cystacanths

Cystacanths are the encysted larval stage of thorny-headed worms, or acanthocephalans (phylum Acanthocephala). They develop within the tissues of an intermediate host, typically a crustacean such as a crayfish, amphipod, or isopod, and are the infective form that passes to the definitive vertebrate host when the intermediate host is consumed. The cystacanth represents a mature larval form that is ready to establish infection in the next host.

Morphology and development: A cystacanth is enclosed in a tough cyst wall and contains a fully formed larval worm with a retractable proboscis bearing hooks. The proboscis is pressed against or within the cyst wall and becomes functional when released in the gut of the definitive host. The cyst wall protects the larva as it passes through the environment and during passage to the next host.

Life cycle: The life cycle begins with eggs released by the definitive host into the environment. The eggs hatch into an oncosphere, which penetrates the wall of an intermediate host and develops through stages (acanthor, acanthella) until a cystacanth is formed. When the intermediate host is ingested by the definitive host, the cystacanth excysts, the proboscis everts, and the parasite attaches to the intestinal wall to mature into an adult worm.

Ecology and significance: Cystacanth infections can influence the behavior of intermediate hosts in ways that increase predation by the definitive host. They are of interest in parasite ecology and can affect ecosystem dynamics, host populations, and, in some cases, aquaculture. They are distinct from cysticercoids seen in other parasites, such as cestodes, and are identified by their encysted, armed larval form within crustacean or insect hosts.