Antibody detection tests are the
only means of confirmation of a clinical diagnosis of visceral larva migrans (VLM), ocular
larva migrans (OLM), and covert toxocariasis (CT), the most common clinical syndromes
associated with Toxocara infections. The currently recommended serologic
test for toxocariasis is enzyme immunoassay (EIA) with larval stage antigens extracted
from embryonated eggs or released in vitro by cultured infective larvae. The latter,
Toxocara excretory-secretory (TES) antigens, are preferable to larval extracts
because they are convenient to produce and because an absorption-purification step is not
required for obtaining maximum specificity. Evaluation of the true sensitivity and
specificity of serologic tests for toxocariasis in human populations is not possible
because of the lack of parasitologic methods to detect Toxocara parasites.
These inherent problems result in underestimations of sensitivity and specificity.
Evaluation of the Toxocara EIA in groups of patients with presumptive diagnoses
of VLM or OLM indicated sensitivity of 78% and 73%, respectively, at a titer of >1:32.
When the cutoff titer for OLM cases was lowered to 1:8, sensitivity was increased to
90%. Further confirmation of the specificity of the serologic diagnosis of OLM can
be obtained by testing aqueous or vitreous humor samples for antibodies. Specificity
has been reported to be >90% at a titer of >1:32. When interpreting the
serologic findings, clinicians must be aware that a measurable titer does not necessarily
indicate current clinical Toxocara canis infection. In most human
populations, a small number of those tested have positive EIA titers that apparently
reflect the prevalence of asymptomatic toxocariasis. In the United States, 2.8%
(cutoff titer 1:32) of nearly 9,000 persons tested showed such positive
reactions, but the percentage
but varied significantly according to age, race, and socioeconomic status.
Smith HV. Antibody
reactivity in human toxocariasis. In: Lewis JW, Maizels RM, editors. Toxocara and toxocariasis: clinical, epidemiological, and molecular perspectives.
London, UK: Institute of Biology and the British Society for Parasitology;
1993. p. 91-109.