Development of a recombinant nucleocapsid protein-based indirect ELISA for the detection of antibodies against porcine reproductive and respiratory syndrome virus in swine sera

Using recombinant nucleocapsid protein GST-N of porcine reproductive and respiratory syndrome virus (PRRSV)as antigen,an indirect ELISA for the detection of antibodies against PRRSV was developed.The optimal coat- ing concentration of antigen was determined by checkerboard titration and amounted to 400 ng of GST-N fusion protein per well,the serum sample for testing was diluted to 1:100 for detection and the cutoff was determined to be 0.43 D_(pos)+0.57 D_(neg).A total of 364 serum samples originating from 14 swine herds were detected by this recombinant nucleocapsid protein- based indirect ELISA,and the positive rate was 51.4%,slightly higher than that(49.7%)detected by HerdChek ELISA. Comparing this indirect ELISA with HerdChek ELISA kit,its sensitivity and specificity were 94.5%,91.3 % respectively, the correlation between these two ELISA methods was 92.9%,and the Youden index was 0.86.All data in this study demonstrated that the recombinant nucleocapsid protein-based indirect ELISA would be a very useful tool for routine diag- nostics,epidemiological surveys and outbreak investigations.