Use of sentinel serosurveillance of mules and donkeys in the monitoring of West Nile virus infection

A cross-sectional study was carried out on clinically normal mules and donkeys in a region of southern Spain to assess the seroprevalence of West Nile virus (WNV) following detection of infection in contiguous horse and human populations. Antibodies against WNV were detected by a blocking ELISA and micro-virus neutralisation test in 12/165 (7.3%; CI95% 4.3–11.3) of the animals sampled. Even though the individual seroprevalence was higher in mules (9.6%; 8/83) than in donkeys (4.9%; 4/82), the difference was not statistically significant. Nine of 90 herds (10.0%; CI95% 3.8–16.2) contained at least one seropositive animal. Antibodies against WNV were also detected in 1/4 (25%) donkeys tested on three farms where WNV cases had been confirmed in horses. None of 26 potential explanatory variables was identified as a risk factor for seropositivity. Such serosurveillance of sentinel mules or donkeys may be a useful tool in the epidemiological monitoring of WNV in regions where horses are vaccinated.