metacestodes

Metacestodes are the larval stages of cestodes (tapeworms) that develop in the tissues of intermediate hosts after the oncosphere migrates from the gastrointestinal tract. They represent the developmental bridge between the larval forms that infect tissues and the adult worm that resides in the intestine of the definitive host. In most life cycles, the metacestode persists in tissues such as the liver, lungs, or muscle, forming a cyst-like structure that contains the larval scolex. When a definitive host ingests the metacestode, the scolex evaginates and the adult tapeworm then develops in the host’s gut.

Metacestode forms include cysticercus, hydatid cyst, coenurus, and strobilocercus. A cysticercus is a fluid-filled cyst with a single protoscolex. A hydatid cyst (produced by Echinococcus species) is typically large and may be multilocular, containing brood capsules and numerous protoscolices. A coenurus contains multiple scolices within a single cyst; a strobilocercus is a more segmented form in which developing proglottids or a chain of scolices form within the cyst in some Taenia species.

Metacestodes are clinically important in both human and veterinary medicine because they give rise to cysticercosis and echinococcosis, depending on the genus involved. Neurocysticercosis, caused by Taenia solium cysticerci in the human brain, is a major cause of seizures in endemic areas. Echinococcosis, caused by Echinococcus spp., forms hydatid cysts in the liver and lungs and can be life-threatening if a cyst ruptures. Diagnosis relies on imaging and serology; treatment includes antiparasitic drugs and, for large or complicated cysts, surgery or aspiration. Prevention focuses on hygiene, control of definitive hosts, and safe food practices.