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Strongyloidiasis is an intestinal parasitic infection caused
by the roundworm Strongyloides stercoralis. Strongyloides has
a unique ability to replicate within its host and behave as a potentially
fatal opportunistic pathogen in patients who are immunocompromised,
particularly in those receiving corticosteroids. The global prevalence
of strongyloidiasis is estimated to be 30 to 100 million cases.1Strongyloides infections
are endemic in most tropical regions of the world, with hyperendemic
areas in Brazil and central Africa. Endemic foci are also found
in certain temperate areas such as the south-central United States (especially
eastern Kentucky and rural Tennessee) and both western and eastern
Europe. Most infections diagnosed in temperate climates were acquired
by travel in the tropics. Humans are the principal host, but dogs,
cats, and other animals may be reservoirs.2
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Infection is acquired when third-stage filariform larvae, which
are usually found in contaminated soil or in human stool, penetrate
the human skin, enter a blood or lymphatic vessel, and proceed to
the lungs (eFig. 330-1).
Larvae break into the alveolar spaces and migrate through bronchi,
trachea, esophagus, and stomach to reach the duodenum, where female
worms complete their maturation. Males are nonparasitic and pass
with the stools after migration is completed. Adult females establish
themselves in the lamina propria of the small intestine, where they
lay a small number of eggs that hatch to produce sluggishly moving
rhabditiform larvae (Fig. 330-1). In a favorable
external environment, the rhabditiform larvae molt
again into the long, slender, and swift filariform larvae, which
is the skin-penetrating, infective form of the parasite (Fig. 330-2). While still in the intestine,
rhabditiform larvae become filariform and repenetrate the colonic
mucosa or perianal skin, thus starting a new parasitic generation
within the same host. This endogenous cycle, known as autoinfection, allows
the virtually indefinite persistence of the parasite in its host.
In the presence of certain conditions, particularly corticosteroid
therapy and profound malnutrition, the internal replication of parasites
may increase dramatically (ie, hyperinfection), and large numbers
of filariform larvae, as well as adults, may disseminate to extraintestinal
sites and produce a fatal outcome.
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Clinical Manifestations
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Strongyloidiasis is usually characterized by marked eosinophilia,
and the diagnosis should be considered in any child from endemic
areas with unexplained eosinophilia. Many infections are asymptomatic.
Initial skin penetration by filariform larvae may produce transient
pruritic papules at the site of penetration, especially on the feet.
Larval migration through the lungs may cause pneumonitis with wheezing,
dyspnea, and blood-streaked sputum, resulting in Löeffler
pneumonia. Like other enteric parasitoses, strongyloidiasis classically
has been associated with a variety of gastrointestinal ailments,
ranging from dyspepsia and postprandial bloating to diarrhea and
malabsorption with a protein-losing
enteropathy. Abdominal pain is particularly prevalent among pediatric patients.3 More
severe gastrointestinal presentations have been reported, including
upper and lower intestinal bleeding and perforation, emphysematous
gastritis, appendicitis, granulomatous hepatitis, and eosinophilic
ascites, with symptoms mimicking ulcerative colitis. Filariform
larvae excreted in the stool may penetrate the skin of the perianal
area, buttock, and thigh, resulting in migrating serpiginous, erythematous,
and pruritic tracks called larva currens.
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Patients with strongyloidiasis receiving corticosteroids and
certain other immunosuppressed patients, especially those with profound malnutrition,
may develop a characteristic and usually fatal disseminated hyperinfection. Symptoms
may include intractable bloody diarrhea, gram-negative sepsis, hemorrhagic
pneumonitis, meningitis, brain abscess, and generalized purpura.
Disseminated strongyloidiasis has not been described as a common infection
among patients with AIDS, although rare cases of disseminated strongyloidiasis
with immune reconstitution syndrome in AIDS patients starting antiretroviral
therapy have been reported.4 Hyperinfection may
develop even with mild corticosteroid-induced immunosuppression.
The groups at highest risk for infection are patients with altered
cellular immunity, patients on long-term steroid treatment, patients
with lymphomas, kidney allograft recipients, travelers to endemic
areas, and prisoners or other institutionalized persons.
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Diagnosis and Treatment
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In contrast to most helminth infections, which are diagnosed
by identifying eggs in the stool, diagnosis of strongyloidiasis
requires visualization of larvae in stool specimens (Fig.
330-2). The most common stage of S stercoralis identified
in feces is the rhabditiform larva, but in occasional patients filariform
larvae, adult females, and even eggs have been seen. The sensitivity
of a single stool examination performed in clinical laboratories
is low (30–60%). Alternative techniques to detect
larvae in stool samples include direct smear in saline-Lugol iodine
stain, Baermann concentration, formalin-ethyl acetate concentration,
Harada-Mori filter paper strip culture, and nutrient agar plate
cultures.5 In the agar culture method, a method
that is estimated to increase sensitivity fourfold over direct smear,
the stool is placed on a nutrient agar plate for 2 days or longer,
and as larvae crawl on the agar they carry bacteria with them and
create visible streaks. All of these are more sensitive than single
stool smears, but they are rarely available in clinical laboratories.
The concentration method of Baermann allows the examination of a
larger volume of feces (up to several grams) and is more sensitive
than direct microscopy.
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If no special techniques are available when the diagnosis is
strongly suspected on clinical grounds, careful examination of several
specimens, collected on different days, is necessary before strongyloidiasis
can be excluded with reasonable confidence. Although the examination
of duodenal aspirate (obtained by intubation or string test capsule)
reportedly is very sensitive, this method is less commonly used,
being primarily recommended in pediatric patients when necessary
to achieve a rapid demonstration of parasites, as in the immunocompromised
child with suspected overwhelming infection. In disseminated infections,
larvae and adult parasites have been found in specimens of sputum
and bronchoalveolar lavage, ascitic fluid, pancreatic aspirates,
and cerebrospinal fluid.
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The only hematologic abnormality found in children with chronic,
uncomplicated strongyloidiasis is an elevated peripheral eosinophil
count, which might also be associated with elevated serum IgE. Although extremely
elevated eosinophil counts (ie, > 30% of the total white
count) may rarely occur, 70% to 80% of patients
in most series have values between 6% and 15% (or
500–1500 eosinophils/μL), although
day-to-day variation is common. Because patients with disseminated
strongyloidiasis often receive immunosuppressive drugs capable of
reducing the eosinophilic response, their peripheral eosinophilia
might be suppressed.
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Serologic tests to detect serum antibodies against filariform
larvae or their antigenic products are available in a few reference
laboratories. The most commonly used tests include the indirect
immunofluorescence test and the enzyme-linked immunoassay, which are
positive in about 85% of cases. Apparent false-positive
results are found in some patients with filariasis and Ascaris
lumbricoides infections, limiting the specificity of the
assay in areas where these infections are prevalent. Serologic tests
may be more reliable for international travelers from nonendemic
areas, who are unlikely to be previously exposed to other helminthes.
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The drug of choice for both uncomplicated and disseminated strongyloidiasis
is ivermectin.6,7 The recommended dose for both
adults and children is 200 μg per kilogram per
day for 2 days. Ivermectin was shown to have higher cure rates than
albendazole, an alternative therapeutic drug, in several clinical
trials (ivermectin 83–100% cure rate versus albendazole
38–77% cure rate). Cure rates with ivermectin
were similar to those observed with thiabendazole, the former drug
of choice until the mid 1990s, but side effects from thiabendazole
were more frequent and more severe. Side effects reported from ivermectin were
mostly mild and transient.