercariae leave the snail host and move through the aquatic or marine environment using a whip-like tail. Cercariae are infective to second host in the life cycle, and infection may occur passively (e.g., a fish consumes a cercaria) or actively (the cercaria penetrates the fish).

The life cycles of some digeneans include only two hosts, the second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which, by the same token, is in this case also the definitive host.

In other groups, infection of a third host, from still another vertebrate species, is required before the parasite matures sexually. In the second intermediate host, cercariae of these taxa develop into a resting stage, the metacercaria, which is usually encapsulated in a cyst of host and/or parasite origin. This stage is infective to the definitive host. Infection of the definitive host occurs when the intermediate host is eaten by the definitive host. Metacercariae excyst in the definitive host’s gut in response to a variety of mechanisms, such as gut pH levels, digestive enzymes, temperature, etc. Once excysted, flukes migrate to more or less specific sites in the definitive host and the life cycle repeats.

Adapted from the Wikipedia article "Life cycles", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki