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*Infection is by ingestion of the embryonated egg (egg in which a larva has formed).
*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.
*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.