Toxocara (Toxocariasis) – Antibody

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Background
This page provides antibody (serology) testing information for toxocariasis at Public Health Ontario (PHO). The main causative agents of toxocariasis are the parasitic larval nematodes Toxocara canis (canid definitive host) and Toxocara cati (felid definitive host). The infectious potential of other related nematodes (e.g. Toxascaris leonina) is not yet established.Infections occur worldwide, particularly in children and young adults, following ingestion of infectious eggs from the environment or ingestion of larvae in undercooked paratenic hosts (e.g. livestock, poultry). It takes at least 1 to 4 weeks after canid or felid feces deposition in the environment for eggs to become infectious.Toxocariasis manifestations include neural larva migrans (NLM), visceral larva migrans (VLM), and ocular larva migrans (OLM). NLM manifestations include eosinophilic meningoencephalitis, myelitis, vasculitis, or radiculitis. Non-specific hypodense or ring-enhancing lesions may be seen on brain imaging. VLM manifestations depend on the organ affected and may include multiple ill-defined visceral granulomatous lesions, eosinophilia, urticarial rash, fever, hepatitis, pneumonitis, myalgia, or carditis. OLM manifestations may include uveitis, diffuse unilateral subacute neuroretinitis (DUSN), or endophthalmitis. Leukocoria, granulomas, vitreous strands, and/or retinal fold may be seen on fundoscopy.

Updates

Addition of further clarification on the acceptance criteria (including symptoms and eosinophilia/eye lesion considerations), along with the assay's performance characteristics and limitations.

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Testing Indications

Serology testing may be considered for the diagnosis of individuals with compatible exposure history and clinical presentation for toxocariasis. Due to their small size and trailing post-migratory inflammation, larvae are rarely detectable by tissue microscopy.

Acute meningitis, including NLM, is a Disease of Public Health Significance (DOPHS) under the Health Protection and Promotion Act (HPPA). Additional testing and management recommendations are stated in the Ontario Ministry of Health Infectious Disease Protocol Appendix 1 for Disease: Meningitis.

Note: Toxocara PCR is not available at PHO.

Acceptance/Rejection Criteria

Testing will only be performed when all the following criteria are documented on the requisition form:

Clinical signs or symptoms consistent with toxocariasis
Eosinophilia, eosinophilic pleocytosis, or ocular lesion(s)

Specimen Collection and Handling

Timing of Specimen Collection

For serology: If a recent infection is suspected following exposure, repeat collection of a convalescent (2-3 weeks later) serum sample may be beneficial since serology may be negative in the acute phase of illness.

Limitations

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

Storage and Transport

Centrifuge tube if using serum separator tube (SST).

Place specimen container in a biohazard bag and properly seal the bag. Specimens should be stored at 2-8°C and shipped to PHO on ice packs within 48 hours of collection.

All clinical specimens must be shipped in accordance to the Transportation of Dangerous Good Act.

Requisitions and Kit Ordering

Test Frequency and Turnaround Time (TAT)

Toxocara antibody testing is referred out to the National Reference Centre for Parasitology (NRCP) in Montreal. Turnaround time for antibody testing is up to 30 calendar days from receipt at PHO’s laboratory.

Test Methods

Toxocara antibody testing is performed at the NRCP using an enzyme-linked immunosorbent assay (ELISA) based on Toxocara lavae excretory-secretory (TES) antigens.

Performance and Limitations:
Toxocara antibody testing performance is reported to have 87% sensitivity and 93% specificity by the NRCP. In other antibody assays, sensitivity has been reported to range from 60-98% and specificity from 35-80%. Cross-reactivity with other helminth infections is common. Test performance may vary by Toxocara species and clinical presentation (e.g. OLM versus VLM). The accuracy of antibody testing on ocular or CSF samples is not established but detection has been reported in some cases. The assay may remain positive for years after infection and may not be able to distinguish between active, resolved, or recurrent infection.^1-4

Interpretation

For Toxocara antibody testing results (“Toxocara by EIA – Referred out”):

Toxocariasis ELISA Sample Value	Result	Interpretation
≥ 1.20	High Positive	Toxocara antibodies detected. Does not distinguish between species. Cross-reactivity with other helminth infections is common. May not distinguish between active, resolved, or recurrent infection.
0.35 to 1.19	Low Positive	Toxocara antibodies detected. Does not distinguish between species. Cross-reactivity with other helminth infections is common. May not distinguish between active, resolved, or recurrent infection.
0.25-0.34	Equivocal	Toxocara antibody status inconclusive. Clinical/exposure correlation required and repeat testing may be considered if clinically indicated.
< 0.25	Negative	Toxocara antibodies NOT detected. If recent infection is suspected, collection of a convalescent serum sample 2-3 weeks later is recommended to assess for seroconversion.

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.

CSF specimens that are positive for Toxocara are reported to the Medical Officer of Health as per Health Protection and Promotion Act.

References

Ma G, Holland CV, Wang T, Hofmann A, Fan CK, Maizels RM, Hotez PJ, Gasser RB. Human toxocariasis. Lancet Infect Dis. 2018 Jan;18(1):e14-e24. doi: 10.1016/S1473-3099(17)30331-6. Epub 2017 Aug 3. PMID: 28781085.
Jin Y, Shen C, Huh S, Sohn WM, Choi MH, Hong ST. Serodiagnosis of toxocariasis by ELISA using crude antigen of Toxocara canis larvae. Korean J Parasitol. 2013 Aug;51(4):433-9. doi: 10.3347/kjp.2013.51.4.433. Epub 2013 Aug 30. PMID: 24039286; PMCID: PMC3770874.
Ahn SJ, Ryoo NK, Woo SJ. Ocular toxocariasis: clinical features, diagnosis, treatment, and prevention. Asia Pac Allergy. 2014 Jul;4(3):134-41. doi: 10.5415/apallergy.2014.4.3.134. Epub 2014 Jul 29. PMID: 25097848; PMCID: PMC4116038.
Sánchez SS, García HH, Nicoletti A. Clinical and Magnetic Resonance Imaging Findings of Neurotoxocariasis. Front Neurol. 2018 Feb 8;9:53. doi: 10.3389/fneur.2018.00053. PMID: 29472889; PMCID: PMC5809457.