An update on metacercarial excystment of trematodes.

This review updates an earlier one by Fried (Adv Parasitol 33:91-144, 1994) on metacercarial excystation of trematodes. Our coverage of the salient literature is mainly from 1995 to 2008 and includes significant studies on echinostomatid and non-echinostomatid trematodes. Most of the echinostomatid studies are on species of 37-collar spined Echinostoma, although one study is concerned with a species of Echinochasmus. The non-echinostomatid coverage includes work on species of Paragonimus, Haplorchis, Apatemon, Microphallus, Clonorchis, Riberoia, Maritema, Opisthorchis, and Zygocotyle. Most of the studies were concerned with methods used to excyst the metacercariae of the species being investigated. However, some studies were concerned with metacercarial excystation in order to use the excysted larvae for in vitro cultivation or morphometric studies. Other studies used the excysted larvae for physiological, biochemical, or behavioral work. Several studies were also concerned with elucidating both intrinsic and extrinsic factors which mediate chemical excystation of the metacercarial stage of digenetic trematodes.