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General Appearance
- Non-bursate.
- Big fleshy worms; typically 5-40cm long (depending on age, sex and species).
- Three lips around the mouth.
- A single-bulbed pharynx.
General Life-Cycle
- Adult females in small intestine lay eggs.
- Eggs have a thick protective shell.
- Some also have a sticky outer albuminoid coat.
- Eggs passed in faeces.
- L1 → infective L2 inside the egg.
- Infection is by ingestion of the embryonated egg (egg in which a larva has formed).
- Eggs can remain viable in humid environment for up to 5years.
- After hatching in the intestine, the larvae of most species (but not all) undergo hepato-tracheal migration:
- egg hatches in intestine → larva penetrates intestinal mucosa → hepatic portal blood → liver → venous blood → heart → lung capillaries → alveoli → ascends trachea → swallowed → small intestine (where the adults develop).
NOTE: there are important variations on this theme; for example, the migratory larvae of some species can cross the placenta or enter the mammary glands (examples of vertical transmission); while some species will utilise paratenic or intermediate hosts.
Occlusion of pancreatic ducts sometimes happens Pancreas - parasitic
In Peritoneal Cavity Parasitic - Pathology
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