Cestodes (Tapeworms): PM603 Spring 2025

General Information

• Lecture 4 covers Cestodes (tapeworms) for the Spring 2025 PM603 Parasitology and Virology course.
• Helminths (worms) are classified into Platyhelminthes (flatworms) and Nemathelminthes (cylindrical worms). Platyhelminthes include Trematoda (flukes) and Cestoda (tapeworms).

Characteristics of Cestodes

• Tapeworms are Platyhelminthes, known for their flattened, ribbon-like, segmented bodies and hermaphroditic nature.
• The body of a tapeworm consists of a rounded head called Scolex, and a flat body consisting of segments called proglottids.
• Segments (proglottids) are added below the scolex as the worm grows
• No alimentary canal is present.
• The oldest proglottids produce eggs and are found at the distal ends of the worm.
• Cattle, pigs and fish can become hosts after eggs are excreted into faeces and transmitted.

Morphology of Cestodes

• Scolex (head): Equipped with muscular suckers and a crown of hooks (rostellum) for fixation.
• Neck region: Serves as the generative part.
• Strobila (body): Consists of segments (proglottids) that are immature, mature, or gravid.
• Eggs are non-operculated and contain a six-hooked (hexacanth) embryo or onchosphere, however eggs are not found in E. granulosus.

General Life Cycle

• Adult worms mature in the human intestine and eggs are shed via faeces into the environment.
• Grazing animals ingest the eggs, and larval forms encyst in their tissues.
• Humans get infected when they eat the meat of infected animals with encysted larvae.

Classification of Medically Important Cestodes

• Intestinal Cestodes: Taenia spp. (T. saginata, T. solium) and Hymenolepis spp. (H. nana).
• Tissue Cestodes: Echinococcus granulosus.

Main Intestinal Cestodes

• Taenia saginata: Beef tapeworm, measures 4-12m.
• Taenia solium: Pork tapeworm, measures 2-7m.

Life Cycle of Taenia saginata and Taenia solium

• Taenia spp' life cycles involves oncospheres hatching, penetrating the intestinal wall, and circulating to musculature, where they develop into cysticerci.
• Humans get the parasite from raw or undercooked infected meat.
• Taenia saginata is transmitted by swallowing Cysticercus bovis larvae in beef, which causes Taeniasis.
• Taenia solium transmission occurs by the swallowing of Cysticercus cellulosae larvae in pork which causes Taeniasis.
• T. solium can also be transmitted via auto infection which causes Cysticercosis.
• Diagnostic stage includes eggs of T. solium in faeces or when swallowed by men.
• The adult lives solely in the small intestine of humans.
• Taenia saginata is cosmopolitan in areas where cattle are consumed, while Taenia solium is cosmopolitan in areas where pigs are consumed.

Taeniasis pathogenesis and clinical features

• Most infections are asymptomatic, intestinal disturbances, obstruction, loss of weight, and appendicitis may occur.
• T. solium scolex may cause severe intestinal disturbance, but symptoms are usually mild.

Diagnosis and Treatment of Taeniasis

• Clinical picture, presence of eggs in faeces, and gravid proglottids in faeces are all indicative of infection
• Praziquantel is used for treatment, Niclosamide and Albendazole also used.

Pathogenesis & Clinical Picture of Cysticercosis

• Neurocysticercosis, eye inflammation, bulging eye are symptoms that depend of the organ affected due to ingesting the eggs of T. solium
• It can trigger seizures, headaches, stiff neck, nausea, and vomiting.
• Larval stages of the Cysticercus cellulosae develop in extra intestinal tissue.
• Adult-onset seizures are triggered by Cysticercosis.

Diagnosis and Treatment of Cysticercosis

• Biopsy, X-rays, CT scans, and serological methods are used for diagnosis.
• Anthelminthic therapy, anticonvulsants, corticosteroids, and surgical removal are used in treatment.

Hymenolepis nana (Dwarf Tapeworm)

• The adult worm is the smallest tapeworm in humans (15-40mm) and causes Hymenolepiasis.
• Infections are common in children in areas of poor sanitation and is cosmopolitan.
• It lives in the small intestine of rodents, rats and mice.

Transmission of Hymenolepis nana

• Via the ingestion of eggs by a 2-host cycle, with rodents as definitive hosts and grain beetles, fleas, or other insects as intermediate hosts.
• Via the human-to-human oral-anal cycle, as well as internal autoinfection.

Clinical Features and Pathogenicity of Hymenolepis nana

• GIT upset, weight loss, dizziness, and convulsion are symptoms and manifestations.
• Diagnosis is through the presence of eggs in faeces.
• Niclosamide and Praziquantel are used for treatment.

Tissue Cestodes

• Cestodes are typically Intestinal helminthes, however its larvae can exist in tissues of men
• Cysticercus cellulosae (larvae of Taenia solium) and Hydatid cysts (larvae of Echinococcus granulosus)

Echinococcus granulosus

• Adult worms are 2-7mm long, and have a scolex with a rostellum, and three attached segments (immature, mature & gravid).
• This causes Hydatid disease and is transmitted through ingestion of eggs in food or by hands which were contaminated by feces of dogs.
• Cysts are known to be found primarily in the liver, followed by the lungs and spleen.

Clinical Features and Pathogenicity of Echinococcus granulosus

• Composed of a sac full of protoscolices in hydatid fluid is called hydatid sand
• Slow growth of hydatid cysts can cause death, obstructions or pressure on organs.
• Cysts may cause anaphylactic shock, pulmonary symptoms like sputum containing blood and cyst fluid if ruptured and is used to destroy bones

Diagnosis and Treatment for Echinococcus granulosus

• CT and ultrasound scanning, X-rays, and the Intradermal casoni test may be helpful.
• For treatment, surgical removal of Hydatid cysts is used.