Tapeworms

1
Tapeworms

• Cestodes

2
Diphyllobathrium latum

• Also called the broad fish tapeworm, common in
  fish-eating carnivores
• Adult worm may get to 10 m long and can shed up
  to 1 million eggs a day
• Shed spent segments of tapeworm as an indicator
  of infection
• Infection results from eating second intermediate
  or paratenic hosts infected with plerocercoids in
  raw or undercooked fish
• Symptoms range from none to fatigue diarrhea
• However, approximately 2 of infected individuals
  develop megaloblastic anemia, due to the worms
  unique affinity for vitamin B12
• Treat with niclosamide or praziquantel

3
Life Cycle

• adult in intestine of definitive host
  (piscivorous mammals) ? eggs in feces ? 1-several
  weeks for coracidium (larva with a ciliated
  epithelium) hatches from egg ? eaten by
  freshwater crustacean (1st intermediate host) ?
  procercoid in hemocoel ? eaten by fish/frog (2nd
  intermediate host) ? plerocercoid in muscles ?
  eaten by fish/snake/swine (paratenic hosts) ?
  plerocercoid in muscles ? eaten by definitive
  host ? adult

4
D. latum Adult

• Scolex showing groove-like bothrium (arrow) used
  to grasp the host mucosa
• Mature proglottid wider than long
• Eggs are released through the uterine pore, eggs
  in uterus

5
D. latum Eggs

• A million eggs/day may be shed in the feces from
  each worm
• Egg has an operculum, symmetrically oval, and
  sometimes have an abopercular knob (arrow)
• Structurally they are similar to a trematode egg
• The embryo is undeveloped when the egg is
  released from the uterus, and must complete its
  development to the coracidium stage in fresh water

6
Taenia pisiformis

• Tapeworm of dogs,
• Scolex and neck (where immature proglottids are
  formed), showing armed rostellum and 4 acetabula
  (suckers)
• Scolex of T. solium would appear similar, whereas
  T. saginata lacks hooks
• Higher magnification of rostellum, showing circle
  of hooks

7
T. pisiformis

• Strobila (body) with immature proglottids being
  formed from neck region

8
Taenia saginata and solium

• T. saginata is considered non-pathogenic
• T. solium is pathogenic because of the cysticerci
  that can cause serious problems with the human
  host

9
T. saginata vs. T. solium

• Species distinguished on the basis of their
  scolex, and by counting the number of side
  branches of the gravid uterus, numbering gt14 in
  T. saginata and lt14 in T. solium
• Scolex of T. saginata is absent of hooks
• Scolex of T. solium with rostellum and armed with
  hooks
• Human infections with the adult worm are acquired
  by eating undercooked beef (T. saginata) or pork
  (T. solium) infected with cysticerci
• Infections with adults usually cause mild or no
  symptoms
• However, T. solium is quite hazardous, because
  its eggs (unlike those of T. saginata) if
  ingested by humans release an oncosphere that
  develops into a bladder-like cysticercus larva,
  which can cause severe disease in the brain or
  eye
• Also, gravid proglottids may rupture in the
  intestinal lumen, resulting in overwhelming
  cysticercosis
• Treat with niclosamide or praziquantel

10
Life Cycle

• adult in small intestine ? gravid proglottid in
  feces ruptures ? embryonated eggs ? ingested by
  cow (T. saginata), pig (T. solium), or human (T.
  solium) ? oncosphere hatches from egg ?
  penetrates gut, enters blood ? cysticercus in
  internal organs ? ingested by human ? adult

11
T. saginata vs. T. solium
Proglottids
Scolex
12
Cysticercus of T. solium

• Infected pork, showing 4 cysticerci, each
  consisting of a fluid-filled bladder containing
  an invaginated, introverted (inside-out) scolex
• Note the absence of inflammation around viable
  cysticerci
• Dead cysticerci elicit a strong inflammatory
  response, which can be fatal if occurring in the
  brain
• Cysticerci can lodge in any organ, and may be
  visible as surface swellings if occurring
  subcutaneously
• Depending on numbers and location, cysticerci in
  the brain may cause symptoms of epilepsy

13
Taeniid eggs

• The oncosphere is surrounded by a 2-layered
  embryophore, and the thicker outer layer has
  prominent radial striations, a diagnostic feature
• The surrounding gelatinous layer and capsule
  (arrow, top left photograph) are flimsy and often
  detach from the striated embryophore, which then
  becomes the outermost layer
• Eggs of various taeniids (e.g., Echinococcus,
  Multiceps, and Taenia spp.) are indistinguishable
  from one another and therefore further diagnostic
  tests would be advisable, e.g., examination of
  scolices and gravid proglottids recovered from
  feces
• Taeniid eggs on the ground may remain infective
  for over 5 months

14
Hymenolepis nana

• Dwarf tapeworm
• Can infect humans, especially children
• 1 in southern US and as high as 97 in Moscow
• Intermediate host is optional which is unlike any
  other tapeworm
• Humans become infected by ingesting eggs from
  rodent or human feces, or by ingesting
  intermediate hosts infected with cysticerci
• Light infections are asymptomatic
• However, because eggs may hatch in the intestine
  of the definitive host, autoinfection can produce
  heavy parasite burdens (thousands of worms)
  leading in humans to diarrhea and symptoms of
  toxicity
• Treat with praziquantel

15
H. nana

• Adult with enlarged scolex and neck
• The rostellum is retractable into a sac and is
  armed with hooks, not visible
• Mature proglottids

16
H. nana Eggs

• Oncosphere with hooks (thin arrow), surrounded by
  an embryophore that has polar filaments at either
  end (thick arrows)
• The outermost capsule is separated from the
  embryophore by a gelatinous granular layer
• These eggs are infective not only for an insect
  intermediate host, but also for the rodent or
  human definitive host