A two-graph Receiver Operating Characteristic analysis was done to determine the optimal cut-off value of an indirect enzyme-linked immunosorbent assay (ELISA) for detection of antibodies to Wesselsbron disease (WSL) virus. When ELISA and haemagglutination-inhibition (HI) results of WSL-positive and WSL-negative control sheep sera were compared, the sensitivity of ELISA was 97,9% and that of HI, 87,5%, while the specificity of ELISA and HI were 95,7% and 100 %, respectively. The ELISA's superior sensitivity was confirmed by the results of the two assays performed on a simulated range of low-positive sera, which showed that the ELISA was able to detect WSL-antibody levels at least ten times lower than those the HI could. The ELISA was also less cross-reactive than the HI to guinea-pig antisera against nine flaviviruses relevant to southern Africa. The combination of the ELISA's ability to test untreated sera in single dilution and its superior sensitivity and lower crossreactivity as compared to that of the traditional HI, makes it the assay of choice for automation and large-scale screening of animals for antibodies to WSL virus.
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