West Nile Virus in Birds

Overview and Taxonomy of West Nile Virus in Birds

West Nile virus (WNV) is a single-stranded, positive-sense RNA virus belonging to the genus Orthoflavivirus (family Flaviviridae), within the Japanese encephalitis serocomplex [13, 24]. First isolated in 1937 from a febrile woman in the West Nile District of Uganda, the virus has since emerged as a globally significant zoonotic pathogen with a complex enzootic cycle that relies primarily on ornithophilic mosquitoes as vectors and diverse avian species as amplifying hosts [5, 9, 12]. Birds are the definitive vertebrate reservoir; they sustain viral amplification and transmission to mosquitoes, which then may infect incidental dead-end hosts such as humans and horses [13, 20, 25]. The expanding geographic range of WNV, from its origins in sub-Saharan Africa to North America, Europe, the Middle East, and parts of Asia, has been driven by the movement of migratory birds, climate change, and anthropogenic land-use alterations [5, 7, 24]. Understanding the taxonomic framework and the avian–virus interface is foundational to predicting outbreak dynamics, reservoir competence, and the potential for viral evolution.

Taxonomic Classification and Phylogenetic Diversity

WNV is classified within the Orthoflavivirus genus and exhibits a high degree of genetic diversity, most commonly partitioned into two principal lineages (lineage 1 and lineage 2), although additional lineages have been recognized at lower prevalence [15, 31]. Lineage 1 is further subdivided into clades 1a (encompassing most European, Middle Eastern, North American, and Australian strains) and 1b (Kunjin virus, a naturally attenuated subtype circulating in Australasia) [5, 17]. The New York 1999 (NY99) strain, which ignited the Western Hemisphere epizootic, is a lineage 1a virus closely related to a goose isolate from Israel in 1998 [5]. During its spread across North America, NY99 acquired a mutation in the envelope gene (E protein) that enhanced infectivity in mosquito vectors, while a separate mutation in the nonstructural protein NS3 increased virulence in birds [5]. Lineage 2, once considered confined to sub-Saharan Africa and Madagascar, began emerging in Europe around 2004 and has since become the dominant lineage in many parts of continental Europe, accounting for over 73% of publicly available sequences in the region [15, 36]. Sub-lineage WNV-2a has spread to at least 14 European countries, exhibiting high dispersal velocities (88–215 km/yr) that correlate strongly with bird movement patterns [15]. In Central Europe, a monophyletic Berlin-specific clade of lineage 2 has been identified, indicating local endemic maintenance [19]. The genomic stability of these lineages is notable: Italian strains of WNV-2 collected over several years cluster tightly, suggesting in situ evolution rather than repeated reintroduction [37]. Conversely, a new lineage 1 strain emerged in northern Italy in 2021–2022, co-circulating with lineage 2 and showing evidence of greater neuroinvasiveness in birds and humans [32, 36]. This lineage-level diversity has direct implications for avian pathogenesis, for example, the NS2B/NS3/NS4B/NS5 genomic region mediates attenuation in mammals, while NS4B/NS5/3′UTR influences vector competence in Culex pipiens [23].

Avian Host Range and Reservoir Competence

The avian host range of WNV is extraordinarily broad: over 300 bird species have been reported as susceptible to infection [5]. However, not all species contribute equally to viral amplification. Reservoir competence, a metric derived from viremia intensity, duration, and infectiousness to mosquitoes, varies dramatically among taxa [3]. Experimental infection studies using the NY99 strain in 25 North American bird species demonstrated that passeriform and charadriiform birds are the most competent reservoirs [3]. The five most competent species were all passerines: Blue Jay (Cyanocitta cristata), Common Grackle (Quiscalus quiscula), House Finch (Carpodacus mexicanus), American Crow (Corvus brachyrhynchos), and House Sparrow (Passer domesticus) [3]. These species develop viremias sufficiently high to infect feeding mosquitoes, and they frequently shed virus orally and cloacally, enabling direct contact transmission [3]. In Europe, the Eurasian magpie (Pica pica) has been shown to harbor WNV lineage 2 strains with the same genetic virulence determinants (His249Pro NS3 mutation, E-glycosylation motif) found in the 2010 Greek outbreak [35]. The ecological importance of the American Robin (Turdus migratorius) in North America is well established: mathematical models indicate that seasonal outbreaks are driven primarily by competent migratory robins with low WNV-induced mortality, even though highly competent but short-lived resident crows are more conspicuous mortality indicators [21]. In southern Europe, the Common Blackbird (Turdus merula) has been identified as an important amplifying host in urban settings, with high seroprevalences detected inside villages where human cases occurred [29]. Raptors (birds of prey) are disproportionately susceptible to WNV infection, developing higher seroprevalences and more frequent neurological signs than many other avian groups [14]. This heightened susceptibility may arise from predation on infected prey, amplifying the oral route of infection [14, 37]. Indeed, wintertime WNV transmission in Italy has been attributed to bird-to-bird oral transmission in predatory raptors, helping the virus overwinter when mosquito activity is minimal [37].

Ecological and Epidemiological Significance

WNV circulates in an enzootic cycle that is exquisitely sensitive to avian community composition, mosquito vector dynamics, and environmental conditions. Molecular and serological surveys across diverse regions consistently identify passerines as the primary reservoir hosts, but the specific species involved shift with geography. In Germany, national surveillance since 2018 has revealed WNV lineage 2 circulating in eastern states, with seroprevalences in birds reaching >20% in the endemic zone by 2022 [1, 26]. Antibodies against WNV have been detected in a wide array of species, including both resident and migratory birds, and the virus has expanded westward and southward within Germany over successive years [26]. Co-circulation with Usutu virus (USUV), a closely related flavivirus sharing mosquito vectors and avian hosts, complicates serological interpretation due to intense cross-reactivity [2, 13, 30]. In Switzerland and Germany, co-infections with WNV and USUV have been documented in dead birds, and neutralizing antibodies to both viruses are frequently found in zoo birds and wild raptors [2, 10]. The presence of USUV can prime or interfere with the immune response to WNV, affecting vaccine efficacy in captive collections [10]. Migratory birds are considered the primary mechanism for long-distance WNV introduction and dissemination. Long-distance migrants that breed in the Palearctic and winter in sub-Saharan Africa, such as the Eurasian Reed Warbler, Barn Swallow, and Willow Warbler, are capable of carrying WNV across the Sahara and into Europe [7, 8, 18]. Phylogeographic analyses indicate that the spread of WNV in Europe is strongly associated with migratory bird flyways and with the coverage of wetlands that provide stopover habitat [4, 7, 15]. In the Danube Delta Biosphere Reserve, a major migratory stopover, WNV seropositivity in wild birds reached 11.8% in 2016, with juvenile birds contributing to local transmission [22]. The risk of WNV introduction into WNV-free regions such as Great Britain has been quantified using models that predict a high probability of infected passerines arriving from continental wetlands like the Camargue, especially if the virus becomes established further north in France [28]. The arrival of viremic birds alone, however, is not sufficient; establishment requires overlapping phenology of competent native mosquito vectors, such as Culex torrentium in central and northern Europe [11, 27].

Avian WNV infection can be subclinical or lethal, and the outcomes are highly species- and lineage-dependent. In highly susceptible species such as corvids and raptors, infection often leads to fatal neuroinvasive disease with systemic lesions involving the brain, heart, kidneys, and intestines [14, 25]. In contrast, many passerines (e.g., the House Sparrow) survive acute infection and become persistently infected, shedding virus for weeks from tissues including the skin, kidney, and spleen [3, 33]. This persistence may facilitate viral overwintering and re-emergence in spring [3]. The presence of pre-existing antibodies from previous exposure or from vaccination with inactivated equine vaccines can modulate infection outcome; in zoo birds, pre-existing antibodies against WNV or USUV significantly influenced the antibody response to vaccination [10]. Surveillance of dead and live birds remains the cornerstone of early-warning systems for WNV outbreaks in both North America and Europe. In California, from 2003–2018, over 23,000 dead birds were tested for WNV, providing an early indicator of enzootic activity well before human cases were detected [34]. In Italy’s Emilia-Romagna region, active testing of corvids (minimum 3.8 per 100 km²) from mid-June to mid-August allowed detection of WNV circulation in 92.4% of epidemic seasons, and the virus was first identified in birds in 72.1% of seasons before the first human case [6]. These findings underscore the critical role of avian surveillance in a One Health framework, especially as climate change extends the transmission season and expands the geographic envelope of WNV into previously non-endemic temperate regions [5, 24]. The taxonomy of WNV, and its close relationship with USUV and other flaviviruses, must be precisely understood to deploy species-specific diagnostic tools and to interpret serological data in regions where multiple flaviviruses co-circulate [16, 30].

Molecular Pathogenesis and Host-Virus Interactions in Avian Species

The molecular pathogenesis of West Nile virus (WNV) in avian species represents a complex interplay between viral genetic determinants, host immune responses, and ecological factors that collectively determine infection outcomes, reservoir competence, and transmission dynamics. As a zoonotic orthoflavivirus maintained in an enzootic cycle between ornithophilic mosquitoes and avian amplifying hosts, WNV exhibits remarkable heterogeneity in pathogenicity across bird taxa, ranging from subclinical infections to rapid, fatal neuroinvasive disease [3, 25]. Understanding the molecular underpinnings of these differential outcomes is critical for predicting spillover risk to humans and horses, both of which are incidental dead-end hosts [12, 20].

Viral Genetic Determinants of Avian Pathogenicity

The WNV genome, a single-stranded positive-sense RNA molecule of approximately 11 kb, encodes three structural proteins (capsid [C], pre-membrane [prM], and envelope [E]) and seven non-structural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B, NS5) [9, 17]. Specific genetic signatures have been strongly associated with enhanced virulence in avian hosts. The most well-characterized determinant is the NS3 helicase domain substitution at position 249, where a proline-to-histidine mutation (Pro249His) was identified in the NY99 strain that emerged in New York in 1999 [5]. This mutation, which is also present in WNV strains responsible for outbreaks in Romania (1996) and Russia (1999), significantly increases viremia magnitude and mortality in corvids and other passeriform species [5, 25]. The molecular mechanism involves enhanced NS3 protease activity, leading to more efficient cleavage of the viral polyprotein and consequently higher viral replication rates in avian cells.

Comparative genomic analyses of lineage 1 and lineage 2 strains have revealed additional virulence determinants. The envelope glycoprotein, which mediates receptor binding and membrane fusion, contains a glycosylation motif at positions N154–S156 that is critical for neuroinvasiveness [35]. Glycosylated E protein facilitates viral entry into avian neurons and enhances replication in the central nervous system (CNS). Studies using reverse genetics systems, including infectious subgenomic amplicons (ISA) and reporter replicons, have demonstrated that the NS4B/NS5/3′UTR genomic region modulates vector competence in Culex pipiens mosquitoes, while the NS2B/NS3/NS4B/NS5 region influences attenuation in mammalian models [23]. These findings suggest that viral genetic determinants of virulence are partially host-specific, with distinct molecular requirements for efficient replication in avian versus mammalian cells.

The emergence of WNV lineage 2 in Europe has been associated with particularly severe avian mortality. Phylogenetic analyses indicate that the European WNV-2a sub-lineage, which accounts for 73% of all European WNV sequences, has evolved into two major co-circulating clusters originating from Central Europe [15]. These clusters exhibit distinct dynamic histories and transmission patterns, with dispersal velocities of 88–215 km/year that correlate strongly with bird movement patterns [15]. Notably, the Italian outbreak of 2022 demonstrated that a newly introduced WNV lineage 1 strain exhibited higher infection rates in wild birds and more rapid geographic expansion compared to co-circulating lineage 2 strains, suggesting that viral genetic factors can drive differential amplification in avian reservoirs [32, 36].

Host Cellular Receptors and Entry Mechanisms

The initial step in avian WNV infection involves viral attachment to host cell surface receptors. While the specific receptor(s) for WNV in avian cells remain incompletely characterized, studies using avian cell lines, including duck embryo fibroblasts (DEF) and chicken hepatoma (LMH) cells, have demonstrated that WNV enters cells via clathrin-mediated endocytosis following interaction with glycosaminoglycans, particularly heparan sulfate [17]. The envelope protein domain III (EDIII) contains the primary receptor-binding motif, and variations in EDIII sequence among WNV strains correlate with differences in avian cell tropism.

Dendritic cell-specific ICAM-3 grabbing non-integrin (DC-SIGN) and related C-type lectin receptors have been implicated in flavivirus entry in mammalian systems, but their role in avian infection is less clear. The presence of mannose-specific lectins on avian macrophages may facilitate viral uptake through interaction with E protein glycans. Once internalized, the acidic environment of the endosome triggers conformational changes in the E protein, leading to fusion of viral and endosomal membranes and release of the nucleocapsid into the cytoplasm [9].

Innate Immune Responses and Viral Countermeasures

Avian species possess a robust innate immune system that includes pattern recognition receptors (PRRs), interferon (IFN) signaling pathways, and natural killer (NK) cell responses. The retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) serve as cytoplasmic sensors of viral RNA, triggering activation of interferon regulatory factors (IRFs) and subsequent production of type I interferons (IFN-α/β) [20]. However, WNV has evolved sophisticated countermeasures to evade these responses. The viral NS1 protein inhibits TLR3-mediated signaling, while NS2A, NS4A, and NS4B collectively antagonize IFN signaling by blocking JAK-STAT phosphorylation and nuclear translocation of STAT1/STAT2 complexes [20].

The differential capacity of avian species to mount effective IFN responses correlates strongly with reservoir competence. Highly competent reservoir species, such as the American Robin (Turdus migratorius) and House Sparrow (Passer domesticus), exhibit relatively weak and delayed IFN responses, allowing high-titer viremia without severe pathology [3, 21]. In contrast, highly susceptible species like American Crows (Corvus brachyrhynchos) and Blue Jays (Cyanocitta cristata) develop robust but ultimately ineffective IFN responses that contribute to immunopathology, including widespread apoptosis of neurons and glial cells [3, 25]. This dichotomy highlights the evolutionary trade-off between viral control and immunopathology in determining infection outcomes.

The complement system also plays a critical role in avian anti-WNV defense. Birds possess a well-developed alternative and lectin complement pathways, and opsonization of viral particles by complement components C3b and C4b enhances phagocytosis by macrophages and dendritic cells. However, WNV NS1 protein has been shown to recruit complement regulatory proteins, including factor H and C4b-binding protein, thereby inhibiting complement-mediated neutralization [20].

Pathogenesis of Neuroinvasive Disease in Avian Species

The neurotropic nature of WNV is a hallmark of its pathogenesis in susceptible avian species. Following peripheral inoculation via mosquito bite, the virus undergoes initial replication in Langerhans cells and keratinocytes at the bite site, followed by migration to draining lymph nodes where amplification occurs [25]. Viremia ensues, with viral titers in highly competent passeriform species reaching 10⁸–10¹⁰ plaque-forming units (PFU)/mL of serum [3]. This high-titer viremia is sufficient to infect feeding mosquitoes, thereby perpetuating the enzootic cycle.

Neuroinvasion occurs via multiple routes, including direct infection of cerebral microvascular endothelial cells, transport across the blood-brain barrier (BBB) via infected leukocytes (the "Trojan horse" mechanism), and retrograde axonal transport from peripheral neurons [25]. Once within the CNS, WNV preferentially infects neurons, particularly those in the brainstem, cerebellum, and hippocampus. Histopathological examination of infected birds reveals neuronal necrosis, perivascular cuffing with mononuclear cells, gliosis, and microglial nodule formation [25]. In corvids and raptors, the severity of neuropathology correlates with viral load and is characterized by diffuse encephalitis with prominent involvement of the Purkinje cell layer of the cerebellum [14, 25].

The molecular basis for neuronal tropism involves the expression of putative WNV receptors on neuronal surfaces, including αvβ3 integrin and the laminin receptor. Additionally, neurons are particularly susceptible to WNV-induced apoptosis due to their high metabolic activity and limited regenerative capacity. The viral NS3 protein has been shown to activate caspase-3 and caspase-8 pathways, leading to programmed cell death [9]. Furthermore, the NS2B-NS3 protease complex cleaves host proteins involved in innate immunity, including STING (stimulator of interferon genes), thereby dampening the antiviral response and promoting viral replication [20].

Extraneural Pathology and Viral Dissemination

While neuroinvasion is the most clinically significant aspect of WNV pathogenesis, extraneural pathology contributes substantially to morbidity and mortality in avian hosts. Systemic infection involves viral replication in multiple organ systems, including the heart, kidneys, liver, spleen, and gastrointestinal tract [25]. Myocarditis is a frequent finding in fatal avian cases, characterized by multifocal necrosis of cardiac myocytes with associated inflammatory infiltrates. This cardiac involvement may contribute to acute death through arrhythmias or pump failure.

Renal pathology is also prominent, with WNV antigen detected in tubular epithelial cells and glomerular endothelium. Interstitial nephritis and tubular necrosis have been documented in experimentally infected birds, and persistent viral shedding in urine may contribute to environmental contamination and oral transmission [3, 25]. The detection of WNV RNA in cloacal and oral swabs from experimentally infected birds indicates that horizontal transmission through fecal-oral routes or direct contact is possible, particularly in communal roosting or nesting sites [3].

Splenic and hepatic involvement is characterized by lymphoid depletion, necrosis of splenic white pulp, and multifocal hepatic necrosis. The spleen serves as a major site of viral amplification, and the degree of splenic pathology correlates with viremia magnitude [25]. In raptors, which appear particularly susceptible to WNV infection, hepatic necrosis is often severe and may be the primary cause of death in some individuals [14].

Viral Persistence and Overwintering Mechanisms

One of the most intriguing aspects of WNV pathogenesis in birds is the capacity for viral persistence. Experimental infections have demonstrated that WNV RNA can be detected in tissues of surviving birds for weeks to months after initial infection, particularly in the spleen, kidney, and brain [3]. This persistent infection may serve as a mechanism for overwintering, allowing the virus to survive periods when mosquito vectors are inactive. The detection of WNV lineage 2 in a goshawk in Italy during January, when mosquito activity is minimal, supports the hypothesis of bird-to-bird transmission as an overwintering strategy [37]. Oral transmission through predation or scavenging of infected carcasses may be particularly important in raptors and corvids, which frequently consume infected prey [14, 37].

The molecular mechanisms underlying viral persistence involve modulation of the host immune response, including downregulation of MHC class I expression on infected cells, thereby evading cytotoxic T lymphocyte recognition. Additionally, WNV has been shown to establish latency in hematopoietic progenitor cells, providing a reservoir for reactivation under conditions of immunosuppression or stress [20]. The NS5 protein, which functions as the viral RNA-dependent RNA polymerase, exhibits error-prone replication that generates quasispecies diversity, allowing the virus to adapt to changing host environments and potentially escape immune pressure [9].

Species-Specific Variation in Reservoir Competence

The concept of reservoir competence, defined as the ability of a host species to transmit WNV to feeding mosquitoes, is central to understanding enzootic transmission dynamics. Experimental infection studies have systematically evaluated reservoir competence across 25 North American bird species, revealing that passeriform and charadriiform birds are significantly more competent than other taxonomic groups [3]. The five most competent species identified were all passerines: Blue Jay, Common Grackle (Quiscalus quiscula), House Finch (Haemorhous mexicanus), American Crow, and House Sparrow [3].

The molecular basis for this variation in reservoir competence is multifactorial. High-competence species typically exhibit: (1) prolonged high-titer viremia (≥10⁵ PFU/mL for 3–5 days), which is the threshold required to infect feeding Culex mosquitoes; (2) minimal clinical disease, allowing continued activity and exposure to vectors; and (3) efficient viral shedding through cloacal and oral routes [3, 21]. In contrast, low-competence species, such as many waterfowl and galliform birds, develop low or undetectable viremia and rapidly clear the infection.

Phylogenetic analyses have demonstrated that reservoir competence is evolutionarily conserved to some degree, with certain avian orders and families exhibiting consistently high or low competence [40]. Macroecological studies incorporating serological prevalence, molecular prevalence, and mortality data across 949 bird species from 39 countries have identified phylogeny as a significant predictor of infection metrics [40]. This phylogenetic signal suggests that fundamental aspects of avian immunology, including the structure and function of PRRs, IFN signaling pathways, and MHC diversity, are shaped by evolutionary history and contribute to species-specific susceptibility.

Role of Migratory Birds in Viral Dissemination

Migratory birds play a dual role in WNV ecology: they serve as amplifying hosts during the breeding season and as vectors for long-distance viral dissemination during migration [7, 18]. The Afro-Palaearctic flyway, which connects sub-Saharan Africa with Europe, is particularly important for the introduction of WNV strains into naive populations [7, 31]. Phylogeographic analyses have demonstrated that WNV lineage 2 was introduced into Europe from Africa on multiple occasions, with the Danube Delta region of Romania serving as a major entry point [42].

The molecular pathogenesis of WNV in migratory birds is influenced by the physiological demands of migration, including elevated corticosteroid levels, altered metabolic states, and temporary immunosuppression. These factors may enhance viral replication and prolong viremia, increasing the probability that infected birds will introduce the virus into new areas [7, 18]. Mathematical modeling has demonstrated that the early growth rate of seasonal WNV outbreaks is more strongly influenced by the influx of susceptible migratory birds than by resident bird populations [21]. This finding has important implications for surveillance, as the arrival of migratory birds in the spring may serve as an early warning signal for impending outbreaks.

Serological surveys of migratory birds in Europe have revealed that long-distance migrants, particularly those wintering in sub-Saharan Africa, exhibit higher WNV seroprevalence than resident species [8, 38]. In Germany, WNV neutralizing antibodies were predominantly detected in long-distance, partial, and short-distance migrants, while USUV antibodies were more common in resident species [38]. This pattern suggests that migratory behavior facilitates exposure to diverse WNV strains across the geographic range, and that migrants may serve as sentinels for viral circulation in remote areas.

Co-infections with Usutu Virus and Other Flaviviruses

The co-circulation of WNV with Usutu virus (USUV), another mosquito-borne orthoflavivirus, presents unique challenges for understanding pathogenesis and diagnosis. Both viruses share overlapping geographic ranges, vector species, and avian hosts, leading to frequent co-infections [2, 13]. In Germany, co-infections with WNV and USUV were detected in six dead birds collected in 2018 and 2019, with WNV lineage 2 and USUV lineages Africa 2, Africa 3, and Europe 2 identified through genomic sequencing [2]. Serological analysis of zoo birds revealed that some individuals had high neutralizing antibody titers against both viruses, indicating sequential or concurrent exposure [2, 10].

The molecular interactions between WNV and USUV during co-infection are poorly understood but may involve competition for cellular resources, heterologous interference, or immune modulation. Preliminary attempts to co-propagate both viruses in vitro failed, suggesting that direct competition may limit dual replication in individual cells [2]. However, the detection of dual-positive birds in the field indicates that co-infection can occur at the organismal level, potentially through sequential mosquito bites or oral transmission.

The antigenic cross-reactivity between WNV and USUV, as well as with other flaviviruses such as Japanese encephalitis virus (JEV) and dengue virus, complicates serological diagnosis [16, 30]. In regions where multiple flaviviruses co-circulate, such as Southeast Asia and sub-Saharan Africa, distinguishing between infections requires highly specific neutralization tests [16, 30]. The presence of cross-reactive antibodies may also influence pathogenesis through antibody-dependent enhancement (ADE), a phenomenon documented for dengue virus but less well-characterized for WNV in avian hosts.

Implications for Surveillance and Control

Understanding the molecular pathogenesis and host-virus interactions of WNV in avian species has direct implications for surveillance and control strategies. The identification of highly competent reservoir species, such as American Robins and House Sparrows, allows targeted surveillance efforts to detect viral circulation before spillover to humans [3, 21]. In Europe, corvids have proven to be effective sentinel species, with active surveillance of magpies and crows detecting WNV circulation weeks before the onset of human cases [6]. The World Organisation for Animal Health (WOAH) and the World Health Organization (WHO) recognize the importance of avian surveillance as a component of integrated One Health approaches to WNV management [6, 39].

Vaccination strategies for zoo birds and endangered species have been developed using inactivated WNV vaccines originally licensed for horses [10]. Field studies in German zoos have demonstrated that vaccination induces neutralizing antibody responses in a variety of avian species, with the magnitude of response influenced by pre-existing immunity to WNV or USUV [10]. While no human WNV vaccine is currently available, the development of peptide-based inhibitors, monoclonal antibodies, and small molecules targeting viral NS proteins holds promise for future therapeutic interventions [9].

The Centers for Disease Control and Prevention (CDC) and the European Centre for Disease Prevention and Control (ECDC) recommend integrated surveillance programs that combine mosquito monitoring, avian serosurveys, and human case reporting to provide early warning of WNV activity [34, 41]. The molecular characterization of circulating WNV strains through genomic sequencing is essential for tracking viral evolution, identifying virulence determinants, and predicting outbreak potential [15, 32]. As climate change expands the geographic range of Culex vectors and alters avian migration patterns, the need for robust molecular surveillance of WNV in avian reservoirs will only intensify [5, 24].

Epidemiology and Spatial-Temporal Dynamics of West Nile Virus in Avian Populations

The epidemiology of West Nile virus (WNV) in avian populations represents a complex interplay between host ecology, vector biology, viral genetics, and environmental drivers that collectively determine the spatial and temporal patterns of virus transmission. As the primary amplifying hosts in the enzootic cycle, birds are not merely passive recipients of infection but active participants in the maintenance, amplification, and geographic dissemination of WNV across continental and intercontinental scales. Understanding the epidemiological dynamics within avian communities is therefore fundamental to predicting outbreak risk, implementing surveillance strategies, and mitigating spillover to humans and other incidental hosts.

Global Patterns of Avian WNV Circulation and Lineage Distribution

WNV circulates globally across six continents, with birds serving as the obligate reservoir hosts in virtually all ecosystems where the virus is endemic. Molecular epidemiological investigations have revealed that the global distribution of WNV is characterized by multiple genetic lineages with distinct geographic ranges and epidemiological behaviors. In sub-Saharan Africa, where WNV was first isolated in Uganda in 1937, lineage 1 and lineage 2 strains circulate endemically, with evidence suggesting that the region functions as an ancestral reservoir from which viruses have repeatedly emerged [31]. The movement of WNV out of Africa has been historically mediated by migratory birds traversing the Afro-Palaearctic flyway, a corridor that connects wintering grounds in sub-Saharan Africa with breeding grounds across Europe and western Asia [7, 31].

The introduction of WNV into North America in 1999, specifically the NY99 strain of lineage 1, represents one of the most dramatic examples of viral emergence in a naive ecosystem. Phylogenetic analyses demonstrated that this strain was closely related to a goose virus isolated in Israel in 1998, implicating either infected migratory birds or transported mosquitoes in the transatlantic introduction [5, 18]. Within five years of its detection in New York City, the virus had swept across the continental United States, reaching California by 2003 and establishing enzootic cycles that persist to the present day [34]. The rapid continental spread was facilitated by the diversity of susceptible avian species, over 300 bird species have been documented as susceptible to WNV infection in North America alone [5], and the presence of competent Culex mosquito vectors across diverse ecological zones.

In Europe, the epidemiological landscape has been shaped primarily by the expansion of WNV lineage 2, which emerged from sub-Saharan Africa and began causing significant outbreaks in Central and Eastern Europe after 2004 [14, 15]. Phylogeographic analyses have mapped the dispersal of WNV lineage 2 sub-lineage 2a (WNV-2a) across at least 14 European countries, with dispersal velocities estimated between 88 and 215 km per year that correlate strongly with bird movement patterns [15]. The virus has evolved into at least two major co-circulating clusters, both originating from Central Europe but exhibiting distinct dynamic histories and transmission patterns [15]. In Germany, which was considered WNV-free until 2018, the virus has established an endemic focus in the eastern part of the country, with seroprevalence rates in birds reaching 14–16% in the endemic region by 2020 [26]. Serological data from 2021–2022 indicate expanding circulation westward and southward of the known hotspots, with more than 20% of birds in eastern Germany carrying neutralizing antibodies against WNV in 2022 [1].

Avian Species Composition and Reservoir Competence

Not all bird species contribute equally to WNV transmission dynamics. Reservoir competence, the ability of a host species to become infected, sustain viremia sufficient to infect feeding mosquitoes, and survive long enough to contribute to transmission, varies dramatically across taxonomic groups. Experimental infection studies have provided critical quantitative data on species-specific reservoir competence. Komar et al. [3] exposed 25 North American bird species to WNV through infectious mosquito bites and systematically measured viremia titers, clinical outcomes, viral shedding, and seroconversion. The five most competent species were all passerines: Blue Jay (Cyanocitta cristata), Common Grackle (Quiscalus quiscula), House Finch (Carpodacus mexicanus), American Crow (Corvus brachyrhynchos), and House Sparrow (Passer domesticus). Passeriform and charadriiform birds demonstrated significantly higher reservoir competence than other taxa tested, with the highest competence observed in species that maintained high viremia titers without rapid mortality [3].

The mortality impact of WNV infection varies enormously among avian species. In the same experimental study, death occurred in eight of the 25 species tested, with American Crows and other corvids being particularly susceptible [3]. This differential mortality has profound epidemiological implications: highly susceptible species that die rapidly from infection may contribute less to transmission than moderately susceptible species that survive to infect multiple mosquitoes over an extended period. Mathematical modeling has shown that the early growth rates of seasonal WNV outbreaks are more influenced by the migratory bird population than by resident populations, and that although the death of highly competent resident birds such as American Crows serves as an excellent indicator of virus presence, these species have less impact on the basic reproductive number than competent migratory birds with low WNV-associated mortality, such as American Robins (Turdus migratorius) [21].

In Europe, the ecological role of different avian species has been elucidated through serosurveys and experimental studies. In the Guadalquivir River region of Spain, where a major WNV outbreak occurred in 2020, serological testing of wild birds confirmed that blackbirds (Turdus merula) exhibited a high prevalence of WNV antibodies, suggesting this species plays an important role in virus amplification in urban and peri-urban settings [29]. Raptors appear particularly susceptible to WNV infection, often exhibiting neurological signs and mortality, and may serve as important sentinel species for surveillance programs [14]. Birds of prey also potentially amplify infection through predation of infected prey, providing an additional transmission route that may contribute to viral maintenance [14].

A macroecological analysis of WNV infection data from 949 bird species across 39 countries revealed that molecular prevalence and mortality metrics are good predictors of reservoir competence, and that the variability in these metrics is attributable to phylogeny, spatiotemporal factors, and sample size [40]. This suggests that phylogenetic relatedness can be used to predict reservoir competence in untested species, providing a framework for risk assessment in regions where empirical data are lacking.

Migratory Birds as Drivers of Spatial Spread

The role of migratory birds in the intercontinental and intracontinental spread of WNV has been extensively documented through phylogeographic, serological, and ecological studies. Bird migration functions as the primary mechanism for viral introduction into new geographic areas, with the Afro-Palaearctic flyway being particularly well-characterized as a conduit for WNV movement between Africa and Europe [7, 18, 31]. García-Carrasco et al. [7] identified 61 bird species that potentially contribute to the intercontinental spread of WNV or its variants, demonstrating that WNV-risk areas are interconnected through bird migration patterns. The timing of migration relative to viremic periods is critical: most passerine migrants cross the Sahara Desert in spring (February–May) and autumn (August–November), and the duration of viremia in birds is typically 3–7 days, meaning that transcontinental movement of the virus requires either viremic birds that migrate rapidly or a relay mechanism in which infected birds transmit to local mosquitoes at stopover sites, which then infect subsequent waves of migrants [31].

The risk of WNV introduction into new regions via migratory birds has been quantified using mathematical modeling. Bessell et al. [28] estimated that if WNV were circulating in the Camargue region of France during the spring migration period, the probability that one or more migrating passerine birds becomes infected and lands in Great Britain while still viremic is 0.881, with 0.384 birds arriving in areas with suitable vector habitat. Crucially, if WNV became established in wetland areas further north in France, the model predicts that at least one infected bird would reach Great Britain [28]. This quantitative risk assessment underscores the vulnerability of temperate regions to WNV introduction through the northward movement of infected migratory birds, particularly under scenarios of climate warming that may expand the geographic range of competent vectors and extend the transmission season [11, 24].

In Germany, the initial detection of WNV in 2018 was preceded by years of serological surveillance that provided early warning of viral incursion. Michel et al. [38] found WNV-neutralizing antibodies predominantly in long-distance, partial, and short-distance migrants during 2014–2016, before WNV RNA was detected in any German bird or mosquito sample. By 2018, WNV-specific neutralizing antibodies were observed in resident and short-distance migratory birds in eastern Germany, providing the first evidence of local WNV circulation [43]. This pattern, serological evidence preceding molecular detection, has been observed in multiple European countries and highlights the utility of bird serosurveillance as an early warning system.

Within continents, migratory birds also facilitate rapid spread. The dispersal of WNV-2a across Europe at velocities of 88–215 km/year is correlated with bird movement patterns, and the virus has reached at least 14 European countries through a combination of migratory introduction and local enzootic establishment [15]. In the United States, the initial spread of WNV from New York across the continent was similarly attributed to migratory birds, with the pattern of outbreaks in southern Europe providing a precedent for the potential of viremic migratory birds to drive geographic expansion [18].

Environmental and Anthropogenic Drivers of Avian WNV Dynamics

The spatial and temporal patterns of WNV circulation in avian populations are strongly modulated by environmental factors, including temperature, precipitation, land use, and habitat characteristics. Temperature exerts a particularly powerful influence on transmission dynamics by affecting mosquito vector competence, viral replication rates within vectors, and the duration of the extrinsic incubation period. Experimental infection of European Culex mosquitoes with WNV demonstrated that transmission occurs only at incubation temperatures of 24°C or 27°C, with highest transmission efficiency rates observed at the higher temperature range [27]. This temperature dependence explains the strong seasonality of WNV transmission in temperate regions, where most infections occur between July and October [34, 41], and the northward expansion of the virus under climate warming scenarios [5, 24].

The relationship between temperature and WNV incidence has been quantified in human epidemiological studies that are relevant to bird-mosquito transmission cycles. In the United States during 2001–2005, a 5°C increase in mean maximum weekly temperature was associated with a 32–50% higher incidence of WNV infection [24]. European outbreaks have similarly been linked to unusually warm summers: the first WNV cases in Germany in 2018 coincided with an exceptionally warm summer that created ideal conditions for viral replication in mosquitoes [39], and the 2022 outbreaks in Italy followed an unprecedented spring heatwave [32]. In Ontario, Canada, temperature, precipitation, and crow abundance were positively correlated with human WNV cases, while robin abundance showed a negative correlation, suggesting that the relative abundance of different avian species modulates spillover risk [49].

Land use and agricultural practices significantly influence WNV exposure risk in bird populations. In Spain, Casades-Martí et al. [4] found that flavivirus exposure in wild birds was higher on farms (9%) than in neighboring areas (5.6%) or wild areas (5.8%), and that bird diversity was the most relevant predictor of exposure risk. The dominance of passerines in on-farm bird communities, combined with the availability of peridomestic habitats that support mosquito breeding, creates conditions that maximize virus amplification [4]. At continental scale, phylogeographic analyses have demonstrated that agricultural land use, including coverage of cropland, pasture, cultivated vegetation, and livestock density, is positively associated with both the direction and velocity of WNV spread in Europe [15]. This suggests that agricultural landscapes function as ecological amplifiers that sustain higher transmission rates, potentially by providing abundant food resources for passerine birds and breeding habitats for mosquitoes.

Wetlands and river basins represent particularly important ecological settings for WNV transmission in birds. In the Iberian Peninsula, the spatial patterns of favorable areas for WNV outbreaks correspond to major hydrographic basins, and outbreaks tend to start in lower parts of river basins before spreading upstream [45, 47]. Wetlands of international importance for birds account for WNV detection across all animal components (reservoirs, vectors, and dead-end hosts) in Africa [51]. The Danube Delta Biosphere Reserve in Romania, a major stopover site for migratory birds traveling between Europe and Africa, has repeatedly demonstrated evidence of WNV circulation in wild birds, with seropositivity rates of 11.8% detected in 2016 [22, 42]. These wetland ecosystems concentrate both avian hosts and mosquito vectors, creating ecological hotspots for WNV amplification and dissemination.

Seasonal Dynamics and Overwintering Mechanisms

WNV transmission in temperate regions exhibits pronounced seasonality, with most infections occurring during the summer and early autumn months when mosquito populations peak and temperatures support viral replication. In the Emilia-Romagna region of northern Italy, testing of 22,314 corvids over 11 years revealed that WNV is generally first detected in birds in July, with sample prevalence peaking between August and September, while human cases peak in August [6]. This temporal pattern is consistent with observations from California, where peak WNV activity occurs from July through October in the Central Valley and southern California [34], and from across Europe, where most human cases are reported between June and October [41].

The ability of WNV to persist through winter months when mosquito activity is minimal or absent is a critical determinant of viral endemicity. Several overwintering mechanisms have been proposed, including vertical transmission in mosquitoes, persistence in chronically infected birds, and direct bird-to-bird transmission. Experimental evidence has demonstrated that Culex pipiens pallens can preserve WNV under low-temperature conditions simulating overwintering, with rapid viral replication upon temperature increase and subsequent transmission to susceptible animals during blood feeding [48]. This mechanism could allow infected overwintering mosquitoes to serve as a viral reservoir that initiates transmission in spring.

Direct evidence for bird-to-bird transmission during winter has emerged from case reports in Italy. In January 2022, WNV lineage 2 was detected in a goshawk in Umbria, Italy, during the cold season when mosquitoes are usually inactive, suggesting that a non-vector transmission route, potentially oral transmission through predation, supported viral maintenance [37]. This observation is consistent with experimental data demonstrating that WNV can be transmitted through oral and contact routes in multiple avian species [3]. Alternative overwintering mechanisms include transmission through chronically infected birds that maintain viral RNA in tissues for extended periods; Komar et al. [3] found persistent WNV infections in tissues of 16 surviving birds following experimental infection. Additionally, the potential role of non-avian hosts such as American alligators in WNV overwintering has been demonstrated experimentally, with alligators capable of developing viremias sufficient to infect mosquitoes and transmitting virus through water [52].

Co-Circulation with Other Flaviviruses

WNV frequently co-circulates with Usutu virus (USUV), another flavivirus transmitted by Culex mosquitoes and amplified by birds, particularly in Europe. In Germany, a nationwide bird surveillance network has documented the co-circulation of both viruses, with USUV present nationwide while WNV has been restricted primarily to the central-east [1, 26]. Co-infections with WNV and USUV have been detected in birds, with genomic sequencing revealing WNV lineage 2 and USUV lineages Africa 2, Africa 3, and Europe 2 in co-infected individuals [2]. The immunological cross-reactivity between these viruses complicates serological surveillance, as antibodies to one virus can neutralize the other, leading to potential misclassification of exposure [30]. However, serological evidence from zoological gardens in Germany has demonstrated that individual birds can mount detectable neutralizing antibody responses to both viruses simultaneously [2].

The epidemiological significance of co-circulation remains incompletely understood. One hypothesis is that prior infection with one flavivirus may provide partial cross-protection against the other, modulating transmission dynamics. Alternatively, the presence of multiple flaviviruses in the same avian-vector system could increase the overall force of infection, potentially driving higher population-level immunity that dampens outbreak intensity. Understanding these interactions is critical for interpreting serosurveillance data and predicting outbreak risk, particularly in regions like southern Europe where both viruses are expanding their geographic ranges [13].

Surveillance Systems and Early Warning

Wild bird surveillance has proven to be a highly effective early warning system for WNV detection, often preceding human case detection by weeks or months. In the Emilia-Romagna region of Italy, active surveillance of corvids detected WNV circulation before the onset of human cases in 72.1% of epidemic seasons over 11 years, while virus was first or only notified in humans in only 27.8% of seasons [6]. The efficacy of bird surveillance depends critically on sampling intensity and timing: testing a minimum of 3.8 corvids per 100 km² provides satisfactory timeliness, but early detection requires sampling between mid-June and mid-August, before the peak of human cases [6].

Sentinel bird programs using domestic birds, particularly chickens, have been implemented in multiple countries as cost-effective surveillance tools. In California, over 7,340 sentinel chickens were tested for WNV antibodies from 2003–2018, with annual enzootic detection typically preceding detection in humans and prompting enhanced vector control interventions [34]. In Madagascar, domestic birds (ducks, chickens, geese, turkeys, and guinea fowl) showed seroprevalence rates of 29.4% in one district and 16.7% in another, highlighting the utility of poultry as surveillance sentinels in peridomestic settings [46]. Similarly, serosurveillance of domestic birds in Morocco detected WNV neutralizing antibodies in 2.2% of samples, confirming active circulation in a region where human cases had been reported [44].

The predictive power of bird surveillance extends to anticipating human outbreak risk. In Germany, where WNV has established endemic circulation in eastern regions since 2018, seroprevalence in birds reached over 20% in the endemic area by 2022, with serological data indicating expanding circulation westward and southward [1]. This pattern suggests that human infection risk is increasing and that wild bird monitoring serves as a capable early warning system within a One Health framework [1]. The World Health Organization (WHO) and the European Centre for Disease Prevention and Control (ECDC) have recognized the importance of integrating animal surveillance data with human case reporting to improve outbreak preparedness and response [41, 50].

Emerging Threats and Future Directions

The geographic range of WNV in avian populations continues to expand, driven by climate change, land use modification, and viral evolution. In Europe, the 2018 transmission season was unprecedented in scale, with 1,993 human cases reported

Diagnostic Approaches: Serological and Molecular Detection of West Nile Virus in Birds

The accurate and timely detection of West Nile virus (WNV) in avian populations is the cornerstone of effective surveillance, outbreak prediction, and the implementation of control measures within a One Health framework. Given that birds serve as the primary amplifying hosts for WNV, their infection status provides the earliest indication of viral circulation, often preceding human and equine cases by weeks [6, 34]. The diagnostic landscape for WNV in birds is bifurcated into two principal methodologies: serological assays, which detect the host’s immune response to the virus, and molecular techniques, which identify the viral genome itself. The selection and interpretation of these assays are profoundly influenced by the transient nature of viremia in many avian species, the extensive antigenic cross-reactivity within the Flaviviridae family, and the specific objectives of the surveillance program, whether it be detecting acute infection, historical exposure, or viral lineage characterization [30].

Serological Detection: Uncovering Historical and Recent Exposure

Serology remains the most widely employed diagnostic approach for large-scale avian surveillance due to its relative cost-effectiveness, ability to process high sample volumes, and capacity to detect past exposure even after viremia has cleared. The cornerstone of serological screening is the enzyme-linked immunosorbent assay (ELISA), which has been adapted for use in taxonomically diverse avian species [53]. The indirect immunoglobulin G (IgG) ELISA, as pioneered by Ebel et al., has proven invaluable for detecting anti-WNV antibodies across a broad range of bird orders, demonstrating its utility in epizootiological studies [53]. However, the most commonly used format in contemporary surveillance is the competitive ELISA (cELISA), often referred to as a blocking ELISA. This assay uses a monoclonal antibody that competes with avian antibodies for a specific viral epitope, offering the significant advantage of being species-independent, a critical feature when sampling a wide variety of wild birds [4, 44, 55, 56]. The ID Screen® West Nile Competition Multi-species ELISA (ID VET) is a prominent commercial example, widely used in studies from Malaysia to Morocco [44, 55, 56].

While cELISA provides a high-throughput screening method, its primary limitation is the potential for cross-reactivity with other flaviviruses, particularly Usutu virus (USUV), Japanese encephalitis virus (JEV), and St. Louis encephalitis virus, which share antigenic epitopes [16, 30]. This is a critical challenge in regions where multiple flaviviruses co-circulate, such as Europe (WNV and USUV) and Southeast Asia (WNV and JEV) [1, 2, 16]. Consequently, positive results from screening ELISAs must be confirmed by the gold-standard virus neutralization test (VNT), also known as the micro-virus neutralization test or focus reduction neutralization test (FRNT) [4, 16, 22, 59]. The VNT is highly specific, as it measures the functional ability of antibodies to neutralize live virus, thereby differentiating between closely related flaviviruses. For instance, in a study in Cambodia, initial flavivirus-positive sera from domestic birds were subjected to FRNT, revealing that while many had neutralizing antibodies (nAb) against JEV, a distinct subset (10.7%) had nAb exclusively against WNV, providing the first serological evidence of WNV circulation in the region in decades [16]. Similarly, in Germany, where WNV and USUV co-circulate, VNT is indispensable for distinguishing between antibodies elicited by these two viruses, with studies showing that a significant proportion of seropositive birds have nAb against both, indicating co-exposure or cross-reactivity [2, 10, 26].

The interpretation of serological data is nuanced and depends on the immunoglobulin class detected. The presence of IgG antibodies generally indicates past infection and can persist for years, making it a marker of historical exposure and endemic circulation [54, 59]. In contrast, the detection of IgM antibodies is indicative of recent or active infection, as IgM responses appear early and wane more rapidly. However, commercial IgM capture ELISAs for birds are less standardized than IgG assays [59]. Seroprevalence studies have been instrumental in identifying key epidemiological patterns. For example, studies in Illinois found that captive and urban birds had higher seropositivity than those from natural areas, suggesting that peri-domestic environments facilitate transmission [54]. In Germany, serological data from wild birds have been crucial for mapping the westward and southward expansion of WNV from its initial endemic focus in the east, with seroprevalence rates exceeding 20% in some endemic regions by 2022, serving as an early warning system for human exposure risk [1, 26]. Furthermore, the detection of WNV nAb in resident and short-distance migratory birds in eastern Germany in 2018, prior to the detection of WNV RNA, provided the first signs of local viral circulation [43]. The use of sentinel birds, such as corvids in Italy, has demonstrated that active serological surveillance can detect WNV circulation weeks before the onset of human cases, with a minimum testing density of 3.8 corvids per 100 km² recommended for timely detection [6].

Molecular Detection: Direct Identification of Acute Infection

Molecular diagnostics, primarily reverse transcription polymerase chain reaction (RT-PCR), offer the advantage of directly detecting viral RNA, thereby confirming active infection and enabling viral characterization through sequencing. This is critical for identifying the specific viral lineage (e.g., lineage 1 vs. lineage 2) and tracking its phylogeographic spread [15, 19, 36, 55]. Real-time RT-PCR (RT-qPCR) is the most common molecular platform, providing high sensitivity, specificity, and quantification of viral load [22, 43, 57]. These assays typically target conserved regions of the viral genome, such as the envelope (E) gene or the non-structural protein 5 (NS5) gene [55, 57]. The primary limitation of molecular detection in birds is the short and often low-titer viremic period. In many passerine species, viremia peaks within a few days post-infection and clears rapidly, creating a narrow window for RNA detection [3, 33]. This explains why many surveillance studies report low molecular prevalence rates even when seroprevalence is high. For instance, in a study in Spain, only 0.8% of birds were PCR-positive for flaviviruses, whereas 6.9% were seropositive [4]. Similarly, in the Danube Delta, despite serological evidence of WNV circulation (11.8% by ELISA), all mosquito pools tested negative for WNV RNA, highlighting the challenge of detecting the virus during inter-epidemic periods [22].

Despite this limitation, molecular detection is indispensable for confirming outbreaks and characterizing emergent strains. The first detection of WNV in Germany in 2018 was achieved via RT-PCR in birds, marking the virus's introduction into the country [39, 43]. Subsequent molecular surveillance has revealed the dominance of WNV lineage 2 in central Europe, with a specific subcluster accounting for 95% of generated sequences in Germany [1]. In Italy, molecular analysis of bird tissues has been pivotal in documenting the re-emergence of WNV lineage 1 after a decade-long absence and its co-circulation with lineage 2, with evidence suggesting that lineage 1 may be associated with a higher risk of neuroinvasive disease in humans [36, 58]. The detection of WNV RNA in organs from dead birds, particularly the brain, heart, and kidney, is a highly sensitive method for post-mortem diagnosis and is a cornerstone of passive surveillance programs [3, 25, 26, 37]. For example, in the German national surveillance network, over 2900 bird carcasses were tested, providing crucial data on the geographic spread and lineage distribution of both WNV and USUV [26]. The ability to sequence viral RNA from these samples allows for detailed phylogenetic analyses, revealing introduction events and transmission pathways. Studies have shown that WNV lineage 2 strains in Europe likely originated from Central Europe and spread at a velocity of 88–215 km/year, correlating with bird movement patterns [15]. Furthermore, molecular detection in oropharyngeal and cloacal swabs has demonstrated that birds can shed WNV orally and cloacally, providing a potential route for direct (non-vector) transmission, which may be particularly important for overwintering and spread among raptors [3, 37].

Integrated Diagnostic Strategies and the Challenge of Co-Infections

Given the complementary strengths and weaknesses of serological and molecular methods, contemporary surveillance programs invariably employ an integrated approach. A typical strategy involves initial screening of blood samples by cELISA, followed by confirmation of positives with VNT, and parallel testing of all samples (or a subset) by RT-qPCR to detect acute infections [1, 26, 43]. This dual approach maximizes the probability of detecting both recent and historical infections. The challenge of co-infections, particularly with WNV and USUV, has become increasingly apparent in Europe. Studies in Germany have documented co-infections in dead birds, with WNV lineage 2 and USUV lineages Africa 2, Africa 3, and Europe 2 detected simultaneously [2]. Serologically, this presents a diagnostic dilemma, as birds can have high neutralizing antibody titers against both viruses, making it difficult to determine which infection occurred first or which is currently active [2, 10]. Advanced molecular techniques, such as next-generation sequencing (NGS) and lineage-specific RT-PCR assays, are essential for resolving these co-infections and accurately tracking the dynamics of each virus [2, 58].

The choice of sample type is also critical. For live birds, whole blood or serum is used for both serology and RNA extraction, while for deceased birds, organ homogenates (brain, heart, kidney, spleen) offer the highest sensitivity for RNA detection [3, 26, 37]. The World Organisation for Animal Health (WOAH) and the Centers for Disease Control and Prevention (CDC) provide standardized protocols for these diagnostic procedures, emphasizing the need for validation in avian species. The development of reverse genetics systems for WNV, such as those for the Kunjin subtype, has further advanced diagnostic capabilities by providing tools for generating reporter viruses and pseudoviruses, which can be used in highly specific neutralization assays and antiviral screening without the need for live, highly pathogenic virus [17]. In conclusion, the robust diagnosis of WNV in birds requires a sophisticated, multi-pronged laboratory approach that integrates high-throughput serological screening with highly specific confirmatory tests and sensitive molecular detection, all interpreted within the context of flavivirus ecology and host biology. This integrated framework is not merely an academic exercise but a practical necessity for providing the actionable intelligence required to protect both avian and human health.

Transmission Ecology: Vector-Host Dynamics and Environmental Drivers

The transmission ecology of West Nile virus (WNV) represents one of the most complex and multifaceted vector-borne disease systems known to contemporary infectious disease ecology. As an orthoflavivirus maintained primarily within an enzootic cycle between ornithophilic mosquito vectors and avian reservoir hosts, WNV exhibits remarkable plasticity in its transmission dynamics, influenced by a constellation of biotic and abiotic factors operating across multiple spatiotemporal scales [5, 9, 13]. The virus’s successful establishment across temperate, subtropical, and tropical ecosystems, from its initial isolation in Uganda in 1937 to its pandemic emergence in the Western Hemisphere in 1999, underscores the extraordinary adaptive capacity of its transmission machinery [5, 13, 18]. A comprehensive understanding of these dynamics requires integration of vector bionomics, avian host competence, environmental forcing, and the emergent properties of their interactions.

Vector-Host Dynamics: The Core Enzootic Cycle

Mosquito Vector Assemblages and Their Biting Behavior

The fundamental transmission unit of WNV is the mosquito-bird-mosquito cycle, with mosquitoes of the genus Culex serving as the primary vectors globally [5, 13, 20]. However, the specific vector species involved vary dramatically across geographic regions, each with distinct ecological traits that shape transmission intensity. In North America, Culex pipiens (the northern house mosquito), Culex tarsalis, and Culex quinquefasciatus constitute the primary enzootic vectors, with Cx. pipiens exhibiting a pronounced ornithophilic feeding preference that maintains the avian-mosquito cycle [20, 21]. Critically, the host-feeding patterns of these mosquitoes are not fixed; they demonstrate considerable plasticity influenced by host availability, with populations shifting from primarily avian to mammalian feeding during late summer when bird fledglings disperse and mosquito densities peak, thereby facilitating spillover to humans and horses [5, 20]. This behavioral plasticity is central to understanding the bridge vector capacity that links enzootic cycles to epidemic transmission.

In Europe, the vector landscape is dominated by Culex pipiens biotype pipiens and the morphologically similar but ecologically distinct Culex torrentium [27]. Experimental infection studies have demonstrated that Cx. torrentium exhibits substantially higher transmission efficiency for WNV than Cx. pipiens under identical temperature regimes, with transmission efficiency rates reaching 17% at 24°C and 24% at 27°C, compared to a maximum of only 3% for Cx. pipiens biotypes [27]. This finding has profound implications for Central and Northern Europe, where Cx. torrentium is the dominant ornithophilic mosquito, suggesting that these regions may possess greater vector competence for WNV than previously recognized [27]. In southern Europe, Culex perexiguus emerges as the primary enzootic vector, particularly in Mediterranean wetlands and agricultural landscapes, where it maintains intense transmission cycles among passerine birds [29, 60]. The 2020 outbreak in Andalusia, Spain, was preceded by a dramatic increase in Cx. perexiguus abundance, with WNV-positive mosquito pools detected approximately one month before the first human cases were identified [29]. This species’ pronounced ornithophily and its abundance in natural and agricultural areas make it the principal driver of enzootic amplification, while Culex pipiens serves as the bridge vector in urban settings once the enzootic cycle has been established [29].

Avian Host Competence and Reservoir Potential

Birds are the definitive amplifying hosts for WNV, but enormous variation exists across avian taxa in their capacity to sustain transmission [3, 33, 40]. Reservoir competence, a composite measure incorporating viremia magnitude, duration, and the probability of transmission to feeding mosquitoes, varies by orders of magnitude among bird species [3, 40]. Experimental infection studies of 25 North American bird species revealed that passeriform and charadriiform birds exhibited the highest reservoir competence, with the five most competent species being Blue Jays (Cyanocitta cristata), Common Grackles (Quiscalus quiscula), House Finches (Carpodacus mexicanus), American Crows (Corvus brachyrhynchos), and House Sparrows (Passer domesticus) [3]. These species develop viremias exceeding 10⁵ plaque-forming units (PFU)/mL of serum, the threshold considered sufficient to infect feeding mosquitoes, and maintain these titers for 4–7 days [3, 33]. In contrast, many non-passerine species, including Columbiformes (pigeons and doves) and Galliformes (pheasants and chickens), develop substantially lower viremias and contribute less to transmission [3, 33].

The ecological importance of particular avian species in driving transmission is not solely determined by their intrinsic competence but also by their abundance, longevity, and interactions with mosquito vectors. In North America, the American Robin (Turdus migratorius) has been identified as the keystone reservoir species, despite having moderate individual-level competence [21, 49]. Robins are highly abundant, widely distributed, and preferentially fed upon by Culex mosquitoes, resulting in their outsized contribution to transmission [21]. Mathematical modeling of WNV dynamics demonstrates that seasonal outbreaks are more influenced by the migratory robin population than by highly competent but less abundant resident species like American Crows [21]. This finding has profound implications for surveillance: although crow die-offs are sensitive indicators of viral activity, the ecological drivers of transmission are better captured by monitoring abundant, moderately competent species that sustain the majority of mosquito infections [3, 21].

In Europe, the Blackbird (Turdus merula) appears to play an analogous role to the American Robin. Following the 2020 outbreak in Andalusia, serological surveys revealed a high prevalence of WNV antibodies in blackbirds sampled within affected villages, suggesting this species was centrally involved in urban amplification [29]. Similarly, in Germany, the rapid expansion of WNV lineage 2 since 2018 has been associated with high seroprevalence in several passerine species, with over 20% of sampled birds in the endemic eastern region carrying neutralizing antibodies by 2022 [1, 26]. The detection of WNV RNA in House Sparrows, Eurasian Magpies, and raptors has confirmed the role of these species in sustaining local transmission [1, 38, 43].

Environmental Drivers of Transmission Intensity

Temperature-Dependent Transmission Dynamics

Temperature is arguably the single most important environmental driver of WNV transmission, exerting effects on virtually every component of the transmission cycle [5, 24, 27]. The virus’s extrinsic incubation period (EIP), the time required from mosquito infection to the development of infectious viral titers in the salivary glands, is highly temperature-dependent. At temperatures below 21°C, WNV replication in Culex mosquitoes is severely constrained, and transmission to vertebrate hosts becomes unlikely [27]. Experimental studies with German Culex populations demonstrated that WNV was detected in mosquito saliva only at incubation temperatures of 24°C or 27°C, and predominantly after an extended extrinsic incubation period of 21 days [27]. These temperature thresholds explain the geographic restriction of intense WNV transmission to regions where summer temperatures consistently exceed 21°C, such as the Mediterranean Basin, the Danube Delta, and the Central Valley of California [5, 27].

Climate change is altering these thermal regimes, expanding the geographic envelope within which WNV can circulate at high intensity [5, 24, 41]. A seminal analysis of 16,298 WNV cases in the United States during 2001–2005 demonstrated that a 5°C increase in mean maximum weekly temperature was associated with a 32–50% increase in WNV infection incidence [24]. In Europe, the unprecedented heatwave of 2018 created conditions for WNV to establish endemic circulation in Germany for the first time, with the virus detected in birds, horses, and humans in regions that had previously been considered non-endemic [1, 26, 39]. The exceptionally warm summer allowed mosquitoes to reach high population densities and shortened the EIP sufficiently to permit sustained transmission [39]. Similarly, the early onset of WNV transmission in Italy during 2022, with detection in mosquitoes by early June, was attributed to an early-season heatwave that accelerated mosquito population growth and viral replication [32, 50]. This temporal shift has critical public health implications, as it extends the transmission season and increases cumulative exposure risk [41].

Precipitation, Drought, and Hydrological Regimes

The relationship between precipitation and WNV transmission is complex and non-linear, mediated through effects on mosquito breeding habitat and host community structure [24, 47]. In temperate regions, the highest transmission risk often occurs following periods of drought, interspersed with heavy rainfall events [24]. Drought conditions concentrate bird and mosquito populations around remaining water sources, increasing contact rates and amplifying transmission [24, 49]. Moreover, drought-induced organic enrichment of standing water in urban catch basins and drainage systems creates ideal larval habitat for Culex mosquitoes, which are highly adapted to organically polluted water [24]. This phenomenon explains the paradoxical observation that WNV outbreaks frequently intensify during dry summers in North America and Europe [24, 49].

At larger spatial scales, river basins and wetlands emerge as powerful determinants of WNV distribution [15, 45, 47, 51]. In the Iberian Peninsula, human WNV cases during the 2020 outbreak were concentrated along the Guadalquivir River basin, with spatial analysis revealing that the most favorable areas for transmission correspond to the major hydrographic basins of Portugal and Spain [45]. Similarly, in Europe more broadly, river basins were identified as the fundamental operational geographic units for WNV management, with outbreaks typically initiating in lower basin reaches and propagating upstream [47]. These patterns reflect the congregation of both avian hosts and mosquito vectors in riparian habitats, where water availability, high primary productivity, and abundant perching sites create optimal conditions for transmission [47, 51]. In Africa, wetlands of international importance for waterbirds were identified as the primary environmental driver of WNV presence across all components of the transmission cycle, reservoirs, vectors, and dead-end hosts, highlighting the conservation-epidemiology nexus [51].

Land Use, Agriculture, and Anthropogenic Landscape Modification

Land use transformation profoundly influences WNV transmission by altering host and vector community composition and increasing contact rates [4, 15, 47]. Agricultural landscapes, particularly those supporting livestock, create high-risk environments for WNV amplification [4, 15]. Phylogeographic analyses of WNV in Europe have demonstrated that factors related to agricultural intensity, including cropland coverage, pasture extent, managed vegetation, and livestock density, are positively associated with both the direction and velocity of viral spread [15]. This relationship is multifaceted: livestock provide abundant blood meals for mosquitoes, increasing mosquito population densities; agricultural irrigation creates stable larval habitats; and farm environments attract high densities of granivorous passerine birds that serve as competent reservoirs [4, 15].

The gradient of wildlife-livestock interaction is a critical determinant of WNV exposure risk [4]. Studies in Mediterranean ecosystems found that wild bird seroprevalence was highest in farm environments (9%), intermediate in adjacent natural areas (5.6%), and lowest in more remote wild areas (5.8%) [4]. Passerine birds dominated the on-farm bird community, and bird diversity emerged as the most relevant predictor of exposure risk [4]. This finding suggests that agricultural intensification, by simplifying bird communities and concentrating competent reservoir species, may increase transmission efficiency. Importantly, the implementation of management practices that deter passerines from farm environments, such as exclusion netting or habitat modification, has been proposed as a cost-effective strategy to reduce WNV amplification risk in agricultural landscapes [4].

Urbanization creates distinct transmission dynamics [19, 49, 54]. In Illinois, captive and urban birds exhibited higher WNV seropositivity than birds from natural areas, indicating that peridomestic settings concentrate transmission [54]. However, the relationship is nuanced: in Spain, WNV neutralizing antibodies were detected in House Sparrows captured in rural and natural areas but not in urban centers, suggesting that the enzootic cycle was primarily maintained outside dense human settlements [60]. Nevertheless, once amplification occurs in rural or peri-urban areas, bridge vectors can introduce the virus into urban settings, as demonstrated in Berlin, where autochthonous WNV lineage 2 established an endemic cycle in the city, with infected Culex pipiens mosquitoes and human cases documented across multiple years [19].

Migratory Birds as Drivers of Geographic Spread

Intercontinental Virus Translocation

Bird migration represents the primary mechanism for WNV introduction into new geographic regions, enabling the virus to bridge continents and traverse ecological barriers that would otherwise restrict its range [7, 18, 28, 31]. The Afro-Palaearctic flyway, connecting sub-Saharan Africa with Europe through the Mediterranean Basin, has been implicated in the repeated introduction of WNV strains into southern Europe [7, 31, 35]. Phylogenetic analyses of WNV lineage 2 in Europe demonstrate that the virus was independently introduced from Africa into the Danube Delta and into Greece during the 1990s and early 2000s, with subsequent spread facilitated by avian movements [15, 35, 42]. The role of migratory birds in these introductions is supported by the detection of WNV neutralizing antibodies in long-distance migrants sampled upon their arrival in Europe, as occurred in Greece prior to the 2010 outbreak, when migratory birds showed evidence of past exposure before local transmission was detected [35].

The timing of migration relative to virus transmission is critical for successful introduction [21, 28]. Modeling studies of northward migration from southern Europe to Great Britain demonstrate that the probability of an infected migratory bird arriving while still viremic is highly dependent on the force of infection at the departure site and the duration of the migratory flight [28]. If WNV is circulating in the Camargue region of France during a single migratory season, the probability that at least one infected bird reaches Great Britain while still infectious is 0.881 [28]. However, if WNV were to establish further north in France, in wetland areas such as the Grand Brière National Park or La Brenne Regional Park, the model predicts that at least one infected bird will continue to Great Britain annually, substantially increasing the risk of introduction [28]. These findings underscore the dynamic nature of risk, which evolves as the virus expands its geographic range.

Within Europe, the expansion of WNV lineage 2 has been characterized by high dispersal velocities ranging from 88 to 215 km per year, which are correlated with bird movement patterns [15]. The spread from Central Europe, particularly from Hungary and Austria,

Clinical Manifestations, Pathology, and Species-Specific Susceptibility in Birds

The clinical presentation of West Nile virus (WNV) infection in birds spans an exceptionally broad spectrum, ranging from completely subclinical infections to peracute mortality, with the outcome profoundly influenced by viral lineage, host species, age, and environmental co-factors. Understanding this diversity is not merely an academic exercise; it is fundamental to designing effective surveillance programs, predicting outbreak emergence, and elucidating the ecological drivers of viral perpetuation across the enzootic cycle.

Clinical Manifestations: A Spectrum from Subclinical to Peracute Mortality

A vast majority of WNV infections in wild birds are clinically inapparent. Serosurveys across multiple continents consistently reveal high seroprevalence rates in apparently healthy individuals, particularly in long-established endemic regions. For instance, in the German endemic region, over 20% of birds carried neutralizing antibodies against WNV without evidence of disease [1], while in the Danube Delta of Romania, 11.8% of wild birds were seropositive by ELISA, with many being juveniles captured during active banding operations [22]. Similarly, seroprevalence rates of 18.7% were documented in wild birds in Malaysia, again in the absence of any observed clinical disease during trapping [55]. These data underscore the efficiency with which many avian species can control WNV replication, mounting a robust humoral immune response that clears the virus without triggering overt pathology.

However, when clinical disease does manifest, it is often dramatic and highly species-dependent. The most frequently reported clinical signs in highly susceptible species, particularly members of the Corvidae family (e.g., American Crows, Blue Jays, Magpies), include profound lethargy, anorexia, weight loss, and an inability to fly or perch. Neurological signs are the hallmark of severe disease, reflecting the virus’s profound neurotropism. Affected birds frequently exhibit ataxia, head-tilt, torticollis, tremors, circling, disorientation, and seizures. In the landmark experimental infection study by Komar et al. [3], exposing 25 North American bird species to WNV via infectious mosquito bite, mortality was observed in eight species, with American Crows demonstrating a nearly 100% fatality rate. Affected crows in that study became moribund within 3–5 days post-infection, often dying without exhibiting any prodromal signs, a pattern of peracute death that has been repeatedly confirmed in field surveillance [3, 5, 25].

Raptors, including hawks, owls, and falcons, are another group known for pronounced clinical susceptibility. In Germany, WNV RNA was detected in birds of prey presenting with severe neurological dysfunction, including an adult female goshawk in Umbria, Italy, that was found unable to fly and died 15 days later after exhibiting progressive neurologic decline [37]. Birds of prey often display wing droop, paresis, and profound muscle wasting in addition to more classic encephalitic signs [14, 37]. Interestingly, co-infections with Usutu virus (USUV) have been documented in raptors and other birds, adding complexity to clinical presentations, as dual infections may exacerbate neurological disease or alter immune kinetics [2, 26]. The clinical picture in snowy owls co-infected with WNV and USUV in German zoos included profound weakness and neurological deficits, though neutralizing antibody titers against both viruses were detected in survivors [2].

Passerines such as House Sparrows, House Finches, and Common Grackles typically exhibit less severe clinical signs but can still succumb to infection. In the experimental setting, House Sparrows showed a transient period of illness characterized by ruffled feathers and inactivity, but mortality was variable (approximately 40–50% in some studies) [3]. Importantly, many passerines that survive the acute phase of infection may harbor persistent virus in tissues, a phenomenon that has significant implications for viral maintenance and potential reactivation [3, 33].

One of the most critical, yet often overlooked, clinical findings is the shedding of virus. Komar et al. [3] demonstrated that cloacal shedding occurred in 17 of 24 bird species experimentally infected, and oral shedding was detected in 12 of 14 species. This observation is profoundly important for two reasons. First, it suggests that horizontal transmission, both direct and via environmental contamination, may be a more significant route of spread than currently appreciated, particularly in dense roosting or nesting colonies. Second, it implies that surveillance programs relying solely on blood or organ samples from dead birds may underestimate the prevalence of active infection. The detection of WNV RNA in oropharyngeal swabs from wild birds in Malaysia (15.2% molecular prevalence) confirms that oral shedding is a real-world phenomenon that can be leveraged for non-invasive surveillance [55].

Pathological Findings: From Gross Lesions to Molecular Pathogenesis

The pathology of WNV in birds is dominated by a triad of findings: encephalitis, myocarditis, and visceral necrosis, all of which reflect the virus’s ability to infect and destroy a wide range of cell types.

At gross necropsy in highly susceptible species like American Crows, the most consistent finding is a pale, mottled appearance of the myocardium, often accompanied by splenomegaly and hepatomegaly. The brain may appear grossly normal or slightly congested, but the histopathological lesions are unmistakable. Microscopic examination reveals a non-suppurative encephalitis and meningoencephalitis, characterized by perivascular cuffing with mononuclear cells (primarily lymphocytes and macrophages), microglial nodule formation, and neuronal necrosis [25, 37]. The virus exhibits a particular tropism for the brainstem, cerebellum, and thalamus, which explains the characteristic head-tilt, ataxia, and tremors observed clinically. WNV antigen can be demonstrated in neurons and glial cells by immunohistochemistry, confirming direct viral cytopathology as the primary driver of neurological dysfunction [25].

Cardiac pathology is equally prominent and likely a major contributor to death in peracute cases. Myocarditis with myofiber degeneration, necrosis, and a mixed inflammatory infiltrate is frequently observed [25]. This can lead to arrhythmias and acute heart failure, which may explain the rapid demise of birds that seem healthy one day and are found dead the next. The heart is also a known site of persistent infection; Komar et al. [3] isolated virus from the hearts of surviving birds up to 28 days post-infection, suggesting that the myocardium may act as a sanctuary site for the virus.

Visceral pathology includes severe necrosis of the spleen and kidney. Splenic necrosis can be widespread, disrupting the organ’s immune architecture and likely contributing to immunosuppression. Renal involvement, characterized by tubular necrosis and interstitial inflammation, may lead to electrolyte imbalances and uremia, further complicating the clinical picture [25, 37]. Hepatic lesions, while less pronounced, can include multifocal necrosis and steatosis.

A critical aspect of WNV pathogenesis in birds is the role of the non-structural protein NS3. The New York 1999 strain (NY99) carries a specific mutation (NS3-249Pro) that has been experimentally linked to dramatically increased virulence in birds compared to ancestral Old World strains [5, 23]. This mutation enhances viral replication in avian cells, leading to higher viremia titers, more rapid systemic dissemination, and increased lethality. The presence of this mutation in strains isolated from Romania (1996) and Russia (1999), as well as the NY99 strain, suggests a convergent evolution event that has driven the emergence of hypervirulent lineages capable of causing mass mortality in bird populations [5]. More recent work by Fiacre et al. [23] using chimeric WNV strains demonstrated that the NS2B/NS3/NS4B/NS5 genomic region is a major determinant of virulence in mammals (mice), while the NS4B/NS5/3'UTR regions influence vector competence in Culex pipiens mosquitoes. This highlights the complex genetic interplay between virulence in different host types.

Another pathological hallmark is the potential for chronic sequelae. While acute death is dramatic, surviving birds may suffer long-term neurological deficits, including persistent ataxia or blindness, which render them vulnerable to predation and starvation [25]. The recognition that WNV can establish persistent infection in multiple organ systems, heart, kidney, brain, and spleen, raises the possibility of reactivation under stress (e.g., migration, food scarcity, extreme weather). This mechanism could be crucial for virus overwintering in temperate regions, as proposed for the goshawk found infected in January in Italy, when mosquito activity is absent [37].

Species-Specific Susceptibility and Reservoir Competence

The variation in susceptibility among avian species is not random; it is shaped by evolutionary history, ecological niche, and immune system architecture. Critically, the species that suffer the highest mortality are often not the most important for virus amplification. Reservoir competence is defined by the ability of a host to produce a viremia of sufficient magnitude and duration to infect feeding mosquitoes. In the seminal experimental study by Komar et al. [3], the five most reservoir-competent species were all passerines: Blue Jay, Common Grackle, House Finch, American Crow, and House Sparrow. The Blue Jay, in particular, generated extraordinarily high viremia titers (up to 10^10 PFU/mL), a level that would infect nearly any feeding mosquito.

Corvids (crows, jays, magpies, ravens) are universally recognized as highly susceptible and highly competent. Their dual status as sentinel species is invaluable for surveillance. In the Italian region of Emilia-Romagna, testing of over 22,000 corvids over 11 years demonstrated that WNV was detected first or only in birds in 72.1% of epidemic seasons, preceding human case detection in most scenarios [6]. This sentinel function is so reliable that mathematical models have explicitly incorporated crow abundance as a positive predictor of human case risk [49]. The high mortality rate in corvids, however, means that these populations can be severely depleted during outbreaks, potentially reducing local transmission in subsequent years, a phenomenon that has been modeled as a density-dependent feedback loop [61, 62].

Raptors constitute a second group of exceptionally high susceptibility and epidemiological relevance. As top predators, they may become infected through predation of viremic prey, oral transmission (consumption of infected carrion), in addition to mosquito bites [14, 37]. This oral route of infection is a unique feature that may accelerate viral spread within raptor populations and circumvent the need for vector activity. Clinical disease in raptors is frequently severe, with neurological signs prominent, and mortality can be high [14]. The detection of WNV in a goshawk in January 2022 in Italy, when temperatures precluded mosquito activity, strongly suggests that oral or direct transmission among raptors is a viable overwintering strategy [37]. Furthermore, raptors are known to migrate long distances, potentially introducing virus into new regions along the Afro-Palaearctic flyway [7, 14]. The seroprevalence in raptors is often elevated compared to other guilds, likely reflecting their high exposure risk through both vector bites and prey consumption.

Paradoxically, the American Robin (Turdus migratorius), which experiences minimal to no mortality from WNV infection, is considered the most ecologically important amplifying host in North America. Mathematical modeling by Bergsman et al. [21] demonstrated that migratory robins, with their low death rates and high population turnover, have a far greater impact on the basic reproductive number (R₀) than highly competent but mortality-prone corvids. The robin’s high abundance, frequent feeding, and strong association with peridomestic habitats make it a super-spreader species [5, 21]. Similarly, in the Iberian Peninsula, the Common Blackbird (Turdus merula) was identified as a key amplifying host in urban settings during the 2020 outbreak in Andalusia, with high antibody prevalence detected inside affected villages [29].

The role of domestic birds (poultry, ducks, geese) is variable and context-dependent. In general, chickens and turkeys are considered relatively resistant to clinical disease, developing low viremia and seroconverting without overt signs. This makes them ideal sentinel species; sentinel chicken flocks have been used extensively in California to provide early warning of WNV circulation [34]. However, ducks, especially mallards, can develop moderate viremia and may play a role in transmission. In Madagascar, a seroprevalence of 29.4% in domestic birds (ducks, chickens, geese, guinea fowl) was documented, and WNV RNA was detected directly in a chicken, demonstrating that domestic birds can be active participants in the enzootic cycle [46]. In Morocco, domestic birds showed a seroprevalence of 4.3% [44], and in Cambodia, ducks had significantly higher seropositivity than chickens (odds ratio 3.01), suggesting species-specific differences in exposure or susceptibility [16]. The use of poultry as a sentinel tool is particularly valuable in resource-limited settings where wild bird sampling is logistically challenging.

The influence of bird community diversity on WNV transmission is a critical macroecological pattern. A landmark study in southwestern Spain revealed that higher phylogenetic diversity in bird communities was associated with a lower amplification potential, consistent with the dilution effect hypothesis [8]. This finding suggests that species-rich avian communities, which include many incompetent or refractory hosts, may dampen transmission by diverting mosquito bites away from competent reservoirs. Conversely, simplified bird communities dominated by a few highly competent passerines (e.g., House Sparrows, European Starlings) can amplify virus to high levels [4, 8, 60]. This ecological principle has direct implications for land management: agricultural intensification and urbanization tend to reduce avian biodiversity, favoring generalist passerines that are often highly competent WNV hosts [4, 15]. The presence of wetlands and irrigated agricultural lands further compounds risk by providing ideal mosquito breeding habitat, creating a synergistic effect that drives outbreak intensity [15, 47, 51].

From a global surveillance perspective, the macroecological analysis by Tolsá et al. [40] reviewed data from 949 bird species across 39 countries and concluded that molecular prevalence and mortality rates are strong predictors of reservoir competence. This meta-analytical approach allows for the identification of potential new competent reservoirs in under-studied regions, a critical need given the constant threat of WNV emergence in naive areas, as exemplified by the recent detection of WNV lineage 1 in Germany and the Netherlands [5, 19, 26, 39].

In summary, the clinical and pathological presentation of WNV in birds is a complex interplay of viral genetics, host species biology, and ecological context. The extreme vulnerability of corvids and raptors makes them invaluable sentinels, while the silent amplification by robins, blackbirds, and sparrows drives the enzootic cycle. This nuanced understanding is the foundation upon which effective One Health surveillance is built, enabling public health authorities to predict spillover risk and implement timely vector control. The persistence of WNV in avian hosts across seasons and the potential for oral transmission in raptors further complicate eradication efforts and underscore the need for integrated, multidisciplinary approaches to disease management.

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