Neutralizing antibodies for sandfly fever Naples virus in human sera on the island of Mljet.

Sera from apparently healthy residents of various age from different localities of the Adriatic island of Mljet were tested by the plaque reduction neutralization test (PRNT) for antibodies to sandfly fever Naples virus (SFN). An average of 51.4% of the sera examined was found to have neutralizing antibodies. Reactors to SFN virus were found in all the age groups examined and the antibody prevalence appeared to increase with age. The presence of SFN neutralizing antibodies in all age groups indicates that the virus must have been present endemically and that it is still active on the island of Mljet.