Cestodes: Tapeworm Body and Structure

• Cestodes are also known as Platyhelminthes or Flatworms.
• Tapeworms were named this because their bodies are flattened from anterior to posterior.
• Living worms are white and yellowish, and adult forms are flat and ribbon-like.

Cestode Body

• The cestode body consists of a scolex, neck, and strobila/proglottids.

Scolex

• The scolex is the anterior attachment organ (head).
• The head contains suckers or acetabula.
• Expulsion of the scolex is a criteria for a cure.

Neck

• The neck is considered the region of growth.

Strobila or Proglottids

• The strobila is the chain of segments that comprises the body.
• Segments can be immature, mature, or gravid, filled with eggs.
  • Immature segments are nearest to the neck.
  • Mature segments are far from the neck.
  • Gravid segments are found in the middle and filled with eggs.
• This can be best demonstrated by use of India Ink.

Orders Under Family Cestoda

• Order Pseudophyllidea:
  • Also known as False Tapeworm.
  • Diphyllobothrium latum is an example.
• Order Cyclophyllidea:
  • Also known as True Tapeworm.
  • Examples: Taenia solium, Taenia saginata, Dipylidium caninum, and Hymenolepis nana.

Pseudophyllidean vs. Cyclophyllidean

• Scolex:
  • Pseudophyllidean: Spoon-shaped with slit-like sucking grooves (bothria), no hooklets.
  • Cyclophyllidean: Quadrate with four cuplike suckers (rostellum with or without hooklets).
• Strobila:
  • Pseudophyllidean: Anapolytic (not shedding segments).
  • Cyclophyllidean: Apolytic (shedding segments).
• Ova:
  • Pseudophyllidean: Oval, operculated, immature.
  • Cyclophyllidean: Spherical, non-operculated, embryonated.
• Larval stages:
  • Pseudophyllidean: Coracidium>procercoid>plerocercoid.
  • Cyclophyllidean: Cysticercoid, cysticercus, hydatid.
• Intermediate Host:
  • Pseudophyllidean: 2 intermediate hosts: Crustaceans (copepods), fresh water fish.
  • Cyclophyllidean: Only one, usually lower forms of mammal or arthropod.

Taenia Saginata

• Taenia saginata is known as the BEEF Tapeworm.
• Humans are definitive hosts, not intermediate hosts.
• Epidemiology, prevention and control will be considered jointly with T. solium.

Parasite Biology (Adult)

• Adult worms inhabit the upper jejunum and may live up to 25 years.
• Adults measure 25 meters in length with 1000-2000 proglottids.
• The cuboidal scolex contains four prominent acetabula.
• Mature proglottids contain mature male and female reproductive organs, thus also called Hermaphroditic worms.
  • Monoecious worms contain both male and female reproductive organs in one organism.
  • Diecious worms have separate male and female adults.
• The scolex has no crown of hooklets called as rostellum (Unarmed).
• Gravid proglottids are distended with ova and 15-20 lateral branches (dichotomous or tree-like).
• Proglottids undergo apolysis and are either passed out with the feces or actively crawl out of the bowel to the external environment.

Parasite Biology (Larva)

• The larva of Taenia saginata is called Cystercus bovis.
• Human cysticercosis does not occur in T. saginata infections because humans serve only as the definitive host.

Parasite Biology (Ovum)

• Taenia spp. ova are spherical or subspherical, measuring 30-35 um in diameter.
• Ova are brownish and have a thick embryophore which appears striated.
• The eggshell contains the oncosphere or embryo provided with 3 pairs or 6 hooklets, commonly called a Hexacanth embryo.

Taenia Saginata Life Cycle

• Definitive Host: Man.
• Infective stage to man: Cysticercus bovis.
• Intermediate host: Cattle.
• Usually, one adult tapeworm is present in T. saginata infections.
• Adults may be irritated by alcohol, and passage of proglottids sometimes results after a drinking bout.
• Gravid proglottids are passed with feces.
• Eggs are released and viable in the soil for weeks.
• Cattle ingest T. saginata eggs, and the oncosphere is released.
• The oncosphere actively penetrates the intestinal mucosa and enters a venule to be carried to other parts of cattle.
• It develops into the infective stage called Cysticercus bovis in the muscle fiber.
• Humans are infected by ingestion of raw or improperly cooked beef.
• Larva is digested out of the meat and mature into adult in the small intestine.

Pathogenesis & Clinical Manifestations - Taenia Saginata

• T. saginata infection with adults is termed Taeniasis.
• The most common chief complaint is the passage of proglottids or segments in the stool.
• T. saginata causes mild irritation at the site of attachment.
• T. saginata proglottids are actively motile and have been documented to cause obstruction in the bile and pancreatic ducts. Rarely, tangled proglottids may result in intestinal obstruction.

Taenia Saginata - Diagnosis

• Recovery of gravid proglottids, either single or in chains.
  • The first specimen is usually specimen brought in by patients.
  • They are passed out with feces or recovered in the patient's undergarments.
  • Gravid proglottids are pressed or flattened in between two glass slides and are examined against light.
  • Injection of India ink through the genital pore will help make an accurate account of the lateral branches.
• Concentration techniques like formalin-ether concentration technique for demonstration of eggs.
• Perianal swabs for recovery of eggs that are left in the perianal skin as the gravid segments squeeze out of the anal opening.
• Criteria for cure:
  • Recovery of the scolex.
  • A negative stool examination 3 months after treatment.

Taenia Solium

• Taenia solium is known as the Pork tapeworm.
• Humans may serve as both a definitive host and intermediate host, therefore both intestinal and tissue infections occur in humans.

Parasite Biology (Adult - Taenia Solium)

• Adults inhibit the upper small intestine.
• Shorter than T. saginata (7 meters in length) and has less number of proglottids (<1000).
• The scolex has four acetabula and carries a cushion-like rostellum with a double crown of 25-30 large and small hooks, which is absent in T. saginata.
• Mature proglottids contain mature male and female reproductive organs, thus also called Hermaphroditic worms.
• Gravid proglottids characteristically contain 7 to 15 lateral branches (dendritic or fingerlike).
• Gravid proglottids also undergo apolysis to eventually release its eggs, which remain viable for weeks.
• Proglottids are relatively less active than T. saginata.

Parasite Biology (Larva - Taenia Solium)

• The larva of T. solium is called Cysticercus cellulosae.
• Larvae can cause infection in humans because they serve as both the intermediate and definitive host.

Parasite Biology (Ovum - Taenia Solium)

• The eggs of T. solium are indistinguishable from that of T. saginata.
• They measure 30-45 um and have thick brown striated embryophore surrounding a hexacanth embryo (embryo provided with 3 pairs/6 hooklets).

Taenia Solium - Life cycle

• Both Intermediate and Definitive Host: Man
• Infective stage to man: Cysticercus cellulosae
• Intermediate host: Pigs.
• Gravid proglottids undergo apolysis to eventually release eggs.
• Eggs are ingested by hogs, and the oncosphere is released in the intestines.
• The oncosphere penetrates the intestinal mucosa to typically encyst in muscles as Cysticercus cellulosae.
  • Infected tissues include muscles, tongue, heart, diaphragm, liver, spleen, and mesentery.
  • Infected meat is often called "measly pork".
• Humans ingest the improperly cooked infected meat that are then liberated and scolex attach to the intestinal mucosa.
• Larva develop into adults.
• Humans may be an intermediate host of T. solium. The oncosphere hatches in the duodenum and spreads to different organs through the bloodstream. The Cysticercus in humans may travel to vitreous humor and in the brain.

Taenia Solium - Pathogenesis and Clinical Manifestations

• Infection with larva is termed Cysticercusis cellulosae.
• Infection with adults is termed Taeniasis.
• Pathology is due to the larva, and the adult produces negligible pathology.
• Intestinal infection results in mild non-specific abdominal complaints where obstruction of the bile duct and pancreatic duct is unlikely.
• Cysticerci can develop in any organ or tissue but are most commonly located in striated muscles and in the brain

Neurocysticercosis (NCC)

• Considered one of the most serious zoonotic diseases worldwide.
• Cysticerci containing a scolex may be found in the brain parenchyma or floating freely in the ventricles.

Calcification

• The end result of the cellular reaction
• Death of the larva leads to inflammation of the affected region.

Ocular cysticersosis/Presence of “rice grain" in the eye

• Cysticerci in the eyes are often retinal or subretinal in location.
• They may float freely in the vitreous humors.
• The patient complains or intraorbital pain and blurring or loss of vision.

Taenia Solium - Diagnosis

• Computed Axial tomography is utilized.
• Nuclear magnetic resonance imaging (MRI).
• Serologic Tests: - DOT ELISA - a good screening test for cysticercosis; uses crude antigen from cysticerci obtained from pigs. - Western Blot.

Taenia Saginata vs. Taenia Solium

• Common name:
  • T. saginata is known as beef tapeworm.
  • T. solium is known as pork tapeworm.
• Intermediate Host:
  • T. saginata: Cattle
  • T. solium: Pig
• Scolex:
  • T. saginata: None, no rostellar hooks
  • T. solium: With rostellum armed with 2 rows or large and small hooklets.
• Length:
  • T. saginata: 25 meters
  • T. solium: 7 meters
• Number of Proglottids:
  • T. saginata: 1000-2000
  • T. solium: Less than 1000
• Gravid proglottid:
  • T. saginata: 15 to 20 lateral branches (dichotomous or tree-like).
  • T. solium: 7 to 13 lateral branches (dendritic or fingerlike).
• Eggs:
  • T. saginata: Spherical, striated, inside is an embryo with 6 hooklets.
  • T. solium: Spherical, striated, inside is an embryo with 6 hooklets.
• Larva:
  • T. saginata: Cysticercus bovis
  • T. solium: Cysticercus cellulosae
• Infective stage:
  • T. saginata: Cysticercus bovis
  • T. solium: Cysticercus cellulosae
• Pathogenesis:
  • T. saginata: Taeniasis saginata
  • T. solium: Taeniasis solium, cysticercosis

Hymenolepis Nana

• H. nana is known as the dwarf tapeworm because it is the smallest tapeworm infecting humans.
• The parasite is the only human tapeworm that can complete its entire lifecycle in a single host, indicating that it does not require an obligatory intermediate host.
• Humans can harbour both the adult and larval stages of this parasite.

Parasite Biology (Adult - H. Nana)

• A delicate strobili measuring from 25 to 45 mm in length and 1 mm in width.
• The scolex is subglobular with four cup-shaped suckers with a rostellum armed with a ring of 20-30 Y-shaped hooklets.
• Mature proglottids contain three ovoid testes (1 left and 2 right) and one ovary.

Parasite Biology (Ovum - H. Nana)

• Eggs are spherical to subspherical measuring 30 to 47 um in diameter.
• Eggs contain an oncosphere enclosed in an inner membrane with two polar thickenings, from each of which arise 4 to 8 polar filaments.

H. Nana - Life Cycle

• The lifecycle has a dual pathway: A. DIRECT or B. INDIRECT.

A. Direct Transmission

• Transmission: MAN TO MAN.
• Infective stage to man: Egg.
• Humans ingest eggs which hatch in the duodenum.
• The liberated embryos penetrate the mucosal villi and develop into the infective cysticercoid larvae.
• The larvae break out of the villi and attach to the intestinal mucosa to develop into adults.

B. Indirect Transmission

• Involvement of arthropods such as fleas, rice, and flour beetles (Intermediate Host)
• Infective stage to man: Cysticercoid larva
• Eggs are ingested by arthropods.
• In the arthropod, eggs develop into Cysticercoid larva.
• Accidental ingestion of infected arthropod intermediate hosts occurs.
• The cysticercoid larva are released and will eventually develop into the adult tapeworms in the intestine of the host.

H. Nana - Arthropod Intermediate Hosts

• Flour beetles includes Tribolium contusum and Tenebrio molitor.
• Rat fleas includes Xenopsylla cheopis.
• Dog fleas include Ctenophalides canis.
• Cat fleas include Ctenophalides felis.

H. Nana - Pathogenesis and Clinical Manifestations

• Light worm burden is generally asymptomatic.
• Clinical manifestations include headache, dizziness, anorexia, pruritus of the nose and anus, diarrhea, abdominal pain, and pallor.

H. Nana - Diagnosis

• Specific diagnosis is made by demonstration of the characteristic eggs in the patient's stools.

Hymenolepis Diminuta

• H. diminuta is a parasite primarily of rats, hence the name Rat Tapeworm.
• Accidental human infections do occur, resulting in hymenolepiasis.
• H. diminuta differs from H. nana because it requires an intermediate host.

Parasite Biology (Adult - H. Diminuta)

• Adult tapeworms are larger than H. nana, measuring 60 cm in length.
• The scolex differs from that of H. nana by having a rudimentary unarmed rostellum.
• Mature proglottids contain three ovoid testes (1 left and 2 right) and one ovary.

Parasite Biology (Ovum - H. Diminuta)

• H. diminuta eggs are circular and are bile stained.
• Eggs contain an oncosphere enclosed in an inner membrane with two polar thickenings but lack bipolar filaments.
• The hooklets usually have fan-like arrangement.

H. Diminuta - Life Cycle

• Eggs are ingested by a wide range of adult and larval insects like fleas, beetles, cockroaches, mealworms, and earwigs and develop into the infective cysticercoid larvae.
• Infected insects are ingested by the rat or accidentally ingested by humans.
• Larva is released and develops into adult worm.

H. Diminuta - Pathogenesis and Clinical Manifestations

• The H. diminuta clinical manifestations are minimal and non-specific.

Hymenolepis nana vs Hymenolepis diminuta

• Common name:
  • H. nana is known as dwarf tapeworm
  • H. diminuta is known as rat tapeworm
• Scolex:
  • H. nana has With rostellum armed with a ring of 20-30 spines.
  • H. diminuta has an unarmed rostellum in the scolex.
• Eggs:
  • H. Nana: Eggs contain an oncosphere enclosed in an inner membrane with two polar thickenings, from each of which arise 4 to 8 polar filaments.
  • H. Diminuta: Eggs contain an oncosphere enclosed in an inner membrane with two polar thickenings but without filaments.
• Infective stage:
  • H. Nana: Eggs (direct), Cysticercoid larva (indirect)
  • H. Diminuta: Cysticercoid larva
• Intermediate host:
  • H. Nana: Only human tapeworm that does not require an I.H. to complete its natural life cycle, however insects may act as an intermediate host in some occasions.
  • H. Diminuta: Insects
• Final host:
  • H. Nana: Man
  • H. Diminuta: Rat

Dipylidium Caninum

• Also known as Dog Tapeworm or Double-pored Tapeworm
• A common intestinal parasite of dogs and cats worldwide
• Dipylidiasis in humans is accidental

Parasite Biology (Adult - D. Caninum)

• Pale reddish worm measures 10 to 70 cm in length.
• Small, globular scolex with four deeply cupped suckers and a rostellum armed with 1 to 7 rows of rose-thorn shaped hooklets.
• Proglottids have a double set of reproductive organs and genital pore on each side of the lateral margin.
• Mature and gravid proglottids shaped like melon seeds, pumpkin seeds, or rice grains.

Parasite Biology (Ovum - D. Caninum)

• Eggs are spherical and thin-shelled with a hexacanth embryo.

D. Caninum - Life Cycle

• Gravid segments detach and pass via feces
• Ova released at disintegration outside the gut'
• Larval fleas (intermediate hosts) ingest ova while feeding on epidermal debris
• In the fleas body, the ovum develops into cysticercoid larva
• When the insect is ingested by a mammalian host (dogs, cats, humans) the cysticercoid transforms to the adult

D. Caninum - Arthropod Intermediate Hosts

• Larval stage of:
  • Ctenophalides canis (dog flea)
  • Trichodectes canis (dog louse)
  • Ctenophalides felis (cat flea)
  • Pulex irritans (human flea)

Pathogenesis and Clinical Manifestations

• Slight intestinal discomfort, diarrhea, anal pruritus, and allergic reactions.

Diphyllobothrium Latum

• Also known as the Broad or Fish Tapeworm.
• Broad tapeworm because the proglottid is broader than long
• False tapeworm because the scolex is not similar to other tapeworms
• Largest tapeworm of man

Parasite Biology (Adult - D. Latum)

• The scolex is spatulate/spoon-shaped with two sucking organs or grooves called bothria, located dorsally and ventrally.
• Uterus has a rosette formation in the middle of gravid proglottid.
• Proglottids of D. latum disintegrate only when the segment has completed its reproductive function (anapolytic).
• Identifying physiological feature is a Coiling of the uterus (seen as rosette arrangement).

Parasite Biology (Larva - D. Latum)

• Coracidium > Procercoid > Plerocercoid are the Larval Stages:
  • Coracidium – a ciliated embryo
  • Plerocercoid – aka sparganum

Parasite Biology (Ovum - D. Latum)

• The ova are ellowish brown with a moderately thick shells. An inconspicuous operculum.

D. Latum - Intermmediate Hosts

• Requires two intermediate hosts:
  • First (1st) Copepods (genera Cyclops and Diaptomus).
  • Second (2nd) Fresh water fish (perch, trout, salmon and pike).

D. Latum - Definitive Host

• Humans

D. Latum - Life Cycle

• Unembryonated ova are discharged from the gravid proglottid.
• The ova becomes embryonated in water and release the free-swimming coracidium, a ciliated embryo.
• The coracidium is then ingested by freshwater copepods and developed into procercoid larvae.
• The copepod is in turn ingested by fish and developed into plerocercoid larvae in the tissues and muscles of fish.
• Fish with infective plerocercoid larva is ingested raw by the definitive host like humans and other mammals.

Pathogenesis and Clinical Manifestations

• Infections results from adults (Dyphyllobothriasis) and larvae (Sparganoses).
• Infection due to larva will lead to SPARGANOSES: - Usually acquired through drinking of water with copepods (harbor procercoid larva). - With ingestion of copepods, the procercoid released and develops into a plerocercoid that migrates to muscle
• Infection as plerocercoid larva described by edema, intense pain and swelling occurs - If open, larva are exposed - In the eyes, possibly with excessive lacrimation - In subcutaneous tissue w/ pustuels - In lymphatic w/ elephantiasis.
• Infection with adults may lead to FISH TAPEWORM ANEMIA or VITAMIN B12 Deficiency Anemia or MEGALOBLASTIC ANEMIA : - Anemia similar to B12 deficiencies, mistaken for pernicious anemia. - Worms compete effectively for B12 - RBCs described as normochromic and macrocytic

Diagnosis

• Definite diagnosis by recognizing operculated eggs in stool.
• Possibly may be vomited.

Echinococcus Granulosus

• Hydatid worm producing hydatid disease.
• Shortest tapeworm, with only three segments
• Dogs and other canines commonly serve as final host
• Humans are accidental host only.

Parasite Biology (adult)

• Is 3 to 6 mm in length
• Has a pyriform scolex
• Three proglottids -One immature -One mature -One gravid

Life Cycle

• Eggs released outside the host
• Eggs swallowed by intermediate hosts like goats, sheep, camels, horses
• In duodenum eggs hatch
• Oncospheres then penetrate intestinal wall migratin to mesenteric venules, leading to various tissues & organs