Chapter 18 - Digeneans:
Plagiorchiformes and
Opisthorchiformes
Order Plagiorchiformes
• Adults of this order show little similarity to one another; however,
larval stages exhibit many similarities
Family Dicrocoeliidae
• A fluke that can parasitize
humans, but is more common
among a variety of domestic
mammals
• Parasitize the gall bladder,
pancreas, intestine, and liver and
its associated ducts
• All are parasites of terrestrial or
semi-terrestrial vertebrates and
use land snails as first
intermediate hosts
Dicrocoelium dendriticum
Dicrocoelium dendriticum
Life cycle
• Eggs are passed with the feces of
the host sheep
• Eggs containing miracidia are
consumed by snails; mother
sporocysts give rise to daughter
sporocysts which in turn give rise to
cercariae
• Cercariae accumulate in the mantle
cavity of the snail and act as an
irritant
• They become encased in mucous
and are deposited from the snail as a
slime ball
Life cycle cont.
• Slime balls are eaten by ants
with most of the resulting
metecercariae encysting in the
hemocoel of the ant
• A few metecercariae encyst in
the subesophogeal ganglia of the
host ant
• Behavior of the ant becomes
altered (cling to blades of grass in
the evening) and ants are readily
consumed by sheep
• Metacercaria excyst in the
intestine of the sheep host and
migrate to the bile duct and liver
Family Haematoloechidae
• Parasites in the lungs of frogs and toads
Life Cycle of Haematoloecus medioplexus
• Eggs are deposited in the lungs where they hatch
• Miracidia move from the respiratory system to the intestine and are
carried out with the feces
• In water, they are consumed by snails and transform to sporocysts,
which eventually give rise to cercariae
• Cercariae are sucked up by the
rectal branchial chamber of
dragonfly nymphs and here they
encyst as metacercariae
• When adult dragonflies are eaten
by frogs, the metacercariae excyst in
their digestive tract and migrate to
the lungs to become adults
Family Prosthogonidae
• Most are parasites in the oviduct,
bursa of Fabricius, or the gut of birds
• Prosthogonimus macrorchis often
parasitizes the oviducts of domestic
waterfowl and can cause considerable
damage to organs, even preventing egg
laying in some cases
Prosthogonimus macrorchis
Order Opisthorchiformes
Clonorchis sinensis
• Clonorchis sinensis is known as the
oriental or Chinese liver fluke because it is
distributed throughout Japan, Korea, China,
Taiwan, and Vietnam
• It is estimated that this species infects more
than 30,000,000 humans in these areas.
• The parasite also infects a number of other
animals, including dogs, cats, pigs, and
rodents, and these animals serve as
reservoirs of infection.
Life Cycle
Pathology
• The parasite causes thickening of the lining of the bile duct and
an inflammatory response in the surrounding liver tissue.
• In heavy infections the bile duct epithelium can also be eroded,
and the parasite's eggs will enter the liver tissue; in such cases the
eggs are surrounded by a fibrotic capsule (granuloma).
• Heavy infections can also result in stenosis (narrowing or
blockage) of the bile ducts.
• Since the parasites can live for years and the number of parasites
tends to increase as a person ages, the damage to the liver and bile
duct tends to accumulate over time and can result in death.