TABLE OF CONTENTS
Types of Metacestodes (Bladder Worm Stages) in Tapeworms with Examples
Metacestodes, also known as bladder worm stages, are the larval forms of tapeworms (cestodes) that develop in intermediate hosts. Different cestode species produce distinct types of metacestodes, which vary in their morphology, number of scolices, and developmental characteristics.
The major types of metacestodes include cysticercus, cysticercoid, strobilocercus, coenurus, hydatid cyst, procercoid, plerocercoid, and tetrathyridium.
1. Cysticercus
A cysticercus consists of a single invaginated scolex within a fluid-filled cavity.
Examples:
- Cysticercus cellulosae – larval stage of Taenia solium.
- Cysticercus bovis – larval stage of Taenia saginata.
2. Cysticercoid
A cysticercoid consists of a single non-invaginated scolex enclosed within a small vesicle. It has little to no cavity.
Example: Larval stage of tapeworms infecting ruminants and poultry.
3. Strobilocercus
A strobilocercus consists of a single scolex that is not invaginated when fully developed and is attached to the bladder by a long neck. Example: larval stage of T. taeniaeformis.
4. Coenurus
A coenurus is a large fluid-filled bladder containing numerous invaginated scolices attached to its inner wall.
Example: Larval stage of T. multiceps.
5. Hydatid Cyst
A hydatid cyst is a large fluid-filled bladder that produces daughter cysts called brood capsules, within which scolices develop. Mature brood capsules may detach and float freely in the cyst fluid, forming what is known as hydatid sand.
Example: Larval stage of Echinococcus granulosus.
6. Procercoid
Procercoid is the larval stage of cotylodan tapeworms and occurs in the first intermediate host (I/H). It has a solid body and possesses hooks on the posterior region.
7. Plerocercoid
Plerocercoid is also a larval stage of cotylodan tapeworms and occurs in the second intermediate host (I/H). It is elongated, solid, and possesses a scolex similar to that of the adult worm.
8. Tetrathyridium
Tetrathyridium is the larval stage of Mesocestoides lineatus. The body is elongated, solid, and possesses a deeply invaginated scolex.
