Tapeworms

• Characteristics of cestodes (adult tapeworms):

  • Obligate parasites in vertebrates
  • More than 5000 species
  • Flat, segmented, and elongated body with scolex and chain of proglottids
  • Hermaphrodites only
  • 2-3 host life cycles
• Location within host:

  • Definitive Host (DH): in the gut, specifically the small intestine attached to mucosa by scolex
  • Intermediate Host (IMH): in different organs as one type of metacestode
• Digestion:

  • No digestive tract, absorption by integument
  • Feed on intestinal content
• Organs of attachment (on the scolex):

  • Diphyllobothriidea: two longitudinal muscular grooves (bothria)
  • Cyclophyllidea: 4 muscular suckers and rostellum with one or more concentric rows of hooks
• Reproduction:

  • Each mature segment contains own set of gonads (ovaries and testes) = hermaphrodites
  • Fertilization occurs between different segments and/or different individuals
• Life cycle:

  • Indirect, 2-3 host cycles
  • Diphyllobothriidea life cycle: egg → 6-hooked oncosphere (coracidium) → procercoid (in 1st IMH) → plerocercoid (in 2nd IMH) → adult tapeworm (in DH)
  • Cyclophyllidea life cycle: egg → 6-hooked oncosphere → metacestode (infective larval stage in IMH) → adult tapeworm (in DH)
• Types of METACESTODES (tapeworms' larval stages)
• Humans as hosts of cestodes:

  • Definitive hosts: adult tapeworms in small intestine (e.g., Taenia saginata, T. solium, T. asiatica, Diphyllobothrium latum, Hymenolepis nana)
  • Intermediate hosts: larval stages (metacestodes) in various organs (e.g., Echinococcus granulosus, E. multilocularis, T. solium)
• Family Diphyllobothriidae (Order Pseudophyllidea):

  • 2 longitudinal muscular grooves (bothria) on scolex
  • 2 intermediate hosts
  • Example species: Dibothriocephalus latus (former Diphyllobothrium latum)
  • Disease: diphyllobothriasis
  • Zoonosis affecting millions of people
• Family Taeniidae:

  • Definitive hosts: carnivores and humans
  • Genera: Taenia and Echinococcus
  • 2-host life cycle (mammal-mammal, carnivore-herbivore)
  • Intermediate hosts: mammals with development of cysticercus, hydatid cyst
  • Length of adults: Taenia (1-10 m), Echinococcus (1-7 mm)
  • Example species: Taenia solium
• Genus Echinococcus:

  • Humans as accidental intermediate hosts
  • Localization of metacestodes in liver/lungs
  • Slow development of metacestodes
  • Untreated infections are often fatal
• Echinococcus granulosus = cystic echinococcosis (CE)/hydatidosis
• Echinococcus multilocularis = Alveolar echinococcosis (AE)
• Echinococcus multilocularis:

  • Sylvatic cycle involving red fox, Arctic fox, and Arvicolidae (voles)
• Hepatic alveolar echinococcosis in humans, with a case study of a 62-year-old Swiss patient with cysts approximately 1.5 cm in diameter
• Echinococcus multilocularis in Sweden (2016) = Alveolar echinococcosis
• Echinococcosis in humans:

  • Incubation period: 5 - 20 years
  • Accidental findings during special examinations
  • Diagnosis:
  • Ultrasound
  • Magnetic resonance, computed tomography, X-rays
  • ELISA
  • Fatal if not treated
  • Control:
  • Surgery
  • Cestocide therapy: albendazole
• Taenia solium (pork tapeworm)

  • DH: humans
  • IMH: pigs
  • Larva of Cysticercus cellulosae in swine muscles/organs
  • Size of adult: 2 - 4 m x 8 - 10 mm
  • Scolex: rostellum with two rows of hooks
  • Human as accidental intermediate host -> cysticercosis (neurocysticercosis)
• Taenia saginata (beef tapeworm)

  • DH: humans (small intestine) (worldwide distribution)
  • IMH: cattle, buffalo
  • Scolex without rostellum and hooks, tapeworms: 3 - 5 m (event. 25 m), strobila with 2000 proglotids
  • Larva named “Cysticercus bovis” in the past (beef muscles)
• Taenia saginata – life cycle:

  • Cysticercosis in cattle:
  • Cysticercus bovis at predilection sites: tongue, masseters, heart
  • Size: up to 10 mm
  • May survive 1 - 3 years -> calcification
  • Treatment and prevention:
  • Control of taeniosis in humans: praziquantel, niclosamide
  • Prevention:
  • Meat inspection
  • Properly cooked beef
  • Urban sanitation, waste treatment
• Taenia solium – one parasite, two diseases:

  • Cysticercosis = cysticerci in brain, eyes, muscles, subcutaneous tissue
  • Taeniosis = tapeworm in intestine of humans
  • Infection acquired by eggs (faecal contamination, clothes, self-infection)
• Taenia solium – cysticercosis in pigs:

  • Metacestodes (cysticercus) in muscles (diaphragm, heart, tongue), brain, liver, lungs
  • ~10 mm diameter bladder with invaginated protoscolex
  • Meat inspection
• Taenia solium – cysticercosis in humans:

  • Neurocysticercosis = neglected disease
  • Most important helminth disease affecting CNS in humans
  • Major infectious cause of acquired epilepsy
  • 50,000 people die annually due to neurocysticercosis
• Characteristics of Taenia spp. infections:

  • Taenia solium:
  • DH: human
  • IMH: pig
  • Disease in humans: taeniosis, cysticercosis
  • Source of infection for humans: pork muscles
  • Taenia saginata:
  • DH: human
  • IMH: cattle
  • Disease in humans: taeniosis
  • Source of infection for humans: beef muscles
  • Taenia asiatica:
  • DH: human
  • IMH: pig (cattle, goat, wild boar)
  • Disease in humans: taeniosis
  • Source of infection for humans: beef muscles, livers
• Family Hymenolepididae:

  • Hymenolepis nana
  • Hymenolepis diminuta
  • Hymenolepiasis
  • Common cestodes in humans
  • Low pathogenic
• Hymenolepis nana vs. Hymenolepis diminuta:

  • Hymenolepis nana (dwarf tapeworm):
  • DH: mice, rats, humans
  • IMH: beetles, fleas
  • Size of adults: 15 - 40 mm
  • Size of eggs: 30 - 43 μm
  • Hymenolepis diminuta (rat tapeworm):
  • DH: rats, humans, dogs
  • IMH: grain beetle
  • Size of adults: 20 - 60 cm
  • Size of eggs: 60 - 80 μm
• To remember:

  • Humans can serve both as definitive and intermediate hosts of tapeworms
  • Adult tapeworms colonize the small intestine -> harmless
  • Larvae (metacestodes) invade organs -> pathogenic
  • Development of metacestodes by Echinococcus sp. and T. solium in humans can cause severe disease
• Global distribution of Taenia solium cysticercosis/taeniosis:

  • Endemic (full life cycle)
  • Suspected endemic
  • No information available/no evidence
  • Imported cases (human cysticercosis transmission)
• Taenia asiatica:

  • Sister species of T. saginata
  • Discovered in Taiwan (1987)
  • DH: humans
  • IMH: pigs, wild boars, cattle, goats
  • Taeniosis in humans after consumption of undercooked swine livers
• Hymenolepis nana vs. Hymenolepis diminuta:

  • Hymenolepis nana (dwarf tapeworm):
  • DH: mice, rats, humans
  • IMH: beetles, fleas
  • Size of adults: 15 - 40 mm
  • Size of eggs: 30 - 43 μm
  • Does not require IMH to complete life cycle
  • Hymenolepis diminuta (rat tapeworm):
  • DH: rats, humans, dogs
  • IMH: grain beetle
  • Size of adults: 20 - 60 cm
  • Size of eggs: 60 - 80 μm
• To remember:

  • Humans can serve both as definitive and intermediate hosts of tapeworms
  • Adult tapeworms colonize the small intestine -> harmless
  • Larvae (metacestodes) invade organs -> pathogenic
• Development of metacestodes by Echinococcus sp. and T. solium in humans can cause severe disease
• Thanks for your attention!

  Any questions?