Cross-reactivity of Japanese encephalitis virus-vaccinated horse sera in serodiagnosis of West Nile virus.

Flavivirus-infected sera are known to show cross-reactions in serodiagnoses of heterologous flavivirus infections. Japanese encephalitis virus (JEV) is endemic in Asia and, in Japan, many horses are vaccinated against JEV. However, the cross-reactivity level of JEV-vaccinated horse sera in the serodiagnosis of West Nile virus (WNV) has not been clarified. The antibody cross-reactivity of JEV-vaccinated horse sera in WNV serological tests, such as the plaque reduction neutralization test (PRNT), IgG indirect ELISA (IgG-ELISA) and hemagglutination inhibition (HI) test, was examined. All JEV-vaccinated horse sera were positive for JEV antibodies with JEV PRNT at both 90% and 50% plaque reductions. In WNV PRNT, 16.7% of the horses were positive at 90% plaque reduction, and 50% of the horses were positive at 50% plaque reduction. All the JEV-vaccinated horse sera showed positive-to-negative (P/N) ratios of over 2.0 with JEV IgG-ELISA, and half of them had P/N ratios of over 2.0 with WNV IgG-ELISA. There was little difference between the JEV HI and WNV HI titers in individual horses. These results indicate that in serosurveillance of WNV, JEV-vaccinated horses can produce false-positive results in WNV IgG-ELISA, HI and PRNT.