Insight on Bluetongue virus transmission in small ruminants in Senegal.

Bluetongue (BT) is an infectious, arthropod-borne viral disease of domestic and wild ruminants. The disease causes animal mortality, production decrease and commercial limits for herds. Despite the active circulation of the disease in the world, few studies have been carried out in Senegal. The objective of this study was to assess the current prevalence of BT in small ruminants and the serotypes circulating in Senegal. A cross-sectional study was conducted in the fourteen regions of Senegal. After the sampling campaign, sera collected in sheep and goats herds were screened for the presence of Bluetongue virus (BTV) specific antibodies using c-Elisa. The whole blood of seropositive animals was further analyzed by RT-qPCR and positive samples were typed to identify BTV serotypes. Analysis of several risk factors such as age, sex and species of animals was performed using logistic regression. The overall seroprevalence of BTV in Senegal was 72.6% (95% CI: 70.3-74.9%) with 75.9% (95% CI: 72.2-79.5%) in goat and 70.6% (95% CI: 67.5-73.6%) in sheep. Female (prevalence=77.1%) and adult (prevalence=80%) animals showed the highest seropositivity to BTV compared respectively to male (55.7%, p=6.133e-09) and young (49.4%, p < 2.2e-16). The RT-qPCR results showed the presence of BT viral genome in 359 small ruminants. The results obtained from serological and genotyping studies showed an active spread of the Bluetongue virus in domestic ruminants and phylogenetic analysis showed that the BTV-2 is one of the circulating serotypes in Senegal. This study allows having baseline information for controlling Bluetongue in Senegal.

Development and Validation of an ELISA for the Detection of Bluetongue Virus Serotype 4-Specific Antibodies.

In this article, we describe the development and evaluation of a double antigen sandwich enzyme-linked immunosorbent assay (ELISA) able to detect serotype 4-specific antibodies from BTV-4 infected or vaccinated animals using a recombinant BTV-4 VP2 protein. The coding sequence of VP2 was inserted into a pVote plasmid by recombination in the Gateway® cloning system. Vaccinia virus (VacV) was used as a vector for the expression of the recombinant VP2. After production in BSR cells, recombinant VP2 was purified by immunoprecipitation using a FLAG tag and then used both as the coated ELISA antigen and as the HRP-tagged conjugate. The performance of the ELISA was evaluated with 1186 samples collected from BTV negative, infected or vaccinated animals. The specificity and sensitivity of the BTV-4 ELISA were above the expected standards for the detection of anti-BTV-4 VP2 antibodies in animals reared in Europe or in the Mediterranean basin. Cross-reactions were observed with reference sera for serotypes 10 and 20, and to a lesser extent with serotypes 12, 17 and 24, due to their genetic proximity to serotype 4. Nevertheless, these serotypes have never been detected in Europe and the Mediterranean area. This ELISA, which requires only the production of a recombinant protein, can be used to detect BTV serotype 4-specific antibodies and is therefore an attractive alternative diagnostic method to serum neutralization.

The VP3 Protein of Bluetongue Virus Associates with the MAVS Complex and Interferes with the RIG-I-Signaling Pathway.

Bluetongue virus (BTV), an arbovirus transmitted by Culicoides biting midges, is a major concern of wild and domestic ruminants. While BTV induces type I interferon (alpha/beta interferon [IFN-α/ß]) production in infected cells, several reports have described evasion strategies elaborated by this virus to dampen this intrinsic, innate response. In the present study, we suggest that BTV VP3 is a new viral antagonist of the IFN-ß synthesis. Indeed, using split luciferase and coprecipitation assays, we report an interaction between VP3 and both the mitochondrial adapter protein MAVS and the IRF3-kinase IKKε. Overall, this study describes a putative role for the BTV structural protein VP3 in the control of the antiviral response.

The Genome Segments of Bluetongue Virus Differ in Copy Number in a Host-Specific Manner.

Genome segmentation is mainly thought to facilitate reassortment. Here, we show that segmentation can also allow differences in segment abundance in populations of bluetongue virus (BTV). BTV has a genome consisting in 10 segments, and its cycle primarily involves periodic alternation between ruminants and Culicoides biting midges. We have developed a reverse transcription-quantitative PCR (RT-qPCR) approach to quantify each segment in wild BTV populations sampled in both ruminants and midges during an epizootic. Segment frequencies deviated from equimolarity in all hosts. Interestingly, segment frequencies were reproducible and distinct between ruminants and biting midges. Beyond a putative regulatory role in virus expression, this phenomenon could lead to different evolution rates between segments.IMPORTANCE The variation in viral gene frequencies remains a largely unexplored aspect of within-host genetics. This phenomenon is often considered to be specific to multipartite viruses. Multipartite viruses have segmented genomes, but in contrast to segmented viruses, their segments are each encapsidated alone in a virion. A main hypothesis explaining the evolution of multipartism is that, compared to segmented viruses, it facilitates the regulation of segment abundancy, and the genes the segments carry, within a host. These differences in gene frequencies could allow for expression regulation. Here, we show that wild populations of a segmented virus, bluetongue virus (BTV), also present unequal segment frequencies. BTV cycles between ruminants and Culicoides biting midges. As expected from a role in expression regulation, segment frequencies tended to show specific values that differed between ruminants and midges. Our results expand previous knowledge on gene frequency variation and call for studies on its role and conservation beyond multipartite viruses.

"Frozen evolution" of an RNA virus suggests accidental release as a potential cause of arbovirus re-emergence.

The mechanisms underlying virus emergence are rarely well understood, making the appearance of outbreaks largely unpredictable. Bluetongue virus serotype 8 (BTV-8), an arthropod-borne virus of ruminants, emerged in livestock in northern Europe in 2006, spreading to most European countries by 2009 and causing losses of billions of euros. Although the outbreak was successfully controlled through vaccination by early 2010, puzzlingly, a closely related BTV-8 strain re-emerged in France in 2015, triggering a second outbreak that is still ongoing. The origin of this virus and the mechanisms underlying its re-emergence are unknown. Here, we performed phylogenetic analyses of 164 whole BTV-8 genomes sampled throughout the two outbreaks. We demonstrate consistent clock-like virus evolution during both epizootics but found negligible evolutionary change between them. We estimate that the ancestor of the second outbreak dates from the height of the first outbreak in 2008. This implies that the virus had not been replicating for multiple years prior to its re-emergence in 2015. Given the absence of any known natural mechanism that could explain BTV-8 persistence over this long period without replication, we hypothesise that the second outbreak could have been initiated by accidental exposure of livestock to frozen material contaminated with virus from approximately 2008. Our work highlights new targets for pathogen surveillance programmes in livestock and illustrates the power of genomic epidemiology to identify pathways of infectious disease emergence.

Evaluation of a commercial ELISA for detection of epizootic haemorrhagic disease antibodies in domestic and wild ruminant sera.

Bluetongue (BT) and epizootic haemorrhagic disease (EHD) are vector-borne viral diseases affecting domestic and wild ruminants. Both are notifiable under OIE rules. BT and EHD viruses (BTV and EHDV) are closely related Orbiviruses with structural, antigenic and molecular similarities. Both viruses can produce analogous clinical signs in susceptible animals. Serological tests are commonly used for BT and EHD diagnosis and surveillance. Competitive ELISA (c-ELISA) is the most widely used serological test for the specific detection of BTV or EHDV viral protein 7 (VP7) antibodies (Abs). The specificity and sensitivity of the BTV c-ELISA kits available on the market are recognized for the detection of BTV Abs. Concerning EHD, a single commercial EHDV c-ELISA kit (ELISA A kit) commonly used for diagnosis in Europe and Africa was available between 2011 and 2018 but is now no longer on the market. In this study, we evaluated a new commercial c-ELISA to detect ruminant EHDV VP7 Abs in 2,199 serum samples from cattle, sheep, goats, wild deer and zoo animals. The results showed that this ELISA kit is specific and can detect the presence of IgG anti-EHDV VP7 with a very good diagnostic specificity and a satisfactory sensitivity in domestic ruminants, zoo animals and wild deer. Therefore, the evaluated c-ELISA can detect the introduction of EHDV into an area where BTV-seropositive domestic animals are present. The performance of this kit is similar to that of the c-ELISA A kit and can thus be used for diagnosis.

Clinical cases of Bluetongue serotype 8 in calves in France in the 2018-2019 winter.

Bluetongue virus serotype 8 (BTV-8) caused an epizootic in Europe in 2006/09. Transplacental transmission of BTV-8 was demonstrated leading to abortions, congenital malformations or nervous clinical signs in newborn calves. BTV-8 re-emerged in France in 2015. Although the re-emergent strain is nearly genetically identical to the one that had circulated in 2006/2009, it has caused very few clinical cases. However, from mid-December 2018 to April 2019, cases of calves with congenital malformations or displaying nervous clinical signs occurred in some departments (French administrative unit) in mainland France. Blood samples from these animals were sent to local laboratories, and the positive ones were confirmed at the French Bluetongue reference laboratory (BT-NRL). Out of 580 samples found positive at the local laboratories, 544 were confirmed as RT-PCR BTV-8 positive. The 36 samples found positive in the local laboratories and negative in the BT-NRL were all at the limit of RT-PCR detection. Hundred eighty-eight of the confirmed samples were also tested for the presence of Schmallenberg virus (SBV) and bovine virus diarrhoea virus (BVDV) infection: 4 were found positive for BVDV and none for SBV. The main clinical signs recorded for 244 calves, for which a reporting form was completed by veterinarians, included nervous clinical signs (81%), amaurosis (72%) and decrease/ no suckling reflex (40%). Hydranencephaly and microphthalmia were reported in 19 calves out of 27 in which a necropsy was practiced after death or euthanasia. These results indicate that the re-emergent strain of BTV-8 can cross the transplacental barrier and cause congenital malformations or nervous clinical signs in calves.

Red deer (Cervus elaphus) Did Not Play the Role of Maintenance Host for Bluetongue Virus in France: The Burden of Proof by Long-Term Wildlife Monitoring and Culicoides Snapshots.

Bluetongue virus (BTV) is a Culicoides-borne pathogen infecting both domestic and wild ruminants. In Europe, the Red Deer (Cervus elaphus) (RD) is considered a potential BTV reservoir, but persistent sylvatic cycle has not yet been demonstrated. In this paper, we explored the dynamics of BTV1 and BTV8 serotypes in the RD in France, and the potential role of that species in the re-emergence of BTV8 in livestock by 2015 (i.e., 5 years after the former last domestic cases). We performed 8 years of longitudinal monitoring (2008-2015) among 15 RD populations and 3065 individuals. We compared Culicoides communities and feeding habits within domestic and wild animal environments (51,380 samples). Culicoides diversity (>30 species) varied between them, but bridge-species able to feed on both wild and domestic hosts were abundant in both situations. Despite the presence of competent vectors in natural environments, BTV1 and BTV8 strains never spread in RD along the green corridors out of the domestic outbreak range. Decreasing antibody trends with no PCR results two years after the last domestic outbreak suggests that seropositive young RD were not recently infected but carried maternal antibodies. We conclude that RD did not play a role in spreading or maintaining BTV in France.

Bluetongue Virus in France: An Illustration of the European and Mediterranean Context since the 2000s.

Bluetongue (BT) is a non-contagious animal disease transmitted by midges of the Culicoides genus. The etiological agent is the BT virus (BTV) that induces a variety of clinical signs in wild or domestic ruminants. BT is included in the notifiable diseases list of the World Organization for Animal Health (OIE) due to its health impact on domestic ruminants. A total of 27 BTV serotypes have been described and additional serotypes have recently been identified. Since the 2000s, the distribution of BTV has changed in Europe and in the Mediterranean Basin, with continuous BTV incursions involving various BTV serotypes and strains. These BTV strains, depending on their origin, have emerged and spread through various routes in the Mediterranean Basin and/or in Europe. Consequently, control measures have been put in place in France to eradicate the virus or circumscribe its spread. These measures mainly consist of assessing virus movements and the vaccination of domestic ruminants. Many vaccination campaigns were first carried out in Europe using attenuated vaccines and, in a second period, using exclusively inactivated vaccines. This review focuses on the history of the various BTV strain incursions in France since the 2000s, describing strain characteristics, their origins, and the different routes of spread in Europe and/or in the Mediterranean Basin. The control measures implemented to address this disease are also discussed. Finally, we explain the circumstances leading to the change in the BTV status of France from BTV-free in 2000 to an enzootic status since 2018.

Novel Function of Bluetongue Virus NS3 Protein in Regulation of the MAPK/ERK Signaling Pathway.

Bluetongue virus (BTV) is an arbovirus transmitted by blood-feeding midges to a wide range of wild and domestic ruminants. In this report, we showed that BTV, through its nonstructural protein NS3 (BTV-NS3), is able to activate the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway, as assessed by phosphorylation levels of ERK1/2 and the translation initiation factor eukaryotic translation initiation factor 4E (eIF4E). By combining immunoprecipitation of BTV-NS3 and mass spectrometry analysis from both BTV-infected and NS3-transfected cells, we identified the serine/threonine-protein kinase B-Raf (BRAF), a crucial player in the MAPK/ERK pathway, as a new cellular interactor of BTV-NS3. BRAF silencing led to a significant decrease in the MAPK/ERK activation by BTV, supporting a model wherein BTV-NS3 interacts with BRAF to activate this signaling cascade. This positive regulation acts independently of the role of BTV-NS3 in counteracting the induction of the alpha/beta interferon response. Furthermore, the intrinsic ability of BTV-NS3 to bind BRAF and activate the MAPK/ERK pathway is conserved throughout multiple serotypes/strains but appears to be specific to BTV compared to other members of Orbivirus genus. Inhibition of MAPK/ERK pathway with U0126 reduced viral titers, suggesting that BTV manipulates this pathway for its own replication. Altogether, our data provide molecular mechanisms that unravel a new essential function of NS3 during BTV infection.IMPORTANCE Bluetongue virus (BTV) is responsible of the arthropod-borne disease bluetongue (BT) transmitted to ruminants by blood-feeding midges. In this report, we found that BTV, through its nonstructural protein NS3 (BTV-NS3), interacts with BRAF, a key component of the MAPK/ERK pathway. In response to growth factors, this pathway promotes cell survival and increases protein translation. We showed that BTV-NS3 enhances the MAPK/ERK pathway, and this activation is BRAF dependent. Treatment of MAPK/ERK pathway with the pharmacologic inhibitor U0126 impairs viral replication, suggesting that BTV manipulates this pathway for its own benefit. Our results illustrate, at the molecular level, how a single virulence factor has evolved to target a cellular function to increase its viral replication.

Presence of bluetongue and epizootic hemorrhagic disease viruses in Egypt in 2016 and 2017.

BTV and EHDV are closely-related orbiviruses that are transmitted between domestic and wild ruminants via the bites of hematophagous midges. Previous studies have reported seropositivity against BTV antibodies in sheep and goats in two Egyptian governorates (Beni Suef and Menoufia). However, no recent data are available on the BTV serotype(s) circulating in Egypt and the likely presence of EHDV has never been explored. This study investigated the presence of BTV and EHDV among cattle which had been found BTV-seropositive by ELISA method. These cattle living in proximity to sheep and goats previously found BTV-seropositive. These cattle displayed no clinical signs of BT but reproductive problems had been reported in herds. A total of 227 cattle blood samples were therefore collected in 2016 and 2017. Ninety-four of the 227 animals tested by a BTV ELISA were positive for BTV antibodies (41.4%). Of these 94 ELISA-positive cattle, only 83 EDTA-blood samples were available and therefore tested for BTV and EHDV genome detection by RT-PCR and sequencing. Of the cattle sampled in 2016, results revealed that two were RT-PCR-positive for BTV and seven for EHDV. Sequencing showed the presence of EHDV-1 and BTV-3 genome sequences. EHDV-1 S2 shared 99.5% homology with an EHDV-1 S2 from a strain isolated in 2016 in Israel. BTV-3 S2 and S8 sequences shared >99.8% nucleotide similarity with the BTV-3 Zarzis S2 and S8 sequences (Tunisian BTV, also detected in 2016). Of the 66 blood samples tested following their collection in 2017, they were all EHDV-negative by RT-qPCR while five were BTV- positive by RT-qPCR. However, attempts to identify the BTV serotype of these five samples were unsuccessful. Only part of BTV S8 was sequenced and it showed 79% nucleotide similarity with S8 of atypical BTV serotypes (particularly with BTV-26 and another BTV serotype strain isolated from a sheep pox vaccine). Overall, these findings demonstrate that both BTV and EHDV were circulating in Egypt in 2016 and 2017.

Bluetongue virus and epizootic hemorrhagic disease virus survey in cattle of the Galapagos Islands.

Bluetongue virus (BTV) and epizootic hemorrhagic disease virus (EHDV) have both been reported in mainland Ecuador, but their occurrence was unknown in the Galapagos Islands, an Ecuadorian province. We aimed to detect BTV or EHDV in cattle from the 3 main cattle-producing Galapagos Islands at a between-herd design prevalence of 20% and a within-herd design prevalence of 15%. Blood samples were collected from 410 cattle in 33 farms and tested for antibodies against BTV and EHDV by competitive ELISAs. All results were negative, suggesting that BTV and EHDV are not present in the Galapagos Islands.

Experimental infection of calves with seven serotypes of Epizootic Hemorrhagic Disease virus: production and characterization of reference sera.

The aim of this study was to produce reference sera against the seven serotypes of Epizootic hemorrhagic disease virus (EHDV­1, EHDV­2, EHDV­4, EHDV­5, EHDV­6, EHDV­7, and EHDV­8). In a high containment unit, seven Prim 'Holstein calves were inoculated at day 0 (D0) with the selected strains (1 EHDV serotype per calf ). Blood samples (EDTA and whole blood) were periodically taken from D0 until the end of the experiment (D31). Sera were tested with two commercially available EHDV competitive ELISAs (c­ELISA). Viral genome was detected from EDTA blood samples using in­house real­time RT­PCR. Sera taken on D31 post infection (pi) were tested and characterized by serum neutralization test (SNT) and virus neutralization test (VNT) (for calibration of reference sera). Viral RNA was first detected at D2 pi in five calves. All infected animals were RT­PCR positive at D7 pi. Seroconversion was observed between D10 and D23 pi depending on the EHDV serotype. SNT and VNT have allowed to determine the neutralizing antibody titers of each serum and the potential cross­reactions between serotypes. The two c­ELISA used in this study showed similar results. The calibrated sera are now available for the serological identification of an EHDV isolated on tissue culture or to be used as positive control in seroneutralization assay.

Evaluation of an IGM-specific ELISA for early detection of bluetongue virus infections in domestic ruminants sera.

Competitive-ELISA (c-ELISA) is the most widely used serological test for the detection of Bluetongue virus (BTV) viral protein 7 (VP7) antibodies (Ab). However, these BTV c-ELISAs cannot to distinguish between IgG and IgM. IgM Ab are generated shortly after the primary immune response against an infectious agent, indicating a recent infection or exposure to antigens, such as after vaccination. Because the BTV genome or anti-VP7 Ab can be detected in ruminant blood months after infection, BTV diagnostic tools cannot discriminate between recent and old infections. In this study, we evaluated an IgM-capture ELISA prototype to detect ruminant anti-BTV VP7 IgM on 1,650 serum samples from cattle, sheep, or goats. Animals were BTV-naive, infected, or/and vaccinated with BTV-1, -2, -4, -8, -9, -16, or -27, and we also included 30 sera from cattle infected with the Epizootic haemorrhagic disease virus (EHDV) serotype 6. Results demonstrated that this ELISA kit is specific and can detect the presence of IgM with satisfactory diagnostic specificity and sensitivity from 1 to 5 weeks after BTV infection in domestic ruminants (for goats and cattle; for sheep, at least up to 24 days). The peak of anti-VP7 IgM was reached when the level of infectious viruses and BTV RNA in blood were the highest. The possibility of detecting BTV-RNA in IgM-positive sera allows the amplification and sequencing of the partial RNA segment 2 (encoding the serotype specific to VP2) to determine the causative BTV serotype/strain. Therefore, BTV IgM ELISA can detect the introduction of BTV (or EHDV) in an area with BTV-seropositive domestic animals regardless of their serological BTV status. This approach may also be of particular interest for retrospective epidemiological studies on frozen serum samples.

Evidence of bluetongue and Epizootic Haemorrhagic disease circulation on the island of Mayotte.

A cross-sectional study was conducted to explore the epidemiological situation in Mayotte regarding two orbiviruses: Bluetongue virus (BTV) and Epizootic Haemorrhagic Disease virus (EHDV). In all, 385 individual asymptomatic cattle were blood-sampled (one EDTA and one serum tube per animal) between February and June 2016. Antibody (ELISA) and genome prevalence (PCR) was assessed. Almost all the selected cattle showed antibodies against both BTV and EHDV, at 99.5% (CI95% [98.00, 100]) and 96.9% (CI95% [94.5, 98.3]), respectively. Most of the cattle acquired antibodies in their first years of age. EHDV and BTV genomes were detected in 25.2% (CI95% [21.1, 29.8]) and 18.2% (CI95% [14.6, 22.4]) of samples, respectively. Coinfection with BTV and EHDV was observed in 9.4% of samples (CI95% [6.8, 12.7]). Cattle under three years old were more frequently reported as positive for genome detection by PCR than older cattle. Five serotypes of BTV and one serotype of EHDV were identified from eight samples: BTV-4, BTV-9, BTV-11, BTV-15, BTV-19 and EHDV-6, of which some were reported in neighbouring areas. BTV and EHDV both circulate in Mayotte and in its surrounding territories.

Emergence of bluetongue virus serotype 4 in mainland France in November 2017.

In November 2017, a 15-day-old calf located in France (Haute-Savoie department) was found positive for bluetongue virus (BTV) RNA by RT-PCR. Laboratory investigations allowed the isolation and identification of the serotype: BTV-4. The analysis of the full viral genome showed that all the 10 genome segments were closely related to BTV-4 strains involved in a large BT outbreak in the Balkan Peninsula, in Italy since 2014 and in Corsica since the end of October 2016. These results together with epidemiological data suggest that BTV-4 has been introduced to mainland France from Corsica or Italy where BTV-4 outbreaks have been reported in summer and autumn 2016. This is the first report of the introduction of BTV-4 in mainland France.

Fetopathic effects of experimental Schmallenberg virus infection in pregnant goats.

Schmallenberg virus (SBV) is an emerging virus responsible for congenital malformations in the offspring of domestic ruminants. It is speculated that infection of pregnant dams may also lead to a significant number of unrecognized fetal losses during the early period of gestation. To assess the pathogenic effects of SBV infection of goats in early pregnancy, we inoculated dams at day 28 or 42 of gestation and followed the animals until day 55 of gestation. Viremia in the absence of clinical signs was detected in all virus-inoculated goats. Fetal deaths were observed in several goats infected at day 28 or 42 of gestation and were invariably associated with the presence of viral genomic RNA in the affected fetuses. Among the viable fetuses, two displayed lesions in the central nervous system (porencephaly) in the presence of viral genome and antigen. All fetuses from goats infected at day 42 and the majority of fetuses from goats infected at day 28 of gestation contained viral genomic RNA. Viral genome was widely distributed in these fetuses and their respective placentas, and infectious virus could be isolated from several organs and placentomes of the viable fetuses. Our results show that fetuses of pregnant goats are susceptible to vertical SBV infection during early pregnancy spanning at least the period between day 28 and 42 of gestation. The outcomes of experimental SBV infection assessed at day 55 of gestation include fetal mortalities, viable fetuses displaying lesions of the central nervous system, as well as viable fetuses without any detectable lesion.

Ring trial 2016 for Bluetongue virus detection by real-time RT-PCR in France.

Since the unexpected emergence of BTV-8 in Northern Europe and the incursion of BTV-8 and 1 in France in 2006-2007, molecular diagnosis has considerably evolved. Several real-time RT-PCR (rtRT-PCR) methods have been developed and published, and are currently being used in many countries across Europe for BTV detection and typing. In France, the national reference laboratory (NRL) for orbiviruses develops and validates 'ready-to-use' kits with private companies for viral RNA detection. The regional laboratories network that was set up to deal with a heavy demand for analyses has used these available kits. From 2007, ring tests were organized to monitor the performance of the French laboratories. This study presents the results of 63 regional laboratories in the ring trial organized in 2016. Blood samples were sent to the laboratories. Participants were asked to use the rtRT-PCR methods in place in their laboratory, for detection of all BTV serotypes and specifically BTV-8. The French regional laboratories are able to detect and genotype BTV in affected animals. Despite the use of several methods (i.e. RNA extraction and different commercial rtRT-PCRs), the network is homogeneous. The ring trial demonstrated that the French regional veterinary laboratories have reliable and robust BTV diagnostic tools for BTV genome detection.

Nonstructural Protein NSs of Schmallenberg Virus Is Targeted to the Nucleolus and Induces Nucleolar Disorganization.

Schmallenberg virus (SBV) was discovered in Germany in late 2011 and then spread rapidly to many European countries. SBV is an orthobunyavirus that causes abortion and congenital abnormalities in ruminants. A virus-encoded nonstructural protein, termed NSs, is a major virulence factor of SBV, and it is known to promote the degradation of Rpb1, a subunit of the RNA polymerase II (Pol II) complex, and therefore hampers global cellular transcription. In this study, we found that NSs is mainly localized in the nucleus of infected cells and specifically appears to target the nucleolus through a nucleolar localization signal (NoLS) localized between residues 33 and 51 of the protein. NSs colocalizes with nucleolar markers such as B23 (nucleophosmin) and fibrillarin. We observed that in SBV-infected cells, B23 undergoes a nucleolus-to-nucleoplasm redistribution, evocative of virus-induced nucleolar disruption. In contrast, the nucleolar pattern of B23 was unchanged upon infection with an SBV recombinant mutant with NSs lacking the NoLS motif (SBVΔNoLS). Interestingly, unlike wild-type SBV, the inhibitory activity of SBVΔNoLS toward RNA Pol II transcription is impaired. Overall, our results suggest that a putative link exists between NSs-induced nucleolar disruption and its inhibitory function on cellular transcription, which consequently precludes the cellular antiviral response and/or induces cell death. IMPORTANCE: Schmallenberg virus (SBV) is an emerging arbovirus of ruminants that spread in Europe between 2011 and 2013. SBV induces fetal abnormalities during gestation, with the central nervous system being one of the most affected organs. The virus-encoded NSs protein acts as a virulence factor by impairing host cell transcription. Here, we show that NSs contains a nucleolar localization signal (NoLS) and induces disorganization of the nucleolus. The NoLS motif in the SBV NSs is absolutely necessary for virus-induced inhibition of cellular transcription. To our knowledge, this is the first report of nucleolar functions for NSs within the Bunyaviridae family.