plerocerci

A plerocercoid is the larval stage of certain tapeworms, specifically those belonging to the order Tetraphyllidea. This larval form is characterized by being a solid, unsegmented plerocercoid larva. It develops from a oncosphere, the first larval stage, which hatches from the tapeworm egg and infects an intermediate host. The plerocercoid is essentially a pre-scolex stage, meaning it contains a developed scolex (the head of the tapeworm) but lacks a strobila (the segmented body).

Plerocercoids are typically found in the tissues or body cavity of a second intermediate host, which may be a fish, amphibian, or reptile, depending on the specific tapeworm species. If the second intermediate host is consumed by the definitive host, usually a fish-eating mammal or bird, the plerocercoid will mature into an adult tapeworm within the definitive host's intestine.

In some instances, humans can become infected with plerocercoids if they consume raw or undercooked fish or amphibians that harbor the larvae. This can lead to a condition known as sparganosis, where the plerocercoids develop and grow in human tissues. While often asymptomatic, sparganosis can cause localized inflammation, pain, and the formation of palpable nodules. The diagnosis is usually made based on clinical signs and the identification of the plerocercoid larva through biopsy or surgical removal. Treatment typically involves surgical excision of the plerocercoid.