1. Chapter 17 - Digeneans: Echinostomatiformes

2. 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

3. Genus Echinostoma Life Cycle
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

4. 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

5. 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

6. 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 snail’s 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

7. They enter the liver and begin to feed on tissue
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)

8. 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