The eggs are excreted unembryonated in the sputum, or
alternately they are swallowed and passed with stool
In the external environment, the eggs become embryonated
miracidia hatch and seek the first intermediate host, a snail, and
penetrate its soft tissues
through several developmental stages inside the snail
, with the latter giving rise to many
emerge from the snail. The cercariae invade the second intermediate host, a crustacean such as a crab or
crayfish, where they encyst and become metacercariae. This is the infective stage for the mammalian
Human infection with P. westermani occurs by eating inadequately cooked or pickled crab or crayfish that
harbor metacercariae of the parasite
The metacercariae excyst in the duodenum
, penetrate through the
intestinal wall into the peritoneal cavity, then through the abdominal wall and diaphragm into the lungs,
where they become encapsulated and develop into adults
(7.5 to 12 mm by 4 to 6 mm). The worms can also
reach other organs and tissues, such as the brain and striated muscles, respectively.
However, when this
takes place completion of the life cycles is not achieved, because the eggs laid cannot exit these sites.
Time from infection to oviposition is 65 to 90 days.