West Nile Neuroinvasive Disease: Current Management, Emerging Therapies and Future Clinical Trial Priorities

ABSTRACT

West Nile virus (WNV) is a climate‐sensitive, mosquito‐borne flavivirus that has become endemic in Europe, with Italy, particularly the Lazio region, emerging as a significant epicentre. While most infections are asymptomatic, less than 1% of cases progress to West Nile neuroinvasive disease (WNND), which carries a case fatality rate of up to 17% in vulnerable populations. Despite the rising global incidence, clinical management remains primarily supportive, as no antiviral agents or vaccines are currently licenced for human use. Pharmacological research has yielded inconsistent results. Corticosteroids show no significant survival benefit, and ribavirin is discouraged due to potential toxicity and lack of in vivo efficacy. Although interferon‐α2b and intravenous immunoglobulin (IVIG) demonstrated promising results in preclinical models, human clinical trials have been inconclusive. More recently, remdesivir and repurposed drugs like nitazoxanide and teriflunomide have emerged as potential candidates, though they require validation through controlled trials. Future therapeutic success likely depends on early intervention (within 7–10 days of symptom onset) and multitargeted combination approaches that address viral replication, immune modulation, and neuroprotection. There is an urgent need for adequately powered clinical trials and the acceleration of human vaccine development to mitigate the impact of this expanding public health threat.

Abbreviations

BBB

Blood‐Brain Barrier

DENV

Dengue Virus

DIII

Domain III (of envelope protein)

ETAR

1‐β‐D‐ribofuranosyl‐3‐ethynyl‐[1,2,4]triazole

FASN

Fatty Acid Synthase

ICU

Intensive Care Unit

IFN

Interferon

IFN‐α

Interferon alpha

IFN‐β

Interferon beta

IL‐10

Interleukin 10

IVIG

Intravenous Immunoglobulin

JEV

Japanese Encephalitis Virus

MAbs

Monoclonal Antibodies

MIF

Macrophage Migration Inhibitory Factor

MMPs

Matrix Metalloproteinases

NS1

Non‐Structural Protein 1

NS5

Non‐Structural Protein 5

PMOs

Phosphorodiamidate Morpholino Oligomers

RdRp

RNA‐Dependent RNA Polymerase

RDV

Remdesivir

RNAi

RNA Interference

siRNA

Small Interfering RNA

WNF

West Nile Fever

WNND

West Nile Neuroinvasive Disease

WNV

West Nile Virus

YFV

Yellow Fever Virus

1. Introduction

West Nile virus (WNV) is a mosquito‐borne zoonotic pathogen that has become endemic with recurrent epidemic in Europe, with outbreaks reported in southern, eastern, and western countries [1]. First isolated in 1937 from a woman in Uganda, WNV is an enveloped, positive‐sense, single‐stranded RNA virus belonging to the genus Flavivirus, within the Flaviviridae family and the Japanese encephalitis serocomplex [2]. To date, eight phylogenetic lineages of WNV have been identified [3], although only lineages 1 and 2 are currently known to be pathogenic to humans [4]. WNV is the most widely distributed arbovirus worldwide, with confirmed circulation in North and South America, Africa, Europe, Asia, and Oceania. Transmission occurs primarily through the bite of infected mosquitoes, especially species of the Culex genus, within an enzootic cycle that involves birds as the main amplifying hosts. In contrast, humans and other mammals are considered incidental hosts, as they do not develop high viral load in blood to sustain mosquito infection and virus perpetuation.

WNV is classified as a climate‐sensitive vector‐borne emerging infectious disease whose transmission dynamics are closely shaped by environmental and climatic conditions. Climate change and ecosystem alterations driven by biodiversity loss and land‐use modifications influence mosquito vector abundance, distribution, and seasonality, as well as reservoir host populations, thereby modulating viral circulation and the risk of human infection [5]. In addition, rising temperatures and extreme weather events not only favour vector expansion but also directly affect host susceptibility by altering immune system function. Heat stress has been associated with both heightened inflammatory responses and immunosuppression, impairing key adaptive immune mechanisms, while also compromising epithelial barriers and promoting microbial dysbiosis. These combined effects, together with the increasing frequency and intensity of heatwaves and other climate‐related events, contribute to the geographic spread and clinical impact of WNV, with projections indicating a growing risk of outbreaks in Europe under future warming scenarios [6, 7]. As of 3rd December 2025, a total of 1112 locally acquired human cases with known sites of exposure have been reported across nine European countries. Italy, historically the most affected country, accounted for approximately 779 of these cases (70%), and continues to be a major focus of surveillance efforts by the European Centre for Disease Prevention and Control [8]. Among Italian regions, Lazio reported around 263 cases, with a marked clustering in the province of Latina, which has emerged as one of the most significant epicentres of the ongoing epidemic [9].

The clinical spectrum of WNV infection in humans is broad. The majority of infections remain asymptomatic, and among symptomatic cases, approximately 20% develop West Nile fever (WNF), a mild, self‐limited, influenza‐like illness characterised by fever, headache, myalgia, fatigue, and sometimes gastrointestinal symptoms. Symptoms generally resolve within 3–6 days [10]. In less than 1% of infected individuals, the virus progresses to West Nile neuroinvasive disease (WNND), which may manifest as encephalitis, meningitis, or acute flaccid paralysis resembling poliomyelitis [11].

Among cases of WNND, encephalitis is the most frequently reported clinical presentation, observed in approximately 50%–71% of patients, and is often associated with muscle weakness and peripheral nervous system involvement. Meningitis occurs in about 15%–35% of cases and generally has a more favourable prognosis. Less commonly, acute flaccid paralysis develops in 3%–19% of patients and typically presents as sudden limb weakness in the absence of systemic signs. In rare instances, WNV infection has been associated with Guillain–Barré syndrome and other demyelinating neuropathies. The case fatality rate for WNND may reach 17%, particularly in vulnerable populations [12].

Several host‐related risk factors are associated with an increased likelihood of developing severe or neuroinvasive disease. These include advanced age (over 60 years), diabetes mellitus, hypertension, chronic kidney disease, cancer and haematologic malignancies, alcohol abuse, and genetic or immunologic predispositions [11]. Despite the increasing global burden of WNV infection, therapeutic options remain limited and are supported by heterogeneous and often inconclusive clinical evidence. This gap highlights the need for a critical and clinically oriented synthesis of available data. The aim of this narrative review is to provide a comprehensive and updated overview of current and emerging therapeutic strategies for WNV infection, integrating evidence from preclinical and clinical studies, and identifying key gaps to inform future research and clinical practice.

2. Methods

This narrative review was conducted to provide an updated overview of therapeutic strategies for West Nile virus (WNV) infection, with a particular focus on clinical and translational evidence.

A literature search was performed in major biomedical databases, including MEDLINE (via PubMed) and Scopus, up to December 2025. The search strategy combined Medical Subject Headings (MeSH) and free‐text terms related to WNV and therapeutic interventions. Key search terms included ‘West Nile virus,’ ‘WNV,’ ‘West Nile neuroinvasive disease,’ ‘treatment,’ ‘therapy,’ ‘antiviral,’ ‘immunotherapy,’ ‘monoclonal antibodies,’ ‘IVIG,’ ‘interferon,’ and ‘vaccine’

Relevant articles were selected based on their pertinence to the topic, prioritising clinical studies, randomized trials, observational studies, and high‐quality preclinical research. Additional sources were identified through manual screening of reference lists of selected articles.

No formal inclusion or exclusion criteria were predefined, consistent with the narrative nature of the review; however, studies were selected based on relevance, scientific rigour, and contribution to the understanding of therapeutic approaches to WNV infection. When multiple studies addressed similar topics, preference was given to the most recent and comprehensive evidence.

No artificial intelligence tools were used in the screening or selection of the literature.

The review was conducted in accordance with principles aimed at enhancing transparency and quality in narrative reviews, including those outlined in the SANRA (Scale for the Assessment of Narrative Review Articles).

3. Pathophysiology and Therapeutic Targets

These pathogenic mechanisms provide a rationale for therapeutic strategies targeting viral replication, modulation of the host immune response, and preservation of blood–brain barrier integrity. Following inoculation through the bite of an infected mosquito, WNV initially replicates in skin‐resident dendritic cells, which subsequently migrate to regional lymph nodes, facilitating primary viral amplification. The virus then enters the bloodstream, causing a transient viraemia that allows dissemination to peripheral organs, including the spleen and kidneys. In the majority of individuals, this early phase elicits a protective innate and adaptive immune response, resulting in viral clearance without further complications. However, in a subset of individuals, particularly those with immunocompromised status, advanced age, or pre‐existing comorbidities, WNV may breach the blood‐brain barrier (BBB) and establish infection within the CNS, resulting in neuroinvasive disease. The precise mechanisms by which the virus crosses the BBB are not yet fully understood, though several hypotheses have been proposed. These include increased BBB permeability due to pro‐inflammatory cytokine release, direct infection of endothelial cells, and ‘Trojan horse’ mechanisms in which infected monocytes or other immune cells serve as carriers into the CNS [13, 14]. Once inside the CNS, WNV displays a strong neurotropism, with a preference for anterior horn motor neurons, Purkinje cells, and hippocampal neurons, leading to cell death through a combination of direct viral cytopathic effects and indirect immune‐mediated injury. Neuropathological studies have revealed extensive neuronal necrosis, perivascular lymphocytic infiltration, microglial nodules, and astrogliosis, particularly in the brainstem, cerebellum, and spinal cord. While the immune response is essential for viral clearance, excessive activation of microglia and infiltration by cytotoxic T cells can exacerbate neuronal damage via the release of pro‐inflammatory cytokines and reactive oxygen species [15, 16].

To date, no antiviral agents have been approved for the treatment of WNV infection, and clinical management remains supportive, focussing on the control of complications such as seizures, elevated intracranial pressure, or respiratory failure. Several therapeutic strategies have been investigated in preclinical models and early‐phase human trials. Antiviral agents such as interferon‐α, ribavirin, and various nucleoside analogues have been tested, but results have been inconsistent or lacked efficacy in clinical settings [17, 18]. More recent efforts have focused on passive immunotherapy, particularly the use of monoclonal antibodies targeting the viral envelope (E) protein, which mediates viral entry and membrane fusion. Neutralising antibodies have demonstrated protective effects in animal models, and some candidates are under evaluation in early‐stage clinical trials [19, 20]. In parallel, significant progress has been made in vaccine development. Several platforms, including inactivated whole‐virus vaccines, live‐attenuated strains, and viral vector‐based constructs, have shown promising immunogenicity and protection in animal models. Some candidates have progressed to phase I/II human trials, but no vaccine is currently licenced for human use, primarily due to the sporadic and unpredictable nature of outbreaks, which complicates large‐scale efficacy studies [21]. These pathogenic mechanisms provide a rationale for therapeutic strategies targeting viral replication, modulation of the host immune response, and preservation of blood–brain barrier integrity.

4. Therapeutics Strategies

Therapeutic strategies for West Nile virus (WNV) infection remain poorly defined, reflecting the limited and fragmented nature of the available evidence (Figure 1). Despite extensive investigation, no treatment has consistently demonstrated clinical efficacy, and most data derive from preclinical studies or small, heterogeneous clinical series. Current approaches can be broadly categorised into supportive care, antiviral therapies targeting viral entry or replication, and interventions aimed at modulating the host immune response. These strategies collectively seek to limit viral dissemination, reduce immune‐mediated damage, and prevent progression to neuroinvasive disease.

FIGURE 1.

Therapeutic Targets for West Nile Virus infection. ETAR, 1‐β‐D‐ribofuranosyl‐3‐ethynyl‐[1,2,4]triazole; IVIG, intravenous immunoglobulin; MAbs, monoclonal antibodies; WNND, West Nile neuroinvasive disease; WNV, West Nile Virus.

4.1. Supportive Care and Adjunctive Therapies

Despite significant research efforts, the management of WNV infection remains predominantly supportive, as no antiviral or disease‐specific treatment has demonstrated consistent clinical benefit. Standard supportive care focuses on ensuring sufficient hydration, managing symptoms, and monitoring neurological decline, with particular attention to progression towards WNND.

Patients with uncomplicated WNF generally require only symptomatic management, including antipyretics and fluid replacement to address dehydration caused by fever, nausea, or vomiting. In contrast, individuals who develop WNND may require intensive care unit (ICU) admission, primarily for airway protection, intracranial pressure monitoring, and management of seizures.

Particular vigilance is required in WNND, where the risk of acute neuromuscular respiratory failure is considerable. Early indicators, such as dysarthria and dysphagia, should prompt timely evaluation for mechanical ventilation. Evidence drawn from poliovirus‐associated paralysis suggests that excessive physical activity during the acute febrile phase and in the first 48–72 h following weakness onset may worsen neurological outcomes. Consequently, physical and occupational rehabilitation should be initiated only after the acute phase has resolved, with interventions tailored to the individual's neurological status [15, 22].

Although rare, ventilator‐dependent respiratory failure due to bilateral diaphragmatic paralysis has been reported. Anecdotal evidence suggests that theophylline may improve diaphragmatic strength in such cases [23], although robust clinical data are lacking. Furthermore, impaired autonomic regulation may contribute to excessive bronchial secretions, complicating ventilatory support in affected patients.

Severe WNV infection can place a considerable burden on healthcare systems. Case series have described prolonged ICU stays, in some instances exceeding 100 days, and extended mechanical ventilation, which raises complex prognostic and ethical considerations, including decisions regarding long‐term care and life‐sustaining treatment [24].

4.2. Corticosteroids

Corticosteroids have been investigated for use in WNND because of their anti‐inflammatory effects, with the goal of reducing immune‐related neuronal damage. However, the evidence supporting their efficacy remains limited and inconclusive. A recent systematic review and meta‐analysis evaluating corticosteroid use in viral encephalitis, including WNV and related flaviviruses, found no significant survival benefit associated with steroid administration [25].

Corticosteroids are occasionally prescribed by physicians, particularly when there are significant inflammatory markers or imaging shows cerebral oedema. Proinflammatory cytokine levels are upregulated during WNV infection and may be related with disease severity. However, this approach remains controversial due to the potential for immunosuppression and enhanced viral replication, particularly in the early viraemic phase.

Retrospective analyses provide mixed results. In one multicentric study including 65 patients with WNND, the use of dexamethasone (mean daily dose of 13.6 mg) did not significantly reduce in‐hospital mortality or improve neurological outcomes at discharge. Interestingly, patients who did not receive corticosteroids had a shorter length of hospital stay [26]. Similarly, case reports have described inconsistent neurological improvement following high‐dose corticosteroid therapy, without a clear association with dosage, formulation, or duration [27, 28, 29]. Clinical studies about corticosteroids against WNV are summarised in Table 1.

TABLE 1.

Clinical studies on corticosteroids, monoclonal antibodies and IVIG for west nile virus infection.

Agent studied	Subjects	Study type	Clinical presentation	Endpoints	Results	Notes
Corticosteroids (Colaneri et al., 2023) [26]	65 patients 32 no steroids 33 mean dose of dexamethasone 13.6 mg/day, mean age 68.1 y, 73% M	Multicentric retrospective observational study	Neuroinvasive disease	Primary: Survival, Secondary: Reducing LOS, neuro Logical sequelae	No benefit observed: CSI did not reduce mortality or neurologic sequelae, and patients without steroids had shorter hospital stays.	Within the steroid group: 21 dexamethasone (64%), 4 methylprednisolone (12%), and 8 prednisone (24%).
Corticosteroids (Kal et al., 2022) [27]	M, 61 years no medical history	Case report	Neuroinvasive	Survival	Marked improvement: Spontaneous eye opening by day 2, extubated by day 4, and speech returned after treatment completion.	Solumedrol 1 g IV daily for 5 days
Corticosteroids (Leis & Sinclair, 2019)	F, 71year no medical history	Case report	Neuroinvasive	Survival	Rapid recovery: Increased movement by day 1, fully responsive by day 2, with continued improvement leading to transfer to standard care.	Methylprednisolone 1 g IV daily × 5 days
Corticosteroids (Pyrgos & Younus, 2004) [28]	M, 68 years no medical history	Case report	Neuroinvasive	Survival	Patient improved quickly: Speech returned within 24 h, and by 48 h upper limb strength and continence were fully restored.	Methylprednisone 500 mg IV daily x 4 days
MGAWN1 (ClinicalTrials.gov: NCT00927953, unpublished data) [40]	13 patients 7 placebo 6 single infusion of MGAWN1	Phase I/II randomized trial	Neuroinvasive	Survival	Outcomes were limited: Two of six patients on MGAWN1 died versus one on placebo; the study ended early due to poor enrolment.	30 mg/kg single infusion IV of MGAWN1
IVIG (Gnann et al., 2019) [71]	62 patients randomized3:1:1 to receive Omr‐IgG‐am [30], standard IVIG [12], placebo [13]; mean age 56.2 years, 66.1% M	Phase I/II randomized double‐blind, placebo‐controlled trial	Neuroinvasive and mild to severe disease	Primary: Safety and tolerability Secondary: Survival, therapeutic effects	No safety issues, but no efficacy was shown. A non‐significant trend suggested better clinical and survival in the placebo group.	Single dose 0.5 g/kg of Omr‐IgG‐am (OMRIX Bio pharmaceuticals, Israel); single dose of standard IVIG (Polygam S/D; Baxter).
IVIG (Malone et al., 2022) [65]	M, 63 years DM II, hyperlipidaemia	Case report	Neuroinvasive	Survival	Improvement in respiratory function, mental status, and movement.	Plasma exchange and IVIG for 4 days
IVIG (Hamdan et al., 2002) [72]	M, 42 years Lung transplantation 6 months before	Case report	Neuroinvasive	Survival	Rapid improvement: Fully oriented by day 1, symptoms resolved within 48 h, and discharged 2 days later without sequelae.	400 mg/kg Omr‐IgG‐am
IVIG (Hébert et al., 2017) [73]	F, 63 years no medical history	Case report	Neuroinvasive (opsoclonus‐myoclonus syndrome)	Survival	No adverse effects. Within 24 h, myoclonus improved and opsoclonus nearly resolved. Full neurological remission by 8 weeks.	1 g/kg standard IVIG per day for 2 days
IVIG (Shimoni et al., 2001) [75]	F, 70 years Chronic lymphatic leukaemia on follow‐up	Case report	Neuroinvasive	Survival	Initial condition stable for 2 days, then gradual improvement. Within 5 more days, consciousness fully returned to normal.	400 mg/kg Omr‐IgG‐am
IVIG (Morelli et al. 2010)	F, 25 years liver transplant recipient	Case report	West Nile virus infection	Survival	Not neurological complications developed	Convalescent plasma (300–600 mL/day) for 10 days and stopped due to limited supply; Omr‐IgG‐am was administered 0.4 g/kg for additional 10 days.

Abbreviations: CSI, corticosteroids; DM II, diabetes mellitus type II; IV, intravenous; IVIG, intravenous immunoglobulin; LOS, Length of Stay.

Current evidence suggests that corticosteroids do not significantly alter clinical outcomes in WNND. Their role remains undefined, and their use should be limited to selected cases within the context of individualised care, ideally within clinical trials or well‐structured observational studies. This uncertainty is largely attributable to the heterogeneity of study designs, small sample sizes, and variability in timing, dosing, and patient selection.

4.3. Antiviral Agents

Over the past 3 decades, significant progress has been made in the development of antiviral therapies against flaviviruses. In the case of WNV, numerous compounds, including several already approved for other infectious diseases, have been tested in vitro to assess their potential inhibitory effects. A recent systematic review has summarised the antiviral compounds that inhibit the replication of various flaviviruses, including WNV, in vitro or in animal models [31], conversely the agents discussed in this section represent the principal antiviral candidates administered to humans in the context of WNV infection (Table 2).

TABLE 2.

Clinical studies on antiviral treatments for West Nile Virus infection.

Agent studied	Subjects	Study type	Clinical presentation	Endpoints	Results	Notes
INFα2b (Sayaho 2004)	7 patients (only 3 treated with INFα2b) #1 73 years‐old woman with hypertension, DM II; #2 71 years‐old man with DM II, cardiac bypass, AF; #3 50 years‐old man with no chronic diseases	Case series	Mild to severe disease	Survival	All survived	INFα2b start in the first 10d No AD reported
INFα2b (Wehbeh 2004) [38]	23 patients with WNV; 15 received IFNα‐2b and 8 received BSC	Unblinded randomized clinical trial	Neuroinvasive diseases	Neurologic improvement	Preliminary results: Greater neurologic improvement in INFα2b group	Final results never published AD: Neutropenia and hepatitis (resolved after treatment discontinuation)
INFα2b (Chan‐Tack 2005) [30]	1 patient # 76 years‐old man with hypertension, TURP	Case report	Neuroinvasive disease, ARDS	Survival	Died without improved neurological functions	IFNα2b given after 17 days
INFα2b (Kalil 2005) [35]	2 patients (#1: 43 years, man, history of LL treated with stem cell transplant #2: 54 years, woman, rheumatoid arthritis treated with MTX)	Case series	Neuroinvasive disease	Survival	Survived without major sequelae	INFα2b after 6 and 9d
INFα2b (Lewis 2007) [36]	1 patient 83 years‐old man	Case report	Neuroinvasive disease	Survival	Survived Last neurological sequala resolved after 3 months	INFα2b Administered after 3 weeks from onset
INFα2b (Winston 2014) [37]	3 patients With SOT	Case series	Neuroinvasive disease	Survival	2 survived, 1 died	INFα2b Administered with human IG
IFN‐β1a (NCT06510426)	100 patients (estimated)	Double blind placebo‐control trial Ongoing (expected results 2025.12.31)	Mild to severe confirmed disease (older than 18 years‐old)	Therapeutic effects Prevalence of anti‐type‐I‐IFN autoantibodies in patients with WNV	—	IFN‐β1a Administered every 48h for 3 doses. Preliminary data not already published
Ribavirin (Chowers 2001) [44]	233 patients; 37 treated with ribavirin	Observational retrospective study	Mild to severe disease (57.9% encephalitis)	Survival	Ribavirin independent death factor (OR 6.7 CI 3‐15.2; p 0.0001)	Patients treated with RBV: Alive 22 versus dead 15
Ribavirin (Spiegel 2002) [43]	1 patient # 4 years‐old child with LH)	Case report	Neuroinvasive disease	Survival	Discharge after 28 days Complete recovery after 8 months	RBV 200 mg every 6 h for 14 days WNV infection during chemotherapy against LH
Remdesivir (Esposito 2025) [55]	12 patients (4 treated with RDV vs. 8 treated with BSC)	Case series	Neuroinvasive disease	Survival Neurological sequelae	Lower median hospitalisation (12d IQR 8‐12 vs. 16d IQR 8‐34) Lower in‐hospital mortality (0% vs. 25%) Reduced need of rehabilitation (12% vs. 66%)	Only 5 confirmed cases No statistically significant differences No AD reported
Remdesivir (Samsher 2024)	1 patient # 46 years‐old man with MS and EDSS score 4.5	Case report	Neuroinvasive disease	Survival	Complete recovery after 2 months	RDV 200 mg first day than 100 mg every 24h for 4 days IG given for the first 2 days of RDV baut discontinued for AD Under ocrelizumab every 6 months for MS

Abbreviations: AD, adverse events; AF, atrial fibrillation; BSC, best supportive care; DM II, diabetes mellitus type II; EDSS, expanded disability status scale; IFN, interferon; MS, multiple sclerosis; RBV, ribavirin; RDV, remdesivir; SOT, solid organ transplantation; TURP, trans‐urethral resection of the prostate.

4.3.1. Interferon‐α2b

Interferons (IFNs), particularly type I IFNs such as IFN‐α and IFN‐β, play a central role in the host's innate antiviral response. In vitro studies have demonstrated that IFN‐α2b inhibits replication of several flaviviruses, including WNV. Animal models have further substantiated its protective role: mice lacking receptors for type I IFNs exhibit uniform mortality following WNV infection, whereas prophylactic administration significantly reduces both viral replication and mortality rates [32, 33, 34].

Preclinical studies involving IFN‐α2b, both as monotherapy and in combination with ribavirin, have yielded promising results in reducing viral load and improving survival outcomes [20, 34]. However, translation into human therapy has produced mixed results. Early case reports and small series suggested a possible clinical benefit when IFN‐α2b was administered intravenously (3 million IU on day 1), followed by subcutaneous injections for a 14‐day course. In this setting, initiation of therapy within 10 days of symptom onset was associated with improved outcomes, including among high‐risk populations such as elderly individuals and solid‐organ transplant recipients [35, 36, 37].

Adverse events associated with IFN‐α2b include psychiatric symptoms (e.g., depression, anxiety, insomnia) and systemic effects (e.g., leucopenia, transaminase elevation, gastrointestinal disturbances, arthralgia). However, these events were either absent or mild in the reported cases, likely due to the short treatment duration [30].

Despite these encouraging findings, controlled trials have failed to confirm a clear clinical benefit. Preliminary data from a 2004 open‐label trial in WNND suggested greater neurological improvement in patients receiving IFN‐α2b compared to supportive care alone [38], but full trial data were never published. A subsequent murine study hypothesised that IFN's primary benefit may lie in its ability to reduce blood–brain barrier permeability, thereby limiting viral neuroinvasion when administered early [39].

At present, a randomized, double‐blind, placebo‐controlled trial (NCT06510426) is ongoing in Israel, comparing IFN‐β1a versus placebo in adults with confirmed WNV infection. The study targets both immunocompetent and immunocompromised patients and aims to evaluate the efficacy of three subcutaneous doses of IFN‐β1a administered within 8–10 days of symptom onset. Completion is anticipated by the end of 2025 [40]. Importantly, IFN‐α2b has been withdrawn from the European market since 2021 for commercial reasons, though not due to safety concerns. Overall, despite promising antiviral activity in vitro and in animal models, the clinical efficacy of interferon‐based therapies remains unproven, and their potential benefit appears to be highly dependent on early administration.

4.3.2. Ribavirin and ETAR

Ribavirin, a guanosine analogue with broad‐spectrum antiviral activity, exerts its effect by inhibiting viral RNA‐dependent RNA polymerase. Its application in WNV infection has been disappointing. In vitro studies confirm its ability to inhibit WNV replication, but only at concentrations too high to be safely achieved in vivo [41, 42].

Initial anecdotal reports suggested potential benefit, such as a paediatric case involving a child with non‐Hodgkin lymphoma successfully treated with ribavirin [43]. However, subsequent animal studies revealed that ribavirin not only lacked efficacy but also increased mortality in infected hamsters [34]. A retrospective human study further implicated ribavirin as an independent risk factor for mortality in WNV‐infected patients [44]. To date, no registered clinical trials are evaluating ribavirin for WNV, and its use is discouraged based on available evidence.

A related compound, 1‐β‐D‐ribofuranosyl‐3‐ethynyl‐[1,2,4]triazole (ETAR), has shown superior in vitro activity against WNV at significantly lower concentrations than ribavirin, potentially avoiding the toxicity associated with the latter. Its mechanism also involves inhibition of viral RNA synthesis. However, no animal or human studies have yet been published to validate ETAR's efficacy or safety profile in vivo [45, 46, 47].

4.3.3. Remdesivir

Remdesivir (RDV), a monophosphoramidate prodrug of an adenosine analogue, acts by inhibiting RNA‐dependent RNA polymerase (RdRp) [48]. Initially developed for Ebola virus, RDV has demonstrated broad‐spectrum activity against multiple RNA viruses, including coronaviruses, filoviruses, and flaviviruses [49, 50]. It has since been approved for use in COVID‐19 by both the FDA and EMA, and its mechanism, termination of RNA synthesis following incorporation by viral RdRp, is conserved across viral families [51, 52].

Recent studies have shown that RDV triphosphate inhibits RdRp activity in vitro across various flaviviruses, including WNV [53, 54]. Importantly, no significant drug–drug interactions were noted with commonly used medications in resource‐limited settings, further supporting its safety profile [54].

In 2024, a small case series of 12 patients with confirmed or probable WNV infection showed promising early clinical data. Among four with neuroinvasive disease, those treated off‐label with RDV (200 mg loading dose followed by 100 mg daily for 5–10 days) exhibited improved survival and fewer neurologic sequelae compared to untreated controls, although the differences were not statistically significant [55]. A separate case report involving a patient with multiple sclerosis and B‐cell depletion showed full neurological recovery after RDV administration combined with intravenous immunoglobulins [56]. No severe adverse events or treatment‐limiting toxicities related to RDV were observed in either report.

Data on central nervous system penetration remain limited. However, RDV has demonstrated efficacy in other neuroinvasive viral infections, suggesting at least partial blood–brain barrier permeability.

Despite these promising preliminary observations, the current level of evidence remains insufficient to recommend RDV for routine use in WNV infection. Controlled clinical trials are urgently needed to validate its efficacy and safety in this setting. However, the currently available evidence is limited to small observational studies and case reports, precluding definitive conclusions regarding its efficacy.

4.4. Immunotherapeutic Approaches

4.4.1. Monoclonal Antibodies

Monoclonal antibodies (MAbs) have demonstrated potent antiviral activity in preclinical models and are considered key correlates of protection in vaccine development against WNV. Although in vivo results are promising, no MAb therapy has been approved for WNV infection. Candidates are still in early clinical trials assessing safety and preliminary efficacy [57].

Therapeutic MAbs mainly target the WNV envelope (E) protein, a key component essential for viral entry. Among the three structural domains of the E protein, domain III (DIII), particularly the lateral ridge, is the principal target for neutralising antibodies such as the murine MAb E16, which blocks viral fusion and cell entry [58, 59, 60]. The humanised MAb MGAWN1 also targets the E protein and inhibits the conformational changes required for membrane fusion, showing potent neutralising activity without cross‐reactivity to other flaviviruses [61].

MGAWN1 was tested in a small phase I/II randomized trial involving 13 patients with confirmed WNND. Participants received a single infusion of MGAWN1 or placebo. Two of six treated patients died, compared with one in the placebo arm. The study was terminated prematurely due to low enrolment, and full peer‐reviewed data were never published (ClinicalTrials.gov: NCT00927953). Consequently, no conclusions can be drawn regarding efficacy, and the study's underpowered nature represents a major limitation.

Additional targets beyond the E protein are under investigation. Anti‐NS1 MAbs have shown partial protective effects in murine models, primarily through Fcγ receptor–mediated clearance of infected cells. Although less effective as monotherapy, these agents may provide additional benefit in combination with anti‐E antibodies and may help reduce the emergence of resistant variants [57, 62]. Anti‐prM antibodies have been studied less extensively, with limited evidence of protective efficacy (Table 1).

Broadly neutralising MAbs developed for other flaviviruses have also been evaluated for cross‐protection against WNV. Although some in vitro and in vivo results have been promising, none of them have been proven to cross the blood–brain barrier (BBB), a critical limitation in the context of neuroinvasive diseases.

4.4.2. Intravenous Immunoglobulin (IVIG)

Passive immunotherapy with intravenous immunoglobulin (IVIG) or convalescent plasma has been proposed as a treatment strategy for WNV, particularly in immunocompromised individuals. IVIG preparations contain polyclonal antibodies derived from pooled human plasma, and since the widespread circulation of WNV in the United States and Israel, many commercial lots have contained high titres of anti‐WNV antibodies [63, 64, 65]. Reports varied in both the dosing regimens and timing of IVIG administration, which typically ranged from 400 to 500 mg/kg/day for 4–5 consecutive days [66, 67].

In animal models, the therapeutic efficacy of IVIG has been well documented. In murine studies, administration of WNV‐specific IVIG shortly after viral exposure reduced mortality from approximately 80%–20% [68, 69]. However, the timing of administration is critical: early treatment was associated with a clear survival benefit, whereas delayed therapy, particularly after onset of neurological symptoms, failed to improve outcomes [70]. These findings support the hypothesis that passive antibody transfer may limit viral dissemination and modulate the inflammatory response during the early viraemic phase, but is unlikely to reverse established CNS injury.

Despite these encouraging preclinical results, evidence from human studies remains inconclusive. The largest randomized controlled trial to date, conducted by Gnann et al. [71], enroled 62 WNND patients and randomized them to receive high‐titre anti‐WNV immunoglobulin (Omr‐IgG‐am), standard IVIG, or placebo. While the primary endpoint (safety) was met, no statistically significant differences in mortality or neurological recovery were observed between groups. Paradoxically, a non‐significant trend towards better outcomes was observed in the placebo arm, raising concerns about the therapeutic efficacy of IVIG in established WNND. Treatment delay may explain this, since most patients were enroled only after lab confirmation, when viral entry into the CNS might have already occureed.

Several case reports have described variable responses to IVIG, especially in immunocompromised patients. Some individuals showed clinical improvement following IVIG administration [72, 73], while others experienced no benefit [74, 75]. The heterogeneity of these reports and the lack of standardized protocols limit their generalisability.

Other immunotherapeutic interventions, such as plasma exchange, have been described only anecdotally and cannot be recommended outside experimental settings [65].

In summary, while IVIG has demonstrated efficacy in animal models when administered early, human trials have not confirmed a clear benefit in patients with neuroinvasive disease (Table 1). These findings underscore the importance of early intervention and suggest that the therapeutic window for IVIG is likely restricted to the initial viraemic phase, before central nervous system involvement is established, supporting a potential preventive or adjunctive rather than curative role in advanced WNND.

4.4.3. Vaccines

Despite more than 2 decades of research, no WNV vaccine has yet been licenced for human use. Several candidates have shown immunogenicity and safety in phase I and II clinical trials, but challenges related to dosing, durability of protection, and population‐specific efficacy have limited further development [76, 77].

Among the most promising candidates, the live attenuated chimaeric vaccine ChimeriVax‐WN02 achieved seroconversion rates exceeding 90% after a single dose in phase II trials [78]. Another live attenuated vaccine, rWN/DEN4Δ30, demonstrated variable immunogenicity (55%–75%) depending on dose and schedule [79]. DNA‐based vaccines such as VRC‐WNVDNA017‐00‐VP and VRC‐WNVDNA020‐00‐VP elicited robust neutralising antibody responses (above 96%) following a three‐dose regimen, while recombinant subunit vaccines incorporating truncated E proteins with adjuvants have shown promise in inducing both humoral and cellular immunity in animal studies [80, 81].

Inactivated vaccines like HydroVax‐001 have produced moderate immune responses in humans, which may be enhanced with additional booster doses. However, none of these candidates have progressed to phase III evaluation or licensure [77, 81].

Insights from the development of licenced flavivirus vaccines — notably those targeting dengue — may inform future strategies for WNV, especially in terms of immunogenic epitope targeting and vector‐based delivery [82]. In contrast to the human setting, vaccination of horses has been highly successful: four WNV vaccines are currently approved for veterinary use, each demonstrating excellent efficacy and safety.

Looking forward, next‐generation vaccine platforms under investigation include chimaeric and inactivated formulations, novel adjuvants, and multi‐epitope constructs designed using immunoinformatics tools. These approaches aim to enhance immunogenicity, minimise dosing schedules, and confer cross‐species protection, key factors in the development of an effective human WNV vaccine.

4.5. Emerging and Experimental Therapies

4.5.1. Nitazoxanide and Teriflunomide

Recent advances in the structural and molecular biology of flaviviruses have created new opportunities for antiviral therapy development. However, due to the lengthy process involved in developing novel antiviral drugs, drug repurposing of clinically approved compounds may significantly shorten development timelines. The main repurposed drugs considered for WNV treatment are nitazoxanide and teriflunomide [83, 84].

Nitazoxanide is currently approved for the treatment of Cryptosporidium parvum and Giardia intestinalis infections. It has demonstrated inhibitory effects on the replication of several flaviviruses including Japanese encephalitis virus (JEV), dengue virus (DENV), and yellow fever virus (YFV). The antiviral mechanism of nitazoxanide involves interference with viral protein glycosylation and disruption of mature viral particle production [83].

Teriflunomide is the active metabolite of leflunomide, both immunosuppressive agents used in multiple sclerosis treatment. They inhibit dihydroorotate dehydrogenase, the rate‐limiting enzyme in de novo pyrimidine synthesis. This enzyme has recently attracted interest as a target for broad‐spectrum antiviral drug development. Teriflunomide has been reported to inhibit a wide range of RNA viruses, including influenza, SARS‐CoV‐2, chikungunya virus, and WNV [85].

In a high‐throughput screen by Tang et al., 978 FDA‐approved small molecules were evaluated for anti‐WNV activity. Four compounds, including cilnidipine, mycophenolate mofetil, nitazoxanide, and teriflunomide, efficiently inhibited WNV infection in Vero cells and human neuroblastoma SH‐SY5Y cells. Moreover, both nitazoxanide and teriflunomide improved survival rates in WNV‐infected mice by approximately 20% and 40%, respectively, indicating their potential to protect against WNV disease and lethality [84]. Teriflunomide also exhibits antiviral activity against other RNA viruses such as Zika, Dengue, and influenza, notably in combination with molnupiravir [85].

4.5.2. Favipravir and Galidesivir

Favipiravir and galidesivir are two broad‐spectrum antiviral nucleoside analogues that have shown preclinical activity against WNV [86]. Favipiravir reduces WNV replication in vitro. The mechanism of action of the drug is still not well defined. One recognized mechanism is the action host enzymes upon favipravir converting it to its active form (favipiravir‐4‐ribofuranosyl‐5 = ‐triphosphate) which functions as a purine nucleotide analog and performs direct inhibition versus RNA‐dependent RNA polymerase [87].

Another mechanism is the mutagenic activity in cell culture. Lethal mutagenesis activity increases the virus mutation frequency determining the reduction of viral fitness. This, together with the clinical safety record of favipiravir, which is already licenced in Japan against influenza point to favipiravir as an promising antiviral candidate against WNV [88].

Galidesivir is a broad‐spectrum antiviral nucleoside analog that inhibits the RNA‐dependent RNA polymerase (RdRp) of various RNA viruses [89].

It is active also against WNV causing premature termination of RNA synthesis, leading to suppression of viral replication. A study of Eyer et al. ts demonstrate that galidesivir strongly inhibits in vitro replication of WNV. In this study complete inhibition of viral replication was observed at galidesivir concentrations BCX4430 concentrations of 25e50 mM until the fifth day post infection, indicating strong and stable anti‐WNV activity [90].

4.5.3. Doravirine

Doravirine is an FDA‐approved non‐nucleoside reverse transcriptase inhibitor indicated for the treatment of HIV‐1 infection. Recent in silico studies have suggested that doravirine may have potential multitarget inhibitory activity against WNV proteins, including the core protein and NS5 methyltransferase domain, based on molecular docking and dynamics simulations. These findings indicate favourable binding and stability, supporting further investigation into doravirine as a possible antiviral candidate for WNV [91]. However, these results are limited to computational models and have not been validated in vitro, in vivo, or in clinical trials.

4.5.4. RNA Interference (RNAi)

RNA elements within flavivirus genomes represent promising targets for antiviral therapy. Synthetic short interfering RNAs (siRNAs) have demonstrated inhibition of WNV replication both in vitro and in vivo. The efficacy of siRNA treatment depends on the routes of viral inoculation and siRNA delivery.

Kumar et al. administered both viral inoculation and siRNA delivery intracranially. Mice treated with siRNA 6 h post‐infection exhibited 100% survival, whereas mock‐treated infected mice showed 100% mortality [92]. Similarly, Bai et al. reported partial protection when mice were pretreated with siRNA prior to WNV inoculation [93]. Deas et al. designed phosphorodiamidate morpholino oligomers (PMOs) complementary to RNA elements at the 5′ and 3′ termini of the WNV genome. Among these, PMOs targeting the 5‐terminal 20 nucleotides (5End) and a 3‐terminal sequence involved in genome cyclisation (3CSI) showed the greatest potency, reducing viral titres by 5–6 logs at 5 μM concentration without apparent cytotoxicity [94].

4.5.5. Host‐Targeted Strategies

Due to its minimal genome, WNV relies heavily on host factors for replication, making these host molecules attractive therapeutic targets. These include factors that support viral infection and those involved in pathogenesis and innate immune responses. Several studies have evaluated host molecules as targets in flaviviral infections with the main mechanisms summarised below [95].

4.5.6. Host Metabolic Processes

Lipid metabolism is closely linked to flaviviral infections. Cholesterol, for example, is essential for the infection of cells by both WNV and DENV. Agents such as methyl‐β‐cyclodextrin, which sequesters cholesterol, and 25‐hydroxycholesterol, which modulates cholesterol‐related transcription factors, have been shown to reduce WNV infection in vitro [96]. Additionally, inhibitors of fatty acid synthase (FASN), such as cerulenin and C75, decrease cellular infection by DENV and WNV in vitro [97].

4.5.7. Entry Inhibitors

Several compounds studied primarily for dengue virus entry inhibition may also be effective against WNV. These include heparin/highly sulphated heparan sulphate, pentosan polysulfate, and neolactotetraosylceramide [98].

4.5.8. Pathogenesis Targeting

Multiple host targets have been identified to mitigate WNV‐induced pathogenesis. Targeting macrophage migration inhibitory factor (MIF) with small‐molecule antagonists or antibodies protected against WNV pathogenesis. Similarly, neutralisation of interleukin‐10 (IL‐10) with antibodies conferred protection from lethal WNV infection. WNV infection also causes BBB disruption, with matrix metalloproteinases (MMPs) implicated in this process. This suggests that proteins involved in BBB integrity regulation could be exploited as therapeutic targets to control WNV pathogenesis [99].

Although these approaches are promising, their translation into clinical practice remains limited, and further studies are needed to establish their safety, optimal timing, and potential role in combination strategies.

5. Future Directions and Clinical Trial Priorities

Future therapeutic strategies for WNV infection should increasingly focus on multitarget and combination approaches that simultaneously address viral replication, immune modulation, and neuroprotection. Evidence from preclinical studies indicates that combining direct‐acting antivirals, such as RdRp inhibitors like Remdesivir, with immunomodulatory agents such as interferon‐beta 1a may enhance viral suppression and mitigate neurological damage if administered early in the disease course. Furthermore, integrating neutralising MAbs combined with antivirals holds promise for preventing viral entry and replication, thereby reducing the risk of viral escape and improving CNS penetration. Importantly, the timing of therapeutic intervention appears critical; clinical outcomes are likely to improve when treatment is initiated within the initial 7–10 days of symptom onset, before extensive neuroinvasion occurs. Future trials should therefore prioritise early‐stage intervention, especially in high‐risk populations. Additionally, advancing the measurement of drug penetration into the CNS and the identification of reliable biomarkers of efficacy remain essential to tailor therapies and monitor response in real time. Emerging therapeutic avenues targeting host cell metabolism, such as inhibitors of dihydroorotate dehydrogenase, also warrant investigation in combination with antivirals to enhance efficacy and circumvent resistance. The continued development of antibody‐based therapies should focus on targeting multiple viral epitopes to minimise resistance and on innovative delivery methods to improve CNS bioavailability. Lastly, while no licenced vaccines are currently available for humans, encouraging preliminary results underscore the urgent need to accelerate vaccine development and conduct large‐scale clinical trials to establish effective preventive strategies, especially in vulnerable populations. Taken together, these priorities highlight a comprehensive approach that integrates antiviral, immunological, and preventive measures to improve outcomes in WNV infection.

6. Conclusion

In conclusion, the global incidence of WNV infection is rising, driven by climate change and expanding geographic spread. This increase coincides with a growing population of immunosuppressed individuals and those with risk factors for severe neuroinvasive disease, underscoring an urgent need for effective therapies. Despite promising preclinical data, the paucity of robust clinical trials hampers progress in establishing standard treatments. Addressing this gap requires prioritising well‐designed, adequately powered randomized clinical trials studies focused on early intervention and combination therapies. For clinicians, it is critical to maintain high suspicion in at‐risk patients and consider early referral for experimental therapies within clinical trials. Monitoring for neuroinvasive complications and supportive care remain cornerstones of management in the absence of approved antivirals. From a research perspective, there is a clear unmet need for trials evaluating multi‐targeted and combination approaches, as well as strategies improving CNS penetration of therapeutics. Developing standardized protocols for early intervention and integrating host‐targeted therapies could reshape future treatment paradigms. A coordinated international effort is essential to close these gaps and prepare for the increasing impact of WNV worldwide. Bridging the gap between promising preclinical findings and robust clinical evidence will be essential to improve patient outcomes and address the growing public health impact of WNV.

Author Contributions

A.C.: conceptualisation, study design, writing of the original draft. D.K., C.D.B., M.L. and S.V.: literature search, data analysis, interpretation of results, writing of the original draft. T.A.B., G.M., M.V.T., N.B. and P.D.M.: investigation (literature review), data acquisition, data interpretation. A.C. and D.K.: writing, review and editing, critical revision. C.D.B. and M.L.: validation, writing, review and editing. S.V.: validation, writing, review, editing, supervision and funding acquisition. All authors read and approved the final version of the manuscript and agree to be accountable for all aspects of the work.

Funding

This study was supported by funds allocated to the National Institute for Infectious Diseases ‘Lazzaro Spallanzani’, IRCCS, 00,149, Rome (Italy), from the Italian Ministry of Health (progetto P3, Linea 1 Malattie Infettive tropicali e neglette: aspetti patogenetici, immunologici e diagnostici, 2025–2027).

Consent

The authors have nothing to report.

Conflicts of Interest

The authors declare no conflicts of interest.

Permission to Reproduce Material From Other Sources

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Supporting information

Supporting Information S1

Data Availability Statement

The authors have nothing to report.