Biology: Tapeworms and Cestodes

Cestodes (Tapeworms)

• Lack a digestive system and absorb nutrients through their tegument.
• No digestive tract; absorb all nutrients directly through the tegument.

Tegument

• Syncitium: cytons connected by trabeculae (cytoplasmic bridges) to distal cytoplasm.
• Microtriches (singular microtrix): highly specialized microvilli covering the entire surface of the tegument of cestodes.

Nervous and Sensory System

• Main nerve centre: cerebral ganglion in the scolex.
• Motor and sensory innervation depends on the number and complexity of the scolex.
• Smaller nerves emanate from the commissures to supply the general body muscular and sensory endings.

Reproduction

• True tapeworms are exclusively hermaphrodites (Dioecious): have both male and female reproductive systems in their bodies.
• Reproductive system includes one or many testes, cirrus, vas deferens, and seminal vesicle as male organs, and a single lobed or unlobed ovary with the connecting oviduct and uterus as female organs.

Excretion and Osmoregulation

• Cestodes have the protonephridial flame bulb system typical of flatworms.
• Flame cell protonephridium embedded throughout the parenchyma.
• Excretory ducts are lined with microvilli involved in transport of excretory wastes and may help to regulate ionic concentrations of the excretory fluid.

Developmental Stages of Cestodes

• Egg: two types - Operculated and Non-operculated.
• Larvae: two types - Solid (e.g., Procercoid, Plerocercoid) and Cystic (true bladder) with single scolex (e.g., Cysticercus) or many scolexes and/or daughter cysts (e.g., Hydatid cyst, Coenurus cyst).

Order Cyclophyllidea Characteristics

• Large or small worms consisting of chains of segments.
• Scolex is quadrate with four cup-like round suckers.
• An apical rostellum with hooklets may be present.
• Vitelline glands concentrated in a single mass.
• Common genital pore is marginal (on lateral side of segment).
• No uterine opening for the exit of eggs from the gravid uterus.
• Eggs only escape from the rupture or disintegration of ripe segments.

Order Pseudophyllidea Characteristics

• Large worms consisting of a long chain of segments.
• Scolex has two slit-like sucking grooves (bothria) instead of suckers.
• Vitelline glands are widely scattered in the parenchyma and composed of many acini.
• Genital pores are on the ventral surface of the segment and are not marginal.
• Uterus opens to the exterior through which eggs come out.
• Eggs are operculated and can develop only in water; immature when oviposited (unembryonated) and oncosphere gives rise to ciliated embryo.

Classification

• Order Cyclophyllidea: less medically important, includes Taenia saginata, Taenia solium, Hymenolepis spp., Echinococcus granulosus, and Echinococcus multilocularis.
• Order Pseudophyllidea: includes Diphyllobothrium latum and Spirometra spp.

Life Cycle of Cestodes

• Adult in definitive host.
• Egg: operculated or non-operculated.
• Coracidium: ciliated embryo stage.
• Oncosphere: immature embryo stage.
• Solid larval form: procercoid larva.
• Cystic larval form: plerocercoid larva, cysticercus, coenurus, hydatid cyst, etc.

Diphyllobothrium latum

• Common name: Fish tapeworm.
• Order Pseudophyllidea.
• Adult: the largest tapeworm, up to 10m or more in length, grayish-yellow in color.
• Scolex is elongated, spoon-shaped, with longitudinal slits (bothria) and no rostellum and hooklets.
• Habitat: small intestine.
• Hosts: definitive host - human; reserve host - fish-eating animals; primary intermediate host - Cyclops or diatoms; secondary intermediate host - freshwater fish.
• Mode of infection: ingestion of improperly cooked or smoked fish infected with plerocercoid larvae.
• Clinical features and pathology: abdominal discomfort, loss of weight, loss of appetite, and some malnutrition; anemia and neurological problems associated with vitamin B12 deficiency in heavily infected individuals.

Spirometra mansoni

• Order Pseudophyllidea.
• Morphology: adult worm measures 60-100 cm in length; scolex is elongated and spoon-shaped with two sucking grooves.
• Egg: 52-78 µm × 31-44 µm, light yellow in color, operculated.
• Plerocercoid/Sparganum: tape-like, unsegmented body, white in color, infective stage.
• Definitive host: dog, cat, etc.
• 1st intermediate host: freshwater copepod.
• 2nd intermediate host: frog.
• Transport (paratenic/transfer) host: snake, bird, mammal.
• Mode of infection: drinking contaminated water, eating raw or undercooked meat of infected animals, or through larval transfer (applying flesh as a poultice on the eye or wound).
• Pathogenesis and clinical features: sparganosis in humans is a tissue infection caused by plerocercoid; can cause ocular, subcutaneous, oral, and cerebral sparganosis.