Spread of West Nile Virus and Usutu Virus in the German Bird Population, 2019-2020

Abstract

West Nile virus (WNV) and Usutu virus (USUV) are important flaviviruses circulating in Germany. While USUV was first reported more than 10 years ago, WNV has only reached the country in 2018. Wild birds are important amplifying hosts for both viruses. Therefore, we have been monitoring the bird population in different regions of Germany by a previously established network for many years. This report summarizes the results of molecular and/or serological methods of 2345 blood samples from birds of 22 different orders and over 2900 bird carcasses from 2019 and 2020. USUV RNA circulation was found in different regions of Germany, with emphasis on USUV lineages Europe 3 and Africa 3. Increased evidence of USUV lineage Europe 2 was detected in eastern Germany. WNV RNA was found only in birds from the eastern part of the country. The seroprevalence for USUV was between 3.11% and 7.20% in all three regions investigated, whereas the WNV seroprevalence spanned from 14.77% to 16.15% in eastern Germany, with a noticeable tendency for a westward and southward expansion in both years. Thus, wild bird monitoring for WNV and USUV can serve as an early warning system for a human exposure risk.

Keywords: Germany; Usutu virus; West Nile virus; bird; flavivirus; monitoring.

Figure 1

Total number (n) of blood samples collected in 2019 and 2020 and sampled bird orders per sampling region A to C. Region A: northern and central-western part of Germany; region B: eastern and central-eastern part of Germany; region C: central and southern parts of Germany (ellipses). The main sample collectors (big red stars) and the minor sample collectors (small red stars) of the wild bird network are shown for each region.

Figure 2

Circulation of the different USUV lineages in Germany from 2017 to 2020. The map includes 66 new partial USUV sequences from 2019 and 2020 (blood samples from birds in our first panel = live bird monitoring, and organ samples from the second panel = dead bird monitoring) and 60 sequences from a previous study from 2017 to 2018 published by Michel et al. [22], as well as six full-genome USUV sequences from birds of the co-infection study by Santos et al. [57]. The different USUV lineages are depicted as colored triangles: red = Europe 3, blue = Africa 3, green = Africa 2, and purple = Europe 2. Different shades of each of these colors indicate the years of detection of each USUV lineage.

Figure 3

Distribution of blocking ELISA (bELISA) results across the three different sample regions (A–C) of Germany in 2019–2020. In the pie charts, the negative bELISA results are depicted in green, the reactive but doubtful results in yellow, and positive results in orange. The grey segments represent samples for which an initial bELISA screening was not possible due to small sample volume.

Figure 4

Number of positive bird serum samples verified by WNV and USUV VNT and the calculated seroprevalence for WNV and USUV in the three different study regions (A–C) in Germany in 2019 and 2020. The results for 2019 are shown on the left and for 2020 on the right. Serological results for 2019 include nine clinically affected free-ranging goshawks, which were published previously [56].

Figure 5

Detection of WNV-antibody-positive wild birds (resident and partial migrants) and their distribution in 2019 and 2020 in Germany. Light red stars represent the situation in 2019, dark red stars are the sites in 2020. The geographical distribution of the WNV-affected areas in Germany since the introduction of the virus in 2018 is shown in different shades of orange based on the extent of infection in each district, as defined by WNV-RNA-positive birds and WNV-RNA- and/or IgM-antibody-positive horses for the past three years (2018–2020). The figure includes nine serological results of ten free-ranging goshawks, clinically affected with WNV, which have already been published [56].

Figure 6

Database for USUV RNA detection in birds in Germany from 2019 and 2020. (A) Results from 2019: USUV-RNA-positive birds = 151 (orange dots) and USUV-RNA-negative birds = 1087 (green dots). (B) Results from 2020: USUV-RNA-positive birds = 48 (orange dots) and USUV-RNA-negative birds = 1690 (green dots).