Strongyloidiasis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Aditya Ganti M.B.B.S. [2]

Overview

The diagnosis of strongyloidiasis is made by presence of clinical signs and symptoms, eosinophilia, and positive serological findings. Definitive diagnosis of strongyloidiasis is generally made by the detection of larvae in the stool. Sputum examination may rarely be used to identify organisms in cases of hyperinfection. Agar tracking (detection of larval tracks on agar culture plates) has been shown to be more sensitive than the conventional stool examination. However, agar tracking is usually unavailable on a routine basis in clinical microbiology laboratories. Immunodiagnostic tests for strongyloidiasis are indicated when the infection is suspected and the organism cannot be demonstrated by repeated examinations of stool. Enzyme immunoassay (EIA) is currently recommended because of its greater sensitivity (90%). Antibody test results cannot be used to differentiate between the past and current infection. In disseminated cases of strongyloidiasis, larvae can be detected in sputum by simple wet-mount in fluid from a bronchoalveolar lavage (BAL).

Laboratory Findings

Eosinophilia

Eosinophilia is generally present during the acute and chronic stages but may be absent with dissemination.

Microscopic Stool Examination

• The gold standard for the diagnosis of strongyloidiasis is serial stool examination.
• The diagnosis rests on the microscopic identification of larvae (rhabditiform and occasionally filariform) in the stool.
• Single ova and parasite examination can have negative results in up to 70% of cases.
• Repeated examinations of stool specimens increase the chances of detecting larvae.
• The diagnostic sensitivity increases to 50% with three stool examinations and can approach 100% if seven serial stool samples are examined
• Specialized stool examinations techniques include Baermann concentration, Horadi-Mori filter paper culture, quantitative acetate concentration technique, and nutrient agar plate cultures.
• Diagnostic characteristics:
  • length 200 to 250 µm (up to 380 µm)
  • Short buccal cavity
  • Prominent genital primordium.

Strongyloides stercoralis:The prominent genital primordium in the mid-section of the larva (black arrow) is readily evident. Note also the Entamoeba coli cyst (white arrow) near the posterior end of the larva. Source:https://commons.wikimedia.org/w/index.php?curid=219824

Antibody Detection

• Immunodiagnostic tests for strongyloidiasis are indicated when the infection is suspected and the organism cannot be demonstrated by duodenal aspiration, string tests, or by repeated examinations of stool.
• Antibody detection tests should use antigens derived from Strongyloides stercoralis filariform larvae for the highest sensitivity and specificity.
• Although indirect fluorescent antibody (IFA) and indirect hemagglutination (IHA) tests have been used,
• Immunocompromised persons with disseminated strongyloidiasis usually have detectable IgG antibodies despite their immunosuppression.
• Cross-reactions in patients with filariasis and other nematode infections can occur.
• Antibody test results cannot be used to differentiate between the past and current infection.
• A positive test warrants continuing efforts to establish a parasitological diagnosis followed by antihelminthic treatment.
• Serologic monitoring may be useful in the follow-up of immunocompetent treated patients: antibody levels decrease markedly within 6 months after successful chemotherapy.
• More sensitive and specific serologic tests using recombinant antigens have been and are being developed, and are available at specific laboratories.
• An additional advantage of these serologic tests is that there is typically a significant drop in titer by 6 months after parasite eradication, which may make it possible to use these tests as a check the response to medical therapy.

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Enzyme-linked immunosorbent assay

• Serology can be useful but is not commonly available and can give false-negative results.
• Results for ELISA should be used in conjunction with clinical history and geographical data