(roundworm) Strongyloides stercoralis. Other Strongyloides
include S. fülleborni, which infects chimpanzees and baboons and
may produce limited infections in humans.
Strongyloides life cycle is more complex than that of most nematodes
with its alternation between free-living and parasitic cycles, and its
potential for autoinfection and multiplication within the host. Two
types of cycles exist:
Free-living cycle: The rhabditiform larvae passed in the stool
"Parasitic cycle" below) can either molt twice and become infective
filariform larvae (direct development)
molt four times and become free living adult males and females
mate and produce eggs
which rhabditiform larvae hatch
The latter in turn can either develop
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
"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.
Tropical and subtropical areas, but cases also occur in temperate areas
(including the South of the United States). More frequently found in
rural areas, institutional settings, and lower socio-economic groups.
Frequently asymptomatic. Gastrointestinal symptoms include abdominal
pain and diarrhea. Pulmonary symptoms (including Loeffler’s syndrome)
can occur during pulmonary migration of the filariform larvae.
Dermatologic manifestations include urticarial rashes in the buttocks
and waist areas. Disseminated strongyloidiasis occurs in
immunosuppressed patients, can present with abdominal pain, distension,
shock, pulmonary and neurologic complications and septicemia, and is
potentially fatal. Blood eosinophilia is generally present during the
acute and chronic stages, but may be absent with dissemination.
Diagnosis rests on the microscopic identification of larvae (rhabditiform
and occasionally filariform) in the stool or duodenal fluid.
Examination of serial samples may be necessary, and not always
sufficient, because stool examination is relatively insensitive.
The stool can be
examined in wet mounts:
concentration (formalin-ethyl acetate)
of the larvae by the Baermann funnel technique
by the Harada-Mori filter paper technique
in agar plates
The duodenal fluid
can be examined using techniques such as the Enterotest string or
duodenal aspiration. Larvae may be detected in sputum from patients
with disseminated strongyloidiasis.
comparison with other intestinal parasites
The drug of choice
for the treatment of uncomplicated strongyloidiasis is ivermectin with
albendazole* as the alternative. All patients who are at risk of
disseminated strongyloidiasis should be treated.
*This drug is
approved by the FDA, but considered investigational for this purpose.