Chapter 17 - Digeneans: Echinostomatiformes

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

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

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

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

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

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)

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