An IgG avidity ELISA to discriminate between recent and chronic Neospora caninum infection

The avidity of IgG antibodies directed to Neospora caninum was measured usi ng an IgG avidity enzyme-linked immunosorbent assay (ELISA) employing N. ca ninum proteins incorporated into immunostimulating complexes as antigen, in this ELISA, low-affinity antibodies were eluted by adding an incubation st ep with urea after the serum incubation. The antibody titers obtained with and without incubation with urea were then used to calculate the IgG avidit y values. Analysis of sequential sera collected from experimentally infecte d calves revealed that the avidity increased during the course of infection . Three weeks after infection, the IgG avidity was 9-18%, and 24 weeks late r it had increased to 58-76%. Cattle naturally infected for more than 6 mon ths all had avidities >50%. The results in this study, however preliminary, indicate that the IgG avidity ELISA can be used to discriminate between re cent and chronic N. caninum infections and may therefore be a valuable comp lement to IgG assays in epidemiologic studies of N. caninum infection in ca ttle.