Strongyloidiasis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Laboratory Findings

Eosinophilia

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

Stool Smaples

The 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.

Rhabditiform larvae of Strongyloides stercoralis in wet mounts after fixation in formalin 10%. Diagnostic characteristics: length 200 to 250 µm (up to 380 µm); buccal cavity short, and prominent genital primordium.

A: Strongyloides stercoralis

A: 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.

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, enzyme immunoassay (EIA) is currently recommended because of its greater sensitivity (90%). Immunocompromised persons with disseminated strongyloidiasis usually have detectable IgG antibodies despite their immunodepression. Cross-reactions in patients with filariasis and some other nematode infections may occur. Antibody test results cannot be used to differentiate between 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.