Strongyloides

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http://www.dpd.cdc.gov/dpdx/HTML/Strongyloidiasis.htm

 

 

Strongyloides

 

 

 

[strongyloides stercoralis]

 

 

Causal Agent:

The nematode (roundworm) Strongyloides stercoralis. Other

Strongyloides include S. fülleborni, which infects chimpanzees and

baboons and may produce limited infections in humans.

 

Life Cycle:

 

Life cycle of Strongyloides stercoralis

 

The 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 (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.

 

Geographic Distribution:

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 socioeconomic

groups.

 

 

 

Clinical Features:

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.

 

Laboratory Diagnosis:

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:

 

* directly

* after concentration (formalin-ethyl acetate)

* after recovery of the larvae by the Baermann funnel technique

* after culture by the Harada-Mori filter paper technique

* after culture 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.

 

Diagnostic findings

 

* Microscopy

* Antibody detection

* Morphologic comparison with other intestinal parasites

 

Treatment:

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. For

additional information, see the recommendations in The Medical Letter

(Drugs for Parasitic Infections).

 

*This drug is approved by the FDA, but considered investigational for

this purpose.