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Toxocariasis is a zoonotic disease in that IgM titers can remain high for long periods making difficult to determine the stage of the disease. The aim of this study is to investigate the applicability of indirect ELISA, associated with urea, to discriminate between the acute and chronic toxocariasis. IgG avidity was evaluated in 25 BALB/c mice experimentally infected with 1000 Toxocara canis eggs. Blood samples were collected, and sera treated with 6 M urea and assayed by ELISA every two weeks. The percent IgG avidity was determined using the mean absorbance of sera treated with urea, divided by the mean absorbance of untreated sera. In the first 15 days post-inoculation, was observed a low percentage, between 7.25 and 27.5%, IgG avidity, characteristic of an acute infection. After 60 days of infection, all the mice showed between 31.4 and 58% IgG avidity, indicating a chronic infection.

Visceral toxocariasis is a neglected zoonosis caused by Toxocara canis larvae in unusual hosts. In dogs, the definitive host, the infection occurs mainly through transplacental and transcolostral transmission. Studies on experimental models have shown that vertical transmission may result from acute infections. Considering that toxocariasis is characterized as a chronic infection, with possible reactivation of larvae present in the brain, this study evaluated the presence of larvae in the brain of female BALB/c mice and their offspring with chronic infection during three successive pregnancies. ELISA-TES was used to evaluate the antibody levels. T. canis larvae were detected in the brain tissue of the mice during the three successive generations evaluated. The offspring's IgG level gradually decreased, and mean absorbance (ABS) above the cutoff point (0.070) was observed only at 30 (0.229) and 50 (0.096) days of age, while IgM was not detected. The infections in the offspring confirmed that vertical transmission of T. canis larvae occurred during chronic toxocariasis in three successive generations of mice.