Taenia saginata - Introduction, Classification, History, Habitat, Morphology

Introduction of Taenia saginata

Taenia saginata is a large zoonotic tapeworm causing intestinal taeniasis in humans. It is also known as the beef tapeworm. Infection is acquired orally by ingesting raw or undercooked beef containing the larvae (Cysticercus bovis) of the parasite.

Cattles are intermediate hosts where larval development takes place while the definitive host of Taenia saginata is the human host harboring the adult tapeworm. The parasite does not cause cysticercosis and the infection is harmless and asymptomatic in most cases.

Taenia saginata strongly resembles other human tapeworms such as Taenia asiatica and Taenia solium, in structure and biology. However, Taenia saginata can be differentiated on the basis of the following properties:

• typically larger and longer than Taenia asiatica and Taenia solium

• more proglottids, more testes, and higher branching of the uteri

• lacks an armed scolex

Classification of Taenia saginata

Kingdom: Animalia

Phylum: Platyhelminthes

Class: Cestoda

Order: Cyclophyllidea

Family: Taeniidae

Genus: Taenia

Species: T. saginata

History of Taenia saginata

In 1782, Goeze differentiated Taenia saginata from T. solium. Leuckart, in 1863, demonstrated cattle to be the intermediate host of the tapeworm.

Habitat of Taenia saginata

In the definitive host, the adult tapeworm can be found in the small intestine (upper jejunum). The parasite remains attached to the intestinal mucosa with the help of suckers.

Morphology of Taenia saginata

Adult worm

• white, ribbon-like, flattened, and segmented worm

• measures 4 meters to 10 meters but sometimes may reach upto 15 meters

• contains head (scolex), neck, and body (strobila) which consists of a chain of segments (proglottids)

Scolex

• quadrate in shape

• measures 2mm in diameter

• contains four cup-shaped muscular suckers

• suckers may be pigmented

• hooks or rostellum is absent

Neck

• short and fragile

• measures 3mm to 7mm in length

Strobila

• measures 4 meters to 10 meters but sometimes may reach upto 15 meters

• consists of 1000-2000 proglottids or segments

• the proglottids of Taenia saginata are arranged in a linear sequence of immature, sexually mature, and gravid

• each proglottid consists of numerous testes and bi-lobed ovary

• on the lateral part of each segment, common genital pores are present with alternative irregularly between the right and left margin

• sexually mature segments are opaque, and white and measure 2cm in length and 5mm to 7mm n breadth

• gravid segments are rectangular measuring 20mm by 5mm to 7mm and are found in the posterior part of the parasite

• as gravid segments uterus is fully developed, it consists of 15-30 lateral branches

These gravid proglottids routinely break off from the strobila and are passed along with the feces.

The proglottids of Taenia saginata are different from that of T. solium as Taenia saginata:

• has twice the number of testes i.e. 300-400 testes than present in T. solium

• have developed vaginal sphincter

• lacks accessory ovarian lobe

• typically larger and longer than Taenia asiatica and Taenia solium

• more proglottids, more testes, and higher branching of the uteri

• lacks an armed scolex

Egg

• round or oval in shape

• measures 33μm to 43μm in diameter

• bile stained

• each egg has two membranes or shells- an outer membrane and an inner membrane

  * outer shell, also called the egg capsule, is thin and transparent and is present when passed fresh from proglottids but is absent in eggs from old feces

  * inner shell, also called embryophore, is a thick, brown roughly structured wall enclosing the embryo

  * embryo (oncosphere) measures 14μm to 20μm in diameter, contains six hooklets (hexacanth embryo), and is only infective to cattle

• since eggs are similar in size and shape to that of T. solium, Echinococcus, and Multiceps, light microscopy cannot identify Taenia saginata eggs

Infective Form

Cysticercus bovis

• the cysticerci larvae (Cysticercus bovis) is the infective form for humans

• oval in shape measuring 5mm x 10mm

• has a translucent cyst filled with a clear fluid

• contains opaque invaginated protoscolex (future scolex)

• they can be found in any organ but is much more common in the heart and muscles of the cattle

• this stage occurs only in cattle and not the humans