West Nile virus: an overview of its spread in Europe and the Mediterranean basin in contrast to its spread in the Americas

Prevalence of West Nile virus neutralizing antibodies in Spain is related to the behavior of migratory birds

West Nile virus (WNV) is a bird flavivirus capable of infecting horses and humans that is transmitted by blood-sucking vectors. In Europe and Africa, sporadic infections and outbreaks causing human illness and deaths have occurred and have led to 2 mutually nonexclusive hypotheses regarding the circulation of WNV in Europe: (1) the occurrence of endemic sylvatic cycles that occasionally result in human or equine infection, or (2) sporadic seeding of WNV by migratory birds from areas where the virus is endemic in Africa or elsewhere that cause local epizootic foci and eventually lead to infection in humans. To investigate these 2 possibilities, we used a micro virus-neutralization test to examine the prevalence of WNV neutralizing antibodies in 574 individuals belonging to 25 species of birds captured in spring 2004 in Seville (southern Spain). Trans-Saharan migrant species had both higher prevalences and antibody titers than resident and short-distance migrants. This result suggests that trans-Saharan migrants spend part of their life cycles in areas with greater circulation of WNV, or a closely related flavivirus, before their arrival in Spain. On the other hand, seroprevalences assessed in resident birds suggest a low level of WNV circulation in the studied locality. Aside from the question of local circulation, it thus seems that the risk for introduction of strains of WNV from Africa by migratory birds merits further field and experimental studies in Spain.

Importance of bird-to-bird transmission for the establishment of West Nile virus

West Nile virus (WNV) is principally considered to be maintained in a mosquito-bird transmission cycle. Under experimental conditions, several other transmission routes have been observed, but the significance of these additional routes in nature is unknown. Here, we derive an expression for the basic reproduction number (R0) for WNV including all putative routes of transmission between birds and mosquitoes to gauge the relative importance of these routes for the establishment of WNV. Parameters were estimated from published experimental results. Sensitivity analysis reveals that R0 is sensitive to transmission between birds via close contact, but not to mosquito-to-mosquito transmission. In seasons or in areas where the mosquito-to-bird ratio is low, bird-to-bird transmission may be crucial in determining whether WNV can establish or not. We explain the use of R0 as a flexible tool to measure the risk of establishment of vector-borne diseases.

Land cover, landscape structure, and West Nile virus circulation in southern France

The transmission of West Nile virus (WNV) is strongly influenced by environmental factors. In France, two endemic areas for WNV have been identified: Camargue and Var. The objective of our study was to test whether landscape characteristics could be associated with the risk of endemic circulation of WNV in these two ecologically different areas. Equine serological data collected during outbreaks in Var (2003) and Camargue (2004) were used. Both areas were marked out in cells of 5 km(2), and data were aggregated for each cell. Sixteen cells, classified as "high" level viral circulation, and 28 cells, classified as "low" level viral circulation were used for the analysis. The Corine Land Cover database (European Environment Agency) was used to evaluate, for each geographic cell, the area covered by 12 land cover classes, as well as the value of 5 landscape metrics (patch richness and density, edge density, the Shannon's diversity index, and interspersion and juxtaposition index (IJI). Multivariate linear generalized regression showed that IJI as well as the surface covered by heterogeneous agricultural areas were significantly higher in high level WNV circulation cells than in low level ones (p = 0.01 and 0.05 respectively). Both variables are indicators of a complex spatial biotope configuration that may favor the co-existence of competent vectors and reservoir hosts: the structure of the landscape thus appeared as a key element in WNV circulation. An internal validation was performed and the model was used to compute a risk map for the French Mediterranean coast. Cells with a probability > 0.8 of having a high level of viral circulation were found near Aix-en-Provence, Beziers, and Perpignan, areas where no serological study has yet been conducted. Equine cases reported in 2006 were all located in the neighborhood of cells having a > 0.8 probability for high WNV circulation status.

The geosimulation of West Nile virus propagation: a multi-agent and climate sensitive tool for risk management in public health

Background

Since 1999, the expansion of the West Nile virus (WNV) epizooty has led public health authorities to build and operate surveillance systems in North America. These systems are very useful to collect data, but cannot be used to forecast the probable spread of the virus in coming years. Such forecasts, if proven reliable, would permit preventive measures to be put into place at the appropriate level of expected risk and at the appropriate time. It is within this context that the Multi-Agent GeoSimulation approach has been selected to develop a system that simulates the interactions of populations of mosquitoes and birds over space and time in relation to the spread and transmission of WNV. This simulation takes place in a virtual mapping environment representing a large administrative territory (e.g. province, state) and carried out under various climate scenarios in order to simulate the effects of vector control measures such as larviciding at scales of 1/20 000 or smaller.

Results

After setting some hypotheses, a conceptual model and system architecture were developed to describe the population dynamics and interactions of mosquitoes (genus Culex) and American crows, which were chosen as the main actors in the simulation. Based on a mathematical compartment model used to simulate the population dynamics, an operational prototype was developed for the Southern part of Quebec (Canada). The system allows users to modify the parameters of the model, to select various climate and larviciding scenarios, to visualize on a digital map the progression (on a weekly or daily basis) of the infection in and around the crows' roosts and to generate graphs showing the evolution of the populations. The basic units for visualisation are municipalities.

Conclusion

In all likelihood this system might be used to support short term decision-making related to WNV vector control measures, including the use of larvicides, according to climatic scenarios. Once fully calibrated in several real-life contexts, this promising approach opens the door to the study and management of other zoonotic diseases such as Lyme disease.

Transmission dynamics and changing epidemiology of West Nile virus

West Nile virus (WNV) is a flavivirus that is maintained in a bird-mosquito transmission cycle. Humans, horses and other non-avian vertebrates are usually incidental hosts, but evidence is accumulating that this might not always be the case. Historically, WNV has been associated with asymptomatic infections and sporadic disease outbreaks in humans and horses in Africa, Europe, Asia and Australia. However, since 1994, the virus has caused frequent outbreaks of severe neuroinvasive disease in humans and horses in Europe and the Mediterranean Basin. In 1999, WNV underwent a dramatic expansion of its geographic range, and was reported for the first time in the Western Hemisphere during an outbreak of human and equine encephalitis in New York City. The outbreak was accompanied by extensive and unprecedented avian mortality. Since then, WNV has dispersed across the Western Hemisphere and is now found throughout the USA, Canada, Mexico and the Caribbean, and parts of Central and South America. WNV has been responsible for >27,000 human cases, >25,000 equine cases and hundreds of thousands of avian deaths in the USA but, surprisingly, there have been only sparse reports of WNV disease in vertebrates in the Caribbean and Latin America. This review summarizes our current understanding of WNV with particular emphasis on its transmission dynamics and changing epidemiology.

Seroconversion in wild birds and local circulation of West Nile virus, Spain

A serosurvey for neutralizing antibodies against West Nile virus (WNV) in common coots (Fulica atra) was conducted in Doñana, Spain. Antibody prevalence was highest in 2003, intermediate in 2004, and lowest in 2005. Some birds seroreverted <1 year after first capture. Seroconversion of birds suggests local circulation of the virus.

Transmission of tropical and geographically restricted infections during solid-organ transplantation

In recent years, the increasing number of donors from different regions of the world is providing a new challenge for the management and selection of suitable donors. This is a worldwide problem in most countries with transplantation programs, especially due to the increase in immigration and international travel. This paper elaborates recommendations regarding the selection criteria for donors from foreign countries who could potentially transmit tropical or geographically restricted infections to solid-organ transplant recipients. For this purpose, an extensive review of the medical literature focusing on viral, fungal, and parasitic infections that could be transmitted during transplantation from donors who have lived or traveled in countries where these infections are endemic has been performed, with special emphasis on tropical and imported infections. The review also includes cases described in the literature as well as risks of transmission during transplantation, microbiological tests available, and recommendations for each infection. A table listing different infectious agents with their geographic distributions and specific recommendations is included.

A global perspective on the epidemiology of West Nile virus

West Nile virus (WNV) (Flavivirus: Flaviviridae) is the most widespread arbovirus in the world. A significant range expansion occurred beginning in 1999 when the virus was introduced into New York City. This review highlights recent research into WNV epizootiology and epidemiology, including recent advances in understanding of the host-virus interaction at the molecular, organismal, and ecological levels. Vector control strategies, vaccines, and antivirals, which now must be considered on a global scale, are also discussed.

Persistent infections of mammals and mammalian cell cultures with West Nile virus

Before 1990, West Nile virus (WNV) was considered to be one of many arthropod-borne viruses that caused mild febrile illness in man. However, in the 1990s, the virus was associated with severe CNS disease that produced mortality in horses and man in Europe. In 1999, WNV was identified as the etiologic agent of an outbreak of human and avian encephalitis in New York City (NY, USA). Like many other Flaviviridae family members, WNV is generally considered to cause acute infections, however, persistent WNV infections have been observed in laboratory-infected animals and in human patients. These persistent infections could be facilitated by changes to the viral genome that allow the virus to evade detection by the host cell, a property that has been studied in cell culture. This review highlights our current knowledge of persistent WNV infections in vitro and in vivo, and speculates on how persistence could influence virus transmission.

Human infections associated with wild birds

Introduction

Wild birds and especially migratory species can become long-distance vectors for a wide range of microorganisms. The objective of the current paper is to summarize available literature on pathogens causing human disease that have been associated with wild bird species.

Methods

A systematic literature search was performed to identify specific pathogens known to be associated with wild and migratory birds. The evidence for direct transmission of an avian borne pathogen to a human was assessed. Transmission to humans was classified as direct if there is published evidence for such transmission from the avian species to a person or indirect if the transmission requires a vector other than the avian species.

Results

Several wild and migratory birds serve as reservoirs and/or mechanical vectors (simply carrying a pathogen or dispersing infected arthropod vectors) for numerous infectious agents. An association with transmission from birds to humans was identified for 10 pathogens. Wild birds including migratory species may play a significant role in the epidemiology of influenza A virus, arboviruses such as West Nile virus and enteric bacterial pathogens. Nevertheless only one case of direct transmission from wild birds to humans was found.

Conclusion

The available evidence suggests wild birds play a limited role in human infectious diseases. Direct transmission of an infectious agent from wild birds to humans is rarely identified. Potential factors and mechanisms involved in the transmission of infectious agents from birds to humans need further elucidation.

Population dynamics of pest mosquitoes and potential malaria and West Nile virus vectors in relation to climatic factors and human activities in the Camargue, France

The Camargue is an extensive wetland in the southeast of France, which is highly influenced by human activities. Large ponds, marshes and irrigated fields provide abundant potential breeding sites for mosquitoes. mosquitoes, which are important in terms of the nuisance they cause to people and animals, the limitations they impose on tourism and their potential threat to human health. Several of the mosquito species present are potential vectors of malaria and West Nile virus. Therefore, the population dynamics of these species were monitored over an entire breeding season during March-October 2005. Mosquito populations were sampled in two study areas once every 2 weeks, using CDC light traps baited with CO(2). Sixteen species were collected. The majority (98.7%) of the catch were Aedes caspius (Pallas) (Diptera: Culicidae), Culex modestus (Ficalbi), Culex pipiens L. and Anopheles hyrcanus (Pallas). The population dynamics of these species varied considerably in relation to the species' biology, climatic conditions (rainfall, temperature and season), water management, implementation of mosquito control campaigns and landscape use.

Innate immune evasion by hepatitis C virus and West Nile virus

Antiviral immunity in mammals involves several levels of surveillance and effector actions by host factors to detect viral pathogens, trigger alpha/beta interferon production, and to mediate innate defenses within infected cells. Our studies have focused on understanding how these processes are regulated during infection by hepatitis C virus (HCV) and West Nile virus (WNV). Both viruses are members of the Flaviviridae and are human pathogens, but they each mediate a very different disease and course of infection. Our results demonstrate common and unique innate immune interactions of each virus that govern antiviral immunity and demonstrate the central role of alpha/beta interferon immune defenses in controlling the outcome of infection.

Effects of climate on West Nile Virus transmission risk used for public health decision-making in Quebec

Background

In 2002, major human epidemics of West Nile Virus (WNV) were reported in five cities in the North East region of North America. The present analysis examines the climatic conditions that were conducive to the WNV epidemic, in order to provide information to public health managers who eventually must decide on the implementation of a preventive larvicide spraying program in Quebec, Canada. Two sets of variables, the first observed in the summer of 2002 and the second in the preceding winter were analysed to study their potential as explanatory variables for the emergence of the virus at epidemic levels.

Results

Results show that the climatic conditions observed in the year 2002 have contributed to the emergence of the virus and can be observed once every forty years on average. The analysis has shown that the 2002 events observed in several North East North American cities are characterized by two main variables: the number of degree-days below -5°C in the winter (DD-5) and the number of degree-days greater than 25°C in the summer (DD25).

Conclusion

In the context of a declining rate of human and aviary infection to WNV, this element contributed to the decision to suspend the use of preventive larvicides in the province of Quebec in 2006 and for the foreseeable future. The second part of this study indicates that it is very important to estimate the risk that extreme values can be observed simultaneously in the summer and in the winter preceding the appearance of the virus. The proposed models provide important information to public health officials, weeks before the appearance of the virus, and can therefore be useful to help prevent human epidemics.

Prevalence of West Nile virus neutralizing antibodies in colonial aquatic birds in southern Spain

The rapid expansion of West Nile virus (WNV) throughout the New World has raised interest in understanding the population dynamics and patterns of dispersal of emerging infectious diseases by wildlife. WNV affects humans, although its main reservoirs are various species of birds. Here we analyse the prevalence of WNV-neutralizing antibodies in nearly full-grown chicks belonging to seven different species of colonial waterbirds at three localities in southern Spain. Chicks with neutralizing antibodies against WNV were detected in three species and at all three localities. However, the low antibody titres suggest the presence of antibodies is probably due to maternal transfer of antibody, presumably from exposure of the adult birds to WNV or a similar flavivirus at some stage of their lives. The analyses of the movements of tagged birds confirmed that all species with antibody visit regions that have had reports of WNV infection over the past decade.

Two Distinct Introductions of the West Nile Virus in Portugal Disclosed by Phylogenetic Analysis of Genomic Sequences

In this report, we describe genomic sequencing and analysis of different West Nile virus strains isolated from mosquitoes in the south of Portugal (Alentejo and Algarve) at two different time points (1971 and 2004, respectively). Phylogenetic analysis indicated different origins for the two recorded introductions of WNV in our country, with strains segregating in different sub-clades within lineage 1a. PTRoxo (isolated in 1971) was found to be very similar to an Egyptian WNV strain isolated in 1951, while the viruses isolated in 2004 formed a statistically well-supported group with WNV strains isolated over the last decade in countries lining the Mediterranean (Morocco, Italy, and France). Analyses of the putative amino acid sequences showed all the main expected features described for viral mature proteins except for the absence of the N-glycosylation site in the envelope glycoprotein of PTRoxo, which may be reflected as attenuated neurovirulence and neuroinvasive phenotypes.

West Nile virus in the United States (1999-2005)

The accidental introduction of West Nile Virus into New York City from the Old World in 1999 resulted in an epidemic in humans, horses, and birds that swept to the west coast in just 3 years. The virus is transmitted by infective mosquitoes among susceptible native birds, which serve as amplifying hosts. Clinical disease occurs in humans and horses, but not enough virus is produced in their blood to infect other mosquitoes; therefore, humans and horses are considered dead-end hosts. Humans can best protect themselves by remaining indoors during periods of high mosquito activity and/or by using recommended repellents. Effective vaccines are available for horses.

First international diagnostic accuracy study for the serological detection of West Nile virus infection

Background

The diagnosis of an acute or convalescent West Nile (WN) virus infection can be confirmed by various serological assays such as enzyme immunoassay (EIA), immunofluorescence assay (IFA), or neutralisation test (NT) which are conducted by a growing number of laboratories. However, as the degree of proficiency may vary between laboratories, quality control measures for laboratory diagnostics are essential.

Methods

We have performed an external quality assurance (EQA) programme for the serological detection of WN virus infection to assess the diagnostic quality of laboratories. The participating laboratories received a proficiency panel of 10 coded lyophilised test samples comprising four antisera positive for WN antibodies as positive controls, three antisera positive for antibodies against other heterologous flaviviruses plus one multireactive unspecific serum as specificity controls, and two negative serum samples.

Results

Twenty-seven laboratories from 20 different countries in Europe, the Middle East, the Americas and Africa participated in this EQA programme. Applying the proficiency criteria of this study, only eight laboratories correctly analysed all samples with their respective EIA, IFA or NT methods. Eighteen laboratories correctly identified between 77.8 and 90% of the samples, and one laboratory identified only 70% correctly with a clear need to eliminate cross-reactivity with other antisera, particularly those elicited by yellow fever virus. Differentiation between the results for IgM and IgG was considered separately and revealed that IgM-antibodies were detected less frequently than IgG-antibodies (p < 0.001). However, the assay used was not a significant technical factor influencing laboratory performance.

Conclusion

The EQA programme provides information on the quality of different serological assays used by the participating laboratories and indicates that most need to improve their assays, in particular to avoid cross-reactions with antibodies to heterologous flaviviruses.

Detection of West Nile virus lineages 1 and 2 by real-time PCR

West Nile virus (WNV) is a Flavivirus attracting worldwide attention because it has spread rapidly across the Americas since its first appearance in New York City in 1999. Several PCR-based diagnostic methods have been developed for the detection of WNV. The focus of these assays has been WNV lineage 1 which can be found worldwide, while lineage 2 viruses were thought to be endemic only in some regions of Africa. However, both lineages may be imported from Africa to Europe by migrating birds. In order to determine the incidence of WNV in Germany, a real-time-based PCR assay was developed, targeting a conserved region of WNV lineages 1 and 2. This assay is a suitable tool for the diagnosis of WNV and for surveillance studies.

Evidence of laboratory vector competence of Culex modestus for West Nile virus

During the past 10 years, West Nile virus (WNV) has been responsible for large and severe human outbreaks and horse epizootics through the Old and the New World. Since WNV was first isolated from field-collected mosquitoes of Culex modestus in 1964 in France, this species, which aggressively feeds on birds and mammals, is considered a putative WNV vector in Europe. We report on the first evidence on the laboratory vector competence of Cx. modestus for WNV. To assess this trait, F3 and F4 females from southern France were fed through a membrane with a strain of WNV isolated from a horse in the Camargue (Rhône River delta) in 2000. On day 14 after virus ingestion, 90% of mosquitoes displayed a disseminated infection. WNV was detected in the saliva from 2 of 5 infected mosquitoes selected for testing. These preliminary results demonstrate that Cx. modestus is capable of experimentally transmitting WNV.

Investigating the spatial risk distribution of West Nile virus disease in birds and humans in southern Ontario from 2002 to 2005

BACKGROUND

The West Nile virus (WNv) became a veterinary public health concern in southern Ontario in 2001 and has continued to threaten public health. Wild bird mortality has been shown to be an indicator for tracking the geographic distribution of the WNv. The purpose of this study was to investigate the latent risk distribution of WNv disease among dead birds and humans in southern Ontario and to compare the spatial risk patterns for the period 2002-2005. The relationship between the mortality fraction in birds and incidence rate in humans was also investigated.

METHODS

Choropleth maps were created to investigate the spatial variation in bird and human WNv risk for the public health units of southern Ontario. The data were smoothed by empirical Bayesian estimation before being mapped. Isopleth risk maps for both the bird and human data were created to identify high risk areas and to investigate the potential relationship between the WNv mortality fraction in birds and incidence rates in humans. This was carried out by the geostatistical prediction method of kriging. A Poisson regression analysis was used to model regional human WNv case counts as a function of the spatial coordinates in the east and north direction and the regional bird mortality fractions. The presence of disease clustering and the location of disease clusters were investigated by the spatial scan test.

RESULTS

The isopleth risk maps exhibited high risk areas that were relatively constant from year to year. There was an overlap in the bird and human high risk areas, which occurred in the central-west and south-west areas of southern Ontario. The annual WNv cause-specific mortality fractions in birds for 2002 to 2005 were 31.9, 22.0, 19.2 and 25.2 positive birds per 100 birds tested, respectively. The annual human WNv incidence rates for 2002 to 2005 were 2.21, 0.76, 0.13 and 2.10 human cases per 100,000 population, respectively. The relative risk of human WNv disease was 0.72 times lower for a public health unit that was 100 km north of another public health unit. The relative risk of human WNv disease increased by the factor 1.44 with every 10 positive birds per 100 tested. The scan statistic detected disease cluster in the bird and human data. The human clusters were not significant, when the analysis was conditioned on the bird data.

CONCLUSION

The study indicates a significant relationship between the spatial pattern of WNv risk in humans and birds.

West Nile Virus in horses, sub-Saharan Africa

To evaluate the presence and extension of West Nile virus where French soldiers are stationed in Africa, specific antibody prevalence was determined by using ELISA and Western blot. Among 245 horses living in close proximity to the soldiers, seroprevalence was particularly high in Chad (97%) and Senegal (92%).

Emerging zoonoses and vector-borne infections affecting humans in Europe

The purpose of this study was to assess and describe the current spectrum of emerging zoonoses between 2000 and 2006 in European countries. A computerized search of the Medline database from January 1966 to August 2006 for all zoonotic agents in European countries was performed using specific criteria for emergence. Fifteen pathogens were identified as emerging in Europe from 2000 to August 2006: Rickettsiae spp., Anaplasma phagocytophilum, Borrelia burgdorferi, Bartonella spp., Francisella tularensis, Crimean Congo Haemorrhagic Fever Virus, Hantavirus, Toscana virus, Tick-borne encephalitis virus group, West Nile virus, Sindbis virus, Highly Pathogenic Avian influenza, variant Creutzfeldt-Jakob disease, Trichinella spp., and Echinococus multilocularis. Main risk factors included climatic variations, certain human activities as well as movements of animals, people or goods. Multi-disciplinary preventive strategies addressing these pathogens are of public health importance. Uniform harmonized case definitions should be introduced throughout Europe as true prevalence and incidence estimates are otherwise impossible.

West Nile virus in Spain: report of the first diagnosed case (in Spain) in a human with aseptic meningitis

We report the first case of illness caused by West Nile virus (WNV) so far diagnosed in Spain. A 21-y-old male presented with clinical and biological signs compatible with viral meningitis. Acute and convalescent serum samples showed IgM and IgG positivity for WNV. These results were confirmed by microneutralization assays.

West Nile virus in Southern Portugal, 2004

West Nile virus (WNV) genomic RNA was detected by reverse transcription-polymerase chain reaction (RT-PCR) in six out of 57 mosquito pools collected in Southern Portugal, during the summer of 2004, yielding an infection rate (IR) of 2.8/1,000 mosquitoes. Phylogenetic analysis of a 217-nucleotide fragment of the NS5 coding region, amplified from Culex pipiens s.l. and Culex univittatus unfed females, demonstrated a close relationship with WNV strains circulating in the Mediterranean basin (Italy, 1998; France, 2000; Morocco, 2003). The data in this short report demonstrate the presence of WNV in mosquitoes in Southern Portugal and the need of permanent surveillance for viral activity within the country.

Pathology and tissue distribution of West Nile virus in North American owls (family: Strigidae)

This study describes the macroscopic and microscopic lesions and the viral antigen distribution in 82 owls (Family: Strigidae) of 11 North American and one Eurasian species that died following natural West Nile virus infection. The range of lesions seen was greater than that previously reported for owls, and involved more organs. Two patterns of antigen distribution were identified: one that involved the blood and all major organs; and a second where antigen was sparse, localized, and absent from the blood. The first pattern was associated with species of northern natural breeding range, while the second was seen in owls of a more southern distribution and appeared to be associated with a more prolonged course of illness. Further differences in lesion and antigen distribution appeared to be either species related or individual. The findings underline the complexity and variability of West Nile virus pathology within birds of a relatively narrow taxonomic group.

Experimental infection of common garter snakes (Thamnophis sirtalis) with West Nile virus

The role of various reptilian species in the infectious cycle of several arboviruses is documented, but their role in that of West Nile virus (WNV) is uncertain. Common garter snakes (Thamnophis sirtalis) were infected subcutaneously with 10(5) plaque forming units (PFU) WNV-Isr 98, five of nine snakes became viremic, and five exhibited persistent low levels of neutralizing antibodies. Four of the parentally infected snakes died and high titers of virus were found in multiple organ samples. In contrast, orally infected garter snakes did not become viremic, but viral RNA was detected in cloacal swabs. Since oral infection of predator birds by WNV is known, their ingestion of infected snakes may also result in their becoming infected.

Encéphalites infectieuses: données et limites du PMSI pour l'étude épidémiologique, France métropolitaine 2000–2002

Many virus and bacteria can cause encephalitis but are rarely identified as the aetiological agent by individual diagnosis. In France, the only continuous source of information about encephalitis is the national hospital medical database (NHMD). Data from the VIH-negative patients recorded in mainland France between 2000 and 2002 with a diagnosis of encephalitis were extracted and analysed according to demographic, geographical and temporal distribution. Hospitalisation details were described. An average of 1200 patients was recorded each year. They were residents of all French districts and equally hospitalized in university hospitals and non university hospitals. Their mean age was 38, and most were men. The aetiological diagnosis was unknown for 80%. The most frequent aetiological diagnosis was herpes simplex virus in adults, and VZV virus in children. These results give us some clues to design a national study on encephalitis. The study will be implemented in mainland France in 2007 and will last one year. We invite all voluntary hospitals to include their encephalitic patients in our study.

Syndromic surveillance in the Netherlands for the early detection of West Nile virus epidemics

West Nile virus (WNV) is an arthropod-borne flavivirus that is endemic in Africa, Europe, and Eastern Asia. The recent introduction and rapid dissemination of the virus in the United States as well as an increase in WNV outbreaks in Europe, has raised concerns for its spread in Europe. A surveillance system was developed to allow timely detection of an introduction of WNV infections in The Netherlands. This program focuses on cases presenting with neurological disease and includes the monitoring of hospital discharge diagnoses, trends in cerebrospinal fluid (CSF) diagnostic requests, laboratory testing of CSF, and monitoring of neurological disease in horses. Retrospective data from the hospital discharge records showed yearly peaks of unexplained meningitis and (meningo)encephalitis in the summer. A total of 781 CSF samples from humans and 71 serum and/or CSF samples from horses presenting with neurological disease of suspected viral etiology tested negative for the presence of specific antibodies to WNV. With a coverage rate of 59% in 2003, the probability that a cluster of five WNV cases presenting with neurological symptoms would have been detected was 99%. We conclude that, from 1999 to 2004, no evidence of WNV infection could be found in either humans or horses in The Netherlands.

Horse-, bird-, and human-seeking behavior and seasonal abundance of mosquitoes in a West Nile virus focus of southern France

After 35 yr of disease absence, West Nile virus (family Flaviviridae, genus Flavivirus, WNV) circulation has been regularly detected in the Camargue region (southern France) since 2000. WNV was isolated from Culex modestus Ficalbi, which was considered the main vector in southern France after horse outbreaks in the 1960s. Recent WNV transmissions outside of the Cx. modestus distribution suggested the existence of other vectors. To study potential WNV vectors, horse- and bird-baited traps and human landing collections of mosquitoes were carried out weekly from May to October 2004 at two Camargue sites: one site in a wet area and the other site in a dry area, both chosen for their past history of WNV transmission. At the wet site, the most abundant species in bird-baited traps were Culex pipiens L. and Cx. modestus; both species also were found in lower proportions on horses and humans. The most abundant species in horse-baited traps and human landing collections were Aedes caspius (Pallas), Aedes vexans (Meigen), and Anopheles hyrcanus (Pallas) sensu lato; some of these species were occasionally collected with avian blood at the end of the summer. Anopheles maculipennis Meigen sensu lato was an abundant horse feeder, but it was rarely collected landing on human bait and never contained avian blood. At the dry site, Cx. pipiens was the most abundant species in bird- and horse-baited traps. The seasonal and circadian dynamics of these species are analyzed, and their potential in WNV transmission in Camargue discussed.

First international proficiency study on West Nile virus molecular detection

Background: West Nile virus (WNV) molecular detection is being conducted by a growing number of laboratories, but the degree of proficiency may vary between them. External quality control is needed.

Methods: We have conducted an international quality assurance study on WNV molecular detection. Participating laboratories tested noninfectious samples inactivated by heat and gamma irradiation. Participants received 7 coded lyophilized samples containing WNV of genetic lineages 1a, 1b, and 2 at 2600 to 18 000 000 RNA copies/mL, 3 samples containing heterologous flaviviruses, and 2 negative samples.

Results: Thirty laboratories participated. The average laboratory achieved 50% detection probability from 7762 copies/mL onward (probit analysis; 95% CI = 1174–24547 copies/mL). Lineages 1a and 1b were detected with equal efficiencies, but the lineage 2 strain (Ug37) was detected at significantly lower rates. Only 27% of participants were able to detect the 6 samples containing ≥1.8 × 10⁴ copies/mL. Three laboratories generated false-positive results in negative samples. Six of 30 laboratories reported correct strain identification in 3 samples containing non-WNV flaviviruses. We observed a significant positive correlation between the capability of detecting non-WNV flaviviruses and detecting WNV lineage 2.

Conclusions: Most participants showed good performance in detecting lineage 1 WNV, the predominant virus in the Northern Hemisphere. The inability of some laboratories to detect even highly concentrated lineage 2 WNV downgraded the overall outcome. The lineage 2 material received through this study will provide laboratories with the necessary template for improving their assays. Such material is otherwise hard to obtain.

Expression of vector-based small interfering RNA against West Nile virus effectively inhibits virus replication

RNA interference is one of the effective emerging anti-viral strategies to inhibit virus infection in cells. In this study, a small interfering RNA expressing vector (pSilencer-NS5) targeting the NS5 gene of West Nile virus (WNV) was employed to target and destroy WNV transcripts. Real-time PCR revealed drastic reduction in WNV RNA transcripts in pSilencer-NS5-transfected Vero cells. The virus infectious titre was also significantly reduced by 90% as determined by plaque assays. The resulting decrease in virus replication was shown to be specific since both scrambled and nucleotide(s) mismatch siRNA against WNV NS5 gene did not have any effect on WNV productive yields. Furthermore, Western immunoblot analysis on the expression of viral NS5 and envelope (E) proteins showed significant down-regulation on the expression of viral NS5 and envelope (E) proteins in virus-infected cells that were pre-transfected with pSilencer-NS5. These data clearly supported the notion that the expression of vector-based siRNA against WNV NS5 gene is able to exert its silencing effect on WNV-infected cells without inducing cytotoxicity, hence holding promise in therapeutic treatment of this important emerging infectious disease.

Lineage 1 and 2 strains of encephalitic West Nile virus, central Europe

Two different West Nile virus (WNV) strains caused lethal encephalitis in a flock of geese and a goshawk in southeastern Hungary in 2003 and 2004, respectively. During the outbreak in geese, 14 confirmed human cases of WNV encephalitis and meningitis were reported in the same area. Sequencing of complete genomes of both WNV strains and phylogenetic analyses showed that the goose-derived strain exhibits closest genetic relationship to strains isolated in 1998 in Israel and to the strain that emerged in 1999 in the United States. WNV derived from the goshawk showed the highest identity to WNV strains of lineage 2 isolated in central Africa. The same strain reemerged in 2005 in the same location, which suggests that the virus may have overwintered in Europe. The emergence of an exotic WNV strain in Hungary emphasizes the role of migrating birds in introducing new viruses to Europe.

Prediction of human cases of West Nile virus by equine cases, Saskatchewan, Canada, 2003

In 2003, an outbreak of West Nile virus (WNV) occurred in Saskatchewan, Canada from July to September. One-hundred thirty-three horse cases and 947 human cases were recorded and data were analyzed retrospectively for evidence of clustering to determine if clinical infection in the horse population could be used to estimate human risk of infection with WNV. Kulldorff's scan statistic was used to identify spatial-temporal clusters in both the human and horse cases. In most areas, human clusters were not preceded by horse clusters. In one area, a significant cluster of horse cases preceded human cases by 1 week; however, 1 week does not provide sufficient time for human-health authorities to act and provide advance warning for the public.

Co-occurrence of West Nile Fever and circovirus infection in a goose flock in Hungary

The authors investigated an outbreak of West Nile Fever characterized by severe neurological symptoms and death in a flock of 3600 6-week-old geese. Ataxia, intermittent torticollis and opisthotonus, incoordination, rhythmic side-to-side movement of the head, wriggling of the neck and abnormal head position were features of the disease. Death occurred within 4 to 5 days after the clinical signs appeared. The average daily mortality was 5 to 15, reaching 14% (in total) over a period of 6 weeks. There were no consistent gross pathological lesions, but in a few cases yellowish-grey foci of 3 to 6 mm in diameter were observed on the surface or transection of the brain. Histopathology revealed perivascular lymphohistiocytic infiltration and glia cell proliferation in the brainstem, cerebellum, cortex and spinal cord as well as degeneration of neural fibres in the spinal cord. In addition to the lesions caused by the West Nile Virus in the brain, characteristics of circovirus infection such as lymphocyte depletion, vacuolization and basophilic intra-cytoplasmic inclusion bodies containing circovirus-like particles were seen by light and electron microscopy in the cloacal bursa. West Nile Virus infection was confirmed by reverse transcriptase-polymerase chain reaction amplification of virus-specific nucleic acid from tissue samples of the brain. Based on the nucleotide sequence analysis of the polymerase chain reaction products, 99% identity was found on the tested NS5 region with the IS-98 ST1 strain isolated from a stork in Israel in 1998, and with West Nile Virus stains emerging in the USA in 1999. Using an indirect fluorescent antibody test, high antibody titres against the virus were detected in the serum samples submitted from the affected flock. In selected sera this was confirmed by neutralization antibody test as well.

DNA microarray technique for detection and identification of seven flaviviruses pathogenic for man

A flavivirus microarray was developed for detection and identification of yellow fever (YF), West Nile, Japanese encephalitis (JE), and the dengue 1-4 viruses, which are causing severe human disease all over the world. The microarray was based on 500-nucleotide probe fragments from five different parts of the seven viral genomes. A low-stringent amplification method targeting the corresponding regions of the viral genomic RNA was developed and combined with hybridization to the microarray for detection and identification. For distinction of the generated virus-specific fluorescence-patterns a fitting analysis procedure was adapted. The method was verified as functional for all seven flaviviruses and the strategy for the amplification, combined with the long probes, provided a high tolerance for smaller genetic variability, most suitable for these rapidly changing RNA viruses. A potentially high detection and identification capacity was proven on diverged strains of West Nile and dengue viruses. The lower limit for detection was equivalent, or better, when compared to routinely used RT-PCR methods. The performance of the method was verified on human patient samples containing dengue viruses, or normal human serum spiked with YF or JE viruses. The results demonstrated the ability of the flavivirus microarray to screen simultaneously a sample for several viruses in parallel, in combination with a good lower limit of detection.

Manifestations of West Nile neuroinvasive disease

Since its introduction to North America in 1999, West Nile virus, an arthropod-borne flavivirus, has become the most significant cause of epidemic encephalitis in the western hemisphere. While most human infections with the virus are asymptomatic and the majority of symptomatic persons experience febrile illness, severe neurologic manifestations, including meningitis, encephalitis, and poliomyelitis may be seen. This review summarizes the virology, epidemiology and pathogenesis of human infection with West Nile virus, and details recent advances in our understanding of the pathophysiology and various clinical manifestations of infection.

A cluster study of predictors of severe West Nile virus infection

OBJECTIVE

To assess the value of multifocal chorioretinitis and of clinical manifestations and biologic parameters in the diagnosis of West Nile virus (WNV) infection.

PATIENTS AND METHODS

We conducted a prospective, controlled case series study during an outbreak of WNV infection between August 15 and October 24, 2003, of 64 consecutive patients who presented with clinical manifestations consistent with WNV disease. In each patient, standardized clinical and biologic data were collected. An ophthalmologic examination searching particularly for multifocal chorioretinitis was performed.

RESULTS

Of 64 patients who presented primarily with meningitis and/or encephalitis, 36 had IgM antibodies against WNV. The WNV-infected patients tended to be older (median age of 54 years vs 46 years in WNV infection and control groups, respectively) and more frequently had diabetes (30% vs 7% in WNV infection and control groups, respectively; P = .03). Multifocal chorioretinitis was found in 75% of WNV-infected patients but in no patient in the control group (P = .001). Blood glucose and amylase levels were higher in WNV-infected patients, whereas serum sodium levels were lower. The cerebrospinal fluid leukocyte count and protein levels were significantly higher in WNV meningitis or encephalitis. Overall, multifocal chorioretinitis had 100% specificity and 73% sensitivity (88% when only patients with meningitis or encephalitis were analyzed) for the diagnosis of WNV. Multivariate analysis disclosed multifocal chorioretinitis as the only predictor of WNV infection (odds ratio, 62; 95% confidence interval, 6-700; P = .001).

CONCLUSION

Multifocal chorioretinitis appears to be a specific marker of WNV infection, particularly in patients who present with meningoencephalitis. An ophthalmologic examination should be part of the routine evaluation of such patients.

Epidemiology and transmission dynamics of West Nile virus disease

Since 1999, >16,000 cases in the United Stateswere transmitted by Culex mosquitoes.

From 1937 until 1999, West Nile virus (WNV) garnered scant medical attention as the cause of febrile illness and sporadic encephalitis in parts of Africa, Asia, and Europe. After the surprising detection of WNV in New York City in 1999, the virus has spread dramatically westward across the United States, southward into Central America and the Caribbean, and northward into Canada, resulting in the largest epidemics of neuroinvasive WNV disease ever reported. From 1999 to 2004, >7,000 neuroinvasive WNV disease cases were reported in the United States. In 2002, WNV transmission through blood transfusion and organ transplantation was described for the first time, intrauterine transmission was first documented, and possible transmission through breastfeeding was reported. This review highlights new information regarding the epidemiology and dynamics of WNV transmission, providing a new platform for further research into preventing and controlling WNV disease.

Infecciones por el virus de Toscana, el virus del Nilo occidental y otros arbovirus de interés en Europa

Arbovirosis, viral infection transmitted by arthropods, is a widespread health problem. Recurrent outbreaks caused by some of these viruses such as dengue or West Nile strains in regions where they do not appear frequently, justify the establishment of global control measures. Tick-borne encephalitis viruses, sand fly fever viruses (Toscana, Naples and Sicily) and occasionally West Nile and Crimean-Congo fever viruses are the most frequent causes of arbovirosis in Europe, although circulation of other potentially pathogenetic viruses such as Chikungunya has also been detected. The only native arbovirosis described in Spain is infection produced by Toscana virus, which causes aseptic, usually benign meningitis. Nevertheless, some West Nile virus-associated meningo-encephalitis cases have been described in France, Portugal and countries in the Magreb region, increasing the risk of sporadic occurrence of these processes in our country. To achieve an accurate diagnosis, high clinical suspicion is required as well as highly specific laboratory techniques, mainly based on IgM detection, RT-PCR and viral culture of CSF and/or serum.

West Nile virus in Morocco, 2003

West Nile virus (WNV) reemerged in Morocco in September 2003, causing an equine outbreak. A WNV strain isolated from a brain biopsy was completely sequenced. On the basis of phylogenetic analyses, Moroccan WNV strains isolated during the 1996 and 2003 outbreaks were closely related to other strains responsible for equine outbreaks in the western Mediterranean basin.

Novel flavivirus or new lineage of West Nile virus, central Europe

Rabensburg virus, isolated from Culex pipiens mosquitoes in central Europe, represents a new lineage of West Nile virus or a novel flavivirus of the Japanese encephalitis virus group.

A flavivirus (strain 97-103) was isolated from Culex pipens mosquitoes in 1997 following floods in South Moravia, Czech Republic. The strain exhibited close antigenic relationship to West Nile virus (WNV) prototype strain Eg-101 in a cross-neutralization test. In this study, mouse pathogenicity characteristics and the complete nucleotide and putative amino acid sequences of isolate 97-103, named Rabensburg virus (RabV) after a nearby Austrian city, were determined. RabV shares only 75%–77% nucleotide identity and 89%–90% amino acid identity with representative strains of WNV lineages 1 and 2. Another RabV strain (99-222) was isolated in the same location 2 years later; it showed >99% nucleotide identity to strain 97-103. Phylogenetic analyses of RabV, WNV strains, and other members of the Japanese encephalitis virus (JEV) complex clearly demonstrated that RabV is either a new (third) lineage of WNV or a novel flavivirus of the JEV group.