The summer of 2018 in Germany was the second hottest and driest on record. These generally extremely favorable climatic conditions most likely triggered the further expansion and the efficient propagation of the zoonotic arthropod-borne West Nile virus in many Southern/Southeastern and even Central European countries. WNV infections were detected for the first time in resident wild and aviary birds, such as common blackbirds, northern goshawks and great grey owls in Eastern and Southeastern Germany. The causative WNV strain belonged to the central European subclade II. Phylogeographic analysis indicated a single introduction event of WNV into Germany, most likely in 2016 from Czech Republic, and also a unique non-synonymous mutation in the NS3 gene. Extraordinary high temperatures in 2018 presumably led to decreased averaged extrinsic incubation period values for WNV in mosquitoes, leading to rapid virus amplification and greater transmission risk for vertebrates in Germany. Blood transfusion services and clinicians in Germany should be aware of these possible WNV infection risks in humans especially during late summer.

In the summer of 2016, Belgium, France, Germany and the Netherlands reported widespread Usutu virus (USUV) activity based on live and dead bird surveillance. The causative USUV strains represented four lineages, of which two putative novel lineages were most likely recently introduced into Germany and spread to other western European countries. The spatial extent of the outbreak area corresponded with R₀ values > 1. The occurrence of the outbreak, the largest USUV epizootic registered so far in Europe, allowed us to gain insight in how a recently introduced arbovirus with potential public health implications can spread and become a resident pathogen in a naïve environment. Understanding the ecological and epidemiological factors that drive the emergence or re-emergence of USUV is critical to develop and implement timely surveillance strategies for adequate preventive and control measures. Public health authorities, blood transfusion services and clinicians in countries where USUV was detected should be aware of the risk of possible USUV infection in humans, including in patients with unexplained encephalitis or other neurological impairments, especially during late summer when mosquito densities peak.

Usutu virus (USUV), one of the most neglected Old World encephalitic flaviviruses, causes epizootics among wild and captive birds and sporadic infection in humans. The dynamics of USUV spread and evolution in its natural hosts are unknown. Here, we present the phylogeny and evolutionary history of all available USUV strains, including 77 newly sequenced complete genomes from a variety of host species at a temporal and spatial scaled resolution. The results showed that USUV can be classified into six distinct lineages and that the most recent common ancestor of the recent European epizootics emerged in Africa at least 500 years ago. We demonstrated that USUV was introduced regularly from Africa into Europe in the last 50 years, and the genetic diversity of European lineages is shaped primarily by in situ evolution, while the African lineages have been driven by extensive gene flow. Most of the amino acid changes are deleterious polymorphisms removed by purifying selection, with adaptive evolution restricted to the NS5 gene and several others evolving under episodic directional selection, indicating that the ecological or immunological factors were mostly the key determinants of USUV dispersal and outbreaks. Host-specific mutations have been detected, while the host transition analysis identified mosquitoes as the most likely origin of the common ancestor and birds as the source of the recent European USUV lineages. Our results suggest that the major migratory bird flyways could predict the continental and intercontinental dispersal patterns of USUV and that migratory birds might act as potential long-distance dispersal vehicles. IMPORTANCE Usutu virus (USUV), a mosquito-borne flavivirus of the Japanese encephalitis virus antigenic group, caused massive bird die-offs, mostly in Europe. There is increasing evidence that USUV appears to be pathogenic for humans, becoming a potential public health problem. The emergence of USUV in Europe allows us to understand how an arbovirus spreads, adapts, and evolves in a naive environment. Thus, understanding the epidemiological and evolutionary processes that contribute to the emergence, maintenance, and further spread of viral diseases is the sine qua non to develop and implement surveillance strategies for their control. In this work, we performed an expansive phylogeographic and evolutionary analysis of USUV using all published sequences and those generated during this study. Subsequently, we described the genetic traits, reconstructed the potential pattern of geographic spread between continents/countries of the identified viral lineages and the drivers of viral migration, and traced the origin of outbreaks and transition events between different hosts.

One year after the first autochthonous transmission of West Nile virus (WNV) to birds and horses in Germany, an epizootic emergence of WNV was again observed in 2019. The number of infected birds and horses was considerably higher compared to 2018 (12 birds, two horses), resulting in the observation of the first WNV epidemy in Germany: 76 cases in birds, 36 in horses and five confirmed mosquito-borne, autochthonous human cases. We demonstrated that Germany experienced several WNV introduction events and that strains of a distinct group (Eastern German WNV clade), which was introduced to Germany as a single introduction event, dominated mosquito, birds, horse and human-related virus variants in 2018 and 2019. Virus strains in this clade are characterized by a specific-Lys2114Arg mutation, which might lead to an increase in viral fitness. Extraordinary high temperatures in 2018/2019 allowed a low extrinsic incubation period (EIP), which drove the epizootic emergence and, in the end, most likely triggered the 2019 epidemic. Spatiotemporal EIP values correlated with the geographical WNV incidence. This study highlights the risk of a further spread in Germany in the next years with additional human WNV infections. Thus, surveillance of birds is essential to provide an early epidemic warning and thus, initiate targeted control measures.

To monitor adult mosquitoes several trapping devices are available. These are differently constructed and use various mechanisms for mosquito attraction, thus resulting in different trapping sensitivities and efficacies for the various species. Mosquito monitoring and surveillance programs in Europe use various types of mosquito traps, but only a few comparisons have been conducted so far. This study compared the performance of four commercial trapping devices, which are commonly used in Europe.

Mosquitoes collected in Germany in 2016, including Culex pipiens pipiens biotype pipiens, Culex torrentium and Aedes albopictus, as well as Culex pipiens pipiens biotype molestus (in colony since 2011) were experimentally infected with Zika virus (ZIKV) at 18 °C or 27 °C. None of the Culex taxa showed vector competence for ZIKV. In contrast, Aedes albopictus were susceptible for ZIKV but only at 27 °C, with transmission rates similar to an Aedes aegypti laboratory colony tested in parallel.