Cestodes Lecture 5

Study Notes

Cestodes Lecture 5

Cestodes are multi-cellular, bilaterally symmetrical, elongated, flat or rounded animals.
They belong to the phylum Platyhelminthes (flatworms)
This phylum is further divided into Trematoda and Cestoidea.
Cestodes are also known as tapeworms.
Cestoidea (tapeworm) is further divided into groups: Pseudophyllidea and Cyclophyllidea.
Examples of Pseudophyllidea are D. latum.
Examples of Cyclophyllidea are Taenia, Echinococcus granulosus, Hymenolepis nana.
Adults are flat and segmented, varying in length from a few mm to several meters.
The body is divided into the head (scolex), neck, and proglottids (strobila).
The scolex has suckers for attachment, sometimes with rostellum and hooks.
The neck is the region of growth.
Proglottids are differentiated into immature, mature, and gravid segments, containing eggs.
Cestodes lack a body cavity and an alimentary tract.
Nutrients are absorbed through the cuticle or tegument.
They are hermaphrodites.
Adults are parasites of the small intestines of humans or animals, with the exception of Hymenolepis nana, which acts as both intermediate and definitive hosts.
Gravid segments or eggs of cestodes are passed in the feces of their definitive hosts.
Taenia saginata scolex is quadrate, unarmed with four strong suckers.
Taenia solium scolex is globular with rostellum armed with double rows of hooks.
Taenia saginata adults measure 4-10 meters long, with 1000-2000 proglottids.
Taenia solium adults are shorter, 3-8 meters long and have fewer proglottids (approximately 1000).
The diagnostic stage consists of gravid segments or mature eggs in the feces of the definitive host.
Taenia eggs are spherical with thick striated brown embryophore surrounding a hexacanth embryo or oncosphere (embryo).
Taenia saginata gravid segments are more elongate and rectangular compared to Taenia solium.
The uterus of Taenia saginata has more lateral branches compared to Taenia solium.
The intermediate hosts for Taenia saginata are cattle; for Taenia solium, pigs.
The infective stage of Taenia saginata is cysticercus bovis in cattle muscles.
The infective stage of Taenia solium is cysticercus cellulosae in pig muscles.
Humans become infected by ingesting undercooked beef (for T. saginata) or pork (for T. solium) containing the infective cysts.

Hymenolepis nana

Hymenolepis nana is a cyclophyllidean cestode also known as the dwarf tapeworm.
Adult worms live in the small intestines of humans.
The scolex is rhomboidal with four suckers and spines.
The neck is long and slender, followed by more than 100 segments, which are broader than long.
The infective stage is eggs.
Diagnostic stage: Eggs in the feces of the definitive host. The eggs are oval or spherical, with two thin shells; the inner shell has two knobs at opposite sides from which four to eight filaments arise.
Mode of infection: by ingestion of eggs in contaminated food or drink, or autoinfection.
Intermediate hosts: rodents are infected when they ingest cysticercoid-infected arthropods.

Echinococcus spp

This is in the Cyclophyllidea group.
Echinococcus granulosus is a type of tapeworm.
Adult worms live in the small intestines of dogs and other carnivorous animals (dogs, foxes).
The scolex is pyriform and has four suckers armed with hooks.
The neck is short and is followed by three proglottids (immature, mature, and gravid).
The infective stage is eggs which are passed in the feces of the definitive host.
The intermediate host is herbivorous animals such as sheep, goats, and man.
The diagnostic stage is a hydatid cyst in different organs.
The mode of infection is by ingestion of food contaminated with eggs.