Seroprevalence of West Nile virus in horses in different Moroccan regions.

West Nile virus-associated disease is one of the most widespread vector-borne diseases in the world. In Morocco, the first cases were reported in horses in 1996 and the disease re-emerged in 2003 and in 2010. The objective of this work was to study the epidemiological situation of WNV-associated infection in Morocco, by quantifying the seroprevalence of anti-WNV IgM and IgG antibodies in horses in different bioclimatic regions-zones of Morocco in 2011. During the months of May, June and July 2011, 840 serum samples were collected from horses in four regions characterized by different environmental and climatic features such as altitude, temperature and precipitation. These environmental-climatic regions are: the Atlantic plateaus of the Gharb and pre-Rif region, the North Atlasic plains and plateaus region, the Atlas Mountains and pre-Atlas region and the plains and plateaus of the Oriental region. All samples were tested for the anti-WNV IgG antibodies by ELISA and positive sera were confirmed by virus neutralization (VN). An anti-WNV antibody prevalence map was developed. A total of 261 samples (31%) were found positive by both techniques. The prevalence of the infection was higher in the Atlantic plateaus of the Gharb and pre-Rif region, in the northern part of the country. Available data concerning the previous WNV-associated disease outbreaks in Morocco and the preliminary results of this serological survey suggest that the Moroccan northwest is the region at highest risk for WNV circulation. In this region, the climate is more humid with higher rainfall than other regions and milder winter temperatures exist. In the same area, the presence of migratory bird settlements may affect the risk of virus introduction and amplification.