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

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 Dicrocoelium dendriticum

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 Life cycle cont.

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.