Usutu virus infection in aviary birds during the cold season.

The mosquito-borne flavivirus Usutu virus (USUV) is responsible for countless deaths in both resident populations and birds kept in outdoor aviaries. Since 2001, USUV outbreaks have attracted increased attention due to the rapid geographical spread of the virus and its close relationship to West Nile virus (WNV), an emerging pathogen in humans and animals. Similar to WNV, the USUV enzootic transmission cycle predominantly involves Culex spp. as vectors, whereas birds serve as amplifying reservoir hosts. In Europe, USUV-associated disease outbreaks in birds are almost exclusively described during late spring and early autumn (early April to late October). Contagiousness of virus particles excreted by infected birds has not yet been proven, so that the role of non-vector-borne transmission, as it is known for the closely related WNV, remains unclear. Here we report the diagnosis of USUV infection in 15 of 24 birds from mortality outbreaks that occurred during the cold season between late October 2018 and early April 2019, in eight different aviaries located in Germany. Detection of USUV was performed using standardized molecular biological methods and immunohistochemistry for verification of the infection. USUV infection in a parrot species, a tropical finch and two estrildid finches are reported for the first time. Further research on the occurrence of USUV infection during the cold season is key to understanding the dynamics of viral transmission as well as for a profound health risk assessment for aviary birds as well as humans.

Toscana, West Nile, Usutu and tick-borne encephalitis viruses: external quality assessment for molecular detection of emerging neurotropic viruses in Europe, 2017.

BackgroundNeurotropic arboviruses are increasingly recognised as causative agents of neurological disease in Europe but underdiagnosis is still suspected. Capability for accurate diagnosis is a prerequisite for adequate clinical and public health response.AimTo improve diagnostic capability in EVD-LabNet laboratories, we organised an external quality assessment (EQA) focusing on molecular detection of Toscana (TOSV), Usutu (USUV), West Nile (WNV) and tick-borne encephalitis viruses (TBEV).MethodsSixty-nine laboratories were invited. The EQA panel included two WNV RNA-positive samples (lineages 1 and 2), two TOSV RNA-positive samples (lineages A and B), one TBEV RNA-positive sample (Western subtype), one USUV RNA-positive sample and four negative samples. The EQA focused on overall capability rather than sensitivity of the used techniques. Only detection of one, clinically relevant, concentration per virus species and lineage was assessed.ResultsThe final EQA analysis included 51 laboratories from 35 countries; 44 of these laboratories were from 28 of 31 countries in the European Union/European Economic Area (EU/EEA). USUV diagnostic capability was lowest (28 laboratories in 18 countries), WNV detection capacity was highest (48 laboratories in 32 countries). Twenty-five laboratories were able to test the whole EQA panel, of which only 11 provided completely correct results. The highest scores were observed for WNV and TOSV (92%), followed by TBEV (86%) and USUV (75%).ConclusionWe observed wide variety in extraction methods and RT-PCR tests, showing a profound absence of standardisation across European laboratories. Overall, the results were not satisfactory; capacity and capability need to be improved in 40 laboratories.

Human Usutu Virus Infection with Atypical Neurologic Presentation, Montpellier, France, 2016.

Infection with Usutu virus (USUV) has been recently associated with neurologic disorders, such as encephalitis or meningoencephalitis, in humans. These findings indicate that USUV is a potential health threat. We report an acute human infection with USUV in France putatively associated with a clinical diagnosis of idiopathic facial paralysis.

Usutu Virus RNA in Mosquitoes, Israel, 2014-2015.

We identified Usutu virus (USUV) RNA in 6 pools of mosquitoes trapped in northern Israel during 2014-2015. These Israeli strains were most similar to strains identified in Senegal and Germany, which further elucidates common ancestry and evolutionary dynamics of USUV. Our findings suggest that human infection with USUV might occur in Israel.

Investigation into Usutu and West Nile viruses in ticks from wild birds in Northwestern Italy, 2012-2014.

To assess the potential role of ticks as carriers of West Nile virus (WNV) and Usutu virus (USUV), we tested 1721 ticks from 379 wild birds in Northwestern Italy between 2012 and 2014. Ticks were analyzed in pools using a pan-flavivirus real-time RT-PCR and positive pools were subjected to RT-PCR for USUV and WNV genome detection. All the tested samples resulted negative, suggesting that Ixodes spp. ticks, at least in our study area, are not competent vectors and not even exploitable sentinels for USUV and WNV.

Lack of usutu virus RNA in cerebrospinal fluid of patients with encephalitis of unknown etiology, Tuscany, Italy.

Usutu virus (USUV) is an African mosquito-borne flavivirus associated with human neurological disorders in Europe. Recently, USUV introduction in Europe has been traced back to Eurasian blackbirds deaths in the Tuscany region of Italy in 1996. Ninety-six cerebrospinal fluid (CSF) samples from patients with encephalitis of unknown etiology diagnosed in 2010-2013 were screened to determine whether USUV circulates in humans in Tuscany. Using real-time polymerase chain reaction, no positive patient was found. USUV does not seem to cause neuroinvasive disorders in humans in Tuscany.

Undetection of vector-borne viruses in equids of Galapagos Islands.

Domestic species, including equids, were introduced in the Galapagos Islands in the XIX century. Equine vector-borne diseases are circulating in South America but their occurrence in the Galapagos Island was unknown. The objective of this study was to detect the occurrence of West Nile virus (WNV), Usutu virus (USUV) and equine infectious anemia virus (EIAV) in the four Galapagos Islands raising equids if they were present at a prevalence >1%. Serum samples were collected from 411 equids belonging to 124 owners from April to July 2019. All the results were negative to the ELISA tests used suggesting that WNV, USUV and EIAV are not circulating in the equine population of the Galapagos Islands.

Vector competence of Swedish Culex pipiens mosquitoes for Usutu virus.

Usutu virus (USUV) is an emerging mosquito-borne flavivirus with increasing prevalence in Europe. Understanding the role of mosquito species in USUV transmission is crucial for predicting and controlling potential outbreaks. This study aimed to assess the vector competence of Swedish Culex pipiens for USUV. The mosquitoes were orally infected with an Italian strain of USUV (Bologna 2009) and infection rates (IR), dissemination rates (DR), and transmission rates (TR) were evaluated over 7 to 28 days post-infection. The study revealed that Swedish Cx. pipiens are susceptible to USUV infection, with a gradual decrease in IR over time. However, the percentage of mosquitoes with the ability to transmit the virus remained consistent across all time points, indicating a relatively short extrinsic incubation period. Overall, this research highlights the potential of Swedish Cx. pipiens as vectors for USUV and emphasizes the importance of surveillance and monitoring to prevent future outbreaks of mosquito-borne diseases.

Development of a live-attenuated chimeric vaccine against the emerging Usutu virus.

Usutu virus (USUV) is an emerging arthropod-borne flavivirus that has expanded into multiple European countries during the past several decades. USUV infection in human has been linked to severe neurological complications, and no vaccine is now available against USUV. In this work, we develop a live-attenuated chimeric USUV vaccine (termed ChinUSUV) based on the full-length infectious cDNA clone of the licensed Japanese encephalitis virus (JEV) vaccine strain SA14-14-2. In vitro studies demonstrate that ChinUSUV replicates efficiently and maintains its genetic stability. Remarkably, ChinUSUV exhibits a significant attenuation phenotype in multiple mouse models even compared with the licensed JEV vaccine. A single immunization with ChinUSUV elicits potent IgG and neutralizing antibody responses as well as T cell response. Passive transfer of sera from ChinUSUV-immunized mice confers significant protection against lethal homologous challenge in suckling mice. Taken together, our results suggest that ChinUSUV represents a potential USUV vaccine candidate that merits further development.

A conserved role for AKT in the replication of emerging flaviviruses in vertebrates and vectors.

One third of all emerging infectious diseases are vector-borne, with no licensed antiviral therapies available against any vector-borne viruses. Zika virus and Usutu virus are two emerging flaviviruses transmitted primarily by mosquitoes. These viruses modulate different host pathways, including the PI3K/AKT/mTOR pathway. Here, we report the effect on ZIKV and USUV replication of two AKT inhibitors, Miransertib (ARQ-092, allosteric inhibitor) and Capivasertib (AZD5363, competitive inhibitor) in different mammalian and mosquito cell lines. Miransertib showed a stronger inhibitory effect against ZIKV and USUV than Capivasertib in mammalian cells, while Capivasertib showed a stronger effect in mosquito cells. These findings indicate that AKT plays a conserved role in flavivirus infection, in both the vertebrate host and invertebrate vector. Nevertheless, the specific function of AKT may vary depending on the host species. These findings indicate that AKT may be playing a conserved role in flavivirus infection in both, the vertebrate host and the invertebrate vector. However, the specific function of AKT may vary depending on the host species. A better understanding of virus-host interactions is therefore required to develop new treatments to prevent human disease and new approaches to control transmission by insect vectors.

Longitudinal Study of the Occurrence of Usutu Virus and West Nile Virus Infections in Birds in a Zoological Garden in Northern Germany.

Usutu virus (USUV) and West Nile virus (WNV) are known to cause diseases and mortalities in bird populations. Since 2010/2011, USUV has circulated in Germany and spread nationwide, while WNV was only introduced into East Germany in 2018. The zoological garden investigated is located in Northern Germany, where USUV infections in wild birds have been detected for several years. In this longitudinal study conducted over a four-year period, zoo birds were sampled biannually and screened for molecular and serological evidence of USUV and WNV. USUV genomes were detected in eight of the sampled birds and whole-genome sequences revealed the circulation of USUV lineages Europe 3 and Africa 3. Of the eight birds infected with USUV during the study period, four died after the infection, while four survived without displaying clinical signs. Furthermore, in a few of the birds, a USUV (re-)infection was confirmed on a serological level with three birds producing USUV-neutralizing antibodies (nAbs) over a period of four years. Nonetheless, in two birds sampled throughout this longitudinal study, neither a USUV nor a WNV infection was evident. In 2022, WNV nAbs were detected for the first time in a juvenile zoo bird, indicating the introduction of the virus into this region.

Serological evidence of circulation of West Nile virus in equids in Algerian eastern drylands and its epidemiological risk factors.

In order to determine the prevalence of equine infectious anemia virus (EIAV), Usutu virus (USUV), and West Nile virus (WNV) in eastern Algerian drylands, 340 sera from distinct equids have been collected from 2015 to 2017. Serological analysis for the presence of antibodies against EIAV and flaviviruses was performed using commercially available ELISAs. Sera detected positive, doubtful, or negative close to the doubtful threshold in flavivirus ELISA were tested by the virus neutralization test (VNT), using WNV and USUV strains. The prevalence of WNV antibodies with ELISA was 11.47% (39/340) against 13.53% (46/340) by WNV VNT. EIAV antibodies were not detected in any samples. WNV seroprevalence varies with species, breed and location of horses. Only, one equid was positive for both WNV and USUV neutralizing antibodies. This is the first screening on equids sera of EIAV and USUV in Algeria. This study indicate that WNV and possibly USUV have circulated/are circulating in the Algerian equine population, unlike EIAV does not seem to be present.

Continuous surveillance of potentially zoonotic avian pathogens detects contemporaneous occurrence of highly pathogenic avian influenza viruses (HPAIV H5) and flaviviruses (USUV, WNV) in several wild and captive birds.

Three avian viral pathogens circulate in Germany with particular importance for animal disease surveillance due to their zoonotic potential, their impact on wild bird populations and/or poultry farms: Highly pathogenic (HP) avian influenza virus (AIV) of subtype H5 (HPAIV H5), Usutu virus (USUV), and West Nile virus (WNV). Whereas HPAIV H5 has been mainly related to epizootic outbreaks in winter, the arthropod-borne viruses USUV and WNV have been detected more frequently during summer months corresponding to peak mosquito activity. Since 2021, tendencies of a potentially year-round, i.e. enzootic, status of HPAIV in Germany have raised concerns that Orthomyxoviruses (AIV) and Flaviviruses (USUV, WNV) may not only circulate in the same region, but also at the same time and in the same avian host range. In search of a host species group suitable for a combined surveillance approach for all mentioned pathogens, we retrospectively screened and summarized case reports, mainly provided by the respective German National Reference Laboratories (NRLs) from 2006 to 2021. Our dataset revealed an overlap of reported infections among nine avian genera. We identified raptors as a particularly affected host group, as the genera Accipiter, Bubo, Buteo, Falco, and Strix represented five of the nine genera, and highlighted their role in passive surveillance. This study may provide a basis for broader, pan-European studies that could deepen our understanding of reservoir and vector species, as HPAIV, USUV, and WNV are expected to further become established and/or spread in Europe in the future and thus improved surveillance measures are of high importance.

Validation of a Reporter Cell Line for Flavivirus Inhibition Assays.

Here, we report the validation of a new reporter cell line, Hec1a-IFNB-Luc, for use in inhibition studies of various flaviviruses relevant to human pathology. The reporter system allows the detection of viral replication after luciferase gene activation driven by an interferon beta (IFN-ß) promoter. We found the reporter cell line to be highly responsive to all 10 flaviviruses tested, including the 4 dengue virus serotypes. The applicability of the Hec1a-IFNB-Luc reporter cell line for serodiagnostic purposes in neutralizing antibody assays was confirmed by comparison of its sensitivity and specificity to those of "gold-standard," clinically applied, cytopathic effect-based assays, showing comparable performances. The reporter cell line used for the assessment of viral inhibition by small-molecule antiviral compounds was also confirmed, and the sensitivity of the Hec1a-IFNB-Luc reporter cell line was compared to those from published data reporting on the activity of the antivirals in various other assays, indicating that the Hec1a-IFNB-Luc reporter cell line allowed the determination of the inhibitory capacity at least as sensitive as alternative assays. By measuring luciferase activity as a proxy for viral replication, the reporter cell line allows early detection, reducing the time to results from often 5 to 7 days to 3 days, without the need for optical inspection of cytopathic effects, which often differ between viruses and cell lines, streamlining the development of flavivirus assays. IMPORTANCE The Hec1a-IFNB-Luc reporter cell line allows the detection of all 10 flaviviruses tested, including the 4 dengue virus serotypes. Its use for serodiagnostic purposes, measuring neutralizing antibody activity in sera, and the assessment of the antiviral activities of small-molecule compounds was confirmed, and it was found to be comparable to clinically applied assays. The Hec1a-IFNB-Luc reporter cell line allows the rapid and quantitative determination of antiviral effects on multiple human pathological flaviviruses using a single protocol.

First Human Usutu Virus Reported in Asti (Piedmont, Italy, August 2022) and Early Follow-Up.

The Usutu virus (USUV) has recently attracted the attention of scientists because of its rapid spread across Europe and its growth over the previous seasons in Italy. Here, we describe the first case of USUV infection in Asti, Piedmont region, Italy. The patient remained asymptomatic in the acute phase and during the early follow-up, despite a mild increase in liver enzymes. The prompt diagnosis in this patient was due to positive qualitative PCR for WNV blood-donor screening with negative RT-PCR of WNV and positive USUV-RNA following the confirmation test. Blood-donor screening and transmission risk monitoring are pivotal in following the spread of this Flavivirus in non-endemic countries, due to the high percentage of asymptomatic carriers.

Expedited retrieval of high-quality Usutu virus genomes via Nanopore sequencing with and without target enrichment.

Usutu virus (USUV) is a mosquito-borne zoonotic virus and one of the causes of flavivirus encephalitis in birds and occasionally in humans. USUV rapidly disperses in a susceptible host and vector environment, as is the case in South and Central Europe. However, compared to other flaviviruses, USUV has received less research attention and there is therefore limited access to whole-genome sequences and also to in-depth phylogenetic and phylodynamic analyses. To ease future molecular studies, this study compares first- (partial sequencing via Sanger), second- (Illumina), and third-generation (MinION Nanopore) sequencing platforms for USUV. With emphasis on MinION Nanopore sequencing, cDNA-direct and target-enrichment (amplicon-based) sequencing approaches were validated in parallel. The study was based on four samples from succumbed birds commonly collected throughout Germany. The samples were isolated from various sample matrices, organs as well as blood cruor, and included three different USUV lineages. We concluded that depending on the focus of a research project, amplicon-based MinION Nanopore sequencing can be an ideal cost- and time-effective alternative to Illumina in producing optimal genome coverage. It can be implemented for an array of lab- or field-based objectives, including among others: phylodynamic studies and the analysis of viral quasispecies.

The envelope protein of Usutu virus attenuates West Nile virus virulence in immunocompetent mice.

West Nile virus (WNV) and Usutu virus (USUV) are the two most widespread mosquito-borne flaviviruses in Europe causing severe neuroinvasive disease in humans. Here, following standardization of the murine model with wild type (wt) viruses, we engineered WNV and USUV genome by reverse genetics. A recombinant virus carrying the 5' UTR of WNV within the USUV genome backbone (r-USUV5'-UTR WNV) was rescued; when administered to mice this virus did not cause signs or disease as wt USUV suggesting that 5' UTR of a marked neurotropic parental WNV was not per se a virulence factor. Interestingly, a chimeric virus carrying the envelope (E) protein of USUV in the WNV genome backbone (r-WNVE-USUV) showed an attenuated profile in mice compared to wt WNV but significantly more virulent than wt USUV. Moreover, except when tested against serum samples originating from a live WNV infection, r-WNVE-USUV showed an identical antigenic profile to wt USUV confirming that E is also the major immunodominant protein of USUV.

A Bacterially-Expressed Recombinant Envelope Protein from Usutu Virus Induces Neutralizing Antibodies in Rabbits.

BACKGROUND: Recently, an emerging flavivirus, Usutu virus (USUV), has caused an epidemic among birds in Europe, resulting in a massive die-off in Eurasian blackbirds. Currently found only in Europe and Africa, it can be envisioned that Usutu virus will follow the path of other flaviviruses, like West Nile virus and Zika virus, and will spread via its mosquito vectors and bird hosts to other parts of the world. Several cases of human infections by Usutu virus have already been published. Anticipating this spread, development of an efficacious vaccine would be highly desirable. METHOD: This study describes the production in E. coli, purification, and refolding of a partial USUV envelope protein. Prior to immunization, the protein was characterized using size exclusion chromatography, transmission electron microscopy and dynamic light scattering, showing the limited presence of virus-like structures, indicating that the protein solution is probably a mixture of mono and multimeric envelope proteins. RESULTS: Immunizations of two rabbits with the refolded E-protein fraction, mixed with a strong adjuvant, resulted in the generation of neutralizing antibodies, as evidenced in an in vitro assay. DISCUSSION: The way forward towards a subunit vaccine against Usutu virus infection is discussed.

Akt Interacts with Usutu Virus Polymerase, and Its Activity Modulates Viral Replication.

Usutu virus (USUV) is a flavivirus that mainly infects wild birds through the bite of Culex mosquitoes. Recent outbreaks have been associated with an increased number of cases in humans. Despite being a growing source of public health concerns, there is yet insufficient data on the virus or host cell targets for infection control. In this work we have investigated whether the cellular kinase Akt and USUV polymerase NS5 interact and co-localize in a cell. To this aim, we performed co-immunoprecipitation (Co-IP) assays, followed by confocal microscopy analyses. We further tested whether NS5 is a phosphorylation substrate of Akt in vitro. Finally, to examine its role in viral replication, we chemically silenced Akt with three inhibitors (MK-2206, honokiol and ipatasertib). We found that both proteins are localized (confocal) and pulled down (Co-IP) together when expressed in different cell lines, supporting the fact that they are interacting partners. This possibility was further sustained by data showing that NS5 is phosphorylated by Akt. Treatment of USUV-infected cells with Akt-specific inhibitors led to decreases in virus titers (>10-fold). Our results suggest an important role for Akt in virus replication and stimulate further investigations to examine the PI3K/Akt/mTOR pathway as an antiviral target.

In Vitro and In Vivo Models to Study the Zoonotic Mosquito-Borne Usutu Virus.

Usutu virus (USUV), a mosquito-borne zoonotic flavivirus discovered in South Africa in 1959, has spread to many European countries over the last 20 years. The virus is currently a major concern for animal health due to its expanding host range and the growing number of avian mass mortality events. Although human infections with USUV are often asymptomatic, they are occasionally accompanied by neurological complications reminiscent of those due to West Nile virus (another flavivirus closely related to USUV). Whilst USUV actually appears less threatening than some other emergent arboviruses, the lessons learned from Chikungunya, Dengue, and Zika viruses during the past few years should not be ignored. Further, it would not be surprising if, with time, USUV disperses further eastwards towards Asia and possibly westwards to the Americas, which may result in more pathogenic USUV strains to humans and/or animals. These observations, inviting the scientific community to be more vigilant about the spread and genetic evolution of USUV, have prompted the use of experimental systems to understand USUV pathogenesis and to boost the development of vaccines and antivirals. This review is the first to provide comprehensive coverage of existing in vitro and in vivo models for USUV infection and to discuss their contribution in advancing data concerning this neurotropic virus. We believe that this paper is a helpful tool for scientists to identify gaps in the knowledge about USUV and to design their future experiments to study the virus.