An overview of Usutu virus

Weak association of Usutu virus and haemosporidian infection in birds collected in Germany

The Usutu Virus (USUV) is a mosquito-borne flavivirus originated in Africa. The virus circulates in Germany since 2010. It is primarily transmitted and maintained in the natural cycle by Culex mosquitoes and primarily affects birds, particularly Eurasian blackbird (Turdus merula), leading to significant mortality. Several studies have reported a high co-infection rate of European birds with both USUV and haemosporidians. Haemosporidians are blood parasites which maintain an enzootic life cycle with birds via different arthropod vectors. This study conducted screenings of birds from Germany received through a citizen's science project for both, USUV and haemosporidians between 2016 and 2021. The prevalence of USUV reached its peak in 2018, when it was first detected throughout most parts of Germany rather than being limited to localised hotspots. Subsequently, USUV prevalence consistently declined. On the other hand, the prevalence of haemosporidians initially declined between 2016 and 2019, but experienced a subsequent increase in the following years, exhibiting a more or less inverse pattern compared to the prevalence of USUV. In 2020, a statistically significant positive association between both pathogens was found, which was also detected across all years combined, indicating if at all a weak relationship between these pathogens.

Duck CD40L as an adjuvant enhances systemic immune responses of avian flavivirus DNA vaccine

Under the dual pressure of emerging zoonoses and the difficulty in eliminating conventional zoonoses, the strategic management of bird diseases through vaccination represents a highly efficacious approach to disrupting the transmission of zoonotic pathogens to humans. Immunization with a DNA vaccine yielded limited protection against avian pathogen infection. To improve its immunogenicity, the extracellular domain of duck-derived CD40L (designated as dusCD40L) was employed as a bio-adjuvant. Our findings unequivocally established the evolutionary conservation of dusCD40L across avian species. Notably, dusCD40L exhibited a compelling capacity to elicit robust immune responses from both B and T lymphocytes. Furthermore, when employed as an adjuvant, dusCD40L demonstrated a remarkable capacity to significantly augment the titers of neutralizing antibodies and the production of IFNγ elicited by a DNA vaccine encoding the prM-E region of an avian flavivirus, namely, the Tembusu virus (TMUV). Moreover, dusCD40L could strengthen virus clearance of the prM-E DNA vaccine in ducks post-TMUV challenge. This research study presents a highly effective adjuvant for advancing the development of DNA vaccines targeting TMUV in avian hosts. Additionally, it underscores the pivotal role of duCD40L as a potent adjuvant in the context of vaccines designed to combat zoonotic infections in avian species.

Virulence and transmission vary between Usutu virus lineages in Culex pipiens

Usutu virus (USUV) is a zoonotic arbovirus infecting mainly wild birds. It is transmitted by ornithophilic mosquitoes, mainly of the genus Culex from birds to birds and to several vertebrate dead-end hosts. Several USUV lineages, differing in their virulence have emerged in the last decades and now co-circulate in Europe, impacting human populations. However, their relative transmission and effects on their mosquito vectors is still not known. We thus compared the vector competence and survival of Culex pipiens mosquitoes experimentally infected with two distinct USUV lineages, EU2 and EU3, that are known to differ in their virulence and replication in vertebrate hosts. Infection rate was variable among blood feeding assays but variations between EU2 and EU3 lineages were consistent suggesting that Culex pipiens was equally susceptible to infection by both lineages. However, EU3 viral load increased with viral titer in the blood meal while EU2 viral load was high at all titers which suggest a greater replication of EU2 than EU3 in mosquito. While their relative transmission efficiencies are similar, at least at low blood meal titer, positive correlation between transmission and blood meal titer was observed for EU3 only. Contrary to published results in vertebrates, EU3 induced a higher mortality to mosquitoes (i.e. virulence) than EU2 whatever the blood meal titer. Therefore, we found evidence of lineage-specific differences in vectorial capacity and virulence to both the vector and vertebrate host which lead to balanced propagation of both viral lineages. These results highlight the need to decipher the interactions between vectors, vertebrate hosts, and the diversity of arbovirus lineages to fully understand transmission dynamics.

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.

Field-based assessments of the seasonality of Culex pipiens sensu lato in England: an important enzootic vector of Usutu and West Nile viruses

BACKGROUND

Usutu virus (USUV), which is closely related to West Nile virus (WNV), sharing a similar ecology and transmission cycle, was first reported in the UK in the southeast of England in 2020. Both USUV and WNV are emerging zoonotic viruses hosted by wild birds. The 2020 finding of USUV in England raised awareness of this virus and highlighted the importance of understanding the seasonality of Culex pipiens sensu lato (Cx. pipiens s.l.), the main enzootic vector of these viruses. Zoos are prime locations for trapping mosquitoes because of their infrastructure, security, and range of vertebrate hosts and aquatic habitats.

METHODS

Three independent zoo-based case studies at four locations that cover the seasonality of Cx. pipiens s.l. in England were undertaken: (i) London Zoo (Zoological Society London [ZSL]) and surrounding areas, London; (ii) Chester Zoo (Cheshire); (ii) Twycross Zoo (Leicestershire); and (iv) Flamingo Land (zoo; North Yorkshire). Various adult mosquito traps were used to catch adult Cx. pipiens s.l. across seasons.

RESULTS

High yields of Cx. pipiens s.l./Culex torrentium were observed in Biogents-Mosquitaire and Center for Disease Control and Prevention Gravid traps in all studies where these traps were used. Mosquito counts varied between sites and between years. Observations of adult Cx. pipiens s.l./Cx. torrentium abundance and modelling studies demonstrated peak adult abundance between late July and early August, with active adult female Cx. pipiens s.l./Cx. torrentium populations between May and September.

CONCLUSIONS

The information collated in this study illustrates the value of multiple mosquito monitoring approaches in zoos to describe the seasonality of this UK vector across multiple sites in England and provides a framework that can be used for ongoing and future surveillance programmes and disease risk management strategies.

A modified BG-Sentinel trap equipped with FTA card as a novel tool for mosquito-borne disease surveillance: a field test for flavivirus detection

Early detection of pathogens in vectors is important in preventing the spread of arboviral diseases, providing a timely indicator of pathogen circulation before outbreaks occur. However, entomological surveillance may face logistical constraints, such as maintaining the cold chain, and resource limitations, such as the field and laboratory workload of mosquito processing. We propose an FTA card-based trapping system that aims to simplify both field and laboratory phases of arbovirus surveillance. We modified a BG-Sentinel trap to include a mosquito collection chamber and a sugar feeding source through an FTA card soaked in a long-lasting viscous solution of honey and hydroxy-cellulose hydrogel. The FTA card ensures environmental preservation of nucleic acids, allowing continuous collection and feeding activity of specimens for several days and reducing the effort required for viral detection. We tested the trap prototype during two field seasons (2019 and 2021) in North-eastern Italy and compared it to CDC-CO2 trapping applied in West Nile and Usutu virus regional surveillance. Collections by the BG-FTA approach detected high species diversity, including Culex pipiens, Aedes albopictus, Culex modestus, Anopheles maculipennis sensu lato and Ochlerotatus caspius. When used for two-days sampling, the BG-FTA trap performed equally to CDC also for the WNV-major vector Cx. pipiens. The FTA cards detected both WNV and USUV, confirming the reliability of this novel approach to detect viral circulation in infectious mosquitoes. We recommend this surveillance approach as a particularly useful alternative in multi-target surveillance, for sampling in remote areas and in contexts characterized by high mosquito densities and diversity.

Case report of Usutu virus infection in an immunocompromised patient in Italy, 2022

Usutu virus (USUV) is an arthropod-borne flavivirus emerged in Africa in 1950s and in Eruope in 1990s causing a massive number of birds' deaths. The role of USUV as human pathogen has been only recently hypothesized and cases of USUV infection in humans remain limited and often related to immunocompromised subjects. Herein, we report a case of USUV meningoencephalitis infection in an immunocompromised patient with no history of previous flavivirus infection. The infection due to USUV evolved rapidly since hospital admission thus resulting fatal in few days after symptoms onset and, although not proven, a suspected bacteria co-infection has been hypothesized. Based on these findings, we suggested that when USUV meningoencephalitis is suspected in countries endemic, careful attention should be applied to neurological syndromes during summer months especially among immunocompromised patients.

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.

Stem-Loop I of the Tembusu Virus 3'-Untranslated Region Is Responsible for Viral Host-Specific Adaptation and the Pathogenicity of the Virus in Mice

Tembusu virus (TMUV), an avian mosquito-borne flavivirus, was first identified from Culex tritaeniorhynchus in 1955. To validate the effects of the 3'-untranslated region (3'UTR) in viral host-specific adaptation, we generated a set of chimeric viruses using CQW1 (duck strain) and MM 1775 (mosquito strain) as backbones with heterogeneous 3'UTRs. Compared with rMM 1775, rMM-CQ3'UTR (recombinant MM 1775 virus carrying the 3'UTR of CQW1) exhibited enhanced proliferation in vitro, with peak titers increasing by 5-fold in duck embryonic fibroblast (DEF) cells or 12-fold in baby hamster kidney (BHK-21) cells; however, the neurovirulence of rMM-CQ3'UTR was attenuated in 14-day-old Kunming mice via intracranial injection, with slower weight loss, lower mortality, and reduced viral loads. In contrast, rCQ-MM3'UTR showed similar growth kinetics in vitro and neurovirulence in mice compared with those of rCQW1. Then, the Stem-loop I (SLI) structure, which showed the highest variation within the 3'UTR between CQW1 and MM 1775, was further chosen for making chimeric viruses. The peak titers of rMM-CQ3'UTRSLI displayed a 15- or 4-fold increase in vitro, and the neurovirulence in mice was attenuated, compared with that of rMM 1775; rCQ-MM3'UTRSLI displayed comparable multiplication ability in vitro but was significantly attenuated in mice, in contrast with rCQW1. In conclusion, we demonstrated that the TMUV SLI structure of the 3'UTR was responsible for viral host-specific adaptation of the mosquito-derived strain in DEF and BHK-21 cells and regulated viral pathogenicity in 14-day-old mice, providing a new understanding of the functions of TMUV 3'UTR in viral host switching and the pathogenicity changes in mice. IMPORTANCE Mosquito-borne flaviviruses (MBFVs) constitute a large number of mosquito-transmitted viruses. The 3'-untranslated region (3'UTR) of MBFV has been suggested to be relevant to viral host-specific adaptation. However, the evolutionary strategies for host-specific fitness among MBFV are different, and the virulence-related structures within the 3'UTR are largely unknown. Here, using Tembusu virus (TMUV), an avian MBFV as models, we observed that the duck-derived SLI of the 3'UTR significantly enhanced the proliferation ability of mosquito-derived TMUV in baby hamster kidney (BHK-21) and duck embryonic fibroblast (DEF) cells, suggesting that the SLI structure was crucial for viral host-specific adaptation of mosquito-derived TMUVs in mammalian and avian cells. In addition, all SLI mutant viruses exhibited reduced viral pathogenicity in mice, indicating that SLI structure was a key factor for the pathogenicity in mice. This study provides a new insight into the functions of the MBFV 3'UTR in viral host switching and pathogenicity changes in mice.

Role of structural disorder in the multi-functionality of flavivirus proteins

INTRODUCTION

The life cycle of a virus involves interacting with the host cell, entry, hijacking host machinery for viral replication, evading the host's immune system, and releasing mature virions. However, viruses, being small in size, can only harbor a genome large enough to code for the minimal number of proteins required for the replication and maturation of the virions. As a result, many viral proteins are multifunctional machines that do not directly obey the classic structure-function paradigm. Often, such multifunctionality is rooted in intrinsic disorder that allows viral proteins to interact with various cellular factors and remain functional in the hostile environment of different cellular compartments.

AREAS COVERED

This report covers the classification of flaviviruses, their proteome organization, and the prevalence of intrinsic disorder in the proteomes of different flaviviruses. Further, we have summarized the speculations made about the apparent roles of intrinsic disorder in the observed multifunctionality of flaviviral proteins.

EXPERT OPINION

Small sizes of viral genomes impose multifunctionality on their proteins, which is dependent on the excessive usage of intrinsic disorder. In fact, intrinsic disorder serves as a universal functional tool, weapon, and armor of viruses and clearly plays an important role in their functionality and evolution.

Analysis of avian Usutu virus infections in Germany from 2011 to 2018 with focus on dsRNA detection to demonstrate viral infections

Usutu virus (USUV) is a zoonotic arbovirus causing avian mass mortalities. The first outbreak in North-Western Germany occurred in 2018. This retrospective analysis focused on combining virological and pathological findings in birds and immunohistochemistry. 25 common blackbirds, one great grey owl, and one kingfisher collected from 2011 to 2018 and positive for USUV by qRT-PCR were investigated. Macroscopically, most USUV infected birds showed splenomegaly and hepatomegaly. Histopathological lesions included necrosis and lymphohistiocytic inflammation within spleen, Bursa fabricii, liver, heart, brain, lung and intestine. Immunohistochemistry revealed USUV antigen positive cells in heart, spleen, pancreas, lung, brain, proventriculus/gizzard, Bursa fabricii, kidney, intestine, skeletal muscle, and liver. Analysis of viral genome allocated the virus to Europe 3 or Africa 2 lineage. This study investigated whether immunohistochemical detection of double-stranded ribonucleic acid (dsRNA) serves as an alternative tool to detect viral intermediates. Tissue samples of six animals with confirmed USUV infection by qRT-PCR but lacking viral antigen in liver and spleen, were further examined immunohistochemically. Two animals exhibited a positive signal for dsRNA. This could indicate either an early state of infection without sufficient formation of virus translation products, occurrence of another concurrent virus infection or endogenous dsRNA not related to infectious pathogens and should be investigated in more detail in future studies.

Seasonal Phenological Patterns and Flavivirus Vectorial Capacity of Medically Important Mosquito Species in a Wetland and an Urban Area of Attica, Greece

Seasonal patterns of mosquito population density and their vectorial capacity constitute major elements to understand the epidemiology of mosquito-borne diseases. Using adult mosquito traps, we compared the population dynamics of major mosquito species (Culex pipiens, Aedes albopictus, Anopheles spp.) in an urban and a wetland rural area of Attica Greece. Pools of the captured Cx. pipiens were analyzed to determine infection rates of the West Nile virus (WNV) and the Usutu virus (USUV). The data provided were collected under the frame of the surveillance program carried out in two regional units (RUs) of the Attica region (East Attica and South Sector of Attica), during the period 2017-2018. The entomological surveillance of adult mosquitoes was performed on a weekly basis using a network of BG-sentinel traps (BGs), baited with CO₂ and BG-Lure, in selected, fixed sampling sites. A total of 46,726 adult mosquitoes were collected, with larger variety and number of species in East Attica (n = 37,810), followed by the South Sector of Attica (n = 8916). The collected mosquitoes were morphologically identified to species level and evaluated for their public health importance. Collected Cx. pipiens adults were pooled and tested for West Nile virus (WNV) and Usutu virus (USUV) presence by implementation of a targeted molecular methodology (real-time PCR). A total of 366 mosquito pools were analyzed for WNV and USUV, respectively, and 38 (10.4%) positive samples were recorded for WNV, while no positive pool was detected for USUV. The majority of positive samples for WNV were detected in the East Attica region, followed by the South Sector of Attica, respectively. The findings of the current study highlight the WNV circulation in the region of Attica and the concomitant risk for the country, rendering mosquito surveillance actions and integrated mosquito management programs as imperative public health interventions.

Structure of Usutu virus SAAR-1776 displays fusion loop asymmetry

Usutu virus (USUV) is an emerging arbovirus in Europe that has been increasingly identified in asymptomatic humans and donated blood samples and is a cause of increased incidents of neuroinvasive human disease. Treatment or prevention options for USUV disease are currently nonexistent, the result of a lack of understanding of the fundamental elements of USUV pathogenesis. Here, we report two structures of the mature USUV virus, determined at a resolution of 2.4 Å, using single-particle cryogenic electron microscopy. Mature USUV is an icosahedral shell of 180 copies of envelope (E) and membrane (M) proteins arranged in the classic herringbone pattern. However, unlike previous reports of flavivirus structures, we observe virus subpopulations and differences in the fusion loop disulfide bond. Presence of a second, unique E glycosylation site could elucidate host interactions, contributing to the broad USUV tissue tropism. The structures provide a basis for exploring USUV interactions with glycosaminoglycans and lectins, the role of the RGD motif as a receptor, and the inability of West Nile virus therapeutic antibody E16 to neutralize the mature USUV strain SAAR-1776. Finally, we identify three lipid binding sites and predict key residues that likely participate in virus stability and flexibility during membrane fusion. Our findings provide a framework for the development of USUV therapeutics and expand the current knowledge base of flavivirus biology.

Pathology and Pathogenesis of Eurasian Blackbirds (Turdus merula) Naturally Infected with Usutu Virus

The Usutu virus (USUV) is a mosquito-borne zoonotic flavivirus. Despite its continuous circulation in Europe, knowledge on the pathology, cellular and tissue tropism and pathogenetic potential of different circulating viral lineages is still fragmentary. Here, macroscopic and microscopic evaluations are performed in association with the study of cell and tissue tropism and comparison of lesion severity of two circulating virus lineages (Europe 3; Africa 3) in 160 Eurasian blackbirds (Turdus merula) in the Netherlands. Results confirm hepatosplenomegaly, coagulative necrosis and lymphoplasmacytic inflammation as major patterns of lesions and, for the first time, vasculitis as a novel virus-associated lesion. A USUV and Plasmodium spp. co-infection was commonly identified. The virus was associated with lesions by immunohistochemistry and was reported most commonly in endothelial cells and blood circulating and tissue mononucleated cells, suggesting them as a major route of entry and spread. A tropism for mononuclear phagocytes cells was further supported by viral labeling in multinucleated giant cells. The involvement of ganglionic neurons and epithelial cells of the gastrointestinal tract suggests a possible role of oral transmission, while the involvement of feather follicle shafts and bulbs suggests their use as a diagnostic sample for live bird testing. Finally, results suggest similar pathogenicity for the two circulating lineages.

Previous Usutu Virus Exposure Partially Protects Magpies (Pica pica) against West Nile Virus Disease But Does Not Prevent Horizontal Transmission

The mosquito-borne flaviviruses USUV and WNV are known to co-circulate in large parts of Europe. Both are a public health concern, and USUV has been the cause of epizootics in both wild and domestic birds, and neurological cases in humans in Europe. Here, we explore the susceptibility of magpies to experimental USUV infection, and how previous exposure to USUV would affect infection with WNV. None of the magpies exposed to USUV showed clinical signs, viremia, or detectable neutralizing antibodies. After challenge with a neurovirulent WNV strain, neither viremia, viral titer of WNV in vascular feathers, nor neutralizing antibody titers of previously USUV-exposed magpies differed significantly with respect to magpies that had not previously been exposed to USUV. However, 75% (6/8) of the USUV-exposed birds survived, while only 22.2% (2/9) of those not previously exposed to USUV survived. WNV antigen labeling by immunohistochemistry in tissues was less evident and more restricted in magpies exposed to USUV prior to challenge with WNV. Our data indicate that previous exposure to USUV partially protects magpies against a lethal challenge with WNV, while it does not prevent viremia and direct transmission, although the mechanism is unclear. These results are relevant for flavivirus ecology and contention.

Competition between Usutu virus and West Nile virus during simultaneous and sequential infection of Culex pipiens mosquitoes

Usutu virus (USUV) and West Nile virus (WNV) are closely related mosquito-borne flaviviruses that are mainly transmitted between bird hosts by vector mosquitoes. Infections in humans are incidental but can cause severe disease. USUV is endemic in large parts of Europe, while WNV mainly circulates in Southern Europe. In recent years, WNV is also frequently detected in Northern Europe, thereby expanding the area where both viruses co-circulate. However, it remains unclear how USUV may affect the future spread of WNV and the likelihood of human co-infection. Here we investigated whether co-infections with both viruses in cell lines and their primary mosquito vector, Culex pipiens, affect virus replication and transmission dynamics. We show that USUV is outcompeted by WNV in mammalian, avian and mosquito cells during co-infection. Mosquitoes that were exposed to both viruses simultaneously via infectious blood meal displayed significantly reduced USUV transmission compared to mosquitoes that were only exposed to USUV (from 15% to 3%), while the infection and transmission of WNV was unaffected. In contrast, when mosquitoes were pre-infected with USUV via infectious blood meal, WNV transmission was significantly reduced (from 44% to 17%). Injection experiments established the involvement of the midgut in the observed USUV-mediated WNV inhibition. The competition between USUV and WNV during co-infection clearly indicates that the chance of concurrent USUV and WNV transmission via a single mosquito bite is low. The competitive relation between USUV and WNV may impact virus transmission dynamics in the field and affect the epidemiology of WNV in Europe.

Animal board invited review: Risks of zoonotic disease emergence at the interface of wildlife and livestock systems

The ongoing coronavirus disease 19s pandemic has yet again demonstrated the importance of the human-animal interface in the emergence of zoonotic diseases, and in particular the role of wildlife and livestock species as potential hosts and virus reservoirs. As most diseases emerge out of the human-animal interface, a better understanding of the specific drivers and mechanisms involved is crucial to prepare for future disease outbreaks. Interactions between wildlife and livestock systems contribute to the emergence of zoonotic diseases, especially in the face of globalization, habitat fragmentation and destruction and climate change. As several groups of viruses and bacteria are more likely to emerge, we focus on pathogenic viruses of the Bunyavirales, Coronaviridae, Flaviviridae, Orthomyxoviridae, and Paramyxoviridae, as well as bacterial species including Mycobacterium sp., Brucella sp., Bacillus anthracis and Coxiella burnetii. Noteworthy, it was difficult to predict the drivers of disease emergence in the past, even for well-known pathogens. Thus, an improved surveillance in hotspot areas and the availability of fast, effective, and adaptable control measures would definitely contribute to preparedness. We here propose strategies to mitigate the risk of emergence and/or re-emergence of prioritized pathogens to prevent future epidemics.

Establishment of Culex modestus in Belgium and a Glance into the Virome of Belgian Mosquito Species

Culex modestus mosquitoes are considered potential transmission vectors of West Nile virus and Usutu virus. Their presence has been reported across several European countries, including one larva detected in Belgium in 2018. In this study, mosquitoes were collected in the city of Leuven and surrounding areas in the summers of 2019 and 2020. Species identification was performed based on morphological features and partial sequences of the mitochondrial cytochrome oxidase subunit I (COI) gene. The 107 mosquitoes collected in 2019 belonged to eight mosquito species, Culex pipiens (24.3%), Cx. modestus (48.6%), Cx. torrentium (0.9%), Culiseta annulata (0.9%), Culiseta morsitans (0.9%), Aedes sticticus (14.0%), Aedes cinereus (9.3%), and Anopheles plumbeus (0.9%), suggesting the presence of an established Cx. modestus population in Belgium. The collection of Cx. modestus mosquitoes at the same locations in 2020 confirmed their establishment in the region. Haplotype network analysis of the COI sequences for Cx. modestus showed that the Belgian population is rather diverse, suggesting that it may have been established in Belgium for some time. The Belgian Cx. modestus population was most closely related to populations from the United Kingdom and Germany. Characterization of the virome of the collected mosquitoes resulted in the identification of at least 33 eukaryotic viral species. Nine (nearly) complete genomes belonging to 6 viral species were identified, all of which were closely related to known viruses. In conclusion, here, we report the presence of Cx. modestus in the surrounding areas of Leuven, Belgium. As this species is considered to be a vector of several arboviruses, the implementation of vector surveillance programs to monitor this species is recommended.

IMPORTANCECulex modestus mosquitoes are considered to be a potential “bridge” vector, being able to transmit pathogens between birds as well as from birds to mammals, including humans. In Belgium, this mosquito species was considered absent until the finding of one larva in 2018 and subsequent evidence of a large population in 2019 to 2020 described here. We collected mosquitoes in the summers of 2019 and 2020 in the city of Leuven and surrounding areas. The mosquito species was identified by morphological and molecular methods, demonstrating the presence of Cx. modestus in this region. The ability of mosquitoes to transmit pathogens can depend on several factors, one of them being their natural virus composition. Therefore, we identified the mosquito-specific viruses harbored by Belgian mosquitoes. As Cx. modestus is able to transmit viruses such as West Nile virus and Usutu virus, the establishment of this mosquito species may increase the risk of virus transmission in the region. It is thus advisable to implement mosquito surveillance programs to monitor this species.

Screening of Mosquitoes for West Nile Virus and Usutu Virus in Croatia, 2015-2020

In the period from 2015 to 2020, an entomological survey for the presence of West Nile virus (WNV) and Usutu virus (USUV) in mosquitoes was performed in northwestern Croatia. A total of 20,363 mosquitoes were sampled in the City of Zagreb and Međimurje county, grouped in 899 pools and tested by real-time RT-PCR for WNV and USUV RNA. All pools were negative for WNV while one pool each from 2016 (Aedes albopictus), 2017 (Culex pipiens complex), 2018 (Cx. pipiens complex), and 2019 (Cx. pipiens complex), respectively, was positive for USUV. The 2018 and 2019 positive pools shared 99.31% nucleotide homology within the USUV NS5 gene and both clustered within USUV Europe 2 lineage. The next-generation sequencing of one mosquito pool (Cx. pipiens complex) collected in 2018 in Zagreb confirmed the presence of USUV and revealed several dsDNA and ssRNA viruses of insect, bacterial and mammalian origin.

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.

Unmasking viral sequences by metagenomic next-generation sequencing in adult human blood samples during steroid-refractory/dependent graft-versus-host disease

BACKGROUND

Viral infections are common complications following allogeneic hematopoietic stem cell transplantation (allo-HSCT). Allo-HSCT recipients with steroid-refractory/dependent graft-versus-host disease (GvHD) are highly immunosuppressed and are more vulnerable to infections with weakly pathogenic or commensal viruses. Here, twenty-five adult allo-HSCT recipients from 2016 to 2019 with acute or chronic steroid-refractory/dependent GvHD were enrolled in a prospective cohort at Geneva University Hospitals. We performed metagenomics next-generation sequencing (mNGS) analysis using a validated pipeline and de novo analysis on pooled routine plasma samples collected throughout the period of intensive steroid treatment or second-line GvHD therapy to identify weakly pathogenic, commensal, and unexpected viruses.

RESULTS

Median duration of intensive immunosuppression was 5.1 months (IQR 5.5). GvHD-related mortality rate was 36%. mNGS analysis detected viral nucleotide sequences in 24/25 patients. Sequences of ≥ 3 distinct viruses were detected in 16/25 patients; Anelloviridae (24/25) and human pegivirus-1 (9/25) were the most prevalent. In 7 patients with fatal outcomes, viral sequences not assessed by routine investigations were identified with mNGS and confirmed by RT-PCR. These cases included Usutu virus (1), rubella virus (1 vaccine strain and 1 wild-type), novel human astrovirus (HAstV) MLB2 (1), classic HAstV (1), human polyomavirus 6 and 7 (2), cutavirus (1), and bufavirus (1).

CONCLUSIONS

Clinically unrecognized viral infections were identified in 28% of highly immunocompromised allo-HSCT recipients with steroid-refractory/dependent GvHD in consecutive samples. These identified viruses have all been previously described in humans, but have poorly understood clinical significance. Rubella virus identification raises the possibility of re-emergence from past infections or vaccinations, or re-infection. Video abstract.

Structural and antigenic investigation of Usutu virus envelope protein domain III

The mosquito-borne flavivirus Usutu virus (USUV) has recently emerged in birds and humans in Europe. Symptoms of a USUV infection resemble those of West Nile virus (WNV); further, the close antigenic relationship of domain III (DIII) of the USUV and WNV envelope (E) proteins has prevented the development of a reliable serological test to distinguish USUV from WNV. To begin to address this deficiency, we identified ten different sequence groups of DIII from 253 complete and 80 partial USUV genome sequences. We solved the DIII structures of four groups, including that of the outlying CAR-1969 strain, which shows an atypical DIII structure. Structural comparisons of the USUV DIII groups and the DIII of WNV bound to the neutralizing antibody E16 revealed why the E16 failed to neutralize all USUV strains tested except for USUV CAR-1969. The analyses allowed predictions to be made to engineer an antibody specific for USUV CAR-1969.

Climatic changes and their role in emergence and re-emergence of diseases

Global warming and the associated climate changes are predictable. They are enhanced by burning of fossil fuels and the emission of huge amounts of CO2 gas which resulted in greenhouse effect. It is expected that the average global temperature will increase with 2-5 °C in the next decades. As a result, the earth will exhibit marked climatic changes characterized by extremer weather events in the coming decades, such as the increase in temperature, rainfall, summertime, droughts, more frequent and stronger tornadoes and hurricanes. Epidemiological disease cycle includes host, pathogen and in certain cases intermediate host/vector. A complex mixture of various environmental conditions (e.g. temperature and humidity) determines the suitable habitat/ecological niche for every vector host. The availability of suitable vectors is a precondition for the emergence of vector-borne pathogens. Climate changes and global warming will have catastrophic effects on human, animal and environmental ecosystems. Pathogens, especially neglected tropical disease agents, are expected to emerge and re-emerge in several countries including Europe and North America. The lives of millions of people especially in developing countries will be at risk in direct and indirect ways. In the present review, the role of climate changes in the spread of infectious agents and their vectors is discussed. Examples of the major emerging viral, bacterial and parasitic diseases are also summarized.

The continued threat of emerging flaviviruses

Flaviviruses are vector-borne RNA viruses that can emerge unexpectedly in human populations and cause a spectrum of potentially severe diseases including hepatitis, vascular shock syndrome, encephalitis, acute flaccid paralysis, congenital abnormalities and fetal death. This epidemiological pattern has occurred numerous times during the last 70 years, including epidemics of dengue virus and West Nile virus, and the most recent explosive epidemic of Zika virus in the Americas. Flaviviruses are now globally distributed and infect up to 400 million people annually. Of significant concern, outbreaks of other less well-characterized flaviviruses have been reported in humans and animals in different regions of the world. The potential for these viruses to sustain epidemic transmission among humans is poorly understood. In this Review, we discuss the basic biology of flaviviruses, their infectious cycles, the diseases they cause and underlying host immune responses to infection. We describe flaviviruses that represent an established ongoing threat to global health and those that have recently emerged in new populations to cause significant disease. We also provide examples of lesser-known flaviviruses that circulate in restricted areas of the world but have the potential to emerge more broadly in human populations. Finally, we discuss how an understanding of the epidemiology, biology, structure and immunity of flaviviruses can inform the rapid development of countermeasures to treat or prevent human infections as they emerge.

Usutu virus: A new threat?

Usutu virus (USUV) is an emerging arbovirus that was first isolated in South Africa in 1959. This Flavivirus is maintained in the environment through a typical enzootic cycle involving mosquitoes and birds. USUV has spread to a large part of the European continent over the two decades mainly leading to substantial avian mortalities with a significant recrudescence of bird infections recorded throughout Europe within the few last years. USUV infection in humans is considered to be most often asymptomatic or to cause mild clinical signs. Nonetheless, a few cases of neurological complications such as encephalitis or meningoencephalitis have been reported. USUV and West Nile virus (WNV) share many features, like a close phylogenetic relatedness and a similar ecology, with co-circulation frequently observed in nature. However, USUV has been much less studied and in-depth comparisons of the biology of these viruses are yet rare. In this review, we discuss the main body of knowledge regarding USUV and compare it with the literature on WNV, addressing in particular virological and clinical aspects, and pointing data gaps.

Transcriptome Analysis Reveals the Neuro-Immune Interactions in Duck Tembusu Virus-Infected Brain

The duck Tembusu virus (DTMUV) is a mosquito-borne flavivirus. It causes severe symptoms of egg-drop, as well as neurological symptoms and brain damage in ducks. However, the specific molecular mechanisms of DTMUV-induced neurovirulence and host responses in the brain remain obscure. To better understand the host-pathogen and neuro-immune interactions of DTMUV infection, we conducted high-throughput RNA-sequencing to reveal the transcriptome profiles of DTMUV-infected duck brain. Totals of 117, 212, and 150 differentially expressed genes (DEGs) were identified at 12, 24, and 48 h post infection (hpi). Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses uncovered genes and pathways related to the nervous system and immune responses in duck brain. Neuro-related genes, including WNT3A, GATA3, and CHRNA6, were found to be significantly downregulated. RIG-I-like receptors (DHX58, IFIH1) and Toll-like receptors (TLR2 and TLR3) were activated, inducing the expression of 22 interferon stimulated genes (ISGs) and antigen-processing and -presenting genes (TAP1 and TAP2) in the brain. Our research provides comprehensive information for the molecular mechanisms of neuro-immune and host-pathogen interactions of DTMUV.

New Insights into the Susceptibility of Immunocompetent Mice to Usutu Virus

Usutu virus (USUV) is a mosquito-borne flavivirus that shares many similarities with the closely related West Nile virus (WNV) in terms of ecology and clinical manifestations. Initially distributed in Africa, USUV emerged in Italy in 1996 and managed to co-circulate with WNV in many European countries in a similar mosquito-bird life cycle. The rapid geographic spread of USUV, the seasonal mass mortalities it causes in the European avifauna, and the increasing number of infections with neurological disease both in healthy and immunocompromised humans has stimulated interest in infection studies to delineate USUV pathogenesis. Here, we assessed the pathogenicity of two USUV isolates from a recent Belgian outbreak in immunocompetent mice. The intradermal injection of USUV gave rise to disorientation and paraplegia and was associated with neuronal death in the brain and spinal cord in a single mouse. Intranasal inoculation of USUV could also establish the infection; viral RNA was detected in the brain 15 days post-infection. Overall, this pilot study probes the suitability of this murine model for the study of USUV neuroinvasiveness and the possibility of direct transmission in mammals.

First serological evidence of West Nile virus infection in wild birds in Northern Algeria

While the epidemiology of Flaviviruses has been extensively studied in most of the Mediterranean basin, little is known about the current situation in Algeria. In order to detect the circulation of West Nile (WNV) and Usutu viruses (USUV) in Kabylia, 165 sera were collected from two wild birds species, namely the long distance migrant Turdus philomelos (song thrush) (n = 92) and the resident Passer domesticus (house sparrow) (n = 73). A total of 154 sera were first analyzed by commercial competition ELISA. WNV and USUV micro-neutralization tests were performed on all c-ELISA positive sera and all samples with poor volume. Overall, 7.8 % (CI95 %: 3.5-11.9) were positive by c-ELISA. Positive results were detected in 12.5 % (CI95 %:5.6-19.4) of song thrushes and 1.5 % (CI95 %: 0.0-4.5) for sparrow. Micro-neutralization tests revealed an overall seroprevalence of 6.7 % for WNV (CI95 %: 2.9-10.3), Neutralizing antibodies were found in 8.7 % (CI95 %: 3.0-14.4) for song thrushes and in 4.1 % (CI95 %: 0.0-8.7) of sparrows. The current study demonstrates significant seroprevalence of WNV antibodies in wild birds in Algeria.

Reemergence of West Nile Virus Infections in Southern Greece, 2017

Greece experienced the largest European West Nile virus (WNV) outbreak in 2010 since the 1996 Romania epidemic. West Nile virus reemerged in southern Greece during 2017, after a 2-year hiatus of recorded human cases, and herein laboratory findings, clinical features, and geographic distribution of WNV cases are presented. Clinical specimens from patients with clinically suspected WNV infection were sent from local hospitals to the Microbiology Department of Medical School, National and Kapodistrian University of Athens, and were tested for the presence of specific anti-WNV antibodies and WNV RNA. From July to September 2017, 45 confirmed or probable WNV infection cases were identified; 43 of them with an acute/recent infection, of which 24 (55.8%) experienced WNV neuroinvasive disease (WNND). Risk factors for developing WNND included advanced age, hypertension, and diabetes mellitus. A total of four deaths (16.7%) occurred, all in elderly patients aged > 70 years. Thirty-nine cases were identified in regional units that had not been affected before (36 in Argolis and two in Corinth, northeastern Peloponnese, and one in Rethymno, Crete). The remaining four cases were reported from previously affected regional units of northwestern Peloponnese. The reemergence of WNV after a 2-year hiatus of recorded human cases and the spread of the virus in newly affected regions of the country suggests that WNV has been established in Greece and disease transmission will continue in the future. Epidemiological surveillance, intensive mosquito management programs, and public awareness campaigns about personal protective measures are crucial to the prevention of WNV transmission.

Surveillance of mosquito vectors in Southern Sweden for Flaviviruses and Sindbis virus

There are three human pathogenic bird-viruses transmitted by Culex mosquitoes in Europe: the alphavirus Sindbis and the flaviviruses West Nile virus and Usutu virus. Cases of Sindbis fever occur in the north while the flaviviruses are reported from southern Europe. In this study, 7933 Culex pipiens/torrentium mosquitoes from southern Sweden were screened by RTqPCR for these viruses. None of the mosquitoes were positive for viral RNA. The importance of mosquito species composition is discussed as a potential explanation to the lack of detection of mosquito-borne viruses in southern Sweden. However, continued surveillance of mosquitoes for Flaviviruses would be valuable as an early warning for public health awareness.

[What are the determinants of viral outbreaks and is it possible to predict their emergence?]

L’émergence d’une maladie virale résulte le plus souvent d’un déséquilibre dans l’interaction entre l’agent infectieux, l’hôte et l’environnement. Après une phase d’introduction de la maladie virale dans un territoire ou une population donnée et une fois que les premières chaînes de transmission sont en place, on peut assister à la diffusion de la maladie, voire sa pérennisation si les mesures de contrôle ne sont pas mises en œuvre ou ne sont pas suffisamment efficaces. S’il est difficile d’anticiper la survenue et l’introduction d’une maladie virale émergente, les trois axes suivants de lutte doivent être développés pour en limiter l’impact : (1) anticipation et préparation ; (2) recherche et (3) veille et surveillance. Pour garantir enfin que les mesures prises soient pertinentes au regard des données disponibles et acceptables par la population, il convient de s’appuyer de manière systématique sur une approche multidisciplinaire qui devra être réévaluée de manière dynamique.

Different dynamics of Usutu virus infections in Austria and Hungary, 2017-2018

Usutu virus (USUV), a mosquito‐borne flavivirus closely related to West Nile virus, emerged in Austria in 2001, when it caused a considerable mass‐mortality of Eurasian blackbirds. Cases in birds increased until 2003 and quickly declined thereafter, presumably due to developing herd immunity. Since 2006, no further cases were recorded, until two blackbirds were tested positive in 2016. In Hungary, USUV first appeared in 2005 and has caused only sporadic infections since then. Initially, the only genetic USUV lineage found across both countries was Europe 1. This changed in 2015/2016, when Europe 2 emerged, which has since then become the prevalent lineage. Due to dispersal of these strains and introduction of new genetic lineages, USUV infections are now widespread across Europe. In 2009, the first cases of USUV‐related encephalitis were described in humans, and the virus has been frequently detected in blood donations since 2016. To monitor USUV infections among the Austrian wild bird population in 2017/2018, 86 samples were investigated by RT‐PCR. In 67 of them, USUV nucleic acid was detected (17 in 2017, 50 in 2018). The majority of succumbed birds were blackbirds, found in Vienna and Lower Austria. However, the virus also spread westwards to Upper Austria and southwards to Styria and Carinthia. In Hungary, 253 wild birds were examined, but only six of them were infected with USUV (five in 2017, one in 2018). Thus, in contrast to the considerable increase in USUV‐associated bird mortality in Austria, the number of infections in Hungary declined after a peak in 2016. Except for one case of USUV lineage Africa 3 in Austria in 2017, Europe 2 remains the most prevalent genetic lineage in both countries. Since USUV transmission largely depends on temperature, which affects vector populations, climate change may cause more frequent USUV outbreaks in the future.

Evidence for West Nile Virus and Usutu Virus Infections in Wild and Resident Birds in Germany, 2017 and 2018

Wild birds play an important role as reservoir hosts and vectors for zoonotic arboviruses and foster their spread. Usutu virus (USUV) has been circulating endemically in Germany since 2011, while West Nile virus (WNV) was first diagnosed in several bird species and horses in 2018. In 2017 and 2018, we screened 1709 live wild and zoo birds with real-time polymerase chain reaction and serological assays. Moreover, organ samples from bird carcasses submitted in 2017 were investigated. Overall, 57 blood samples of the live birds (2017 and 2018), and 100 organ samples of dead birds (2017) were positive for USUV-RNA, while no WNV-RNA-positive sample was found. Phylogenetic analysis revealed the first detection of USUV lineage Europe 2 in Germany and the spread of USUV lineages Europe 3 and Africa 3 towards Northern Germany. USUV antibody prevalence rates were high in Eastern Germany in both years. On the contrary, in Northern Germany, high seroprevalence rates were first detected in 2018, with the first emergence of USUV in this region. Interestingly, high WNV-specific neutralizing antibody titers were observed in resident and short-distance migratory birds in Eastern Germany in 2018, indicating the first signs of a local WNV circulation.

Discovery of Novel Crustacean and Cephalopod Flaviviruses: Insights into the Evolution and Circulation of Flaviviruses between Marine Invertebrate and Vertebrate Hosts

Most described flaviviruses (family Flaviviridae) are disease-causing pathogens of vertebrates maintained in zoonotic cycles between mosquitoes or ticks and vertebrate hosts. Poor sampling of flaviviruses outside vector-borne flaviviruses such as Zika virus and dengue virus has presented a narrow understanding of flavivirus diversity and evolution. In this study, we discovered three crustacean flaviviruses (Gammarus chevreuxi flavivirus, Gammarus pulex flavivirus, and Crangon crangon flavivirus) and two cephalopod flaviviruses (Southern Pygmy squid flavivirus and Firefly squid flavivirus). Bayesian and maximum likelihood phylogenetic methods demonstrate that crustacean flaviviruses form a well-supported clade and share a more closely related ancestor with terrestrial vector-borne flaviviruses than with classical insect-specific flaviviruses. In addition, we identify variants of Wenzhou shark flavivirus in multiple gazami crab (Portunus trituberculatus) populations, with active replication supported by evidence of an active RNA interference response. This suggests that Wenzhou shark flavivirus moves horizontally between sharks and gazami crabs in ocean ecosystems. Analyses of the mono- and dinucleotide composition of marine flaviviruses compared to that of flaviviruses with known host status suggest that some marine flaviviruses share a nucleotide bias similar to that of vector-borne flaviviruses. Furthermore, we identify crustacean flavivirus endogenous viral elements that are closely related to elements of terrestrial vector-borne flaviviruses. Taken together, these data provide evidence of flaviviruses circulating between marine vertebrates and invertebrates, expand our understanding of flavivirus host range, and offer potential insights into the evolution and emergence of terrestrial vector-borne flaviviruses.IMPORTANCE Some flaviviruses are known to cause disease in vertebrates and are typically transmitted by blood-feeding arthropods such as ticks and mosquitoes. While an ever-increasing number of insect-specific flaviviruses have been described, we have a narrow understanding of flavivirus incidence and evolution. To expand this understanding, we discovered a number of novel flaviviruses that infect a range of crustaceans and cephalopod hosts. Phylogenetic analyses of these novel marine flaviviruses suggest that crustacean flaviviruses share a close ancestor to all terrestrial vector-borne flaviviruses, and squid flaviviruses are the most divergent of all known flaviviruses to date. Additionally, our results indicate horizontal transmission of a marine flavivirus between crabs and sharks. Taken together, these data suggest that flaviviruses move horizontally between invertebrates and vertebrates in ocean ecosystems. This study demonstrates that flavivirus invertebrate-vertebrate host associations have arisen in flaviviruses at least twice and may potentially provide insights into the emergence or origin of terrestrial vector-borne flaviviruses.

Usutu virus infection in Dutch blood donors

BACKGROUND

The screening of Dutch blood donations for West Nile virus (WNV) may be imminent, as WNV emerges in nearby countries and more donors travel to WNV-affected regions. Since 2016 the related, mosquito-borne Usutu virus (USUV) causes seasonal mortality in Dutch birds. To what extent will human USUV infections affect Dutch WNV donor screening?

STUDY DESIGN AND METHODS

From April through September 2018, plasma samples from blood donations in blackbird-rich regions were stored. When increased bird mortality was reported in August, samples from July, August, and September were tested for USUV-RNA in pools of eight, using a home-brew combined WNV/USUV-PCR assay. Reactive pools were deconstructed. Original plasma units and samples of previous and follow-up donations of reactive donors were tested for USUV- and WNV-RNA, and for antibody responses.

RESULTS

The number of USUV RNA-positive, WNV RNA-negative donations was 0 of 2688 donations in July, 6 of 4416 in August (1:736), and 1 of 4936 in September. The seven infected donors tested negative for USUV-RNA in preceding and follow-up donations. For 6 donors, seroconversion for USUV-antibodies was demonstrated. All index donations tested positive in a commonly used PCR-assay for WNV donor screening. Three exposed recipients did not show signs of infection. Screening a random subset of 1092 donations from September for USUV-IgG antibodies showed that 22 donors tested reactive; for three donors retrospective testing identified an USUV PCR-positive pre-seroconversion donation.

CONCLUSION

Seasonal USUV infection in Dutch blood donors is common. Cross-reactivity in molecular assays for WNV-screening occurs, but can be resolved using USUV- and WNV-specific PCR-primers and sequencing of viral RNA.

West Nile Virus and Usutu Virus Co-Circulation in Europe: Epidemiology and Implications

West Nile virus (WNV) and Usutu virus (USUV) are neurotropic mosquito-borne flaviviruses that may infect humans. Although WNV is much more widespread and plays a much larger role in human health, the two viruses are characterized by similar envelope antigens, clinical manifestations, and present overlapping in terms of geographic range of transmission, host, and vector species. This review highlights some of the most relevant aspects of WNV and USUV human infections in Europe, and the possible implications of their co-circulation.

First Evidence of Fatal Usutu Virus Natural Infections in an Anatidae, the Common Scoter (Melanitta nigra)

While fatal infections caused by the Usutu virus appeared to concern only passerines (especially the blackbird) and Strigiformes (especially the great gray owl), we report herein that the virus also naturally causes a fatal disease in an anseriforme species, the common scoter (Melanitta nigra).

Prevalence and molecular epidemiology of West Nile and Usutu virus infections in Croatia in the 'One health' context, 2018

In 2018, Croatia reported the largest outbreak of West Nile virus (WNV) infections as well as the re-occurrence of human Usutu virus (USUV) infections. For the first time, fatal WNV and USUV infections were detected in wild birds. We analysed epidemiological characteristics and molecular epidemiology of WNV and USUV infections detected during 2018 transmission season. From April to November, 178 patients with neuroinvasive disease and 68 patients with febrile disease were tested for WNV and USUV. Viral RNA was detected in cerebrospinal fluid (CSF) and urine samples using a real-time RT-PCR. Positive samples were tested by nested RT-PCR and nucleotide sequencing. IgM/IgG antibodies were detected in serum/CSF samples using ELISA with confirmation of cross-reactive samples by virus neutralization test (VNT). WNV neuroinvasive disease was confirmed in 54 and WNV fever in seven patients from 10 continental Croatian counties. Areas affected in 2018 were those in which cases occurred in previous seasons, while in three areas human cases were reported for the first time. Phylogenetic analysis of six strains from patients residing in different geographic areas showed circulation of WNV lineage 2. In three patients, neuroinvasive USUV infection was confirmed by RT-PCR or VNT. Sequence analysis of one detected strain revealed USUV Europe 2 lineage. During the same period, a total of 2,574 horse and 1,069 poultry serum samples were tested for WNV antibodies using ELISA. Acute asymptomatic WNV infection (IgM antibodies) was documented in 20/0.7% horses. WNV IgG antibodies were found in 307/11.9% horses and in 125/12.7% poultry. WNV RNA was detected in two goshawks and USUV RNA was detected in one blackbird from north-western Croatia. In the Zagreb area, 3,670 female mosquitoes were collected. One Culex pipiens pool collected in July tested positive for USUV RNA. Our results highlight the importance of continuous multidisciplinary 'One health' surveillance of these emerging arboviruses.

Flaviviruses in migratory passerines during spring stopover in a desert oasis

Bird migration has long been hypothesized as the main mechanism for long-distance dispersal of flaviviruses, but the role of migratory birds in flaviviruses spillover is not well documented. In this study, we investigated the eco-epidemiology of West Nile virus (WNV) and Usutu virus (USUV) in trans-Saharan passerines during their spring stopover in southern Tunisian oases. To do, we combined oral swab analysis and serological tools to assess whether migratory birds could be reaching these stopover sites while infectious or have been previously exposed to viruses. All sampled birds tested negative for oral swab analysis. However, anti-WNV and anti-USUV antibodies were detected in 32% and 1% of tested birds, respectively. Among WNV-seropositive species, the Golden oriole (Oriolus oriolus) showed the highest anti-WNV occurrence probability. In this species, anti-WNV occurrence was twice larger in males than females. Inter-specific and intraspecific morphological, physiological and behavioural differences could explain these results. Although our findings did not show evidence for passerines migrating while infectious, they did not exclude an existing enzootic WNV transmission cycle in Tunisian oases. Further investigations including larger samples of migratory birds are needed for a better understanding of this issue.

[Mosquito microbiota and its influence on disease vectorial transmission]

Mosquitoes (Diptera: Culicidae) are found worldwide. Around 100 among 3500 mosquito species are known to be vectors of parasites and viruses, responsible for infectious diseases including malaria and dengue. Mosquitoes host diverse microbial communities that influence disease transmission, either by direct interference or via affecting host immunity and physiology. These microbial communities are present within diverse tissues, including the digestive tract, and vary depending on the sex of the mosquito, its developmental stage, and ecological factors. This review summarizes the current knowledge about the mosquito microbiota, defined as a community of commensal, symbiotic or pathogenic microbes harboured by a host. We first describe the current knowledge on the diversity of the microbiota, that includes bacteria, fungi, parasites and viruses and on its modes of acquisition throughout the mosquito life cycle. We then focus on microbial interactions within the mosquito gut, which notably affect vector competence, and on host-microbe interactions affecting mosquito fitness. Finally, we discuss current or potential methods based on the use of microbes or microbial products to interfere with pathogen transmission or to reduce mosquito lifespan and reproduction.

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.

Mosquito-borne epornitic flaviviruses: an update and review

West Nile Virus, Usutu virus, Bagaza virus, Israel turkey encephalitis virus and Tembusu virus currently constitute the five flaviviruses transmitted by mosquito bites with a marked pathogenicity for birds. They have been identified as the causative agents of severe neurological symptoms, drop in egg production and/or mortalities among avian hosts. They have also recently shown an expansion of their geographic distribution and/or a rise in cases of human infection. This paper is the first up-to-date review of the pathology of these flaviviruses in birds, with a special emphasis on the difference in susceptibility among avian species, in order to understand the specificity of the host spectrum of each of these viruses. Furthermore, given the lack of a clear prophylactic approach against these viruses in birds, a meta-analysis of vaccination trials conducted to date on these animals is given to constitute a solid platform from which designing future studies.

Evaluation of a new West Nile virus nucleic acid test for screening of blood donations

BACKGROUND

West Nile virus (WNV) is transmitted to humans through mosquito bites and can be further transmitted to humans through transfusion or transplantation. Because most infected individuals are asymptomatic, blood donor screening is important in areas where WNV is endemic. These studies evaluated the performance of a new test for detection of WNV RNA in blood donations.

STUDY DESIGN AND METHODS

Analytical performance evaluation included sensitivity, specificity, inclusivity, and correlation. A clinical specificity study was conducted at four blood donor testing laboratories in parallel with the cobas TaqScreen WNV Test (Roche Molecular Systems, Inc.).

RESULTS

The 95% and 50% limit of detection for cobas WNV was 12.9 copies/mL (95% confidence interval [CI], 10.8–16.3) and 2.1 copies/mL (95% CI, 1.9–2.4) for WNV lineage 1, respectively, and 6.2 copies/mL (95% CI, 4.8–8.9) and 1.1 copies/mL (95% CI, 0.8–1.3) for WNV lineage 2, respectively. Clinical specificity was 100% in 10,823 donor samples tested individually (95% CI, 99.966%–100%) and 63,243 tested in pools of 6 (95% CI, 99.994%–100%). Samples of other members of the Japanese encephalitis virus serocomplex, including St Louis encephalitis, Japanese encephalitis, Murray Valley encephalitis, Usutu, and Kunjin viruses were detected by cobas WNV.

CONCLUSION

The cobas WNV test for use on the cobas 6800/8800 System, a fully automated test system, demonstrated high sensitivity and specificity and is suitable for the detection of WNV in blood donors.

Favipiravir inhibits in vitro Usutu virus replication and delays disease progression in an infection model in mice

Usutu virus (USUV) is an emerging flavivirus that causes Usutu disease mainly in birds, but infection of mammals such as rodents, bats and horses has also been demonstrated. In addition, human cases (both in immunocompromised and -competent individuals) were also reported. Large outbreaks with other flaviviruses, such as West Nile virus and Zika virus, indicate that one should be vigilant for yet other outbreaks. To allow the identification of inhibitors of USUV replication, we established in vitro antiviral assays, which were validated using a small selection of known flavivirus inhibitors, including the broad-spectrum viral RNA polymerase inhibitor favipiravir (T-705). Next, an USUV infection model in AG129 (IFN-α/β and IFN-γ receptor knockout) mice was established. AG129 mice proved highly susceptible to USUV; an inoculum as low as 10² PFU (1.3 × 10⁵ TCID₅₀) resulted in the development of symptoms as early as 3 days post infection with viral RNA being detectable in various tissues. Treatment of mice with favipiravir (150 mg/kg/dose, BID, oral gavage) significantly reduced viral load in blood and tissues and significantly delayed virus-induced disease. This USUV mouse model is thus amenable for assessing the potential in vivo efficacy of (novel) USUV/flavivirus inhibitors.

[Usutu virus: the phantom menace]

Usutu virus, an arbovirus discovered in Africa in 1959, has spread over a large part of Europe over the last twenty years causing significant bird mortality as reported in France since 2015. The zoonotic risk, associated with this succession of avian epizootics in Europe, deserves to be taken into account even if human cases remain rare to date. Human infections are most often asymptomatic or present a benign clinical expression. However, neurological complications such as encephalitis or meningoencephalitis have been described. In addition, the recent description of an atypical case of facial paralysis in France suggests that the clinical spectrum of infections caused by Usutu virus is not fully characterized. Finally, the recent history of other arboviral outbreaks invites the scientific community to be extremely vigilant.

Usutu virus induced mass mortalities of songbirds in Central Europe: Are habitat models suitable to predict dead birds in unsampled regions?

The Usutu virus (USUV) is a mosquito-borne flavivirus closely related to the better known West Nile virus, and it can cause mass mortalities of song birds. In the present paper, a dataset of georeferenced locations of USUV-positive birds was compiled and then used to map the geographical distribution of suitable USUV habitats in Central Europe. Six habitat models, comprising BIOCLIM, DOMAIN, maximum entropy model (MAXENT), generalized linear model (GLM), boosted regression trees model (BRT), and random forests model (RF), were selected and tested for their performance ability to predict cases of disease in unsampled areas. Suitability index maps, a diagram depicting model performance by the Area Under the Curve (AUC) vs. the True Skill Statistic (TSS), and a diagram ranking sensitivity vs. specificity as well as correct classification ratio (CCR) vs. misclassification ratio (MCR) were presented. Of the models tested GLM, BRT, RF, and MAXENT were shown suitable to predict USUV-positive dead birds in unsampled regions, with BRT the highest predictive accuracy (AUC = 0.75, TSS = 0.50). However, the four models classified major parts of the model domain as USUV-suitable, although USUV was never confirmed there so far (MCR=0.49 to 0.61). DOMAIN and especially BIOCLIM can only be recommended for interpolating point observations to raster files, i.e. for analyzing observed USUV distributions (MCR = 0.10). Habitat models can be a helpful tool for informing veterinary authorities about the possible distribution of a given mosquito-borne disease. Nevertheless, it should be taken in consideration, that the spatial and temporal scales, the selection of an appropriate model, the availability of significant predictive variables as well as the representativeness and completeness of collected species or disease cases may strongly influence the modeling results.

Serological Evidence of Mosquito-Borne Flaviviruses Circulation in Hunting Dogs in Campania Region, Italy

A Flavivirus survey on 183 hunting dogs was conducted in Campania region, Southern Italy. The seroprevalence value of 40.43% (74/183, 95% confidence intervals [CIs] 33.37-47.49) detected in our study using a competitive enzyme-linked immunosorbent serologic assay (cELISA) proves a considerable level of Flavivirus exposition of these animals. Among the 74 cELISA-positive sera, seroneutralization (SN) test showed that 24 sera resulted positive for Usutu virus with an overall prevalence of 13.11% (24/183) (95% CI 8.27-17.95), but none of cELISA-positive samples resulted positive for West Nile virus. Data analysis showed a significant difference of cELISA seropositivity risk factors in case of presence of farm animals in contact with hunting dogs and for dogs living in a rural environment but not for gender, age, management, hunting season, and hunting abroad. A RT-PCR assay was performed to detect the Flavivirus RNA, but none of the blood samples tested positive. This study documents the first report regarding the circulation of Flavivirus in hunting dog in Southern Italy and suggests the dog as an interesting target to monitor Flavivirus circulation.

What Does the Future Hold for Yellow Fever Virus? (II)

As revealed by the recent resurgence of yellow fever virus (YFV) activity in the tropical regions of Africa and South America, YFV control measures need urgent rethinking. Over the last decade, most reported outbreaks occurred in, or eventually reached, areas with low vaccination coverage but that are suitable for virus transmission, with an unprecedented risk of expansion to densely populated territories in Africa, South America and Asia. As reflected in the World Health Organization's initiative launched in 2017, it is high time to strengthen epidemiological surveillance to monitor accurately viral dissemination, and redefine vaccination recommendation areas. Vector-control and immunisation measures need to be adapted and vaccine manufacturing must be reconciled with an increasing demand. We will have to face more yellow fever (YF) cases in the upcoming years. Hence, improving disease management through the development of efficient treatments will prove most beneficial. Undoubtedly, these developments will require in-depth descriptions of YFV biology at molecular, physiological and ecological levels. This second section of a two-part review describes the current state of knowledge and gaps regarding the molecular biology of YFV, along with an overview of the tools that can be used to manage the disease at the individual, local and global levels.

Are tree squirrels involved in the circulation of flaviviruses in Italy?

West Nile virus (WNV), Usutu virus (USUV) and tick-borne encephalitis virus (TBEV) are emerging zoonotic flaviviruses (family Flaviviridae), which have circulated in Europe in the past decade. A cross-sectional study was conducted to assess exposure to these antigenically related flaviviruses in eastern grey squirrels (Sciurus carolinensis) in Italy. Seventeen out of 158 (10.8%; CI_95% : 5.9-15.6) squirrels' sera tested through bELISA had antibodies against flaviviruses. Specific neutralizing antibodies to WNV, USUV and TBEV were detected by virus neutralization tests. Our results indicate that tree squirrels are exposed to Culex and tick-borne zoonotic flaviviruses in Italy. Moreover, this study shows for the first time USUV and TBEV exposure in grey squirrels, broadening the host range reported for these viruses. Even though further studies are needed to define the real role of tree squirrels in the epidemiology of flaviviruses in Europe, this study highlights that serology could be an effective approach for future investigations aimed at broadening our knowledge about the species exposed to these zoonotic infections.