Procercoids

Procercoids are an early larval form of certain tapeworms (cestodes) that develop in the first intermediate host, typically a crustacean such as a copepod. After the oncosphere reaches the copepod, the larva migrates into the body cavity and differentiates into a procercoid, an immature stage with a rudimentary scolex. The procercoid represents a transitional stage between the initial infection of the crustacean and subsequent development in a second intermediate host.

In the common life cycle of many Diphyllobothriidae and related cestodes, the copepod harboring the procercoid is eaten by a second intermediate host, usually a small fish. Within the fish, the larva migrates to tissues and molts into the plerocercoid, the stage that can infect the definitive host when the fish is consumed. The procercoid is typically smaller and simpler than the plerocercoid and does not yet possess a fully formed, functional scolex capable of attachment in the definitive host.

Ecological and medical relevance: procercoids are integral to understanding how certain tapeworms cycle through aquatic food webs. Humans are generally not infected by procercoids; human infection, when it occurs, involves the plerocercoid stage in undercooked or raw fish or, in some lineages, sparganum-like larvae in secondary hosts. Genera in which procercoids have been described include Diphyllobothrium and Spirometra, among others.