Avidity of IgG class antibodies to Francisella tularensis in the course of tularemia in humans

Introduction: Tularemia is a highly infectious zoonotic disease caused by Gram-negative bacterium Francisella tularensis. The microbiological diagnosis of tularemia is based mainly on serological investigations. The present study was undertaken to determine the avidity of IgG class antibodies to Francisella tularensis in the course of tularemia in humans and to evaluate its value for estimation of the phase of diseases. Methods: Fifty two serum samples obtained from 40 patients with tularemia were tested by in-house ELISA in duplicate in the same plate, without and after the 0.5 h incubation with 8M urea. The age of the subjects was between 6 and 77 years. From one patient, a 9-years-old girl with oculoglandular form of tularemia, five serum samples were taken, respectively after 0.5, 1.5, 3, 6 and 12 months from the beginning of the first clinical symptoms. Results: The results of the study showed higher values of the avidity index (AI) of IgG antibodies for F. tularensis, often exceeding the value of 0.9, in children and adolescents than in adults. The examination of serum samples obtained 2-3 times in the course of tularemia from few patients did not show significant differences in the level of avidity index depending on the period of the disease. However, in five serum samples obtained from a 9-years-old girl in the different phases of tularemia the avidity index showed increasing values (0.51, 0.80, 0.92, 0.90 and 0.94, respectively). Conclusions: The avidity index of IgG may be helpful in excluding recent infection, but its usefulness in detecting an active phase of invasion requires further research.