Toxocara – Serology

Testing Indications

This page provides routine serology (antibody) testing information for toxocariasis at Public Health Ontario (PHO). The causative agents of toxocariasis include Toxocara canis (canids definitive hosts) and Toxocara cati (felids definitive hosts). Toxocariasis manifestations include visceral larva migrans (VLM), ocular larva migrans (OLM), and neural larva migrans (NLM).

Testing options:
Serology is the main laboratory diagnostic modality available for toxocariasis. Rarely, larvae may be incidentally found in histologic tissue sections, but biopsies are not routinely recommended for diagnosis as the likelihood of capturing the microscopic larvae (up to 400 µm long and 20 µm wide) is low. Molecular testing is currently not routinely available in clinical laboratories.

Who to test:
Testing is only advised for individuals with compatible exposure history and clinical presentation. Toxocara is found worldwide, particularly in children and young adults, and is usually acquired by ingestion of embryonated eggs from the environment (which requires 1 to 4 weeks in the environment for embryonation) or ingestion of larvae in undercooked paratenic hosts (e.g., beef, lamb, chicken, or duck). Incubation may take weeks to years depending on inoculum size, location of larval migration, and immunologic response. VLM manifestations depend on the organ affected and may include multiple ill-defined visceral granulomatous lesions on imaging, eosinophilia, urticarial rash, fever, anorexia, hepatitis, pneumonitis, myalgia, or carditis. OLM manifestations may include unilateral granulomatous lesions on fundoscopy, uveitis, retinitis, or endophthalmitis. NLM manifestations may include ill-defined lesions on imaging, eosinophilic meningoencephalitis, myelitis, vasculitis, or radiculitis.

Test updates (as of September 6, 2022):
Specimens are now routinely forwarded to the National Reference Centre for Parasitology (NRCP) for testing due to the discontinuation of the commercially available serology assay previously used in-house at PHO.

Specimen Collection and Handling

Specimen Requirements

Test Requested Required Requisition(s) Specimen Type Minimum Volume Collection Kit

Toxocara – Serology

Serum

1.0 ml

5.0 ml blood or 1.0 ml serum

Submission and Collection Notes

1

Complete all fields of the General Test Requisition Form, including:

    1. Test(s) requested and indication for testing
    2. Travel history (if applicable)
    3. Clinical information including symptom onset date
    4. Patient setting/population
2

Label the specimen container 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. Failure to provide this information may result in rejection or testing delay.

Limitations

Haemolysed, icteric, lipemic or microbial contaminated sera are not recommended for testing.

The performance of the NRCP enzyme-linked immunosorbent assay (ELISA) is reported here as 86% sensitivity and 98% specificity for T. canis VLM infections: https://www.mcgill.ca/tropmed/services/national-reference-centre-parasitology/immunodiagnostic-service. Performance characteristics may vary for other clinical presentations (e.g., OLM or NLM) or for less frequently reported Toxocaridae infections (e.g., T. cati, Toxascaris leonina). Titers may also vary depending on the extent of larval burden and immune response. The serology test does not differentiate between Toxocara species. Cross-reaction can occur with other nematode infections (e.g., ascariasis, trichinellosis, filariasis, strongyloidiasis). However, cross-reactivity is not guaranteed, and serology tests specific for these agent(s) must be requested if suspected. Antibody detection may also represent asymptomatic co-infection or past infection, with seroprevalence estimates at 19% worldwide and 6% in Canada (Rostami et al., PloS Negl Trop Dis 2019). Antibodies may also remain positive for years after treatment (Fillaux et al., Vet Parasitol 2013).

Preparation Prior to Transport

Centrifuge tube if using serum separator tube (SST). Place specimen in biohazard bag and seal.

Specimens should be stored at 2-8°C following collection and shipped to the PHO’s laboratory on ice packs as soon as possible.

Requisitions and Kit Ordering

Test Frequency and Turnaround Time (TAT)

Toxocara serology testing is performed once per week at the NRCP.

Turnaround time is up to 10 days from receipt at PHO’s laboratory.

Reporting

Results are reported to the ordering physician or health care provider as indicated on the requisition.

Test Methods

Toxocara serology test is a referred-out test. It is performed at the NRCP using a commercial indirect enzyme immunoassay (ELISA) using Toxocara larvae excretory-secretory (TES) antigens for the detection of IgG antibodies to Toxocara. The assay provides a semi-quantitative value based on the optical density of the detection signal.

Interpretation

Optical Density Result Interpretation
OD <0.25 Negative
OD ≥0.25 and <0.35 Equivocal
OD ≥0.35 and <1.20 Low Positive
OD ≥1.20 High Positive

In clinical cases where there is strong clinical suspicion for toxocariasis and the results are equivocal or low positive, paired serum samples demonstrating a significant rise in antibody level over time may be useful to confirm active infection. Please contact the PHO microbiologist if paired serum interpretation is required.

Mis à jour le 8 sept. 2022