Clinical usefulness of Western blotting and ELISA avidity for the diagnosis of human toxocariasis

The serodiagnosis of human toxocariasis is difficult. Specific IgGs detected routinely with ELISA based on Toxocara excretory-secretory (TES) antigens often persist for years at an elevated level, which does not allow either the differentiation between an active and persistent infection or monitoring of the effect of treatment. Additionally, false-positive results may occur in co-infections with other helminths due to cross-reactions. We evaluated the usefulness of an IgG avidity index (AI) and a Western blotting (WB) IgG in the diagnosis of patients suspected of Toxocara infection. We studied 138 subjects who were submitted to serological testing two or more times. Confirmation of an infection by WB was achieved in 73.2% of patients. A high AI was obtained in 89.1% of patients, and low AI and borderline AI were found in only 10.9%. Low and borderline values of AI remained at similar levels in subsequent studies over 2-3 years. The results showed the necessity of obligatory verification of all ELISA IgG positive and questionable results by WB. The index of IgG avidity may be helpful in excluding recent infection, but its usefulness in detecting an active phase of invasion requires further research.