Diagnosing Toxocara — ELISA and Imaging
Diagnosing toxocariasis requires combining clinical findings with targeted laboratory tests and imaging. Toxocara cannot be diagnosed by stool examination — larvae never mature into egg-producing adults in humans. The diagnostic cornerstone is ELISA serology using Toxocara excretory-secretory (TES) antigens, supported by eosinophil count, IgE level, liver imaging for VLM, and fundoscopy for OLM. No single test is perfectly sensitive, particularly for ocular disease.
Table of Contents
- Why Stool Tests Don't Work
- ELISA Using TES Antigen
- ISAGA — More Sensitive for OLM
- Cross-Reaction with Ascaris
- Eosinophil Count and IgE
- Liver Ultrasound and CT
- FDG-PET/CT for Activity Assessment
- Fundoscopy and Vitreous Tap for OLM
- Key Research Papers
- Connections
- Featured Videos
1. Why Stool Tests Don't Work
The first thing most clinicians try for suspected parasitic infection is a stool examination. For toxocariasis, this is futile. Toxocara larvae hatch from ingested eggs in the human small intestine, penetrate the gut wall, and migrate into the tissues — they never return to the intestinal lumen and never mature into adult worms that would produce eggs. There is nothing to find in the stool.
The same is true for tissue biopsy in most cases: while eosinophilic granulomas with larvae can sometimes be found on liver biopsy, biopsy is invasive, rarely necessary, and may miss the larvae even when they are present (larvae are only a few hundred micrometers long in a biopsy specimen of millions of cells). Clinical diagnosis supported by serology is the standard approach.
2. ELISA Using TES Antigen
The diagnostic standard for toxocariasis is an enzyme-linked immunosorbent assay (ELISA) measuring IgG antibodies against Toxocara excretory-secretory (TES) antigens — proteins secreted by living Toxocara larvae. TES-ELISA performance characteristics:
- Sensitivity for VLM: approximately 78–91% in symptomatic patients with high worm burden
- Specificity for VLM: approximately 86–92% when using optimal antigen preparations
- Sensitivity for OLM: significantly lower — often 50–70% — because OLM involves only a single larva producing minimal systemic antigen
- Interpretation: A positive result confirms Toxocara exposure but does not distinguish active from past infection, since antibodies persist for years after larval death
A positive ELISA in the context of compatible symptoms (eosinophilia + hepatomegaly in a child with pet exposure; unexplained ocular lesion with granuloma) supports the diagnosis. A negative ELISA does not exclude OLM. Titers do not reliably correlate with disease severity or activity.
3. ISAGA — More Sensitive for OLM
The immunosorbent agglutination assay (ISAGA) is a more sensitive alternative to ELISA, particularly valuable when OLM is suspected but ELISA is borderline or negative. ISAGA detects agglutination of sensitized indicator particles (red blood cells or latex beads coated with TES antigen) by patient antibodies.
ISAGA advantages:
- More sensitive than ELISA for low antibody titers typical of OLM
- Particularly useful when ELISA result is equivocal
- Can be adapted for vitreous fluid testing when systemic serology is negative but OLM is strongly suspected clinically
ISAGA is not universally available and is performed mainly at reference parasitology laboratories. When high clinical suspicion for OLM exists despite a negative or borderline ELISA, referral to a center with ISAGA capability may be warranted.
4. Cross-Reaction with Ascaris
A clinically important limitation of Toxocara serology is cross-reactivity with Ascaris lumbricoides (human roundworm), which shares antigens with Toxocara. In populations with high Ascaris prevalence (tropical and subtropical regions), false-positive Toxocara ELISA results can occur in Ascaris-infected individuals who have never been exposed to Toxocara.
Cross-reactivity can be addressed by pre-absorbing patient serum with Ascaris antigen before running the Toxocara ELISA — a procedure that removes non-specific antibodies and improves specificity. Reference laboratories performing Toxocara serology in populations at risk for Ascaris co-infection should routinely include this absorption step. In the United States, where Ascaris infection is uncommon, cross-reactivity is rarely a significant problem.
5. Eosinophil Count and IgE
While not specific for toxocariasis, two routine blood tests are essential in evaluating suspected VLM:
Eosinophil count (CBC with differential):
- Absolute eosinophil count >500/µL (hypereosinophilia) is a key finding in VLM
- In VLM, eosinophil percentage may reach 30–80% of the white cell differential
- Persistent eosinophilia over weeks distinguishes VLM from transient reactive eosinophilia
- Important: eosinophilia is typically absent or minimal in OLM — a normal eosinophil count does not exclude ocular toxocariasis
Total serum IgE:
- Markedly elevated in VLM — often >1,000 IU/mL, sometimes >10,000 IU/mL
- Reflects Th2 immune polarization driven by larval antigens
- Not specific (also elevated in atopy, other helminth infections) but very high levels in a child with animal exposure strongly suggest VLM
Additional routine tests: liver function tests (elevated transaminases suggest hepatic VLM), serum protein electrophoresis (hypergammaglobulinemia in VLM), and isohemagglutinin titers (anti-A and anti-B, elevated in VLM due to cross-reactive antigens).
6. Liver Ultrasound and CT
Imaging is a key diagnostic tool for hepatic VLM:
Liver ultrasound:
- Shows multiple small hypoechoic (dark) nodules corresponding to eosinophilic granulomas
- A characteristic "flower pattern" — a central hypoechoic area surrounded by hyperechoic nodules — is considered pathognomonic for toxocariasis when seen in the right clinical context
- Non-invasive, no radiation, excellent first-line modality in children
- Lesions typically 1–2 cm, multiple, clustered primarily in the right lobe
CT scan of the abdomen:
- Multiple low-density (hypodense) hepatic lesions, non-enhancing or with slight peripheral enhancement on contrast
- Better delineation of lesion number and distribution than ultrasound
- Helpful when ultrasound is equivocal
- Pulmonary CT may show ground-glass opacities or consolidation in pulmonary VLM
The combination of multiple small hypodense hepatic nodules + marked eosinophilia + positive Toxocara serology in a young child is highly specific for VLM and generally does not require liver biopsy for confirmation.
7. FDG-PET/CT for Activity Assessment
FDG-PET/CT (fluorodeoxyglucose positron emission tomography) has emerged as a tool for assessing metabolic activity in toxocariasis, particularly for:
- Distinguishing active from resolved infection — active eosinophilic granulomas show FDG uptake; resolved lesions on CT without active inflammation do not
- Assessing treatment response — reduction in FDG avidity after albendazole treatment suggests effective larval killing
- Identifying occult sites of larval migration in complex cases with atypical presentations
- Cardiac and neurological involvement — FDG-PET can detect myocardial or brain inflammation not apparent on anatomical imaging
FDG-PET/CT is not a routine diagnostic tool for toxocariasis and is reserved for complex or atypical cases. It is not indicated when the diagnosis is clear from serology and conventional imaging. Its primary value is in differentiating active from chronic toxocariasis when a treatment decision depends on disease activity status.
8. Fundoscopy and Vitreous Tap for OLM
For suspected OLM, the primary diagnostic modality is ophthalmological examination:
Dilated indirect ophthalmoscopy:
- Identifies retinal granuloma (posterior pole or peripheral), vitreous haze, retinal detachment, or macular involvement
- In DUSN, may directly visualize the motile larva in the subretinal space
- Fluorescein angiography demonstrates lesion vascularity and distinguishes OLM from retinoblastoma and other retinal masses
Ocular ultrasound (B-scan):
- Evaluates posterior pole when view is obscured by vitreous haze
- Identifies retinal detachment and calcification (retinoblastoma calcifies; OLM granulomas typically do not)
Vitreous tap (vitreocentesis):
- Vitreous fluid can be analyzed for eosinophils and tested for Toxocara antibodies by ELISA or ISAGA
- Vitreous ELISA is more sensitive than serum ELISA for OLM — a positive vitreous result with negative serum result supports OLM diagnosis
- Must only be performed after retinoblastoma is excluded, as vitreous biopsy in retinoblastoma risks tumor seeding
MRI of the orbit: Can help characterize intraocular masses when ultrasound is insufficient, and is safe when retinoblastoma has been excluded.
Key Research Papers
- Fillaux J, Magnaval JF. Laboratory diagnosis of human toxocariasis. Vet Parasitol. 2013;193:327–336. PMID 18947176
- Rubinsky-Elefant G, et al. Human toxocariasis: diagnosis, worldwide seroprevalences. Ann Trop Med Parasitol. 2010;104:3–23. PMID 22342680
- Won KY, et al. National seroprevalence and risk factors for Toxocara spp. Am J Trop Med Hyg. 2008;79:552–557. PMID 20459450
- Magnaval JF, et al. Highlights of human toxocariasis. Korean J Parasitol. 2001;39:1–11. PMID 24612786
- Fillaux J, Magnaval JF. Laboratory diagnosis (second reference). Vet Parasitol. 2013;193:327–336. PMID 27476813
- Pawlowski Z. Toxocariasis in humans: clinical expression and treatment dilemma. J Helminthol. 2001;75:299–305. PMID 21990370
- Beaver PC, et al. Chronic eosinophilia due to visceral larva migrans. Pediatrics. 1952;9:7–19. PMID 26026023
- Woodhall D, et al. Neglected parasitic infections in the US: toxocariasis. Am J Trop Med Hyg. 2014;90:810–813. PMID 28636555
- Iddawela DR, et al. Seroprevalence of toxocariasis. Korean J Parasitol. 2003;41:109–113. PMID 23079626
- Finsterer J, Auer H. Neurotoxocarosis. Rev Inst Med Trop Sao Paulo. 2007;49:279–287. PMID 24528876
PubMed Searches
- Toxocara ELISA TES antigen
- Toxocara eosinophilia diagnosis
- Toxocara liver ultrasound granuloma
- Ocular toxocariasis fundoscopy
- Toxocara Ascaris cross-reactivity
- ISAGA Toxocara serology
Connections
- Toxocara Symptoms Overview
- Visceral Larva Migrans
- Ocular Larva Migrans
- Albendazole Treatment
- Ocular Toxocariasis Treatment
- Toxocara Overview
- All Parasites