Serum antibodies to West Nile virus in naturally exposed and vaccinated horses.

A polyvalent ELISA and plaque reduction neutralization tests (PRNTs) were used to measure serum antibodies to West Nile virus (WNV) in horses naturally exposed to or vaccinated against this flavivirus in Connecticut and New York State, USA. Relying on a PRNT as a 'gold standard', the main objective was to validate a modified ELISA containing a recombinant WNV envelope protein antigen. It was also important to assess specificity by testing sera from horses that had other, undiagnosed illnesses. Sera for the latter study were obtained from 43 privately owned horses during 1995-1996. Analyses by an ELISA and a PRNT confirmed the presence of WNV antibodies in 21 (91%) of 23 sera from naturally exposed horses and in 85% of the 20 vaccinated subjects; overall results for both study groups were highly concordant (91% agreement). Humoral responses of naturally exposed and immunized horses were similar. Both serological tests were useful in confirming past infections with WNV, but there was no evidence that horses with undiagnosed illnesses were exposed to WNV prior to a 1999 outbreak in Connecticut, USA.