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Intestinal Nematodes: Soil-Transmitted Helminths (STH) 87

i = Infective Stage 6 Infective filariform larvae
penetrate the intact skin
d = Diagnostic Stage initiating the infection. 7 The filariform migrate by
various pathways to the
small intestine where they
i become adults.
i
5 The rhabdititorm
larvae develop
into infective
filariform.
8
Adult female
worm in the
intestine.
4
Rhabditiform AUTOINFECTION Autoinfection:
larvae hatch from Development into filariform Rhabditiform larvae
embryonated eggs. 10 in large intestine.
become filariform
larvae, penetrate
intestinal mucosa or
perianal skin, and
migrate randomly to
other organs.
3
Eggs are produced 9
by fertilized
female worms. 1 d Eggs deposited in intestinal mucosa,
hatch, and migrate to lumen.
Rhabditiform larvae
in the intestine are
excreted in stool.
2 Development
into free-living
adult worms.
Fig. 10.6 Life cycle of Strongyloides stercoralis (Reproduced from https://www.cdc.gov/dpdx/
strongyloidiasis/index.html)
trachea and pharynx. (8) They are then swallowed and develop into adults in the
mucosa of the small intestine. (9) The adult female deposits eggs in the intestinal
mucosa. The eggs hatch into L1 larvae which migrate to the intestinal lumen and are
excreted in the stool. (10) The worm may develop internal and external autoinfec-
tion. In external autoinfection, the L1 larvae develop into the infective L3 larvae
during passage down the intestine. These L3 larvae cause reinfection by penetrating
the perianal skin during defaecation. They enter the circulatory system and are car-
ried to the heart and lungs to complete the life cycle. In internal autoinfection seen
commonly in immunosuppressed hosts, the L1 larvae develop into infective L3 lar-
vae in the intestine. The L3 larvae penetrate the intestine and are carried in circula-
tion to the heart and lungs to complete the life cycle.
Pathogenesis and Clinical Features
Strongyloidiasis is generally benign and asymptomatic. Eosinophilia and larvae in
stool are the only indication of infection. In immunocompromised persons, it may
cause clinical manifestations which may be severe and even fatal. The clinical dis-
ease may have cutaneous, pulmonary and intestinal manifestations.