Significance of IgG avidity test in diagnosis of west Nile virus infection

Abstract

Introduction. Serological tests appear to be the method of choice for establishing the diagnosis in the late phase of West Nile virus infection. Long persistence of IgM antibodies against West Nile virus is described and may be a problem for determination of the time of acquisition of West Nile virus infection. The aim of the study was to estimate the significance of IgG avidity determination in establishing the diagnosis of West Nile virus infection. Material and Methods. In a study 56 serum samples seropositive against West Nile virus were included. 24 serum samples were collected in 2012 from healthy residents of South-Backa district and 32 serum samples were collected in 2014 from 124 patients suspected of having West Nile virus infection. Commercial enzyme-linked immunosorbent tests were used for the detection of West Nile virus-specific IgM and IgG antibodies and IgG avidity. Results. Out of 124 patients suspected of having West Nile virus infection, 32 (25.8%) were seropositive for West Nile virus antibodies. Acute infection was laboratory confirmed in 15 (46.9%) cases. All patients with acute infection were West Nile virus IgM positive, 13 (85%) were West Nile virus IgG positive, and 2 (15%) had a borderline result for West Nile virus IgG antibodies. Out of 32 seropositive patients the presence of IgM antibodies was determined in 22 (68.7%). In a group of samples with high IgG avidity values, 6 were IgM positive, while 8 were IgM negative. Conclusion. West Nile virus IgM and IgG antibody serological assays alone are not sufficient for the accurate and reliable diagnosis of WNV infection. West Nile virus IgG avidity testing is necessary to ensure the differential diagnosis of acute from past West Nile virus infection.