Detection and Early Warning of West Nile Virus Circulation in Central Macedonia, Greece, Using Sentinel Chickens and Mosquitoes

Following the West Nile Virus (WNV) epidemic in 2010 in Central Macedonia, Greece, which resulted in 197 human neuroinvasive disease cases, we determined the seasonal appearance and prevalence of the virus in 2011 by testing weekly for WNV genomic RNA in mosquitoes collected in carbon dioxide-baited traps, and for anti-WNV antibodies in sentinel chickens. Preliminary findings of the surveillance program regarding the circulation of Nea Santa-Greece-2010 in sentinel chickens were rapidly communicated to public health authorities. In the present article, the full 2011 data produced by this surveillance program are presented. We detected enzootic circulation of WNV in chickens 1 month prior to the onset of the first human cases in 2011. Culex pipiens and Cx. modestus were abundant throughout the sampling period and at all sites of increased transmission. Molecular identification and phylogenetic analysis of WNV isolates from two chickens and one Cx. pipiens mosquito pool suggested that: (1) the virulent Nea Santa-Greece-2010 WNV lineage 2 strain responsible for the 2010 epidemic was actively circulating in 2011, and (2) all Greek isolates belong to a distinct recent evolutionary clade. In Europe, where numerous strains of different virulence coexist, sequencing information for WNV is important for phylogeography and identification of virulent strains for human health risk assessment.