The antibody cross-reactivity of Japanese encephalitis virus (JEV)-vaccinated horse sera in West Nile virus (WNV) serological tests such as the plaque reduction neutralization test (PRNT), IgG indirect enzyme-linked immunosorbent assay (IgG-ELISA), and hemagglutination-inhibition (HI) test were examined. All JEV-vaccinated horse sera were positive for JEV antibodies in the JEV PRNT at both 90% and 50% plaque reduction. In WNV PRNT, 16.7% and 50% of horse sera were positive at 90% and 50% plaque reduction, respectively. However, the PRNT titer of JEV was at least 5-fold higher than that of WNV in all cross-neutralized sera at both 90% and 50% plaque reduction. All JEV-vaccinated horse sera showed positive-to-negative (P/N) ratios of >2.0 in the JEV IgG-ELISA. In addition, half of them had P/N ratios of >2.0 in the WNV IgG-ELISA. In contrast, there were little differences between the JEV and WNV HI titer in individual horse sera. These results suggest that comparisons of the antibody titers in PRNT might be useful to differentiate between JEV-vaccinated and WNV-infected horses.
(Research Team for Advanced Biologicals, TEL +81-29-838-7708)