Free-living cycle: The rhabditiform larvae passed in the stool
(see "Parasitic cycle" below) can either molt twice and become infective
filariform larvae (direct development)
or molt
four times and become free living adult males and females
that
mate and produce eggs
from
which rhabditiform larvae hatch
. The
latter in turn can either develop
into a
new generation of free-living adults (as represented in
), or
into infective filariform larvae
. The
filariform larvae penetrate the human host skin to initiate the parasitic
cycle (see below)
.Parasitic cycle: Filariform larvae in contaminated soil penetrate the human skin
, and are
transported to the lungs where they penetrate the alveolar spaces; they are
carried through the bronchial tree to the pharynx, are swallowed and then
reach the small intestine
. In the
small intestine they molt twice and become adult female worms
. The
females live threaded in the epithelium of the small intestine and by
parthenogenesis produce eggs
, which
yield rhabditiform larvae. The rhabditiform larvae can either be passed in
the stool
(see "Free-living cycle" above), or can cause autoinfection
. In
autoinfection, the rhabditiform larvae become infective filariform larvae,
which can penetrate either the intestinal mucosa (internal autoinfection) or
the skin of the perianal area (external autoinfection); in either case, the
filariform larvae may follow the previously described route, being carried
successively to the lungs, the bronchial tree, the pharynx, and the small
intestine where they mature into adults; or they may disseminate widely in
the body. To date, occurrence of autoinfection in humans with helminthic
infections is recognized only in Strongyloides stercoralis and
Capillaria philippinensis infections. In the case of Strongyloides,
autoinfection may explain the possibility of persistent infections for many
years in persons who have not been in an endemic area and of hyperinfections
in immunodepressed individuals.