Chapter 17 - Digeneans: Echinostomatiformes

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Chapter 17 - Digeneans Echinostomatiformes
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• Order Echinostomatiformes
• Tegument often has spines
• Some members of this order are among the most
  common trematode parasites encountered
• Some are known to cause devastating losses to
  agriculture

Echinostoma sp.

• Family Echinostomatidae
• Echinostomes are recognized by the double row of
  spines on the circumoral collar
• The size, number and arrangement of these spines
  are important for the taxonomy of the group
• For the most part, these parasites are found in
  the intestinal tract or bile ducts of aquatic
  reptiles, birds, and mammals

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• Genus Echinostoma
• Members of this genus are widespread and
  relatively abundant
• They are not extremely host specific
• Life Cycle
• Eggs hatch in water and miracidia penetrate the
  first intermediate host which typically are
  snails (Physa, Lymnaea, Helisoma)
• The metacercaria often are found with molluscs,
  planaria, fish and tadpoles
• Definitive hosts pick up these parasites from
  ingesting any of these hosts
• Humans will become infected if they eat raw
  mussels or snails
• Review information pertaining to Echinostoma
  revolutum in your book

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• Family Fasciolidae
• Large worms of herbivorous mammals
• Tegument is covered with scale-like spines and
  the acetabulum lies extremely close to the oral
  sucker
• Testes and ovaries are dendritic and the
  vitellaria are extensive
• No second intermediate host in their life cycle
• Metacercaria typically encyst on submerged
  objects in the water

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• Fasciola hepatica
• Rare in humans but relatively common among sheep
  and cattle
• Morphology
• A large fluke, reaching lengths of 30 mm and
  widths of 13 mm
• Testes are extremely branched

Fasciola hepatica
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• Life Cycle
• Adults reside in the bile duct of sheep and
  other ruminants
• Eggs are passed with the bile into the small
  intestine and are released with the feces
• Eggs give rise to free swimming miracidia which
  penetrate a snail host

• Miracidia transform into sporocysts which in turn
  give rise to rediae
• Rediae develop in the snails digestive glands
  and give rise to cercariae
• Cercariae attach to objects in the water and
  undergo encystment as a metacercariae
• Mammals pick up this infective stage when they
  consume vegetation that has encysted metacercariae

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• Life Cycle cont.
• The worms excyst in the small intestine and
  penetrate the gut wall
• Upon entering the coelom they crawl over the
  viscera until they locate the liver
• They enter the liver and begin to feed on
  tissue
• They eventually get into the bile ducts and
  begin producing eggs
• Note
• This parasite has been used to address questions
  concerning how parasites locate sites within
  their hosts a cue used by the worms is found in
  bile (glycocholic acid)

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• Pathology
• Most of the damage results from worms are
  migrating through the liver parenchyma feeding on
  liver cells and blood
• Worms in the bile ducts cause inflammation and
  edema
• This in turn stimulates the production of
  fibrous tissue in the walls of the ducts
• The walls of the ducts become thickened and as a
  consequence can not handle as much bile in
  essence they become less responsive to the needs
  of the liver
• Back pressure can cause atrophy of the liver
  tissue
• With heavy infections the gall bladder can
  become damaged