TAPEWORM

Taenia solium (The pork tapeworm)

· It is highly elongated, tape or ribbon-like, digenetic endoparasite.
· Adult lives in the intestinal mucosa of the small intestine of man (primary or definitive or final host).
· Larva lives in the tissues of the pig, sometimes dog and sheep (secondary or intermediate host). Both hosts are vertebrates.
Both hosts are not vertebrates in the case of flukes, remember it.
· Acoelomate
· Body is dorso-ventrally flattened and is divisible into three distinct parts i.e. an anterior scolex, a short unsegmented neck, and a long segmented proglottid or strobila. It is usually opaque white in colour.
· Scolex is the knob-like or roughly quadrangular structure at the anterior end of the body.

The life cycle of Tapeworm

A. Hexacanth B. Bladderworm C. Cysticercus

D. Adult Tapeworm E. and F. Gravid proglottid

· A conical projection at the top of the scolex is called the rostellum, which is armed with 22-32 curved hooks arranged in two rows around its base.
The hooks of the anterior circle are larger than the posterior.
· The broadest part of the scolex bears four hemispherical highly muscular suctorial organs, the suckers or acetabula.
· Hooks, scolex, and suckers help in attachment hence called organs of adhesion or holdfast organs.
· Neck is the shortest, narrow, and unsegmented part of the body and proliferates (reproduces rapidly) to form numerous proglottids.
· Strobila consists of 800-1000 segments or proglottids (unit part of the body) with the complete set of genitalia.
It is formed in the neck region and pushed backwards thus anterior one is the youngest and the posterior one is the oldest. On the basis of the degree of development (genital organs), strobila includes three kinds of proglottids- immature, mature, and gravid.
· Immature proglottids- about 200 anterior proglottids, without reproductive organs, broader than long, rectangular in shape.
· Mature proglottids- about 450, at the middle of the strobila, self-fertilization occurs as both male and female reproductive organs are present in mature proglottids, Squarish-shaped
· Gravid or ripened proglottids- Oldest and last 150-350 proglottids with branched uterus containing fertilized eggs. Longer than broad in shape
· Formation of different types of proglottids- Strobilization.
It is not true metamerism/segmentation because the formation of proglottids is a mode of reproduction with each proglottid carrying the same structures.
Strobilization, is, therefore, called pseudo-metamerism or pseudo-segmentation.
· Apolysis- Shedding of gravid proglottids

Body wall

· Tapeworm has no cellular or ciliated epidermis. The body wall consists of the tegument, basement membrane, integumentary muscles, and parenchyma.
· Tegument is the outermost thick and resistant layer. It has numerous fine pore canals through which substances (in solution) are absorbed from the host’s intestine. It contains mitochondria and lysosomes. It gives out microvilli-like processes on its outer surface. These microvilli facilitate absorption of the host’s food by increasing the surface area of the body and partially act as holdfast organs by interlocking with microvilli of each lining of the host’s intestine.
· Basement membrane lies below the tegument. Its outer edge is identified while the inner edge merges with the underlying mesenchyme.
· Integumentary muscle: Below the basement membrane, musculature consists of well-developed outer circular and inner longitudinal fibres.
· Mesenchyme or parenchyma forms the packing around the various internal organs. It contains calcareous bodies, tegument secreting cells. It consists of the outer cortex and an inner medulla. Besides skeletal function, it acts as an important transport medium in the absence of a blood vascular system.

Nutrition

· The digestive system or alimentary canal is completely absent. It is a unique parasitic adaptation in tapeworm. It completely depends upon the predigested food absorbed from the host’s gut through the cuticle.
Soluble nutrients like glucose, amino-acid, glycerol, etc diffuse indirectly through the general body surface from the ileum of the small intestine of the host. Reserve food is stored as glycogen and lipoids.

Therapy (Treatment)

· Antihelminthic drugs like carbon tetrachloride, Camoquin, Oleoresin of male Fern Aspidium Sp., etc.

Taenia saginata (Beef tapeworm)

· It is longer than Taenia solium (up to 12 meters or more) and hooks are absent in scolex. Rostellum is also absent.

· It is more prevalent in man than Taenia solium.
· Digenetic parasite, primary host is the man and the intermediate or secondary host is cattle and buffaloes.
· Cosmopolitan, inhabits the beef-eating population
· Strobila contains up to 2,000 proglottids. A gravid proglottid contains about 100,000 eggs. The uterus of gravid proglottid has 15-35 branches on either side.
· Adult lives in the human alimentary canal, proglottids are shed singly with faeces and disintegrate. Scattered eggs on soil, grass, etc are swallowed by grazing cattle.

Eggs hatch in the intestine of cattle freeing 6-hooked oncospheres which encyst in muscles forming cysticerci or bladder worms. Man is infected by eating such measly meat.

Table: Major Differences between Pork Tapeworm and Beef Tapeworm

Character	Taenia solium (Pork tapeworm)	Taenia saginata (Beef tapeworm)
Rostellum	Present, armed	Absent, unarmed
Length	1-5 m	Up to 12 m
Hosts	Primary-man, secondary- pig	Primary-man, secondary- cattle, and buffaloes
Testis	375-575	880-1200
Ovary	Two or three lobes	Two lobes
Number of eggs in each gravid proglottid	30,000-40,000	1,00,000
Number of proglottids	Up to 1000	Up to 2000
Uterine branches	Less numerous (7-13)	Numerous (15-35)
Life span	About 25 years	About 10 years

Diphyllobothrium latum (Fish tapeworm)

· It is the longest/largest tapeworm and is trigenetic with humans as primary and fish and Cyclops as intermediate hosts.

Hymenolepsis nana (Dwarf tapeworm)

· Smallest of human tapeworms measuring about 2 - 4.5 cm long with 100-200 proglottids. Scolex with 4 suckers and rostellum with a single row of 20-30 hooks. There are 3 testes in each proglottid, a simple life cycle without an intermediate host, oncospheres liberated from eggs, live in intestinal villi where they transform into cysticerci. They reenter the lumen of the small intestine to become adults (auto-infection). Unsanitary toilet habits spread infection from person to person.

Echinococcus granulosus (Dog tapeworm or Hydatid worm)

· Smaller tapeworm measuring about 3-6 mm in length
· Adult has three parts-scolex, neck, and strobila of 3-4 proglottids- immature-1, mature-1, gravid-1 or 2, Scolex with 4 suckers, rostellum is protrusible with 2 rows of hooks
· Cats and dogs (primary host), pig, cattle, sheep and goat (intermediate host)
· Carnivorous mammals (dogs) eat these tissues and adult tapeworm develops in the intestine.
· Eggs passed in faeces of the primary host are accidentally eaten by various herbivorous mammals (Pig, sheep, swine, and man). They develop into oncospheres, which infect the secondary host through contaminated food or careless association with dogs, sheep, goats, etc. Eggs then develop into the hydatid cysts in the liver, lungs, and other parts resulting in hydatidosis. Dogs acquire the infection when they feed the faeces of the infected secondary hosts.
· Hydatidosis can be cured by the use of Albendazole.

SOME COMMON TREMATODES

Name	Common name	Primary host	Secondary host	Mode of infection	Site of infection	Pathogenic effect
Fasciola hepatica	Sheep liver fluke	Sheep	Limnea truncatula or Planorbis or Bulinus	Grazing on leaves and grass blades with metacercaria	Liver and bile passages	Liver rot (complete breakdown of the liver) or cirrhosis
Fasciola gigantica	Cattle liver fluke	Cattle
Fasciolopsis buski	Intestinal fluke	Man	Planorbis (Snail)	Metacercaria on water plant	Small intestine	Intestinal inflammation and haemorrhage, ulcers, anaemia etc
Opisthorchis sinensis	Chinese liver fluke	Man	Bithynia or Parafossalurus (Snails)	By eating raw or improperly cooked freshwater fishes infected with metacercariae	Liver	Liver fluke disease
Paragonimus sp.	Lung fluke	Man	Melania (Snail), crabs, and crayfishes	By eating poorly cooked crabs and crayfishes infected with cercariae	Lungs	Paragonimiasis (Cough, fever, and also tuberculosis)
Schistosoma haematobium	Blood fluke	Man	Bulinus or Melania (Snails)	Cercariae in water penetrate the skin when coming in contact	Portal and mesenteric veins	Urinogenital Schistosomiasis
Schistosoma mansoni	Blood fluke	Man	Planorbis (Snail)	Cercariae in water penetrate the skin when coming in contact	Veins near the ileocaecal junction	Severe dysentery and anaemia
Schistosoma japonicum	Blood fluke	Man	Segmentina or Onconelania	Cercariae in water penetrate the skin when coming in contact	Portal and mesenteric veins	Liver enlargement, ulceration, spleen disorders

SOME COMMON CESTODES SUMMARY

Name	Common name	Primary host	Secondary host	Mode of infection	Site of infection	Pathogenic effect
Taenia solium	Pork tapeworm	Intestinal mucosa of man	Pig	By eating improperly cooked Measly pork	Intestine	Taeniasis (Intestinal disorder)
Taenia saginata	Beef tapeworm	Alimentary canal of man	Cattle	By eating improperly cooked beef	Intestine	Intestinal disorders and Anaemia
Diphylobothrium	Broad fish tapeworm or smallest worm		Human, Cyclops and freshwater fishes	By eating infected freshwater fishes	Intestine	Anaemia, Intestinal disorders
Echinococcus granulosus	Dog tapeworm or hydatid worm	Dog	Man	By ingesting eggs contaminated food, drink	Liver, also lungs and muscles, kidneys	Echinococcosiasis (Fibrosis, enlargement of liver, diarrhoea, abdominal pain, eosinophilia)
Hymenolepsis nana	Dwarf tapeworm			Direct	Small intestine	Eosinophilia