Large human outbreak of West Nile virus infection in north-eastern Italy in 2012

Circulation of West Nile Virus and Usutu Virus in Europe: Overview and Challenges

West Nile Virus (WNV) and Usutu Virus (USUV) are both neurotropic mosquito-borne viruses belonging to the Flaviviridae family. These closely related viruses mainly follow an enzootic cycle involving mosquitoes as vectors and birds as amplifying hosts, but humans and other mammals can also be infected through mosquito bites. WNV was first identified in Uganda in 1937 and has since spread globally, notably in Europe, causing periodic outbreaks associated with severe cases of neuroinvasive diseases such as meningitis and encephalitis. USUV was initially isolated in 1959 in Swaziland and has also spread to Europe, primarily affecting birds and having a limited impact on human health. There has been a recent expansion of these viruses' geographic range in Europe, facilitated by factors such as climate change, leading to increased human exposure. While sharing similar biological traits, ecology, and epidemiology, there are significant distinctions in their pathogenicity and their impact on both human and animal health. While WNV has been more extensively studied and is a significant public health concern in many regions, USUV has recently been gaining attention due to its emergence in Europe and the diversity of its circulating lineages. Understanding the pathophysiology, ecology, and transmission dynamics of these viruses is important to the implementation of effective surveillance and control measures. This perspective provides a brief overview of the current situation of these two viruses in Europe and outlines the significant challenges that need to be addressed in the coming years.

Artificial intelligence to predict West Nile virus outbreaks with eco-climatic drivers

Background

In Europe, the frequency, intensity, and geographic range of West Nile virus (WNV)-outbreaks have increased over the past decade, with a 7.2-fold increase in 2018 compared to 2017, and a markedly expanded geographic area compared to 2010. The reasons for this increase and range expansion remain largely unknown due to the complexity of the transmission pathways and underlying disease drivers. In a first, we use advanced artificial intelligence to disentangle the contribution of eco-climatic drivers to WNV-outbreaks across Europe using decade-long (2010-2019) data at high spatial resolution.

Methods

We use a high-performance machine learning classifier, XGBoost (eXtreme gradient boosting) combined with state-of-the-art XAI (eXplainable artificial intelligence) methodology to describe the predictive ability and contribution of different drivers of the emergence and transmission of WNV-outbreaks in Europe, respectively.

Findings

Our model, trained on 2010-2017 data achieved an AUC (area under the receiver operating characteristic curve) score of 0.97 and 0.93 when tested with 2018 and 2019 data, respectively, showing a high discriminatory power to classify a WNV-endemic area. Overall, positive summer/spring temperatures anomalies, lower water availability index (NDWI), and drier winter conditions were found to be the main determinants of WNV-outbreaks across Europe. The climate trends of the preceding year in combination with eco-climatic predictors of the first half of the year provided a robust predictive ability of the entire transmission season ahead of time. For the extraordinary 2018 outbreak year, relatively higher spring temperatures and the abundance of Culex mosquitoes were the strongest predictors, in addition to past climatic trends.

Interpretation

Our AI-based framework can be deployed to trigger rapid and timely alerts for active surveillance and vector control measures in order to intercept an imminent WNV-outbreak in Europe.

Funding

The work was partially funded by the Swedish Research Council FORMAS for the project ARBOPREVENT (grant agreement 2018-05973).

Seroepidemiological Survey of West Nile Virus Infections in Horses from Berlin/Brandenburg and North Rhine-Westphalia, Germany

Following the introduction of the West Nile virus (WNV) into eastern Germany in 2018, increasing infections have been diagnosed in birds, equines, and humans over time, while the spread of WNV into western Germany remained unclear. We screened 437 equine sera from 2018 to 2020, excluding vaccinated horses, collected from convenience sampled patients in the eastern and western parts of Germany, for WNV-specific antibodies (ELISAs followed by virus/specific neutralization tests) and genomes (RT-qPCRs). Clinical presentations, final diagnoses, and demographic data were also recorded. In the eastern part, a total of eight horses were found WNV seropositive in 2019 (seroprevalence of 8.16%) and 27 in 2020 (13.77%). There were also two clinically unsuspected horses with WNV-specific antibodies in the western part from 2020 (2.63%), albeit travel history-related infections could not be excluded. None of the horse sera contained WNV-specific genomes. Eight horses in eastern Germany carried WNV-IgM antibodies, but only four of these showed typical clinical signs. These results underline the difficulty of detecting a WNV infection in a horse solely based on clinical signs. Thus, WNV circulation is established in the horse population in eastern Germany, but not yet in the western part.

West Nile Virus Seroprevalence in the Italian Tuscany Region from 2016 to 2019

Although in humans West Nile virus is mainly the cause of mild or sub-clinical infections, in some cases a neuroinvasive disease may occur predominantly in the elderly. In Italy, several cases of West Nile virus infection are reported every year. Tuscany was the first Italian region where the virus was identified; however, to date only two cases of infection have been reported in humans. This study aimed at evaluating the prevalence of antibodies against West Nile virus in the area of Siena Province to estimate the recent circulation of the virus. Human serum samples collected in Siena between 2016 and 2019 were tested for the presence of antibodies against West Nile virus by ELISA. ELISA positive samples were further evaluated using immunofluorescence, micro neutralization, and plaque reduction neutralization assays. In total, 1.9% (95% CI 1.2–3.1) and 1.4% (95% CI 0.8–2.4) of samples collected in 2016–2017 were positive by ELISA and immunofluorescence assay, respectively. Neutralizing antibodies were found in 0.7% (95% CI 0.3–1.5) of samples. Additionally, 0.9% (95% CI 0.4–1.7) and 0.65% (95% CI 0.3–1.45) of samples collected in 2018–2019 were positive by ELISA and immunofluorescence assay, respectively. The prevalence of neutralizing antibodies was 0.5% (95% CI 0.2–1.3). Although no human cases of West Nile infection were reported in the area between 2016 and 2019 and virus prevalence in the area of Siena Province was as low as less than 1%, the active asymptomatic circulation confirms the potential concern of this emergent virus for human health.

Phylogenetic Analysis of the 2020 West Nile Virus (WNV) Outbreak in Andalusia (Spain)

During recent decades West Nile Virus (WNV) outbreaks have continuously occurred in the Mediterranean area. In August 2020 a new WNV outbreak affected 71 people with meningoencephalitis in Andalusia and six more cases were detected in Extremadura (south-west of Spain), causing a total of eight deaths. The whole genomes of four viruses were obtained and phylogenetically analyzed in the context of recent outbreaks. The Andalusian viral samples belonged to lineage 1 and were relatively similar to those of previous outbreaks which occurred in the Mediterranean region. Here we present a detailed analysis of the outbreak, including an extensive phylogenetic study. As part on this effort, we implemented a local Nextstrain server, which has become a constituent piece of regional epidemiological surveillance, wherein forthcoming genomes of environmental samples or, eventually, future outbreaks, will be included.

Epidemiology of West Nile Virus Infections in Humans, Italy, 2012-2020: A Summary of Available Evidences

In Italy, human cases of West Nile virus (WNV) infection have been recorded since 2008, and seasonal outbreaks have occurred almost annually. In this study, we summarize available evidences on the epidemiology of WNV and West Nile neuro-invasive disease (WNND) in humans reported between 2012 and 2020. In total, 1145 WNV infection cases were diagnosed; of them 487 (42.5%) had WNND. A significant circulation of the pathogen was suggested by studies on blood donors, with annual incidence rates ranging from 1.353 (95% confidence intervals (95% CI) 0.279-3.953) to 19.069 cases per 100,000 specimens (95% CI 13.494-26.174). The annual incidence rates of WNND increased during the study period from 0.047 cases per 100,000 (95% CI 0.031-0.068) in 2012, to 0.074 cases per 100,000 (95% CI 0.054-0.099) in 2020, peaking to 0.377 cases per 100,000 (95% CI 0.330-0.429) in 2018. There were 60 deaths. Cases of WNND were clustered in Northern Italy, particularly in the Po River Valley, during the months of August (56.7%) and September (27.5%). Higher risk for WNND was reported in subjects of male sex (risk ratio (RR) 1.545, 95% CI 1.392-1.673 compared to females), and in older age groups (RR 24.46, 95% CI 15.61-38.32 for 65-74 y.o.; RR 43.7, 95% CI 28.33-67.41 for subjects older than 75 years), while main effectors were identified in average air temperatures (incidence rate ratio (IRR) 1.3219, 95% CI 1.0053-1.7383), population density (IRR 1.0004, 95% CI 1.0001-1.0008), and occurrence of cases in the nearby provinces (IRR 1.0442, 95% CI 1.0340-1.0545). In summary, an enhanced surveillance is vital for the early detection of human cases and the prompt implementation of response measures.

Evidence of Exposure to USUV and WNV in Zoo Animals in France

West Nile virus (WNV) and Usutu virus (USUV) are zoonotic arboviruses. These flaviviruses are mainly maintained in the environment through an enzootic cycle involving mosquitoes and birds. Horses and humans are incidental, dead-end hosts, but can develop severe neurological disorders. Nevertheless, there is little data regarding the involvement of other mammals in the epidemiology of these arboviruses. In this study, we performed a serosurvey to assess exposure to these viruses in captive birds and mammals in a zoo situated in the south of France, an area described for the circulation of these two viruses. A total of 411 samples comprising of 70 species were collected over 16 years from 2003 to 2019. The samples were first tested by a competitive enzyme-linked immunosorbent assay. The positive sera were then tested using virus-specific microneutralization tests against USUV and WNV. USUV seroprevalence in birds was 10 times higher than that of WNV (14.59% versus 1.46%, respectively). Among birds, greater rhea (Rhea Americana) and common peafowl (Pavo cristatus) exhibited the highest USUV seroprevalence. Infections occurred mainly between 2016-2018 corresponding to a period of high circulation of these viruses in Europe. In mammalian species, antibodies against WNV were detected in one dama gazelle (Nanger dama) whereas serological evidence of USUV infection was observed in several Canidae, especially in African wild dogs (Lycaon pictus). Our study helps to better understand the exposure of captive species to WNV and USUV and to identify potential host species to include in surveillance programs in zoos.

West Nile Virus Lineage 2 Vector Competence of Indigenous Culex and Aedes Mosquitoes from Germany at Temperate Climate Conditions

West Nile virus (WNV) is a widespread zoonotic arbovirus and a threat to public health in Germany since its first emergence in 2018. It has become of particular relevance in Germany in 2019 due to its rapid geographical spread and the detection of the first human clinical cases. The susceptibility of indigenous Culex pipiens (biotypes pipiens and molestus) for a German WNV lineage 2 strain was experimentally compared to that of Serbian Cx. pipiens biotype molestus and invasive German Aedes albopictus. All tested populations proved to be competent laboratory vectors of WNV. Culex pipiens biotype pipiens displayed the highest transmission efficiencies (40.0%–52.9%) at 25 °C. This biotype was also able to transmit WNV at 18 °C (transmission efficiencies of 4.4%–8.3%), proving that temperate climates in Central and Northern Europe may support WNV circulation. Furthermore, due to their feeding behaviors, Cx. pipiens biotype molestus and Ae. albopictus can act as “bridge vectors”, leading to human WNV infections.

Early Pathogenesis of Wesselsbron Disease in Pregnant Ewes

Wesselsbron virus (WSLV) is a neglected, mosquito-borne flavivirus that is endemic to the African continent. The virus is teratogenic to ruminants and causes a self-limiting febrile illness in humans. Wesselsbron disease manifests with similar clinical signs and occurs in the same areas under the same climatic conditions as Rift Valley fever, which is therefore included in the differential diagnosis. Although the gross pathology of WSLV infection in pregnant ewes is reported in literature, the pathogenesis that leads to stillbirths, congenital malformations and abortion has remained undescribed. In the present study, pregnant ewes were inoculated with WSLV and subjected to detailed clinical- and histopathology 8 days later. The virus was mainly detected in foetal trophoblasts of the placenta and in neural progenitor cells, differentiated neurons, oligodendrocytes, microglia and astrocytes. Our study demonstrates that WSLV efficiently crosses the maternal-foetal interface and is highly neuroinvasive in the ovine foetus.

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.

Uncovering mechanisms behind mosquito seasonality by integrating mathematical models and daily empirical population data: Culex pipiens in the UK

BACKGROUND

Many mosquito-borne diseases exhibit substantial seasonality, due to strong links between environmental variables and vector and pathogen life-cycles. Further, a range of density-dependent and density-independent biotic and abiotic processes affect the phenology of mosquito populations, with potentially large knock-on effects for vector dynamics and disease transmission. Whilst it is understood that density-independent and density-dependent processes affect seasonal population levels, it is not clear how these interact temporally to shape the population peaks and troughs. Due to this, the paucity of high-resolution data for validation, and the difficulty of parameterizing density-dependent processes, models of vector dynamics may poorly estimate abundances, which has knock-on effects for our ability predict vector-borne disease outbreaks.

RESULTS

We present a rich dataset describing seasonal abundance patterns of each life stage of Culex pipiens, a widespread vector of West Nile virus, at a field site in southern England in 2015. Abundance of immature stages was measured three times per week, whilst adult traps were run four nights each week. This dataset is integrated with an existing delay-differential equation model predicting Cx. pipiens seasonal abundance to improve understanding of observed seasonal abundance patterns. At our field site, the outcome of our model fitting suggests interspecific predation on mosquito larvae and temperature-dependent larval mortality combine to act as the main sources of population regulation throughout the active season, whilst competition for resources is a relatively small source of larval mortality.

CONCLUSIONS

The model suggests that density-independent mortality and interspecific predation interact to shape patterns of mosquito seasonal abundance in a permanent aquatic habitat and we propose that competition for resources is likely to be important where periods of high rainfall create transient habitats. Further, we highlight the importance of challenging population abundance models with data from across all life stages of the species of interest if reliable inferences are to be drawn from these models, particularly when considering mosquito control and vector-borne disease transmission.

A four-year survey (2011-2014) of West Nile virus infection in humans, mosquitoes and birds, including the 2012 meningoencephalitis outbreak in Tunisia

A West Nile virus (WNV) outbreak occurred in Tunisia between mid-July and December 2012. To assess the epidemiological features of the WNV transmission cycle, human cerebrospinal fluid samples from patients with suspected cases (n = 79), Culex pipiens mosquitoes (n = 583) and serum specimens from domestic and migratory birds (n = 70) were collected for 4 years (2011–2014) in the Tunisian Sahel region. Viral testing was performed by polymerase chain reaction (PCR). The WNV genome was detected in 7 patients (8.8%), 4 Culex pipiens pools, and a domestic mallard (Anas platyrhynchos). All PCR-positive samples were from the Monastir region. Phylogenetic analysis revealed that two different WNV strain groups circulated, and isolates from the reservoir (bird), vector (Culex pipiens), and dead-end hosts (humans) were closely related. The Monastir region is a hot-spot for WNV infection, and the reiterative presence of WNV over the years has increased the risk of viral reemergence in Tunisia, which highlights the need for more enhanced and effective WNV surveillance in humans with public awareness campaigns strengthened by monitoring mosquitoes and maintaining avian surveillance for early detection of WNV circulation.

Mosquito community influences West Nile virus seroprevalence in wild birds: implications for the risk of spillover into human populations

Mosquito community composition plays a central role in the transmission of zoonotic vector-borne pathogens. We evaluated how the mosquito community affects the seroprevalence of West Nile virus (WNV) in house sparrows along an urbanisation gradient in an area with the endemic circulation of this virus. We sampled 2544 birds and 340829 mosquitoes in 45 localities, analysed in 15 groups, each containing one urban, one rural and one natural area. WNV seroprevalence was evaluated using an epitope-blocking ELISA kit and a micro virus-neutralization test (VNT). The presence of WNV antibodies was confirmed in 1.96% and 0.67% of birds by ELISA and VNT, respectively. The VNT-seropositive birds were captured in rural and natural areas, but not in urban areas. Human population density was zero in all the localities where VNT-positive birds were captured, which potentially explains the low incidence of human WNV cases in the area. The prevalence of neutralizing antibodies against WNV was positively correlated with the abundance of the ornithophilic Culex perexiguus but negatively associated with the abundance of the mammophilic Ochlerotatus caspius and Anopheles atroparvus. These results suggest that the enzootic circulation of WNV in Spain occurs in areas with larger populations of Cx. perexiguus and low human population densities.

West Nile Virus and Usutu Virus Monitoring of Wild Birds in Germany

By systematically setting up a unique nation-wide wild bird surveillance network, we monitored migratory and resident birds for zoonotic arthropod-borne virus infections, such as the flaviviruses West Nile virus (WNV) and Usutu virus (USUV). More than 1900 wild bird blood samples, from 20 orders and 136 different bird species, were collected between 2014 and 2016. Samples were investigated by WNV and USUV-specific real-time polymerase chain reactions as well as by differentiating virus neutralization tests. Dead bird surveillance data, obtained from organ investigations in 2016, were also included. WNV-specific RNA was not detected, whereas four wild bird blood samples tested positive for USUV-specific RNA. Additionally, 73 USUV-positive birds were detected in the 2016 dead bird surveillance. WNV neutralizing antibodies were predominantly found in long-distance, partial and short-distance migrants, while USUV neutralizing antibodies were mainly detected in resident wild bird species, preferentially with low seroprevalences. To date, WNV-specific RNA has neither been detected in wild birds, nor in mosquitoes, thus, we conclude that WNV is not yet present in Germany. Continued wild bird and mosquito monitoring studies are essential to detect the incursion of zoonotic viruses and to allow risk assessments for zoonotic pathogens.

West Nile virus outbreak in Israel in 2015: phylogenetic and geographic characterization in humans and mosquitoes

OBJECTIVES

West Nile Virus (WNV) is endemic in Israel and was responsible for several outbreaks in the past 16 years. The aim of the present study was to investigate the spatial distribution of WNV acute infections from an outbreak that occurred in 2015 in Israel and report the molecular and geographic characterization of WNV isolates from human cases and mosquito pools obtained during this outbreak.

METHODS

Using a geographical layer comprising 51 continuous areas of Israel, the number of WNV infection cases per 100 000 people in each area and the locations of WNV-infected mosquitoes in 2015 were analysed. Sequencing and phylogenetic analyses followed by geographic localization were performed on 13 WNV human isolates and 19 WNV-infected mosquito pools.

RESULTS

Substantial geographical variation in the prevalence of acute WNV in patients in Israel was found and an overall correlation with WNV-infected mosquitoes. All human patients sequenced were infected only with the Mediterranean subtype of WNV Lineage 1 and resided primarily in the coastal regions in central Israel. In contrast, mosquitoes were infected with both the Mediterranean and Eastern European subtypes of WNV lineage 1; however, only the Mediterranean subtype was found in mosquitoes from the coastal region in central Israel.

CONCLUSION

These results demonstrate differential geographic dispersion in Israel of the two WNV subtypes and may also point to a differential pattern of human infections. As a geographical bridge between Europe, Asia and Africa, analysis of WNV circulation in humans and mosquitoes in Israel provides information relevant to WNV infections in Eurasia.

Emerging arboviral human diseases in Southern Europe

Southern Europe is characterized by unique landscape and climate which attract tourists, but also arthropod vectors, some of them carrying pathogens. Among several arboviral diseases that emerged in the region during the last decade, West Nile fever accounted for high number of human cases and fatalities, while Crimean-Congo hemorrhagic fever expanded its geographic distribution, and is considered as a real threat for Europe. Viruses evolve rapidly and acquire mutations making themselves stronger and naive populations more vulnerable. In an effort to tackle efficiently the emerging arboviral diseases, preparedness and strategic surveillance are needed for the early detection of the pathogen and containment and mitigation of probable outbreaks. In this review, the main human arboviral diseases that emerged in Southern Europe are described.

Noncoding Subgenomic Flavivirus RNA Is Processed by the Mosquito RNA Interference Machinery and Determines West Nile Virus Transmission by Culex pipiens Mosquitoes

Flaviviruses, such as Zika virus, yellow fever virus, dengue virus, and West Nile virus (WNV), are a serious concern for human health. Flaviviruses produce an abundant noncoding subgenomic flavivirus RNA (sfRNA) in infected cells. sfRNA results from stalling of the host 5'-3' exoribonuclease XRN1/Pacman on conserved RNA structures in the 3' untranslated region (UTR) of the viral genomic RNA. sfRNA production is conserved in insect-specific, mosquito-borne, and tick-borne flaviviruses and flaviviruses with no known vector, suggesting a pivotal role for sfRNA in the flavivirus life cycle. Here, we investigated the function of sfRNA during WNV infection of Culex pipiens mosquitoes and evaluated its role in determining vector competence. An sfRNA1-deficient WNV was generated that displayed growth kinetics similar to those of wild-type WNV in both RNA interference (RNAi)-competent and -compromised mosquito cell lines. Small-RNA deep sequencing of WNV-infected mosquitoes indicated an active small interfering RNA (siRNA)-based antiviral response for both the wild-type and sfRNA1-deficient viruses. Additionally, we provide the first evidence that sfRNA is an RNAi substrate in vivo Two reproducible small-RNA hot spots within the 3' UTR/sfRNA of the wild-type virus mapped to RNA stem-loops SL-III and 3' SL, which stick out of the three-dimensional (3D) sfRNA structure model. Importantly, we demonstrate that sfRNA-deficient WNV displays significantly decreased infection and transmission rates in vivo when administered via the blood meal. Finally, we show that transmission and infection rates are not affected by sfRNA after intrathoracic injection, thereby identifying sfRNA as a key driver to overcome the mosquito midgut infection barrier. This is the first report to describe a key biological function of sfRNA for flavivirus infection of the arthropod vector, providing an explanation for the strict conservation of sfRNA production.

IMPORTANCE

Understanding the flavivirus transmission cycle is important to identify novel targets to interfere with disease and to aid development of virus control strategies. Flaviviruses produce an abundant noncoding viral RNA called sfRNA in both arthropod and mammalian cells. To evaluate the role of sfRNA in flavivirus transmission, we infected mosquitoes with the flavivirus West Nile virus and an sfRNA-deficient mutant West Nile virus. We demonstrate that sfRNA determines the infection and transmission rates of West Nile virus in Culex pipiens mosquitoes. Comparison of infection via the blood meal versus intrathoracic injection, which bypasses the midgut, revealed that sfRNA is important to overcome the mosquito midgut barrier. We also show that sfRNA is processed by the antiviral RNA interference machinery in mosquitoes. This is the first report to describe a pivotal biological function of sfRNA in arthropods. The results explain why sfRNA production is evolutionarily conserved.

Epidemiological and clinical aspects on West Nile virus, a globally emerging pathogen

BACKGROUND

Since the isolation of West Nile virus (WNV) in 1937, in Uganda, it has spread globally, causing significant morbidity and mortality. While birds serve as amplifier hosts, mosquitoes of the Culex genus function as vectors. Humans and horses are dead end hosts. The clinical manifestations of West Nile infection in humans range from asymptomatic illness to West Nile encephalitis.

METHODS

The laboratory offers an array of tests, the preferred method being detection of RNA and serum IgM for WNV, which, if detected, confirms the clinical diagnosis. Although no definitive antiviral therapy and vaccine are available for humans, many approaches are being studied.

STUDY

This article will review the current literature of the natural cycle, geographical distribution, virology, replication cycle, molecular epidemiology, pathogenesis, laboratory diagnosis, clinical manifestations, blood donor screening for WNV, treatment, prevention and vaccines.

Evaluation of vector competence for West Nile virus in Italian Stegomyia albopicta (=Aedes albopictus) mosquitoes

West Nile virus (WNV) is a zoonotic arboviral pathogen transmitted by mosquitoes in a cycle that involves wild birds as reservoir hosts. The virus is responsible for outbreaks of viral encephalitis in humans and horses. In Europe, Culex pipiens (Diptera: Culicidae) is considered to be the main vector of WNV, but other species such as Stegomyia albopicta (=Aedes albopictus) (Diptera: Culicidae) may also act as competent vectors of this virus. Since 2008 human cases of WNV disease have been reported in northeast Italy. In 2011, new areas of southern Italy became involved and a first outbreak of WNV lineage 1 occurred on the island of Sardinia. On the assumption that a potential involvement of St. albopicta in WNV transmission cannot be excluded, and in order to evaluate the competence of this species for the virus, an experimental infection of an St. albopicta laboratory colony, established from mosquitoes collected in Sardinia, was carried out. The results were compared with those obtained in a colony of the main vector Cx. pipiens. The study showed St. albopicta collected on Sardinia to be susceptible to WNV infection, which suggests this Italian mosquito species is able to act as a possible secondary vector, particularly in urban areas where the species reaches high levels of seasonal abundance.

West Nile virus infection in children

West Nile virus (WNV) is an emerging flavivirus responsible for an increasing number of outbreaks of neuroinvasive disease in North America, Europe, and neighboring countries. Almost all WNV infections in humans are transmitted through the bite of infected mosquitoes. Transmission during pregnancy and through breastfeeding has been reported, but the risk seems to be very low. West Nile disease in children is less common (1-5% of all WNV cases) and associated with milder symptoms and better outcome than in elderly individuals, even though severe neuroinvasive disease and death have been reported also among children. However, the incidence of WNV infection and disease in children is probably underestimated and the disease spectrum is not fully understood because of lack of reporting and underdiagnosis in children. Infection is diagnosed by detection of WNV-specific antibodies in serum and WNV RNA in plasma and urine. Since no effective WNV-specific drugs are available, therapy is mainly supportive.

First report outside Eastern Europe of West Nile virus lineage 2 related to the Volgograd 2007 strain, northeastern Italy, 2014

BACKGROUND

West Nile virus (WNV) is a Flavivirus transmitted to vertebrate hosts by mosquitoes, maintained in nature through an enzootic bird-mosquito cycle. In Europe the virus became of major public health and veterinary concern in the 1990s. In Italy, WNV re-emerged in 2008, ten years after the previous outbreak and is currently endemic in many areas of the country. In particular, the northeastern part of Italy experience continuous viral circulation, with human outbreaks caused by different genovariants of WNV lineage 1, Western-European and Mediterranean subcluster, and WNV lineage 2, Hungarian clade. Alongside the WNV National Surveillance Program that has been in place since 2002, regional surveillance plans were implemented after 2008 targeting mosquitoes, animals and humans.

FINDINGS

In July and September 2014, West Nile virus lineage 2 was detected in pools of Culex pipiens s.l. mosquitoes from northeastern Italy. Whole genome sequencing and phylogenetic analysis of two representative samples identified the presence of WNV lineage 2 related to the Volgograd 2007 strain (99.3 % nucleotide sequence identity), in addition to WNV lineage 2 Hungarian clade.

CONCLUSIONS

This is the first evidence of the circulation of a WNV lineage 2 strain closely related to the Volgograd 2007 outside Eastern Europe, where it has caused large human outbreaks. This strain may pose a new threat to animal and human health in Italy.

Phylogenetic characterization of Central/Southern European lineage 2 West Nile virus: analysis of human outbreaks in Italy and Greece, 2013-2014

In recent years, West Nile virus (WNV) lineage 2 has been spreading and causing disease outbreaks in humans and animals in Europe. In order to characterize viral diversity, we performed full-length genome sequencing of WNV lineage 2 from human samples collected during outbreaks in Italy and Greece in 2013 and 2014. Phylogenetic analysis showed that these WNV lineage 2 genomes belonged to a monophyletic clade derived from a single introduction into Europe of the prototype Hungarian strain. Correlation of phylogenetic data with geospatial information showed geographical clustering of WNV genome sequences both in Italy and in Greece, indicating that the virus had evolved and diverged during its dispersal in Europe, leading to the emergence of novel genotypes, as it adapted to local ecological niches. These genotypes carried divergent conserved amino acid substitutions, which might have been relevant for viral adaptation, as suggested by selection pressure analysis and in silico and experimental modelling of sequence changes. In conclusion, the results of this study provide further information on WNV lineage 2 transmission dynamics in Europe, and emphasize the need for WNV surveillance activities to monitor viral evolution and diversity.

The challenge of West Nile virus in Europe: knowledge gaps and research priorities

West Nile virus (WNV) is continuously spreading across Europe, and other continents, i.e. North and South America and many other regions of the world. Despite the overall sporadic nature of outbreaks with cases of West Nile neuroinvasive disease (WNND) in Europe, the spillover events have increased and the virus has been introduced into new areas. The high genetic diversity of the virus, with remarkable phenotypic variation, and its endemic circulation in several countries, require an intensification of the integrated and multidisciplinary research efforts built under the 7th Framework Programme of the European Union (FP7). It is important to better clarify several aspects of WNV circulation in Europe, including its ecology, genomic diversity, pathogenicity, transmissibility, diagnosis and control options, under different environmental and socio-economic scenarios. Identifying WNV endemic as well as infection-free areas is becoming a need for the development of human vaccines and therapeutics and the application of blood and organs safety regulations. This review, produced as a joint initiative among European experts and based on analysis of 118 scientific papers published between 2004 and 2014, provides the state of knowledge on WNV and highlights the existing knowledge and research gaps that need to be addressed with high priority in Europe and neighbouring countries.

Identifying the environmental conditions favouring West Nile Virus outbreaks in Europe

West Nile Virus (WNV) is a globally important mosquito borne virus, with significant implications for human and animal health. The emergence and spread of new lineages, and increased pathogenicity, is the cause of escalating public health concern. Pinpointing the environmental conditions that favour WNV circulation and transmission to humans is challenging, due both to the complexity of its biological cycle, and the under-diagnosis and reporting of epidemiological data. Here, we used remote sensing and GIS to enable collation of multiple types of environmental data over a continental spatial scale, in order to model annual West Nile Fever (WNF) incidence across Europe and neighbouring countries. Multi-model selection and inference were used to gain a consensus from multiple linear mixed models. Climate and landscape were key predictors of WNF outbreaks (specifically, high precipitation in late winter/early spring, high summer temperatures, summer drought, occurrence of irrigated croplands and highly fragmented forests). Identification of the environmental conditions associated with WNF outbreaks is key to enabling public health bodies to properly focus surveillance and mitigation of West Nile virus impact, but more work needs to be done to enable accurate predictions of WNF risk.

The global ecology and epidemiology of West Nile virus

Since its initial isolation in Uganda in 1937 through the present, West Nile virus (WNV) has become an important cause of human and animal disease worldwide. WNV, an enveloped virus of the genus Flavivirus, is naturally maintained in an enzootic cycle between birds and mosquitoes, with occasional epizootic spillover causing disease in humans and horses. The mosquito vectors for WNV are widely distributed worldwide, and the known geographic range of WNV transmission and disease has continued to increase over the past 77 years. While most human infections with WNV are asymptomatic, severe neurological disease may develop resulting in long-term sequelae or death. Surveillance and preventive measures are an ongoing need to reduce the public health impact of WNV in areas with the potential for transmission.

Latest developments and challenges in the diagnosis of human West Nile virus infection

West Nile virus (WNV) is a mosquito-borne flavivirus responsible for an increasing number of human outbreaks of neuroinvasive disease in Europe and in North America. Notwithstanding the improvements in the knowledge of virus epidemiology and clinical course of infection and the development of new laboratory tests, the diagnosis of WNV infection remains challenging and many cases still remain unrecognized. WNV genome diversity, transient viremia with low viral load and cross-reactivity with other flaviviruses of the antibodies induced by WNV infection are important hurdles that require the diagnosis to be performed by experienced laboratories. Herein, we present and discuss the novel findings on the molecular epidemiology and clinical features of WNV infection in humans with special focus on Europe, the performance of diagnostic tests and the novel methods that have been developed for the diagnosis of WNV infection. A view on how the field might evolve in the future is also presented.

An outbreak of West Nile Virus infection in the region of Monastir, Tunisia, 2003

BACKGROUND

A West Nile (WN) fever epidemic occurred in the region of Monastir, Tunisia, between August and October 2003.

AIM OF THE STUDY

We attempt to describe the epidemiology, clinical presentation, and outcome of patients with confirmed West Nile virus (WNV) infection.

METHODS

Three groups of specimens were prepared. One was made up of serum only (n  =  43), the other of cerebrospinal fluid (CSF) only (n  =  30), and the third group was made up of both (n  =  40). These specimens were obtained from 113 patients. A serological diagnosis and evidence of WNV genome by nested reverse-transcriptase polymerase chain reaction (nRT-PCR) and TaqMan reverse transcription-polymerase chain reaction (RT-PCR) were carried out.

RESULTS

Thirty-eight cases (33.6%) were serologically positive. Results of nRT-PCR showed a total of 10 positive cases of WNV (8.8%) detected in group 1 (n  =  1/43), group 2 (n  =  5/30), and group 3 (n  =  4/40) whereas the PCR TaqMan showed 18 positive samples (15.9%) found in group 1 (n  =  3/43), group 2 (n  =  9/30), and group 3 (n  =  6/40). All TaqMan PCR positive cases were nRT-PCR positive. In addition, four serologically probable cases were confirmed by TaqMan PCR. The attempts to isolate WNV by cell culture were unsuccessful. Considering the results of TaqMan assay and the serological diagnosis, WNV infection was confirmed in a total of 42 patients. The main clinical presentations were meningoencephalitis (40%), febrile disease (95%), and meningitis (36%). Eight patients (19%) died. The highest case-fatality rates occurred among patients aged ≧55 years. The phylogenetic analysis revealed that isolates of WNV were closely related to the Tunisian strain 1997 (PAH001) and the Israeli one (Is-98).

CONCLUSIONS

West Nile virus is a reemerging global pathogen that remains an important public health challenge in the next decade.

Review of West Nile virus epidemiology in Italy and report of a case of West Nile virus encephalitis

West Nile virus (WNV) is a flavivirus that causes neurological disorders in less than 1 % of infected subjects. Human cases of WNV-associated fever and/or neurological disorders have been reported in Italy since 2008. The first outbreak occurred in the northeastern region of Italy surrounding the Po River and was caused by the Po River lineage 1 strain, and since then, WNV infections have been reported in several regions of central Italy. Although the virus is highly genetically conserved, stochastic mutations in its genome may lead to the emergence of new strains, as was observed in Italy in 2011 with the identification of two new lineage 1 strains, the WNV Piave and WNV Livenza strains. To help further define WNV epidemiology in Italy, we describe a case of an Italian man living in the Po River area who developed fatal encephalitis in 2009 due to infection with the WNV Piave strain. This finding supports the notion that the Piave strain has been circulating in this area of Italy for 2 years longer than was previously believed.

Current developments in understanding of West Nile virus central nervous system disease

PURPOSE OF REVIEW

West Nile virus (WNV) is the most important cause of epidemic encephalitis in the United States. We review articles published in the last 18 months related to the epidemiology, immunology, clinical features, and treatment of this disease.

RECENT FINDINGS

There was a resurgence in WNV disease in the United States in 2012. The WNV strain now predominant in the United States (NA/WN02) differs from the initial emergent isolate in 1999 (NY99). However, differences in the genetics of currently circulating United States WNV strains do not explain variations in epidemic magnitude or disease severity. Innate and acquired immunity are critical in control of WNV, and in some cases pathways are central nervous system specific. The clinical features of infection are now well understood, although nonconfirmed observations of chronic viral excretion in urine remain controversial. There is no specific antiviral therapy for WNV, but studies of antivirals specific for other flaviviruses may identify agents with promise against WNV. Phase I and II human WNV vaccine clinical trials have established that well tolerated and immunogenic WNV vaccines can be developed.

SUMMARY

WNV remains an important public health problem. Although recent studies have significantly increased our understanding of host immune and genetic factors involved in control of WNV infection, no specific therapy is yet available. Development of a well tolerated, immunogenic, and effective vaccine against WNV is almost certainly feasible, but economic factors and the lack of predictability of the magnitude and location of outbreaks are problematic for designing phase III trials and ultimate licensure.

Susceptibility of European jackdaws (Corvus monedula) to experimental infection with lineage 1 and 2 West Nile viruses

Mass bird mortality has been observed in North America after the introduction of West Nile virus (WNV), most notably massive die-offs of American crows (Corvus brachyrhynchos). In contrast, WNV epidemic activity in Europe has been characterized by very low incidences of bird mortality. As the general susceptibility of European corvids to strains of WNV remains in question, European jackdaws (Corvus monedula) were inoculated with WNV strains circulating currently in Greece (Greece-10), Italy (FIN and Ita09) and Hungary (578/10), as well as a North American (NY99) genotype with a demonstrated corvid virulence phenotype. Infection with all strains except WNV-FIN resulted in mortality. Viraemia was observed for birds inoculated with all strains and virus was detected in a series of organs upon necropsy. These results suggested that jackdaws could potentially function as a sentinel for following WNV transmission in Europe; however, elicited viraemia levels might be too low to allow for efficient transmission of virus to mosquitoes.