Use of specific immunoglobulin G antibody avidity in the differential diagnosis of active and chronic Toxocara canis infections

To differentiate between acute and past Toxocara canis infections, a modified enzyme-linked immunosorbent assay, based on the dissociation of antigen-antibody complex with 6 M urea solution was used to measure levels of avidity of specific immunoglobulin G antibody against Toxocara canis excretory-secretory antigen. IgG avidity index was determined in the sera of 212 patients, divided into two groups with various stages of Toxocara infection. Patients were classified according to time period, from the onset of clinical symptoms or first positive serology result. Low IgG avidity index was found in the sera of 6.6% of patients in the acute phase, and all samples from the chronic stage of infection were characterized by high avidity values above 0.5 (mean 0.74). It was documented, that the avidity test for specific IgG antibodies is a valuable diagnostic method that helps to distinguish the early from the later phase of Toxocara canis infection. Indications for anti-parasitic treatment according to immunological activity of Toxocara infection were discussed.