Life Cycle of Taenia solium (Pork tapeworm)

Taenia solium is commonly known as pork tapeworm. It is an endoparasite, the adult lives in the intestine of human beings. It has a cosmopolitan distribution. Taenia solium is a digenetic cestod (class Cestoda under phylum Platyhelminthes) i.e. its life cycle is completed in two hosts, human being is the primary host and pig is the secondary host.

1.0Introduction

• Taenia is found attached to the intestinal mucosa of humans by its scolex (head region) while the rest of the body lies free. 
• It is most common in the pork eating population of tropical and subtropical regions where pork is consumed as food without being properly cooked. 
• The body is very elongated (varies from 3-5 metres), dorsoventrally flattened and looks like a ribbon or tape. 
• The body of a tapeworm is differentiated into three regions–head or scolex, neck and strobila. 
• Scolex bears hooks and suckers for attachment to the host, the neck is very short and proliferative, and strobila consists of a large number of linearly arranged segments-like sections called proglottids.

The life cycle of Taenia solium may be described under the following headings :

Copulation and Fertilization 

• Taenia is hermaphrodite. Each mature proglottid, after the first 200, contains a complete set of male and female reproductive organs. 
• Taenia practices self-fertilization, i.e. eggs are fertilized by sperms from the same proglottid or one proglottid may be inseminated by a proglottid situated anterior to it. 
• Taenia, in fact, is protandrous, and its male reproductive organs develop first in the anterior most mature segments. 
• Thus, the anterior mature proglottids can copulate with the posterior proglottids. This is achieved by the bending of the strobila into folds. 
• The possibility of cross fertilization is remote as no host is in a position to house two adult and large tapeworms at a time. 
• Sperms injected into the vagina swim down to the seminal receptacle where they are stored temporarily till ova are released by the ovary. 
• Fertilization occurs inside the ootype.

2.0Formation of Egg Capsules

• Just after fertilization, the fertilized egg or zygote gets surrounded by yolk cells in the ootype, received from the vitelline glands through the vitelline duct. 
• The fertilized egg and yolk cells are subsequently enclosed in a thick, resistant egg shell secreted by the shell glands. 
• The egg capsule or egg shell then passes into the uterus. 
• The secretion of Mehlis’s gland facilitates the passage of capsules in the uterus. 
• The uterus grows in size and becomes branched laterally as it receives more and more egg capsules. The egg capsules are very small, about 40 µm in diameter.

Development (Oncosphere formation)

• Taenia solium undergoes indirect development with a single larval stage. 
• Fertilized eggs develop in the uterus, starting with unequal cleavage into a large megamere and a small embryonic cell. 
• Megameres form an outer layer, mesomeres create an envelope, and micromeres develop into a central morula. 
• Megameres absorb yolk for nourishment, later forming a syncytial nutritive envelope that gradually disappears. 
• The mesomere layer forms a protective embryophore around the morula. 
• The embryo develops six chitinous hooks, becoming a hexacanth embryo with penetration glands. 
• Surrounded by two membranes, it forms the oncosphere. Once formed, the uterus of a gravid proglottid contains 30,000–40,000 oncospheres.

Infection to secondary (intermediate) host

• The gravid proglottids containing onchospheres at the posterior most part of strobila detach or break off (apolysis) in groups of 4 to 5 and pass out of the body of the host (human) along with the faeces.

• A newly shed proglottid wriggles for some time but eventually disintegrates, setting free thousands of oncospheres. 
• The secondary host pig acquires infection by ingesting the human fecal matter containing the oncospheres. 
• On reaching the stomach of the pig, the egg-shell and the embryophore get digested by the gastric juices and the hexacanth embryo is released. 
• The hexacanth then passes into the small intestine. 
• It is now activated by the presence of bile salts, bores its way through the intestinal epithelium to reach the sub mucosal blood vessels. 
• This is accomplished, jointly by the penetration glands and six hooks. Hooks merely anchor the hexacanth to the intestinal wall, while secretion of penetration glands dissolves the intestinal tissues. 
• Entire process takes about 10 minutes, after which the hooks are of no use and are shed off. Submucosal blood vessel carries the hexacanth to the liver via hepatic portal vein. From the liver it reaches the heart and enters the arterial circulation. It finally reaches the striated muscles in any part of the body. 
• They usually settle in the muscles of the tongue, neck, shoulder, thigh,heart etc. Once settled in muscle each embryo absorbs a large amount of watery fluid from host tissue and grows to spherical, pea-size sac like cyst, called bladder worm or cysticercus larva. The cysticercus of Taenia solium is called cysticercus cellulosae. 
• The flesh of pig or pork containing many cysticerci appear white spotted resembling something that of measles, hence it is characteristically called measly pork.

Cysticercus or Bladder Worm

• The hookless bladder worm gradually becomes encysted by the connective tissue of the host (pig). 
• All cells of the cysticercus larva fall into two layers around the fluid-filled central cavity. Cells of the outer layer coalesce to form a syncytial tegument ( layer of thick syncytial protoplasmic mass). 
• The inner layer of cells is a mesenchymal layer or germinal layer. 
• At the future anterior end (opposite to the side where hooks were present) the wall of the larva thickens and then invaginates into the cavity (Fig. 21), as hollow nob. 
• The invaginated knob develops four suckers on its inner surface and hooks and rostelum are developed at its bottom. 
• Now this inverted knob is called proscolex. In fact, the embryo at this stage is called cysticercus or bladder worm, whose further development does not take place unless it is eaten up by the primary host. 
• Thus fully formed cysticerci or bladder worms are infective to human hosts. 
• They usually survive (in dormant state) in the flesh for several years. It has been seen that one kilogram of measly pork may contain 500 or more cysticerci. 

Infection to primary host (human)

• Pork eating people get the infection of Taenia solium by eating raw or imperfectly cooked measly pork. 
• The cysticercus becomes active on reaching the small intestine. Actually their bladder is digested and the proscolex everts or evaginates (turned inside out) so that the suckers and rostellum come to lie on the outer surface as in the adults. 
• Thus a scolex and a small neck is formed. 
• The scolex anchors itself to the mucous membrane of the intestine by the help of suckers and hooks. 
• The neck begins to proliferate proglottids and gradually a series of proglottids are formed as strobila and ultimately an adult tapeworm is developed. In about 10 to 12 weeks the parasite attains adult condition with gravid proglottids.
• The life cycle of Taenia solium is not so complicated because it does not involve any asexual generation. 
• The complete life cycle may be represented with the help of the following flow-chart : 
• Adult tapeworm in human gut → Fertilized egg in mature proglottids → Egg capsules within gravid proglottids → Oncospheres in gravid proglottids → Gravid proglottids or onc spheres in human faeces → Oncospheres outside human body in faeces → Hexacanths in the gut of pig → Hexacanths in the intestinal blood vessels → Hexacanths in the heart → Hexacanthus in the muscle → Cysticercus in the striated muscles → Measly pork → Cysticercus in the gut of human beings → Adult tapeworm in human intestine.