West Nile virus past infections in the general population of Southern Spain

To analyze the prevalence of past and recent infections by West Nile virus (WNV) and the risk factors associated with WNV exposure in a representative population from southern Spain.

Sample size was established for an estimated prevalence of past WNV infections of 5 +/- 2.5% in 504 subjects. A pre-stratification was performed according to age distribution and place of residence. After random telephone solicitation and acquisition of informed consent, a serum sample was collected and an epidemiologic survey performed on all participating subjects. Samples were tested with ELISA-IgG and MAC-ELISA to detect specific IgG and IgM antibodies; results were confirmed by the plaque reduction neutralization test (PRNT). Multivariate analysis using a forward stepwise logistic regression model was performed to assess potential risk factors associated with WNV exposure.

Prevalence of past WNV infections confirmed by PRNT in the 504 participants was 0.6%, affecting mainly older persons (mean age 65 +/- 23 vs. 34 +/- 22 years; P = 0.018), those living in rural areas (5.4% vs. 0% in urban areas; P = 0.01), and individuals with risk professions (prevalence 2.8% vs. 0%; P = 0.048). None of the five recent infections detected by MAC-ELISA was confirmed by PRNT.

These results strongly suggest past circulation and exposure of the human population to WNV in southern Spain.