Virology, pathology, and clinical manifestations of West Nile virus disease

Powassan and other emerging neuroinvasive arboviruses in North America

PURPOSE OF REVIEW

Arthropod-borne viruses (arboviruses) represent a group of pathogens with increasing global relevance, some of which cause neuroinvasive disease. Transmitted by arthropod vectors and maintained by a variety of primary and amplifying hosts, epidemics are dependent on numerous environmental and anthropogenic factors. This review serves to highlight several important neuroinvasive arboviruses relevant to North America and discuss the neurologic presentations, diagnosis, outcomes, and future trends.

RECENT FINDINGS

Recent shifts in the epidemiology and ecology of arboviruses in North America include the divergence of arboviruses such as dengue and chikungunya from dependence on enzootic cycles, the geographical expansion of Oropouche virus, and the increasing incidence of some established North American arboviruses such as Powassan virus. Accurate identification of the factors contributing to arboviral outbreaks is critical to improve preventive public health measures. Similarly, further elucidating the relevant pathogen and host factors that determine neuroinvasiveness, neurotropism, and neurovirulence will be key to the development of successful vaccines and targeted therapeutics.

SUMMARY

Arboviruses are an important pathogen relevant to human disease. Familiarity with the presentations, diagnostic workup, treatment and preventive strategies, and expected course is critical for clinicians caring for these patients.

First Serologic Evidence of West Nile Virus and Usutu Virus Circulation Among Dogs in the Bulgarian Danube Region and Analysis of Some Risk Factors

This study aimed to assess West Nile virus (WNV) and Usutu virus seroprevalence among the dog population in the Danube region, Bulgaria, to confirm the results of ELISA by the virus neutralisation test (VNT), as well as to analyse several risk factors of seropositivity in dogs. To implement this, a total of 201 blood samples were collected from dogs in four districts bordering the Danube River. All the samples were tested for anti-WNV protein E antibodies using competitive ELISA. Neutralising antibodies against WNV and Usutu virus were tested in all the ELISA-positive samples. The results show a WNV seroprevalence of 45.3% (n = 91, CI = 36.45-55.59) by ELISA, whereas the virus neutralisation test indicated a seroprevalence of 21.9% (n = 44, CI = 15.91-29.39). Neutralising antibodies against Usutu virus were detected for the first time in Bulgaria, with a prevalence of 6% (n = 12, CI = 3.09-10.43). Compared to VNT, ELISA demonstrated 100.0% sensitivity and 70.1% specificity. The region (p < 0.0187), the district (p = 0.0258) and the ages of the dogs (p = 0.0180) were identified as statistically significant risk factors associated with WNV seropositivity. This study provides indirect evidence of WNV and Usutu virus circulation among dogs in the Danube region of Bulgaria, highlighting a potential risk for susceptible hosts in the area.

The Role of Ruminants as Sentinel Animals in the Circulation of the West Nile Virus in Tunisia

Outbreaks of the West Nile Virus (WNV) have increased significantly in recent years in the Mediterranean region, including Tunisia. To understand the risks for animal and human health and to mitigate the impact of future outbreaks, comprehensive viral surveillance in vertebrate hosts and vectors is needed. We conducted the first serosurvey for the WNV in ruminants in southern Tunisia using the ELISA test and confirmed it with the micro-virus neutralization test (VNT). Antibodies were detected by the ELISA test in camels (38/112), sheep (9/155), and goats (7/58), and six samples were doubtful (five camels and one sheep). The ELISA positive and doubtful sera (n = 60) were further analyzed to confirm the presence of specific anti-WNV and anti-Usutu virus (USUV) antibodies using the micro-virus neutralization test (VNT). Out of the 60 sera, 33 were confirmed for specific WNV antibodies, with an overall seroprevalence of 10.15% [95% CI: 7.09-13.96]. The high seroprevalence observed in camels (22.3%) suggests their potential use as sentinel animals for WNV surveillance in southern Tunisia. The viral genome, and consequently active circulation, could not be detected by real-time RT-qPCR in blood samples. Ongoing surveillance of the WNV in animals, including camels, sheep, and goats, may be used for the early detection of viral circulation and for a rapid response to mitigate potential outbreaks in horses and humans.

Tracking the Pathways of West Nile Virus: Phylogenetic and Phylogeographic Analysis of a 2024 Isolate from Portugal

Birds are natural hosts for numerous zoonotic viral pathogens, including West Nile virus, which is transmitted by mosquitoes. During migration, birds can act as vectors for the geographic spread of viruses. WNV is endemic in Portugal, causing annual outbreaks, particularly in horses. Here, we report the first detection of an avian WNV strain isolated from a wild bird (Astur gentilis) collected in Portugal in mid-September 2024. Phylogenetic and phylogeographic analyses were conducted to trace the virus's origin and potential transmission routes, integrating the obtained full-length genomic sequence with a dataset of WNV strains from Africa and Europe (1951-2024). Phylogenetic analysis of 92 WNV sequences spanning lineages 1-5 positioned the 2024 isolate within lineage 1a. Results obtained using phylodynamics-based analysis showed that this isolate likely originated in Africa and reached Portugal via Spain's Cádiz coast, confirming previously described WNV dispersal patterns between Africa and Europe. The data suggest a migratory route from West Africa to Europe, extending through countries such as Senegal, Mauritania, Morocco, Portugal, Spain, Italy, and France, indicating a reciprocal flow of the virus back into Africa. These transmission routes match the migratory paths of Afro-Palearctic bird species, emphasizing the role of migratory birds in the long-distance spread of WNV.

Neuroinvasive West Nile Encephalitis Presenting With Seizures and Acute Flaccid Paralysis: A Case Report

West Nile encephalitis (WNE) is a serious neurological disorder caused by the West Nile virus (WNV), a mosquito-borne flavivirus. The virus is mainly transmitted to humans through bites from infected Culex mosquitoes, with birds acting as the primary reservoir hosts. While most WNV infections are asymptomatic or cause a mild febrile illness (West Nile fever), a small subset of cases progresses to severe neuroinvasive disease, such as encephalitis, meningitis, or acute flaccid paralysis. WNE is marked by brain inflammation, resulting in high fever, headache, neck stiffness, disorientation, tremors, seizures, and paralysis. Advanced age and immunocompromised states are important risk factors for severe disease. There is no specific antiviral treatment for WNE, and management remains supportive. Preventive measures, such as mosquito control and public awareness, are essential for reducing the incidence of this disease. WNV has a global distribution, with outbreaks reported in North America, Europe, Africa, and Asia, making it a significant public health issue.

A review of the mosquito-borne flaviviruses: Dengue virus and West Nile virus in Southern Africa

Dengue virus (DENV) and West Nile (WNV) viruses are important re-emerging mosquito-borne members of the genus Flavivirus that are under-recognized in many parts of Africa. This review aims to evaluate the existing literature on the transmission, epidemiology, diagnostic techniques, clinical presentation and prevention of infection with DENV and WNV in Southern Africa. Literature shows that both DENV and WNV are transmitted by mosquitoes of Aedes spp. and Culex species., respectively, and both viruses are widespread in the Southern African region. Epidemiologically, sporadic outbreaks have been reported of both DENV and WNV in various Southern African countries, indicating the ongoing threat of these viruses. However, the lack of comprehensive surveillance and diagnostic capacity challenges accurate estimation of their true prevalence. Diagnostic techniques for DENV and WNV involve serological tests, molecular tests and viral isolation, enabling prompt diagnosis and differentiation from other febrile illnesses. In Southern Africa, infection with DENV and WNV presents significant public health concerns, with the clinical presentation of both infections ranging from asymptomatic cases to severe manifestations. Symptoms of infection include high fever, myalgia, rash, and, in severe cases, haemorrhagic fever for DENV and neurological complications for WNV. No specific antiviral treatment exists for either virus, underscoring the importance of supportive care and symptom management. To prevent the spread of DENV and WNV in Southern African countries, a combination of prevention and treatment strategies should be employed, including effective mosquito control, continuous monitoring of vector population dynamics, public health education, and surveillance and reporting systems for averting future outbreaks.

A case report of long-latency evoked diaphragm potentials after exposure to acute intermittent hypoxia in post-West Nile virus meningoencephalitis

We present a case report of a 42-year-old female with post-West Nile virus meningoencephalitis who exhibited unique, long-latency diaphragm potentials evoked by transcranial and cervical magnetic stimulation after exposure to acute intermittent hypoxia (AIH). The subject was recruited for a study investigating AIH effects on respiratory motor function in healthy individuals. She had contracted West Nile virus infection 5 years before assessment that resulted in hospitalization and persistent allodynia but was not reported to the research team. During the study, transcranial (TMS) and cervical (CMS) magnetic stimulation were performed before and 30-60 min after a single presentation of AIH [15, 1-min hypoxic episodes (∼9% inspired O2), with 1-min normoxic intervals]. Diaphragm EMG was recorded using chest wall surface electrodes. At baseline, evoked diaphragm potentials were within normal ranges for both TMS (onset latency = 17.0 ± 1.1 ms; peak-to-peak amplitude = 220 ± 27 µV) and CMS (onset latency = 7.8 ± 0.6 ms; peak-to-peak amplitude = 336 ± 8 µV). However, long-latency TMS- and CMS-evoked potentials were observed 30-60 min post-AIH that were not present at baseline nor in healthy subjects. The onset of long-latency potentials ranged from 50 to 808 ms. While AIH is a potentially useful therapeutic strategy to enhance motor function after neurological disease or injury, it may elicit distinct effects in individuals with a history of neuroinfectious disease. Possible explanations for these unusual responses are discussed.NEW & NOTEWORTHY A 42-year-old female with post-West Nile virus meningoencephalitis demonstrated long-latency diaphragmatic potentials evoked by transcranial and cervical magnetic stimulation following exposure to acute intermittent hypoxia that were not present at baseline nor in healthy subjects. Although the cause of long-latency responses is unknown, we discuss possible mechanisms whereby acute intermittent hypoxia could create unique effects on the diaphragm/phrenic motor system in individuals with a history of neuroinfectious disease.

Neuroinvasion via Peripheral Nerves in Epidemic Viral Encephalitis Caused by Enterovirus, Orthoflavivirus and SARS-Coronavirus

Pathogens invade the central nervous system (CNS) and cause infections either through the haematogenous route or via peripheral nerves. Neuroinvasion via peripheral nerves, involving spinal or cranial somatic nerves, is well-established for certain viral encephalitides such as rabies, herpes simplex encephalitis, and poliomyelitis. Advances in understanding emerging and re-emerging viruses that cause epidemic CNS infections have highlighted the growing importance of peripheral nerve pathways in viral neuroinvasion. This review focuses on epidemic viral encephalitides caused by three groups of RNA viruses, viz., enteroviruses (enterovirus A71 and enterovirus D68), orthoflaviviruses (West Nile virus and Japanese encephalitis virus), and severe acute respiratory syndrome coronaviruses (mainly severe acute respiratory coronavirus-2). We examine evidence supporting the hypothesis that peripheral nerve viral transmission may play an increasingly significant if not more critical role than the haematogenous route in neuroinvasion.

Antimicrobial Peptides Against Arboviruses: Mechanisms, Challenges, and Future Directions

This review delves into the potential of antimicrobial peptides (AMPs) as promising candidates for combating arboviruses, focusing on their mechanisms of antiviral activity, challenges, and future directions. AMPs have shown promise in preventing arbovirus attachment to host cells, inducing interferon production, and targeting multiple viral stages, illustrating their multifaceted impact on arbovirus infections. Structural elucidation of AMP-viral complexes is explored to deepen the understanding of molecular determinants governing viral neutralization, paving the way for structure-guided design. Furthermore, this review highlights the potential of AMP-based combination therapies to create synergistic effects that enhance overall treatment outcomes while minimizing the likelihood of resistance development. Challenges such as susceptibility to proteases, toxicity, and scalable production are discussed alongside strategies to address these limitations. Additionally, the expanding applications of AMPs as vaccine adjuvants and antiviral delivery systems are emphasized, underscoring their versatility beyond direct antiviral functions.

Denial

A 51-year-old man presented with decreased vision, fever, confusion, headaches, agitation, nausea, vomiting and diarrhea. Magnetic resonance imaging of the brain demonstrated bilateral T2 hyperintense lesions in the region of the mesial temporal lobe and optic radiations. There was a predominantly polymorphonuclear leukocyte pleocytosis in the cerebrospinal fluid (CSF) with hyperproteinorachia. A meningoencephalitis was diagnosed. Intravenous fluorescein angiography (IVFA) demonstrated a multifocal chorioretinitis that was in a linear pattern in the left eye. CSF enzyme-linked immunosorbent assay was positive for West Nile virus (WNV) IgM. We review the clinical manifestations of WNV disease and highlight the value of IVFA in determining the diagnosis.

Flaviviruses-Induced Neurological Sequelae

Flaviviruses, a group of single-stranded RNA viruses spread by mosquitoes or ticks, include several significant neurotropic viruses, such as West Nile virus (WNV), Japanese encephalitis virus (JEV), tick-borne encephalitis virus (TBEV), and Zika virus (ZIKV). These viruses can cause a range of neurological diseases during acute infection, from mild, flu-like symptoms to severe and fatal encephalitis. A total of 20-50% of patients who recovered from acute flavivirus infections experienced long-term cognitive issues. Here, we discuss these major neurotropic flaviviruses-induced clinical diseases in humans and the recent findings in animal models and provide insights into the underlying disease mechanisms.

Flavivirus NS1-triggered endothelial dysfunction promotes virus dissemination

The Flaviviridae are a family of viruses that include the important arthropod-borne human pathogens dengue virus (DENV), West Nile virus, Zika virus, Japanese encephalitis virus, and yellow fever virus. Flavivirus nonstructural protein 1 (NS1) is essential for virus replication but is also secreted from virus-infected cells. Extracellular NS1 acts as a virulence factor during flavivirus infection in multiple ways, including triggering endothelial dysfunction and vascular leak via interaction with endothelial cells. While the role of NS1 in inducing vascular leak and exacerbating pathogenesis is well appreciated, if and how NS1-triggered endothelial dysfunction promotes virus infection remains obscure. Flaviviruses have a common need to disseminate from circulation into specific tissues where virus-permissive cells reside. Tissue-specific dissemination is associated with disease manifestations of a given flavivirus, but mechanisms dictating virus dissemination are unclear. Here we show that NS1-mediated endothelial dysfunction promotes virus dissemination in vitro and in vivo . In mouse models of DENV infection, we show that anti-NS1 antibodies decrease virus dissemination, while the addition of exogenous NS1 promotes virus dissemination. Using an in vitro system, we show that NS1 promotes virus dissemination in two distinct ways: (1) promoting crossing of barriers and (2) increasing infectivity of target cells in a tissue- and virus-specific manner. The capacity of NS1 to modulate infectivity correlates with a physical association between virions and NS1, suggesting a potential NS1-virion interaction. Taken together, our study indicates that flavivirus NS1 promotes virus dissemination across endothelial barriers, providing an evolutionary basis for virus-triggered vascular leak.

Author Summary

The Flaviviridae contain numerous medically important human pathogens that cause potentially life-threatening infections. Over half of the world's population is at risk of flavivirus infection, and this number is expected to increase as climate change expands the habitats of the arthropod vectors that transmit these flaviviruses. There are few effective vaccines and no therapeutics approved for prevention or treatment of flavivirus infection, respectively. Given these challenges, understanding how and why flaviviruses cause pathogenesis is critical for identifying targets for therapeutic intervention. The secreted nonstructural protein 1 (NS1) of flaviviruses is a conserved virulence factor that triggers endothelial dysfunction in a tissue-specific manner. It is unknown if this endothelial dysfunction provides any benefit for virus infection. Here we provide evidence that NS1-triggered endothelial dysfunction facilitates virus crossing of endothelial barriers and augments infection of target cells in vitro and promotes virus dissemination in vivo . This study provides an evolutionary explanation for flaviviruses evolving the capacity to trigger barrier dysfunction and highlights NS1 and the pathways governing endothelial dysfunction, as therapeutic targets to prevent flavivirus dissemination.

The historical ecological background of West Nile virus in Portugal indicates One Health opportunities

West Nile (WNV) is a zoonotic arbovirus with an expanding geographical range and epidemic activity in Europe. Not having yet experienced a human-associated epidemic, Portugal remains an outlier in the Mediterranean basin. In this study, we apply ecological niche modelling informed by WNV historical evidence and a multitude of environmental variables from across Portugal. We identify that ecological backgrounds compatible with WNV historical circulation are mostly restricted to the south, characterized by a warmer and drier climate, high avian diversity, specific avian species and land types. We estimate WNV ecological suitability across the country, identifying overlaps with the distributions of the three relevant hosts (humans, birds, equines) for public and animal health. From this, we propose a category-based spatial framework providing first of a kind valuable insights for WNV surveillance in Portugal under the One Health nexus. We forecast that near future climate trends alone will contribute to pushing adequate WNV ecological suitability northwards, towards regions with higher human density. This unique perspective on the past, present and future ecology of WNV addresses existing national knowledge gaps, enhances our understanding of the evolving emergence of WNV, and offers opportunities to prepare and respond to the first human-associated epidemic in Portugal.

The Alligator and the Mosquito: North American Crocodilians as Amplifiers of West Nile Virus in Changing Climates

In an age of emerging zoonoses, it is important to understand the intricate system of vectors and reservoirs, or hosts, and their relation to humans. West Nile Virus (WNV) has been detected in a myriad of nonhuman hosts. Transmission of the virus to humans is reliant on amplified seroprevalence within the host, which occurs primarily in birds. However, recent studies have found that other animal groups, including crocodilians, can obtain seroprevalence amplification to levels that make them competent hosts able to transmit WNV to mosquitoes, which can then transmit to humans. Climate change could exacerbate this transmission risk by shifting the distributions of mosquito vectors towards novel geographic ranges. Here, we use maximum entropy models to map the current and future distributions of three mosquito vector species and four crocodilian species in North America to determine the emerging risk of WNV outbreaks associated with changing climates and WNV associated with crocodilians in North America. From our models, we determined that one mosquito species in particular, Culex quinquefasciatus, will increase its distribution across the ranges of all crocodilian species in all tested climate change scenarios. This poses a potential risk to public health for people visiting and living near crocodilian farms and high-density natural crocodilian populations.

Rift Valley Fever Virus Encephalitis: Viral and Host Determinants of Pathogenesis

Rift Valley fever virus (RVFV) is a mosquito-borne virus endemic to Africa and the Middle East. RVFV infection can cause encephalitis, which is associated with significant morbidity and mortality. Studies of RVFV encephalitis following percutaneous inoculation, as would occur following a mosquito bite, have historically been limited by a lack of consistent animal models. In this review, we describe new insights into the pathogenesis of RVFV and the opportunities provided by new mouse models. We underscore the need to consider viral strain and route of inoculation when interpreting data obtained using animal models. We discuss the trafficking of RVFV and the role of host genetics and immunity in modulating the pathogenesis of RVFV encephalitis. We also explore potential strategies to prevent and treat central nervous system disease caused by RVFV and discuss remaining knowledge gaps.

Dengue virus infection and neurological manifestations: an update

Dengue virus is a flavivirus transmitted by the mosquitoes, Aedes aegypti and Aedes albopictus. Dengue infection by all four serotypes (DEN 1 to 4) is endemic globally in regions with tropical and subtropical climates, with an estimated 100-400 million infections annually. Among those hospitalized, the mortality is about 1%. Neurological involvement has been reported to be about 5%. The spectrum of neurological manifestations spans both the peripheral and central nervous systems. These manifestations could possibly be categorized into those directly related to dengue infection, i.e. acute and chronic encephalitis, indirect complications leading to dengue encephalopathy, and post-infectious syndrome due to immune-mediated reactions, and manifestations with uncertain mechanisms, such as acute transverse myelitis, acute cerebellitis and myositis. The rising trend in global dengue incidence calls for attention to a more explicit definition of each neurological manifestation for more accurate epidemiological data. The actual global burden of dengue infection with neurological manifestation is essential for future planning and execution of strategies, especially in the development of effective antivirals and vaccines against the dengue virus. In this article, we discuss the recent findings of different spectrums of neurological manifestations in dengue infection and provide an update on antiviral and vaccine development and their challenges.

New Prognostic Parameter of West Nile Virus: Platelet Distribution Width

Background: West Nile virus (WNV) infection is a viral disease caused by arboviruses. It can cause epidemics of febrile diseases and meningoencephalitis, especially at the end of the summer season. In this study, we aimed to determine the risk factors of WNV encephalitis with a case-control study of the patients followed in our clinic. Materials and Methods: Among the patients who applied to our hospital with sudden onset fever, headache, myalgia, nausea, vomiting, maculopapular rash, viral meningitis, or encephalitis findings in late summer and early autumn, those diagnosed with positive WNV PCR and antibody tests were defined as WNV cases. In the same date range, patients with clinically compatible but negative serological and PCR tests for WNV in our hospital were considered as the control group. Results: WNV infection was diagnosed in 26 of 48 patients who were examined with a preliminary diagnosis of WNV infection, and the other 22 patients were considered as the control group. A statistically significant difference was found between the two groups in C-reactive protein, procalcitonin, 1-h erythrocyte sedimentation rate, alkaline phosphatase, platelet, and platelet distribution width (PDW). PDW >17.85% indicated WNV infection with 82% sensitivity and 91% specificity. PDW percentage >17.85 increased the risk of WNV infection by 6.1 times. The power of the study was calculated as 83%. Conclusion: The most common findings in WNV cases were fever and confusion. WNV infection should be considered in the differential diagnosis in patients with fever and confusion in September and October in settlements on the migration route of birds. The percentage of PDW in whole blood examination can guide the differential diagnosis of WNV cases.

A Spatially Resolved and Environmentally Informed Forecast Model of West Nile Virus in Coachella Valley, California

West Nile virus (WNV) is the most significant arbovirus in the United States in terms of both morbidity and mortality. West Nile exists in a complex transmission cycle between avian hosts and the arthropod vector, Culex spp. mosquitoes. Human spillover events occur when humans are bitten by an infected mosquito and predicting these rates of infection and therefore the risk to humans may be associated with fluctuations in environmental conditions. In this study, we evaluate the hydrological and meteorological drivers associated with mosquito biology and viral development to determine if these associations can be used to forecast seasonal mosquito infection rates with WNV in the Coachella Valley of California. We developed and tested a spatially resolved ensemble forecast model of the WNV mosquito infection rate in the Coachella Valley using 17 years of mosquito surveillance data and North American Land Data Assimilation System-2 environmental data. Our multi-model inference system indicated that the combination of a cooler and dryer winter, followed by a wetter and warmer spring, and a cooler than normal summer was most predictive of the prevalence of West Nile positive mosquitoes in the Coachella Valley. The ability to make accurate early season predictions of West Nile risk has the potential to allow local abatement districts and public health entities to implement early season interventions such as targeted adulticiding and public health messaging before human transmission occurs. Such early and targeted interventions could better mitigate the risk of WNV to humans.

Immunization with different recombinant West Nile virus envelope proteins induces varying levels of serological cross-reactivity and protection from infection

Introduction

West Nile Virus (WNV) is a zoonotic flavivirus transmitted by mosquitoes. Especially in the elderly or in immunocompromised individuals an infection with WNV can lead to severe neurological symptoms. To date, no human vaccine against WNV is available. The Envelope (E) protein, located at the surface of flaviviruses, is involved in the invasion into host cells and is the major target for neutralizing antibodies and therefore central to vaccine development. Due to their close genetic and structural relationship, flaviviruses share highly conserved epitopes, such as the fusion loop domain (FL) in the E protein, that are recognized by cross-reactive antibodies. These antibodies can lead to enhancement of infection with heterologous flaviviruses, which is a major concern for potential vaccines in areas with co-circulation of different flaviviruses, e.g. Dengue or Zika viruses.

Material

To reduce the potential of inducing cross-reactive antibodies, we performed an immunization study in mice using WNV E proteins with either wild type sequence or a mutated FL, and WNV E domain III which does not contain the FL at all.

Results and discussion

Our data show that all antigens induce high levels of WNV-binding antibodies. However, the level of protection against WNV varied, with the wildtype E protein inducing full, the other antigens only partial protection. On the other hand, serological cross-reactivity to heterologous flaviviruses was significantly reduced after immunization with the mutated E protein or domain III as compared to the wild type version. These results have indications for choosing antigens with the optimal specificity and efficacy in WNV vaccine development.

The recombinant truncated envelope protein of West Nile virus adjuvanted with Alum/CpG induces potent humoral and T cell immunity in mice

West Nile virus (WNV) is a mosquito-transmitted flavivirus distributed globally for decades and can cause disease in humans and animals. So far, no WNV vaccine has been licensed for human use. Therefore, the development of novel candidate vaccines and the improvement of vaccination strategies is imperative. As the WNV envelope (E) glycoprotein plays an important role in mediating viral binding to cellular receptors and virus-cell membrane fusion, it is a critical target for the host humoral response. Here, we prepared a recombinant truncated envelope protein of WNV (rWNV-80E) and developed a WNV subunit vaccine formulation with a combination of aluminum hydroxide (alum) and a synthetic oligonucleotide CpG as adjuvants. C57BL/6 mice were immunized twice intramuscularly at 28-day intervals with 5 µg purified rWNV-80E adjuvanted with Alum/CpG. WNV E-specific IgG was detected by enzyme-linked immunosorbent assay and neutralizing antibodies (nAbs) was detected using single-round infectious particles of WNV. Furthermore, T cell immunity was detected by enzyme-linked immunospot assay and intracellular cytokine staining assay. Notably, rWNV-80E was highly immunogenic and elicited potent humoral and cell immunity, as evidenced by significant levels of IFN-γ and TNF-α secretion in the T cells of mice. In summary, the Alum/CpG-adjuvanted rWNV-80E subunit vaccine elicited potent and balanced B- and T-cell immunity in mice, and therefore it is a promising candidate vaccine that warrants further investigation for use in human or veterinary applications.

The Ongoing Epidemic of West Nile Virus in Greece: The Contribution of Biological Vectors and Reservoirs and the Importance of Climate and Socioeconomic Factors Revisited

Emerging infectious diseases have inflicted a significant health and socioeconomic burden upon the global population and governments worldwide. West Nile virus, a zoonotic, mosquito-borne flavivirus, was originally isolated in 1937 from a febrile patient in the West Nile Province of Uganda. It remained confined mainly to Africa, the Middle East, and parts of Europe and Australia until 1999, circulating in an enzootic mosquito-bird transmission cycle. Since the beginning of the 21st century, a new, neurotropic, more virulent strain was isolated from human outbreaks initially occurring in North America and later expanding to South and South-eastern Europe. Since 2010, when the first epidemic was recorded in Greece, annual incidence has fluctuated significantly. A variety of environmental, biological and socioeconomic factors have been globally addressed as potential regulators of the anticipated intensity of the annual incidence rate; circulation within the zoonotic reservoirs, recruitment and adaptation of new potent arthropod vectors, average winter and summer temperatures, precipitation during the early summer months, and socioeconomic factors, such as the emergence and progression of urbanization and the development of densely populated areas in association with insufficient health policy measures. This paper presents a review of the biological and socioenvironmental factors influencing the dynamics of the epidemics of West Nile virus (WNV) cases in Greece, one of the highest-ranked European countries in terms of annual incidence. To date, WNV remains an unpredictable opponent as is also the case with other emerging infectious diseases, forcing the National Health systems to develop response strategies, control the number of infections, and shorten the duration of the epidemics, thus minimizing the impact on human and material resources.

Ecological Niche Modeling of Aedes and Culex Mosquitoes: A Risk Map for Chikungunya and West Nile Viruses in Zambia

The circulation of both West Nile Virus (WNV) and Chikungunya Virus (CHIKV) in humans and animals, coupled with a favorable tropical climate for mosquito proliferation in Zambia, call for the need for a better understanding of the ecological and epidemiological factors that govern their transmission dynamics in this region. This study aimed to examine the contribution of climatic variables to the distribution of Culex and Aedes mosquito species, which are potential vectors of CHIKV, WNV, and other arboviruses of public-health concern. Mosquitoes collected from Lusaka as well as from the Central and Southern provinces of Zambia were sorted by species within the Culex and Aedes genera, both of which have the potential to transmit viruses. The MaxEnt software was utilized to predict areas at risk of WNV and CHIKV based on the occurrence data on mosquitoes and environmental covariates. The model predictions show three distinct spatial hotspots, ranging from the high-probability regions to the medium- and low-probability regions. Regions along Lake Kariba, the Kafue River, and the Luangwa Rivers, as well as along the Mumbwa, Chibombo, Kapiri Mposhi, and Mpika districts were predicted to be suitable habitats for both species. The rainfall and temperature extremes were the most contributing variables in the predictive models.

Comparison of West Nile Virus Disease in Humans and Horses: Exploiting Similarities for Enhancing Syndromic Surveillance

West Nile virus (WNV) neuroinvasive disease threatens the health and well-being of horses and humans worldwide. Disease in horses and humans is remarkably similar. The occurrence of WNV disease in these mammalian hosts has geographic overlap with shared macroscale and microscale drivers of risk. Importantly, intrahost virus dynamics, the evolution of the antibody response, and clinicopathology are similar. The goal of this review is to provide a comparison of WNV infection in humans and horses and to identify similarities that can be exploited to enhance surveillance methods for the early detection of WNV neuroinvasive disease.

Development of recombinant West Nile virus expressing mCherry reporter protein

West Nile virus (WNV) is transmitted to humans and animals by a mosquito and enters the central nervous system, leading to lethal encephalitis. Reporter viruses expressing fluorescent proteins enable detection of infected cells in vitro and in vivo, facilitating evaluation of the dynamics of viral infection, and the development of diagnostic or therapeutic methods. In this study, we developed a method for production of a recombinant replication-competent WNV expressing mCherry fluorescent protein. The expression of mCherry was observed in viral antigen-positive cells in vitro and in vivo, but the growth of the reporter WNV was reduced as compared to the parental WNV. The expression of mCherry was stable during 5 passages in reporter WNV-infected culture cells. Neurological symptoms were observed in mice inoculated intracranially with the reporter WNV. The reporter WNV expressing mCherry will facilitate research into WNV replication in mouse brains.

Patterns of West Nile Virus in the Northeastern United States Using Negative Binomial and Mechanistic Trait-Based Models

West Nile virus (WNV) primarily infects birds and mosquitoes but has also caused over 2,000 human deaths, and >50,000 reported human cases in the United States. Expected numbers of WNV neuroinvasive cases for the present were described for the Northeastern United States, using a negative binomial model. Changes in temperature-based suitability for WNV due to climate change were examined for the next decade using a temperature-trait model. WNV suitability was generally expected to increase over the next decade due to changes in temperature, but the changes in suitability were generally small. Many, but not all, populous counties in the northeast are already near peak suitability. Several years in a row of low case numbers is consistent with a negative binomial, and should not be interpreted as a change in disease dynamics. Public health budgets need to be prepared for the expected infrequent years with higher-than-average cases. Low-population counties that have not yet had a case are expected to have similar probabilities of having a new case as nearby low-population counties with cases, as these absences are consistent with a single statistical distribution and random chance.

Treatment with Granulocyte-Macrophage Colony-Stimulating Factor Reduces Viral Titers in the Brains of West Nile Virus-Infected Mice and Improves Survival

West Nile virus (WNV) is the leading cause of epidemic arboviral encephalitis in the United States. As there are currently no proven antiviral therapies or licensed human vaccines, understanding the neuropathogenesis of WNV is critical for rational therapeutic design. In WNV-infected mice, the depletion of microglia leads to enhanced viral replication, increased central nervous system (CNS) tissue injury, and increased mortality, suggesting that microglia play a critical role in protection against WNV neuroinvasive disease. To determine if augmenting microglial activation would provide a potential therapeutic strategy, we administered granulocyte-macrophage colony-stimulating factor (GM-CSF) to WNV-infected mice. Recombinant human GM-CSF (rHuGMCSF) (sargramostim [Leukine]) is an FDA-approved drug used to increase white blood cells following leukopenia-inducing chemotherapy or bone marrow transplantation. Daily treatment of both uninfected and WNV-infected mice with subcutaneous injections of GM-CSF resulted in microglial proliferation and activation as indicated by the enhanced expression of the microglia activation marker ionized calcium binding adaptor molecule 1 (Iba1) and several microglia-associated inflammatory cytokines, including CCL2 (C-C motif chemokine ligand 2), interleukin 6 (IL-6), and IL-10. In addition, more microglia adopted an activated morphology as demonstrated by increased sizes and more pronounced processes. GM-CSF-induced microglial activation in WNV-infected mice was associated with reduced viral titers and apoptotic activity (caspase 3) in the brains of WNV-infected mice and significantly increased survival. WNV-infected ex vivo brain slice cultures (BSCs) treated with GM-CSF also showed reduced viral titers and caspase 3 apoptotic cell death, indicating that GM-CSF specifically targets the CNS and that its actions are not dependent on peripheral immune activity. Our studies suggest that stimulation of microglial activation may be a viable therapeutic approach for the treatment of WNV neuroinvasive disease. IMPORTANCE Although rare, WNV encephalitis poses a devastating health concern, with few treatment options and frequent long-term neurological sequelae. Currently, there are no human vaccines or specific antivirals against WNV infections, so further research into potential new therapeutic agents is critical. This study presents a novel treatment option for WNV infections using GM-CSF and lays the foundation for further studies into the use of GM-CSF as a treatment for WNV encephalitis as well as a potential treatment for other viral infections.

Recent Advancements in Mosquito-Borne Flavivirus Vaccine Development

Lately, the global incidence of flavivirus infection has been increasing dramatically and presents formidable challenges for public health systems around the world. Most clinically significant flaviviruses are mosquito-borne, such as the four serotypes of dengue virus, Zika virus, West Nile virus, Japanese encephalitis virus and yellow fever virus. Until now, no effective antiflaviviral drugs are available to fight flaviviral infection; thus, a highly immunogenic vaccine would be the most effective weapon to control the diseases. In recent years, flavivirus vaccine research has made major breakthroughs with several vaccine candidates showing encouraging results in preclinical and clinical trials. This review summarizes the current advancement, safety, efficacy, advantages and disadvantages of vaccines against mosquito-borne flaviviruses posing significant threats to human health.

Serosurvey in Two Dengue Hyperendemic Areas of Costa Rica Evidence Active Circulation of WNV and SLEV in Peri-Domestic and Domestic Animals and in Humans

Costa Rica harbors several flaviviruses, including Dengue (DENV), Zika (ZIKV), West Nile virus (WNV), and Saint Louis encephalitis virus (SLEV). While DENV and ZIKV are hyperendemic, previous research indicates restricted circulation of SLEV and WNV in animals. SLEV and WNV seroprevalence and high transmission areas have not yet been measured. To determine the extents of putative WNV and SLEV circulation, we sampled peri-domestic and domestic animals, humans, and mosquitoes in rural households located in two DENV and ZIKV hyperendemic regions during the rainy and dry seasons of 2017-2018 and conducted plaque reduction neutralization test assay for serology (PRNT) and RT-PCR for virus detection. In Cuajiniquil, serological evidence of WNV and SLEV was found in equines, humans, chickens, and wild birds. Additionally, five seroconversion events were recorded for WNV (2 equines), SLEV (1 human), and DENV-1 (2 humans). In Talamanca, WNV was not found, but serological evidence of SLEV circulation was recorded in equines, humans, and wild birds. Even though no active viral infection was detected, the seroconversion events recorded here indicate recent circulation of SLEV and WNV in these two regions. This study thus provides clear-cut evidence for WNV and SLEV presence in these areas, and therefore, they should be considered in arboviruses differential diagnostics and future infection prevention campaigns.

Re-Introduction of West Nile Virus Lineage 1 in Senegal from Europe and Subsequent Circulation in Human and Mosquito Populations between 2012 and 2021

West Nile virus (WNV) is a virus of the Japanese encephalitis antigenic complex and belongs to the family Flaviviridae of the genus flavivirus. The virus can cause infection in humans which in most cases is asymptomatic, however symptomatic cases exist and the disease can be severe causing encephalitis and meningoencephalitis. The virus is maintained in an enzootic cycle involving mosquitoes and birds, humans and other mammals such as horses can be accidental hosts. A mosquito-based arbovirus surveillance system and the sentinel syndromic surveillance network (4S) have been in place since 1988 and 2015 respectively, to better understand the transmission dynamics of arboviruses including WNV in Senegal. Arthropod and human samples have been collected from the field and analysed at Institut Pasteur de Dakar using different methods including RT-PCR, ELISA, plaque reduction neutralization test and viral isolation. RT-PCR positive samples have been analysed by Next Generation Sequencing. From 2012 to 2021, 7912 samples have been analysed and WNV positive cases have been detected, 20 human cases (19 IgM and 1 RT-PCR positive cases) and 41 mosquito pools. Phylogenetic analyzes of the sequences of complete genomes obtained showed the circulation of lineage 1a, with all these recent strains from Senegal identical to each other and very close to strains isolated from horse in France in 2015, Italy and Spain. Our data showed lineage 1a endemicity in Senegal as previously described, with circulation of WNV in humans and mosquitoes. Phylogenetic analyzes carried out with the genome sequences obtained also revealed exchanges of WNV strains between Europe and Senegal which could be possible via migratory birds. The surveillance systems that have enabled the detection of WNV in humans and arthropods should be extended to animals in a one-health approach to better prepare for global health threats.

West Nile Disease Symptoms and Comorbidities: A Systematic Review and Analysis of Cases

West Nile virus (WNV) is a mosquito-borne flavivirus that has emerged as a major cause of viral encephalitis and meningitis, rarely leading to death. Several risk factors have been discussed in the past concerning the severity of the disease, while few reports have focused on precipitating conditions that determine of WNV-related death. Studies on cohorts of patients suffering of West Nile disease (WND) usually encompass low numbers of deceased patients as a result of the rarity of the event. In this systematic review and critical analysis of 428 published case studies and case series, we sought to evaluate and highlight critical parameters of WND-related death. We summarized the symptoms, comorbidities, and treatment strategies related to WND in all published cases of patients that included clinical features. Symptoms such as altered mental status and renal problems presented increased incidence among deceased patients, while these patients presented increased cerebrospinal fluid (CSF) glucose. Our analysis also highlights underestimated comorbidities such as pulmonary disease to act as precipitating conditions in WND, as they were significantly increased amongst deceased patients. CSF glucose and the role of pulmonary diseases need to be revaluated either retrospectively or prospectively in WND patient cohorts, as they may be linked to increased mortality risk.

Modelling the temperature suitability for the risk of West Nile Virus establishment in European Culex pipiens populations

Increases in temperature and extreme weather events due to global warming can create an environment that is beneficial to mosquito populations, changing and possibly increasing the suitable geographical range for many vector-borne diseases. West Nile Virus (WNV) is a flavivirus, maintained in a mosquito-avian host cycle that is usually asymptomatic but can cause primarily flu-like symptoms in human and equid accidental hosts. In rare circumstances, serious disease and death are possible outcomes for both humans and horses. The main European vector of WNV is the Culex pipiens mosquito. This study examines the effect of environmental temperature on WNV establishment in Europe via Culex pipiens populations through use of a basic reproduction number ( R 0 ${R_0}$ ) model. A metric of thermal suitability derived from R 0 ${R_0}$ was developed by collating thermal responses of different Culex pipiens traits and combining them through use of a next-generation matrix. WNV establishment was determined to be possible between 14°C and 34.3°C, with the optimal temperature at 23.7°C. The suitability measure was plotted against monthly average temperatures in 2020 and the number of months with high suitability mapped across Europe. The average number of suitable months for each year from 2013 to 2019 was also calculated and validated with reported equine West Nile fever cases from 2013 to 2019. The widespread thermal suitability for WNV establishment highlights the importance of European surveillance for this disease and the need for increased research into mosquito and bird distribution.

West Nile Virus Seroprevalence and Cross-Neutralization in Sera from Eastern and Central Sudan

Objectives: In regions with co-existing flaviviruses, the diagnosis of previous West Nile virus (WNV) infections is challenging due to cross-reacting antibodies. The aim of the study was to determine the frequency of previous WNV infections in sera from three Sudanese states by excluding potentially dengue virus (DENV) and ZIKV cross-reacting sera and to determine the percentage of WNV cross-neutralizing sera from individuals with previous DENV infection. Methods: Serum samples from Kassala, North Kordofan, and Red Sea state were screened for antibodies against DENV by ELISA. Sera without DENV antibodies (N = 106) and a matched set of sera with DENV antibodies (N = 108) was selected. In all blood samples the frequency of WNV-neutralizing antibodies and the antibody titers were measured with microplate neutralization assays. DENV and Zika virus (ZIKV) microplate neutralization assays were performed with all WNV neutralizing sera of the DENV negative group. Results: A fraction of 30.2% of the DENV antibody negative sera neutralized WNV. The seroprevalence increased with age from 9.5% to 41.7%. Men and women were equally affected. The percentage of DENV positive sera that neutralized WNV was 83.3%. DENV positive sera had higher WNV neutralization titers than DENV negative sera. Conclusions: A significant fraction of the DENV antibody negative sera from three regions in Sudan showed serologic evidence of previous WNV infection. In comparison, the large majority of DENV antibody positive sera had WNV neutralizing antibodies. Studies are needed to identify clinical cases of WNV infection and to determine whether individuals with cross-neutralizing antibodies are protected from WNV disease.

Retrospective Investigation in Horses with Encephalitis Reveals Unnoticed Circulation of West Nile Virus in Brazil

During these past years, several studies have provided serological evidence regarding the circulation of West Nile virus (WNV) in Brazil. Despite some reports, much is still unknown regarding the genomic diversity and transmission dynamics of this virus in the country. Recently, genomic monitoring activities in horses revealed the circulation of WNV in several Brazilian regions. These findings on the paucity of genomic data reinforce the need for prompt investigation of WNV infection in horses, which may precede human cases of encephalitis in Brazil. Thus, in this study, we retrospectively screened 54 suspicious WNV samples collected between 2017 and 2020 from the spinal cord and brain of horses with encephalitis and generated three new WNV genomes from the Ceará and Bahia states, located in the northeastern region of Brazil. The Bayesian reconstruction revealed that at least two independent introduction events occurred in Brazil. The first introduction event appears to be likely related to the North American outbreak, and was estimated to have occurred in March 2013.The second introduction event appears to have occurred in September 2017 and appears to be likely related to the South American outbreak. Together, our results reinforce the importance of increasing the priority of WNV genomic monitoring in equines with encephalitis in order to track the dispersion of this emerging pathogen through the country.

West Nile Virus Seroprevalence in Manisa Province: A Population-based Study

Objective

West Nile Virus (WNV), which causes widespread outbreaks in different parts of the world, is a risk to public health in Turkey, too. Community-based study data are needed to identify measures against possible outbreaks. This study aimed to determine the seroprevalence of community-based WNV in Manisa and to investigate the relationship between sociodemographic and socioeconomic variables.

Methods

We included individuals older than two years of age (N=1,317,917) registered in the Manisa Province Family Medicine Information System. Selected participants (n=1233) were determined by a simple random sampling method. Specific IgG antibodies against WNV were investigated in serum samples using a commercial ELISA test (Euroimmun, Germany). The relationship between age, gender, location, education and income level, occupation, population density, altitude, the location of the toilet in the house, and the presence of hypertension, diabetes mellitus and cardiovascular disease variables were analyzed by chi-square, Fisher's exact test and t-test. Adjusted odds ratio (OR) with95% confidence interval (CI) for each variable were calculated by the logistic regression method to explain potential risks.

Results

WNV IgG antibodies were detected in 47 (3.8%) sera samples by ELISA. Seroprevalence was significantly correlated with independent variables of advanced age, presence of hypertension, diabetes mellitus and cardiovascular disease, low level of education and income, living in low altitude areas and the location of the toilet. In multivariate analysis; age (every one-year increase) (OR:1.05; 95% CI:1.02-1.07; p <0.001), equivalent annual income per capita below 3265 TL (OR:3.21; 95% CI: 1.53-6.73; p=0.002), and living areas below 132 meters altitude (OR=3.21; 95% CI 1.26-8.15; p=0.014) were found to be the risk factors for WNV seropositivity.

Conclusion

In Manisa province, WNV IgG seroprevalence was detected as 3.8% with ELISA method. Older age, low income and living in regions with a low altitude were found to be associated with increased seropositivity significantly.

Depletion of Microglia in an Ex Vivo Brain Slice Culture Model of West Nile Virus Infection Leads to Increased Viral Titers and Cell Death

West Nile virus (WNV) is a major cause of viral encephalitis in the United States. WNV infection of the brain leads to neuroinflammation characterized by activation of microglia, the resident phagocytic cells of the central nervous system (CNS). In this study, depletion of CNS microglia using the CSF1R antagonist PLX5622 increased the viral load in the brain and decreased the survival of mice infected with WNV (strain TX02). PLX5622 was also used in ex vivo brain slice cultures (BSCs) to investigate the role of intrinsic neuroinflammatory responses during WNV infection. PLX5622 effectively depleted microglia (>90% depletion) from BSCs resulting in increased viral titers (3 to 4-fold increase in PLX5622-treated samples) and enhanced virus-induced caspase 3 activity and cell death. Microglia depletion did not result in widespread alterations in cytokine and chemokine production in either uninfected or WNV infected BSCs. The results of this study demonstrated how microglia contribute to limiting viral growth and preventing cell death in WNV infected BSCs but were not required for the cytokine/chemokine response to WNV infection. This study highlighted the importance of microglia in the protection from neuroinvasive WNV infection and demonstrated that microglia responses were independent of WNV-induced peripheral immune responses. IMPORTANCE WNV infections of the CNS are rare but can have devastating long-term effects. There are currently no vaccines or specific antiviral treatments, so a better understanding of the pathogenesis and immune response to this virus is crucial. Previous studies have shown microglia to be important for protection from WNV, but more work is needed to fully comprehend the impact these cells have on neuroinvasive WNV infections. This study used PLX5622 to eliminate microglia in an ex vivo brain slice culture (BSC) model to investigate the role of microglia during a WNV infection. The use of BSCs provided a system in which immune responses innate to the CNS could be studied without interference from peripheral immunity. This study will allow for a better understanding of the complex nature of microglia during viral infections and will likely impact the development of new therapeutics that target microglia.

West Nile and Usutu virus seroprevalence in Hungary: A nationwide serosurvey among blood donors in 2019

In Hungary, West Nile virus (WNV) has been responsible for 459 laboratory confirmed human cases between 2004 and 2019, while the first human Usutu virus (USUV) infection was confirmed only in 2018. A comprehensive serosurvey was conducted among blood donors to assess the WNV and USUV seroprevalence in 2019, one year after the largest European WNV epidemic. Altogether, 3005 plasma samples were collected and screened for WNV and USUV specific Immunoglobulin G (IgG) antibodies by Enzyme-Linked Immunosorbent Assay (ELISA). All reactive samples were further tested for tick-borne encephalitis virus IgG antibodies by ELISA. Indirect immunofluorescence test and microneutralization assay were used as confirmatory methods. Overall, the WNV seroprevalence was 4.32%, and in five blood donors USUV seropositivity was confirmed. The highest seroprevalence was measured in Central, Eastern and Southern Hungary, while the Western part of the country proved to be less affected. There was a statistically strong association between the WNV seroprevalence of 2019 and the cumulative incidence in the period of 2004 and 2019 calculated for every NUTS 3 region. The last WNV serological screening was performed in 2016 and the prevalence of anti-WNV IgG proved to be 2.19%. One year after the 2018 WNV outbreak, a significant increase in seroprevalence was observed in the Hungarian population and evidence for USUV seropositivity was also obtained. The spatial pattern of seroprevalence can support the identification of high-risk areas raising awareness of the need for increased surveillance, such as screening vector, equine, and avian populations. The communication with general practitioners and other professionals in primary health care services can support the early identification of acute human cases. Education and awareness-raising on the importance of protection against mosquito vectors amongst residents are also important parts of preventive measures.

Effects of ivermectin treatment of backyard chickens on mosquito dynamics and West Nile virus transmission

Background

Vector control strategies typically rely on pesticides to target mosquitoes involved in enzootic and zoonotic transmission of West Nile virus (WNV). Nevertheless, increasing insecticide resistance and a desire to reduce pesticide usage provide the impetus for developing alternative strategies. Ivermectin (IVM), an antiparasitic drug which is widely used in human and veterinary medicine, is a potential alternative for targeted control because Culex mosquitoes experience increased mortality following ingestion of IVM in bloodmeals.

Methodology/Principal findings

We conducted a randomized field trial to investigate the impact of treating backyard chicken flocks with IVM in urban neighborhoods across Davis, California on mosquito populations and WNV transmission dynamics. We observed a significant reduction in WNV seroconversions in treated vs. untreated chickens, suggesting a reduction in WNV transmission intensity around treated flocks. We also detected a reduction in parity rates of Cx. tarsalis near treated vs. untreated flocks and increased mortality in wild mosquitoes following a bloodmeal on treated chickens (IVM serum concentration > 5ng/mL) vs. chickens with IVM serum concentrations < 5 ng/mL. However, we did not find a significant difference in abundance or infection prevalence in mosquitoes between treatment groups associated with the reductions in seroconversions. Mosquito immigration from surrounding larval habitat, relatively low WNV activity in the study area, and variable IVM serum concentrations likely contributed to uncertainty about the impact.

Conclusions/Significance

Taken together, our results point to a reduction in WNV transmission due to the impact of IVM on Culex mosquito populations and support the ongoing investigation of oral administration of IVM to wild birds for local control of WNV transmission, although further work is needed to optimize dosing and understand effects on entomological endpoints.

Current mosquito control strategies aimed to prevent pathogen transmission to humans have limited ability to target mosquitoes involved in amplification and spillover transmission of pathogens like West Nile virus (WNV). Additionally, growing prevalence of insecticide resistance in mosquito populations limit the efficacy of these insecticide-based control strategies. Ivermectin (IVM) provides an alternative avenue for control by increasing the mortality of mosquitoes that ingest this drug in bloodmeals. Therefore, IVM treatment of avian species that account for the majority of mosquito bloodmeals during the WNV transmission season could be an effective control strategy. Building on pilot studies indicating the efficacy and feasibility of IVM-deployment for WNV control, we performed a randomized field trial to investigate the impact of IVM-treatment of backyard chickens on local population dynamics of Culex mosquitoes and WNV transmission. We were able to link changes in mosquito populations to reduction in WNV transmission, as measured by chicken seroconversions, through IVM-induced mortality in mosquitoes. However, further work is needed to identify the impact of treatment on mosquito abundance and infection prevalence to fully attribute observed changes to IVM administration. Overall, our results support IVM treatment as a potentially effective alternative to insecticide-based vector control strategies and one that can be used to target WNV transmission on the local scale.

Plant-Derived Recombinant Vaccines against Zoonotic Viruses

Emerging and re-emerging zoonotic diseases cause serious illness with billions of cases, and millions of deaths. The most effective way to restrict the spread of zoonotic viruses among humans and animals and prevent disease is vaccination. Recombinant proteins produced in plants offer an alternative approach for the development of safe, effective, inexpensive candidate vaccines. Current strategies are focused on the production of highly immunogenic structural proteins, which mimic the organizations of the native virion but lack the viral genetic material. These include chimeric viral peptides, subunit virus proteins, and virus-like particles (VLPs). The latter, with their ability to self-assemble and thus resemble the form of virus particles, are gaining traction among plant-based candidate vaccines against many infectious diseases. In this review, we summarized the main zoonotic diseases and followed the progress in using plant expression systems for the production of recombinant proteins and VLPs used in the development of plant-based vaccines against zoonotic viruses.

West Nile virus transmission potential in Portugal

It is unclear whether West Nile virus (WNV) circulates endemically in Portugal. Despite the country’s adequate climate for transmission, Portugal has only reported four human WNV infections so far. We performed a review of WNV-related data (1966–2020), explored mosquito (2016–2019) and land type distributions (1992–2019), and used climate data (1981–2019) to estimate WNV transmission suitability in Portugal. Serological and molecular evidence of WNV circulation from animals and vectors was largely restricted to the south. Land type and climate-driven transmission suitability distributions, but not the distribution of WNV-capable vectors, were compatible with the North-South divide present in serological and molecular evidence of WNV circulation. Our study offers a comprehensive, data-informed perspective and review on the past epidemiology, surveillance and climate-driven transmission suitability of WNV in Portugal, highlighting the south as a subregion of importance. Given the recent WNV outbreaks across Europe, our results support a timely change towards local, active surveillance.

Lourenço et al. review historical data and quantify the transmission potential of West Nile virus in Portugal. They report a North-South divide in infection patterns, a higher ecological capacity in the south, and an increasing positive effect of climate change over the last 40 years.

Serological differentiation of West Nile virus and Usutu virus induced antibodies by envelope proteins with modified cross-reactive epitopes

West Nile virus (WNV) and Usutu virus (USUV) are mosquito-borne viruses belonging to the Japanese encephalitis virus serocomplex within the genus Flavivirus. Due to climate change and the expansion of mosquito vectors, flaviviruses are becoming endemic in increasing numbers of countries. WNV infections are reported with symptoms ranging from mild fever to severe neuro invasive disease. Until now, only a few USUV infections have been reported in humans, mostly with mild symptoms. The serological diagnosis and differentiation between flavivirus infections in general and between WNV and USUV in particular are challenging due the high degree of cross-reacting antibodies, especially of those directed against the conserved fusion loop (FL) domain of the envelope (E) protein. We have previously shown that E proteins containing four amino acid mutations in and near the FL strongly reduce the binding of cross-reactive antibodies leading to diagnostic technologies with improved specificities. Here, we expanded the technology to USUV and analyzed the differentiation of USUV and WNV induced antibodies in humans. IgG ELISAs modified by an additional competition step with the heterologous antigen resulted in overall specificities of 93.94% for WNV Equad and 92.75% for USUV Equad. IgM antibodies against WNV could be differentiated from USUV IgM in a direct comparison using both antigens. The data indicate the potential of the system to diagnose antigenically closely related flavivirus infections. This article is protected by copyright. All rights reserved.

West Nile Virus Chorioretinitis in the Presence of Negative Cerebrospinal Fluid Polymerase Chain Reaction Results

Purpose:

This work aims to evaluate the utility of nucleic acid amplification testing (NAAT) and serology in confirming West Nile Virus (WNV) infection in patients with suspected WNV chorioretinitis.

Methods:

A retrospective cross-sectional study was conducted of a cluster of patients who presented to the Retina Service of Massachusetts Eye and Ear between September and October 2018.

Results:

Three patients were identified with classic WNV chorioretinitis lesions with negative cerebrospinal fluid NAAT and positive serum serology findings. The diagnosis of WNV chorioretinitis was made based on the appearance of the fundus lesions and the presence of characteristic findings on fluorescein angiography as previously described in the literature.

Conclusions:

This report highlights 3 unique cases of WNV chorioretinitis in which NAAT of cerebrospinal fluid failed to identify WNV as the inciting agent. These cases stress the importance of serum serologic testing in diagnosing WNV infection.

Intrinsic Innate Immune Responses Control Viral Growth and Protect against Neuronal Death in an Ex Vivo Model of West Nile Virus-Induced Central Nervous System Disease

Recruitment of immune cells from the periphery is critical for controlling West Nile virus (WNV) growth in the central nervous system (CNS) and preventing subsequent WNV-induced CNS disease. Neuroinflammatory responses, including the release of proinflammatory cytokines and chemokines by CNS cells, influence the entry and function of peripheral immune cells that infiltrate the CNS. However, these same cytokines and chemokines contribute to tissue damage in other models of CNS injury. Rosiglitazone is a peroxisome proliferator-activated receptor gamma (PPARγ) agonist that inhibits neuroinflammation. We used rosiglitazone in WNV-infected ex vivo brain slice cultures (BSC) to investigate the role of neuroinflammation within the CNS in the absence of peripheral immune cells. Rosiglitazone treatment inhibited WNV-induced expression of proinflammatory chemokines and cytokines, interferon beta (IFN-β), and IFN-stimulated genes (ISG) and also decreased WNV-induced activation of microglia. These decreased neuroinflammatory responses were associated with activation of astrocytes, robust viral growth, increased activation of caspase 3, and increased neuronal loss. Rosiglitazone had a similar effect on in vivo WNV infection, causing increased viral growth, tissue damage, and disease severity in infected mice, even though the number of infiltrating peripheral immune cells was higher in rosiglitazone-treated, WNV-infected mice than in untreated, infected controls. These results indicate that local neuroinflammatory responses are capable of controlling viral growth within the CNS and limiting neuronal loss and may function to keep the virus in check prior to the infiltration of peripheral immune cells, limiting both virus- and immune-mediated neuronal damage. IMPORTANCE West Nile virus is the most common cause of epidemic encephalitis in the United States and can result in debilitating CNS disease. There are no effective vaccines or treatments for WNV-induced CNS disease in humans. The peripheral immune response is critical for protection against WNV CNS infections. We now demonstrate that intrinsic immune responses also control viral growth and limit neuronal loss. These findings have important implications for developing new therapies for WNV-induced CNS disease.

Mosquito-Borne Viral Pathogens Detected in Zambia: A Systematic Review

Emerging and re-emerging mosquito-borne viral diseases are a threat to global health. This systematic review aimed to investigate the available evidence of mosquito-borne viral pathogens reported in Zambia. A search of literature was conducted in PubMed and Google Scholar for articles published from 1 January 1930 to 30 June 2020 using a combination of keywords. Eight mosquito-borne viruses belonging to three families, Togaviridae, Flaviviridae and Phenuiviridae were reported. Three viruses (Chikungunya virus, Mayaro virus, Mwinilunga virus) were reported among the togaviruses whilst four (dengue virus, West Nile virus, yellow fever virus, Zika virus) were among the flavivirus and only one virus, Rift Valley fever virus, was reported in the Phenuiviridae family. The majority of these mosquito-borne viruses were reported in Western and North-Western provinces. Aedes and Culex species were the main mosquito-borne viral vectors reported. Farming, fishing, movement of people and rain patterns were among factors associated with mosquito-borne viral infection in Zambia. Better diagnostic methods, such as the use of molecular tools, to detect the viruses in potential vectors, humans, and animals, including the recognition of arboviral risk zones and how the viruses circulate, are important for improved surveillance and design of effective prevention and control measures.

West Nile Virus in the State of Ceará, Northeast Brazil

In June 2019, a horse with neurological disorder was diagnosed with West Nile virus (WNV) in Boa Viagem, a municipality in the state of Ceará, northeast Brazil. A multi-institutional task force coordinated by the Brazilian Ministry of Health was deployed to the area for case investigation. A total of 513 biological samples from 78 humans, 157 domestic animals and 278 free-ranging wild birds, as well as 853 adult mosquitoes of 22 species were tested for WNV by highly specific serological and/or molecular tests. No active circulation of WNV was detected in vertebrates or mosquitoes by molecular methods. Previous exposure to WNV was confirmed by seroconversion in domestic birds and by the detection of specific neutralizing antibodies in 44% (11/25) of equids, 20.9% (14/67) of domestic birds, 4.7% (13/278) of free-ranging wild birds, 2.6% (2/78) of humans, and 1.5% (1/65) of small ruminants. Results indicate that not only equines but also humans and different species of domestic animals and wild birds were locally exposed to WNV. The detection of neutralizing antibodies for WNV in free-ranging individuals of abundant passerine species suggests that birds commonly found in the region may have been involved as amplifying hosts in local transmission cycles of WNV.

Frequency of Zika Virus Immunoglobulin M Antibody in Persons with West Nile Virus Infection

West Nile virus (WNV) and Zika virus (ZIKV) are mosquito-borne viruses in the family Flaviviridae. Residents in, and travelers to, areas where the viruses are circulating are at risk for infection, and both viruses can cause an acute febrile illness. Given known cross-reactivity in flavivirus serologic assays, it is possible a patient with acute WNV infection could be misdiagnosed as having ZIKV infection if appropriate testing is not conducted. To understand how frequently persons with WNV infection have detectable cross-reactive ZIKV immunoglobulin M (IgM) antibody, we used archived serum samples from patients in the United States with recent WNV infection confirmed by a microsphere-based immunoassay test for IgM antibody and neutralizing antibody testing. Samples were tested for ZIKV IgM antibody with the Centers for Disease Control and Prevention (CDC) ZIKV IgM antibody capture enzyme-linked immunosorbent assay. Among 153 sera from patients with acute WNV infection, the ZIKV IgM antibody result was positive in 56 (37%; 95% confidence interval [CI] 29-44%) and equivocal in 28 (18%; 95% CI 13-25%). With 55% of samples having cross-reactive antibodies, it is important for health care providers to request appropriate testing based on the most likely cause of a patient's possible arboviral infection considering their clinical symptoms and signs, travel history, and place of residence. For cases where the epidemiology does not support the preliminary IgM findings, confirmatory neutralizing antibody testing should be performed. These measures will avoid an incorrect diagnosis of ZIKV infection, based on cross-reactive antibodies, in a person truly infected with WNV.

West Nile Virus in Brazil

Background: West Nile virus (WNV) was first sequenced in Brazil in 2019, when it was isolated from a horse in the Espírito Santo state. Despite multiple studies reporting serological evidence suggestive of past circulation since 2004, WNV remains a low priority for surveillance and public health, such that much is still unknown about its genomic diversity, evolution, and transmission in the country. Methods: A combination of diagnostic assays, nanopore sequencing, phylogenetic inference, and epidemiological modeling are here used to provide a holistic overview of what is known about WNV in Brazil. Results: We report new genetic evidence of WNV circulation in southern (Minas Gerais, São Paulo) and northeastern (Piauí) states isolated from equine red blood cells. A novel, climate-informed theoretical perspective of the potential transmission of WNV across the country highlights the state of Piauí as particularly relevant for WNV epidemiology in Brazil, although it does not reject possible circulation in other states. Conclusion: Our output demonstrates the scarceness of existing data, and that although there is sufficient evidence for the circulation and persistence of the virus, much is still unknown on its local evolution, epidemiology, and activity. We advocate for a shift to active surveillance, to ensure adequate preparedness for future epidemics with spill-over potential to humans.

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

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