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

Introduction of Taenia solium

Taenia solium, also known as pork tapeworm, causes intestinal taeniasis. Infection is acquired orally in humans after ingesting raw or undercooked pork containing the larvae (Cysticercus cellulosae) of the parasite. Pigs acquire the infection after consuming human feces contaminated with the tapeworm egg.

Pig is the usual intermediate/secondary host where larval development takes place while the definitive host of T. solium is the human. However, man can also act as an intermediate host making Taenia solium the only cestode with a man acting as both an intermediate/secondary and definitive host.

The Cysticercus cellulosae causes a serious disease called cysticercosis (especially neurocysticercosis), which is absent in T. saginata.

Classification of Taenia solium

Kingdom: Animalia

Phylum: Platyhelminthes

Class: Cestoda

Order: Cyclophyllidea

Family: Taeniidae

Genus: Taenia

Species: T. solium

History of Taenia solium

In 1854, van Beneden first described the life cycle of Taenia solium. He was able to demonstrate the larvae (Cysticercus cellulosae) in the pig muscles after feeding it eggs from human feces.

A year later, in 1855, Kuchenmeister displayed an adult tapeworm in the intestine of humans.

Habitat of Taenia solium

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 hooks or rostellum.

Morphology of Taenia solium

Adult worm

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

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

• measures 2 meters to 3 meters in length

Scolex

• round in shape

• measures 1mm in diameter

• has four suckers

• conspicuous hooks or rostellum is present – hence called armed tapeworm

• rostellum consists of two rows of alternating small and large hooks

Neck

• short and fragile

• measures 5 mm to 10 mm in length

Strobila

• measures 2 meters to 3 meters

• consists of 800-1000 proglottids or segments

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

• immature proglottids are broader than longer

• mature proglottids are also wider than long, similar to that of T. saginata

• gravid proglottids are longer than broad (measure 12mm x 6mm), contain around 30,000 to 50,000 eggs in each segment

• gravid proglottids are grayish-black in color and turn transparent when fully developed

• common genital opening is present laterally in the middle of each segment with alternation between the left and right side

• after breaking off from Strobila, around 5-6 gravid segments are passed passively in chains along with the feces

• Taenia solium can live for 25 years and the most single worm is found infecting a human- multiple infections are rare

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

• presence of 150-200 testes

• the ovary has a third accessory lobe

• the ovary is present on the posterior side of the segment

• vaginal opening lacks a muscular sphincter

• has a median longitudinal uterine stem with 7-13 branches on each side

Egg

• round or oval in shape

• measures 41μm to 43μm in diameter

• infective to pigs as well as humans

Infective Form

Cysticercus cellulosae

• also known as taenia cyst of Taenia solium

• the infective form of the parasites for humans

• this larval form is found in the muscles of both pigs and man

• small, oval in shape measuring 3mm x 15mm

• are fluid-filled milky white bladder-like structures rich in albumin and salts, with a translucent wall through which a single dense white body (invaginated scolex) can be seen

• cross-section of the scolex shows several layers of folded smooth muscles with parts of suckers or hooklets

• cyst wall measures 100μm -200μm and are composed of three layers

  * inner layer consists of longitudinal and circular muscles

  * middle layer consists of pseudoepithelial cells

  * outer cuticular later contains a dentate membrane with a microvillus projection responsible for interaction with host tissues

• cyst and host tissue are separated by a thin collagenous capsule