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Human toxocariasis is a zoonosis caused by infection with larvae of the ascarid nematode Toxocara canis and, less frequently, T. cati. Our study developed a method for distinguishing distant from recent human toxocariasis by assessing the avidity of the IgG antibodies. The avidity of specific antibodies increases with time after antigen challenge and assessment of the degree of avidity can be used to discriminate between recent and distant infections. The relative avidity was measured in 150 sera from children with visceral toxocariasis and in 46 sera from children with ocular toxocariasis. The probable time of infection was estimated on the basis of the medical history and clinical syndrome. Our study showed that 94.2% of positive sera collected from patients reporting infection > 6 months ago had high IgG avidity values, confirming distant toxocariasis, whereas 25.9% of positive sera taken < 6 months after infection showed low indices of IgG avidity. Our results suggest that measurement of the specific IgG avidity may assist in discriminating between recent and distant toxocariasis. The method can be used effectively to rule out (because of high avidity) a recently acquired infection. Low avidity is less reliable in discriminating between recent and distant infections.

The level of specific anti-Toxocara IgG was examined in 343 children from central Poland with suspected Toxocara infection. Based on the presence of specific IgG antibodies and clinical signs and symptoms, toxocariasis was confirmed in 249 patients. The serological results of diagnosed cases were analysed in connection with epidemiological data (contamination of soil around the children's homes and presence of a dog or a cat). A higher prevalence of soil contamination with Toxocara eggs was found in suburban and rural areas (30.9% and 24.6%, respectively) whereas it was lower in urban areas (10.3%). In about 40% of the children whose households were not contaminated, a decrease in the antibody level was observed 2 years after treatment for toxocariasis, while there was no such decline in those living in contaminated places. The logistic regression model employed to determine the correlation between seropositivity in the children and selected epidemiological risk factors showed a statistically significant relationship in connection only with dog ownership (P = 0.0238). The present results have demonstrated the high risk of toxocariasis for children from rural and suburban areas in Poland. The sero-epidemiological investigations indicated that re-infection might be the reason for persistence of seropositivity after treatment.

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