Serosurvey for West Nile virus in horses in southern France

whether there was frequent virus circulation in the surveyarea; if this were the case, the seroprevalence should increasewith the age of the horses. However, for this trend to beobservable, the renewal rate of the animals must be reason-ably low and homogenous. In order to meet these require-ments, only serum samples taken from working horses(arbitrarily defined as horses up to 18 years of age) housedin riding schools were included in the analysis of age-specificseroprevalences. The corresponding dataset contained 504records.Maps were produced with MapInfo software, and the statistical analyses were done using R (Ihaka and Gentleman1996).Of the blood samples taken from 906 horses, 305 were