LIFE HISTORY OF TAPEWORM

1. Early Development

Development of the zygote begins within the worm and by the time the proglottid is shed, has already changed into an embryo. The zygote (Fig. 7.11 A, 7.11 B) undergoes complete but unequal cleavage. It produces a mass of blastomeres of 3 types : 2 or 3 macromeres, 3 to 5 mesomeres and numerous micromeres (Fig. 7.11 C). The micromeres form the embryo with 3 pairs of claw-like hooks at the posteiror end. The 6-hooked embryo is called hexacanth. The mesomeres and macromeres form (Fig. 7.11 G) the inner and outer embryonic membranes respectively round the hexacanth. The inner (Fig. 7.11 D, E) membrane later forms a thick, chitinous, radially striated secondary shell or embryophore (Fig. 7.11 F). The hexacanth with embryophore, outer embryonic membrane and shell or capsule wall is known as the onchosphere (Fig. 7.11 G). (Gr. onchos = hook). It is about 40 μm in diameter. It is actually the onchospheres which are set free when the proglottides disintegrate in the human faeces.

The onchospheres remain viable for several months in the soil. They do not develop further until taken by a pig.

2. Infection of the Pig

A pig gets infection by swallowing onchospheres with the human faeces. The shell wall and embryophore dissolve in the intestine after previous immersion in the acidic juice of the stomach, and the hexacanths are set free. The hexacanths attach themselves to the intestinal mucosa with their hooks. They bore through the intestinal wall with a pair of penetration glands, and get into the blood or lymph vessels. They are carried by the blood to the striated muscles, particularly in the tongue, neck, heart and limbs. In the muscles, hexacanths get lodged, since they are larger than the capillaries. Here, each hexacanth develops into another larval form called the bladder worm or cysticercus.

3. Cysticercus

It is formed from a hexacanth by loss of hooks and rapid growth into an oval, whitish body about 10 mm x 6 mm. It develops a fluid-filled cavity and encysts in a cuticular covering. It now looks like a bladder, hence its name. The wall consists of cuticle and a mesenchyme layer. The fluid in it consists largely of the host's blood plasma. The side of the larva opposite to that originally occupied by the hooks thickens and invaginates into the cavity. Rostellum, hooks and suckers appear at the bottom of the invagination. Thus, an inverted scolex, called the proscolex, is formed. The cysticercus with proscolex is infective and waits for its ingestion by man.

The onchospheres take about 4 months to develop into infective cysticerci. The cysticerci remain alive and infective in the muscles for 5 or 6 years before they die, and are then calcified by the host. There may be about 3,000 cysticerci in 500 grams of infected pork. The infected pork has brownish spots and is called the "measly pork". host. There may be about 3,000 cysticerci in 500 grams of infected pork. The infected pork has brownish spots and is called the "measly pork".

4. Human Infection

Human infection occurs by taking raw or undercooked measly pork. Pink or red appearance of the pork, when cut into slices, is an indication of its being undercooked. In the human intestine, the cyst breaks down and the cysticercus evaginates its proscolex so that the suckers, hooks and rostellum come to lie on the outer surface as in the adult worm. The bladder is now cast off. Thus, a young worm with a scolex and a small neck is formed. It fixes its scolex to the intestinal mucosa by means of hooks and suckers, and starts budding off proglottides from its neck . In about 3 - 4 months, the worm becomes adult, and the first formed proglottides, full of onchospheres, drop off from the end of the strobila to begin the life cycle again.

It may be noted that unlike Fasciola, Taenia has no free larval stage in its life history, and also no invertebrate host. However, like Fasciola, it has enormous power of multiplication but by excessive vegetative growth instead of by polyembryony.