Strongyloides (Strongyloidiasis) – Microscopy and Antibody

Consistent with O. Reg. 671/92 of the French Language Services Act, laboratory testing information on this page is only available in English because it is scientific or technical in nature and is for use only by qualified health care providers and not by members of the public.

Background
This page provides testing information for strongyloidiasis at Public Health Ontario (PHO). The main causative agent of strongyloidiasis in humans is the parasitic nematode Strongyloides stercoralis.

Note: For testing information on the lesser-known species Strongyloides fuelleborni (sub-Saharan Africa, Southeast Asia, Papua New Guinea), refer to the following PHO webpage: Enteric Helminths (Nematodes, Trematodes, Cestodes, Acanthocephalans) and Balantioides – Microscopy.

Updates
This webpage has been merged with the serology testing webpage and updated to include background, testing indications, acceptance criteria, performance and limitations, interpretations, and expected turnaround times based on calendar days instead of business days.

*Uncontrolled print copy. Valid only on day of print: 17 Dec 2025.

Testing Indications

Microscopy and serology are recommended for diagnosing individuals with clinical and epidemiological evidence of strongyloidiasis. Serology should also be used to screen individuals with subclinical strongyloidiasis who are at risk of developing hyperinfection or disseminated disease. Indications for screening are described in the Committee to Advise on Tropical Medicine and Travel (CATMAT) guidelines.

Note: PCR, culture, and antigen tests for Strongyloides are not currently available at PHO. However, some private or hospital laboratories may offer agar plate culture upon request. Contact these laboratories directly if any of these testing is required.

Acceptance/Rejection Criteria

Testing will only be accepted when any of the following are documented on the requisition:

Travel/migration from an endemic area (specify which)
Impaired immune system (e.g. HTLV, transplant, steroids)
Signs or symptoms compatible with strongyloidiasis

Enteric specimens received without sodium acetate, acetic acid, and formalin (SAF) preservation are ineligible and will be cancelled.
Specimens are only accepted if originating from human sources. Specimens submitted from animal sources (e.g. pets) or environmental sources (e.g. water) will be rejected.

Specimen Collection and Handling

Specimen Requirements

Test Requested	Required Requisition(s)	Specimen Type	Minimum Volume	Collection Kit
Strongyloides - Microscopy	General Test Requisition	Enteric specimens (e.g., stool, intestinal biopsy / aspirate, gallbladder stones)	1.0 ml	SAF vial
Strongyloides - Microscopy	General Test Requisition	Other body fluid or tissue specimens (e.g. bronchoalveolar lavage, urine, cerebrospinal fluid, aspirates, biopsies)	1.0 ml (fluid)	SAF or empty sterile vial
Strongyloides - Antibody	General Test Requisition	Blood or serum	5.0 ml blood or 1.0 ml serum	Blood, clotted – vacutainer tubes (SST)

Submission and Collection Notes

Complete all fields on the requisition form.

Important: Specify any of the following testing indications on the requisition. Failure to provide this information may result in rejection:

Travel/migration from an endemic area (specify which)
Impaired immune system (e.g. HTLV, transplant, steroids)
Signs or symptoms compatible with strongyloidiasis

Important: Mix the enteric specimen thoroughly with SAF preservative immediately after collection to ensure proper preservation.

Label the specimen container(s) with the patient’s first and last name, date of collection, and one other unique identifier such as the patient’s date of birth or Health Card Number. For additional information see: Criteria for Acceptance of Patient Specimens. Failure to provide this information may result in rejection or testing delay.

Limitations

For serology: Grossly haemolysed, lipemic, contaminated specimens, and specimens containing anti-coagulant are unsuitable for testing.

Storage and Transport

Place specimen container in a biohazard bag and properly seal the bag. Centrifuge tube if using SST for serum specimens.

SAF specimens can be stored at room temperature (or alternatively 2-8°C) and shipped to PHO within 48 hours of collection. Other unpreserved specimens and serum specimens should be stored at 2-8°C and shipped to PHO within 48 hours of collection. All specimens must be shipped in accordance with the Transportation of Dangerous Good Act.

Special Instructions

For treatment monitoring (within 9 to 14 months only): If repeat serology is to be performed for treatment monitoring, indicate “Follow-up / Convalescent” as the testing indication on the General Test Requisition and contact PHO Laboratory Customer Service to request repeat testing of the pre-treatment serum sample for paired ELISA ratio value comparison. Positive serology samples will only be retained at PHO for up to 14 months.

Requisitions and Kit Ordering

Test Frequency and Turnaround Time (TAT)

Microscopy on enteric specimens is performed daily from Monday to Friday at PHO’s Toronto, Peterborough, Ottawa, and London sites. Turnaround time is up to 7 calendar days from receipt at PHO. Microscopy on other specimens is performed daily from Monday to Friday at PHO’s Toronto site. Turnaround time is up to 7 calendar days from receipt at PHO. If approved, priority testing turnaround time may be up to 1 calendar day from receipt at PHO (see priority instructions below).

Routine serology is performed three times per week at PHO’s Toronto site.
Turnaround time is up to 5 calendar days from receipt at PHO. If approved, priority testing turnaround time may be up to 3 calendar days from receipt at PHO.
In rare circumstances where a positive result is questioned based on the pre-test likelihood (e.g. previously treated infection), serum samples may be forwarded upon request to the National Reference Centre for Parasitology (NRCP) in Montreal for immunoblot confirmation. Turnaround time for immunoblot confirmation may be up to 42 calendar days from receipt at PHO.

STAT and Critical Specimens Testing

If Strongyloides hyperinfection or disseminated disease is suspected, submit specimens for priority testing as per below:

Notify PHO before sending a priority specimen by contacting the Laboratory Customer Service Centre 416-235-6556 / 1-877-604-4567 / customerservicecentre@oahpp.ca.
Packaging:
- Priority specimens must be packaged separate from routine specimen using TDG compliant packaging and labelling.
- Mark ‘Priority’ clearly on the outside of the package to assist with laboratory workflow and identification.
Requisition:

Indicate “Priority” and the suspected organism (Strongyloides) clearly on the requisition.

Shipping:

Ship priority specimens directly to PHO’s laboratory, Toronto site:
Shipping and Receiving Dock
661 University Ave., Toronto, Ontario

For delivery instructions Directions to 661 University Shipping Dock for Clinical Samples.
- Appropriately labelled specimens must be received by 9:00 AM at PHO’s laboratory, Toronto site to meet priority turnaround times.

Priority specimens will be tested and resulted within up to 1 calendar day for microscopy and 3 calendar days for serology. Failure to follow the submission instructions above may result in testing delays.

Test Methods

Microscopy on enteric specimens is performed at PHO using diphasic sedimentation by formalin and ethyl acetate (FEA). Microscopy on other specimens is performed using standard sedimentation.

Serology is performed at PHO using the commercial EUROIMMUN Anti-Strongyloides IgG enzyme-linked immunosorbent assay (ELISA) based on antibody capture using purified Strongyloides papillosus larvae antigen extracts.

If approved by PHO microbiologist, confirmation of positive serological results is performed at the NRCP using a laboratory-developed immunoblot assay.

Performance and Limitations:
Microscopy sensitivity of a single enteric specimen ranges from 5-60% depending on the intensity of infection, time from exposure (i.e. prepatent period), specimen type, and intermittent shedding patterns. Therefore, a single negative microscopy result does not rule out infection. Multiple (e.g., up to 6) specimens may be collected to increase sensitivity. Inadequate specimen volume or delayed mixing of the enteric specimen and SAF fluid in the vial may lead to poor preservation of organism morphology and uninterpretable results. ^1-11

For routine serology by ELISA, sensitivity may be below 50% in the early phase of infection (e.g. acutely symptomatic travelers). During the chronic phase, ELISA sensitivity ranges from 70-98% and specificity from 65-96%. Sensitivity may be lower in individuals with impaired immunity (e.g. HTLV, hematologic malignancies). Cross-reactivity may occur with other helminth infections (including Schistosoma, Echinococcus, Toxocara, Ascaris, Opisthorchis, Taenia, hookworms, and filarial nematodes). Serology may remain positive after treatment but usually with decreasing signal values by 6 to 12 months4. Performance has not been evaluated for Strongyloides fuelleborni infections. For immunoblotting, some assays report a sensitivity of 90-96% and specificity of 96-99%, with rarely reported cross-reactivity (e.g. filarial nematodes).^6-18

Algorithm

Microscopy and routine serology by ELISA will be performed on all patients if the appropriate specimen types are received. If approved, confirmatory serological immunoblot testing will only be performed on positive serology samples by ELISA.

Interpretation

Microscopy:

Helminth Microscopy	Interpretation
Helminth(s) found: Strongyloides species	The species name(s) and stage(s) will be reported. Prompt assessment and treatment is advised. If not already requested, screening for HTLV-1 coinfection may be advised to assess the risk of hyperinfection following CATMAT guidelines.
No helminths found	No evidence of Strongyloides organisms. Does not rule out infection. Due to the limited test sensitivity, testing of additional specimens may be considered if clinically indicated. If not already requested, serology may be advised if clinically indicated.

Serology:

ELISA Ratio Value	Interpretation	Comments
≥ 1.1	Reactive	Strongyloides antibodies detected. Cross-reactivity may occur with other infections. Prompt assessment for clinical correlation and treatment is advised. If not already requested, screening for HTLV-1 coinfection may be advised to assess the risk of hyperinfection following CATMAT guidelines.
0.8 to 1.1	Indeterminate	Strongyloides antibody status inconclusive. Clinical/exposure correlation is required. Treatment and/or repeat testing may be advised depending on the patient’s risk status.
< 0.8	Non-reactive	Strongyloides antibodies NOT detected.

Reporting

Results are reported to the physician, authorized health care provider (General O. Reg 45/22, s.18) or submitter as indicated on the requisition.

References

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