Epidemic Spread of Usutu Virus in Southwest Germany in 2011 to 2013 and Monitoring of Wild Birds for Usutu and West Nile Viruses

Mosquito-borne viruses are becoming an increasing threat for Europe. One of these viruses is Usutu virus (USUV), a single-stranded RNA virus belonging to the Japanese encephalitis virus group within the family Flaviviridae. Since the occurrence of USUV among wild birds in June, 2011, infected Blackbirds (Turdus merula) have frequently been found dead in southwest Germany, cumulating in a massive die-off. Moreover, other bird species (Strigiformes) in this region have been affected. In a first study, 209 of over 600 dead birds (wild birds and birds kept in aviaries) collected from 2011 to 2013 carried USUV, more than 88% of them Blackbirds. USUV had already been detected in 2010, one year before the epizooty, in a mosquito-based surveillance program in Germany. The main epidemic area of the USUV outbreak in wild birds in southwest Germany has been similar for the last three years. In a second study during 2011 to 2013, 902 live migratory and resident birds (representing 87 bird species belonging to 14 bird orders) from four different sampling sites were bled and tested serologically and by qPCR for West Nile virus (WNV) and USUV infections. No USUV or WNV genomes were detected. Some migratory birds (mainly long-distance migrants and some partial migrants) carried neutralizing antibodies against WNV as discriminated by USUV and WNV cross-neutralization tests. Only few resident birds showed relevant USUV-specific neutralizing antibodies. The occurrence of USUV in the Upper Rhine valley area of southwest Germany is a proof of principle for the incursion and spread of other arthropod-borne (arbo)-viruses along these routes. Therefore, monitoring studies in birds and mosquitoes for the presence of arboviruses in these areas are indispensable.