Pigeonpox Virus

Overview and Taxonomy of Pigeonpox Virus

Taxonomic Classification and Phylogenetic Context

Pigeonpox virus (PPV; also abbreviated PGPV) is a member of the genus Avipoxvirus within the subfamily Chordopoxvirinae of the family Poxviridae [6]. The genus Avipoxvirus comprises a diverse group of large, double‑stranded DNA viruses that infect over 300 avian species worldwide [4]. Within this genus, PPV is one of the most economically and ecologically significant species, alongside fowlpox virus (FWPV), canarypox virus, and turkeypox virus [6]. Phylogenetic analyses based on the highly conserved P4b gene (encoding the 4b core protein) have consistently placed PPV in clade A, subclade A2 [7, 8]. This subclade includes all pigeonpox viruses characterized to date, regardless of geographic origin, forming a monophyletic group distinct from fowlpox viruses (subclade A1) and other avipoxviruses [8]. For example, the Tanzanian PPV isolate from Morogoro region clusters within subclade A2, sharing a recent common ancestor with subclade A3 members [8]. Similarly, an Indian isolate (PPV/Pur-Od-4b/01/Ind) was shown by concatenated amino acid phylogeny to be closely related to a feral pigeonpox virus from South Africa [4]. The first complete genome sequencing of a Chinese PPV strain further confirmed its placement in type A2, with 100% identity to other PPV reference strains such as FeP2 [1, 2]. These phylogenetic data underscore the genetic homogeneity of PPV across continents, yet also reveal subtle genomic variations that are critical for understanding host range and virulence.

Genomic Architecture and Unique Features

The PPV genome is a linear, double‑stranded DNA molecule ranging from approximately 280 kbp to 281 kbp, with a GC content of about 29.5% [4]. The Indian complete genome (280,058 bp) contains 270 predicted open reading frames (ORFs) flanked by inverted terminal repeats (ITRs) of 4,689 bp at each terminus [4]. No recombination events were detected in this genome, suggesting a relatively stable genomic structure [4]. Microsatellite sequences are ubiquitously distributed throughout the genome, often located within functional genes, which may influence gene expression or serve as markers for molecular epidemiology [4]. A notable feature of PPV genomes is the frequent integration of nucleic acid sequences from reticuloendotheliosis virus (REV), an avian retrovirus [5, 11]. A Chinese PPV isolate (collected in Taiyuan, Shanxi Province) was found to harbor a full‑length REV genome of 7,942–8,005 bp, belonging to REV type III based on the gp90 gene [5]. In contrast, Egyptian PPV field strains were free of REV‑5′‑LTR integration, whereas co‑circulating FWPV isolates from the same region carried the insert [11]. This differential integration pattern suggests that REV acquisition is not universal in PPV and may depend on co‑infection dynamics or vector–host interactions. The presence of REV sequences can alter virulence, immunogenicity, and vaccine efficacy, posing challenges for disease control [5, 6, 11]. The complete genome of a Chinese PPV strain was also used to develop an attenuated vaccine candidate, highlighting the utility of genomic data for vaccine design [2].

Distinction from Other Avipoxviruses

Accurate taxonomic differentiation of PPV from FWPV and other avipoxviruses is essential for diagnosis, epidemiology, and vaccine development. Historically, differentiation relied on host species, clinical signs, and laborious cross‑protection tests. However, molecular tools now enable precise identification. A multiplex PCR assay targeting the fpv122 gene of FWPV and the HM89_gp120 gene of PPV has been developed, allowing rapid discrimination without sequencing [3]. This assay, combined with pan‑avipoxvirus primers, correctly identified nine clinical samples from chickens, pigeons, and a turkey, confirming the presence of PPV in pigeons and, notably, in one turkey [3]. Such cross‑species infections underscore the need for robust species‑specific diagnostics. Phylogenetic analysis of the P4b gene remains the gold standard for confirmation, as it reliably clusters PPV isolates within subclade A2 [7, 9]. The P4b amplicon (typically 578 bp) can be obtained from scab material or infected chorioallantoic membranes (CAM) after egg inoculation [7, 9]. Sequence identity among global PPV isolates is remarkably high, for example, a Ghanaian isolate showed 100% identity to the reference FeP2 strain [1], while a Tanzanian isolate was 99% identical to Indian, Egyptian, and other PPV sequences [8]. This high degree of conservation indicates a relatively recent common ancestor and limited genetic drift, yet the emergence of novel strains with REV integrations warrants continued surveillance.

Biological and Pathological Characteristics

PPV causes pigeonpox, a contagious disease manifested in two classical forms: cutaneous (dry) and diphtheritic (wet). The cutaneous form presents as nodular proliferative lesions on unfeathered skin (e.g., around the beak, eyelids, legs, and cloaca), whereas the diphtheritic form involves fibronecrotic plaques on the mucous membranes of the oral cavity, pharynx, esophagus, and respiratory tract [1]. Concurrent occurrence of both forms in the same bird, as reported in a 5‑month‑old female pigeon in Ghana, is not uncommon [1]. Histopathological examination of lesions reveals characteristic intracytoplasmic inclusion bodies known as Bollinger bodies, which are pathognomonic for poxvirus infection [1]. The virus replicates in the cytoplasm of epithelial cells, producing mature virions that are released upon cell lysis. Transmission occurs primarily via direct contact, aerosolized virus, or mechanical vectors such as biting insects (e.g., mosquitoes, mites), as described for FWPV [6]. Although specific vector studies for PPV are limited, the epidemiological patterns, with outbreaks often peaking during warm, humid seasons, support a role for arthropod vectors [6, 10]. The disease is associated with reduced growth, decreased egg production, and mortality, particularly in young or immunocompromised birds [2, 6].

Global Distribution and Epidemiological Significance

PPV has a worldwide distribution, with confirmed reports from Africa (Ghana [1], Tanzania [8], Egypt [11]), Asia (India [4], China [2, 5], Bangladesh [9]), and likely many other regions. In Bangladesh, a survey in Mymensingh division found 40% prevalence among pigeons with suspected pox lesions [9]. In China, the virus is endemic across multiple provinces, prompting the development of a homologous attenuated vaccine to replace less effective heterologous vaccines derived from FWPV [2]. The first complete genome from mainland China revealed that the circulating strains belong to type A2 and are closely related to global PPV isolates [2]. The index case in Ghana underscores the need for surveillance of both domestic and wild avian populations to anticipate future outbreaks [1]. Given the economic importance of the pigeon industry (meat, racing, fancy breeds) and the potential for wild birds to act as reservoirs, pigeonpox is a disease of significant concern. The World Organisation for Animal Health (WOAH) includes avian pox among the notifiable diseases in some contexts, though pigeonpox specifically may be subject to regional reporting requirements. Control strategies rely on biosecurity, vector management, and vaccination. However, repeated outbreaks in vaccinated flocks have been associated with emerging FWPV strains carrying REV inserts [6], and similar risks apply to PPV [5]. The development of a safe and immunogenic attenuated PPV vaccine, as demonstrated in China [2], is a critical step toward sustainable disease management. Multivalent vaccines incorporating other avian pathogens may also be feasible using PPV as a vector, given its large genome and abortive replication in mammalian cells [6].

Molecular Pathogenesis and Genomic Characterization of Pigeonpox Virus

Genomic Architecture and Unique Features

The pigeonpox virus (PPV) belongs to the genus Avipoxvirus within the subfamily Chordopoxvirinae of the family Poxviridae. Its genome is a large, linear double-stranded DNA molecule of approximately 280–300 kbp, enclosed by inverted terminal repeats (ITRs) that are essential for replication and packaging. Complete genome sequencing of an Indian isolate (PPV/Pur-Od-4b/01/Ind) revealed a total length of 280,058 bp with a guanine–cytosine (GC) content of 29.51%, flanked by ITRs of 4,689 bp at each terminus [4]. Annotation identified 270 putative open reading frames (ORFs), with no evidence of recombination events in this strain, suggesting a relatively stable genome architecture under natural conditions [4]. Similarly, the first complete genome from mainland China (strain PPV/China/2022) showed analogous features, confirming that PPV genomes are compact and densely packed with coding sequences, many of which are homologous to other avipoxviruses [2].

A defining molecular characteristic of avipoxviruses is the presence of the p4b core protein gene (also referred to as the fpv140 or P4b gene), which encodes a major structural component of the virus core. This gene is highly conserved among avipoxviruses and serves as the primary target for molecular detection and phylogenetic classification [1, 7, 8]. Amplification of a 578-bp fragment of the p4b gene by conventional PCR has been universally adopted for initial diagnosis and species identification [7, 9]. However, to differentiate PPV from the closely related fowlpox virus (FWPV) without sequencing, a multiplex PCR assay targeting species-specific genes has been developed. This assay amplifies the FPV122 gene unique to FWPV and the HM89_gp120 gene unique to PPV, allowing rapid discrimination during outbreaks [3]. Such molecular tools are critical for accurate epidemiological surveillance and vaccine matching, particularly in regions where multiple avipoxvirus species co-circulate, such as Bangladesh, Ghana, and Egypt [1, 9, 11].

Molecular Pathogenesis: From Entry to Dissemination

The molecular pathogenesis of PPV, like that of other avipoxviruses, is initiated by viral entry through abrasions or via mechanical transmission by arthropod vectors (e.g., mosquitoes and mites). Although the specific host receptor for PPV has not been fully characterized, chordopoxviruses generally utilise cell-surface glycosaminoglycans followed by fusion with the plasma membrane. Once inside the host cell, PPV replication occurs entirely in the cytoplasm, employing a virus-encoded DNA-dependent RNA polymerase that transcribes early, intermediate, and late gene classes. The virus produces two distinct infectious forms: the intracellular mature virion (IMV) and the extracellular enveloped virion (EEV), which facilitate cell-to-cell spread and long-distance dissemination within the host.

Clinically, PPV manifests in two forms: cutaneous (dry) and diphtheritic (wet). In the index case from Ghana, both forms were observed simultaneously in a single pigeon, with characteristic nodular lesions on the unfeathered skin (cutaneous) and necrotic plaques in the oral, pharyngeal, and respiratory mucosa (diphtheritic) [1]. The diphtheritic form is particularly severe, as the proliferative lesions can obstruct the airway and cause suffocation. Histopathologically, the presence of Bollinger bodies (intracytoplasmic eosinophilic inclusions) within hyperplastic epithelial cells is considered pathognomonic for avipoxvirus infection. These inclusions represent aggregates of mature virions (termed Bollinger bodies) and are easily identified under light microscopy [1]. The virus induces hyperkeratosis and acanthosis in the epidermis, while in the mucosal form, necrosis and secondary bacterial infections complicate the pathology.

The molecular basis for the differential tissue tropism (cutaneous vs. diphtheritic) is not yet fully understood, but is likely influenced by the expression of specific viral host-range genes and the immune status of the host. Comparative genomics of PPV strains from India and China have revealed the presence of genes encoding ankyrin-repeat proteins, serine/threonine kinases, and C-type lectin-like proteins, which are known in other poxviruses to modulate innate immunity, inhibit apoptosis, and counteract interferon responses [2, 4]. For instance, the PPV genome contains homologs of the vaccinia virus E3L and K3L genes, which interfere with protein kinase R (PKR) activation and interferon-induced antiviral states, thereby enabling replication in the face of host cytokine barriers.

Reticuloendotheliosis Virus Integration: A Molecular Boost for Pathogenicity

One of the most striking genomic features of many avipoxviruses, including PPV, is the frequent integration of full-length or partial proviral sequences of reticuloendotheliosis virus (REV), a gammaretrovirus that can cause immunosuppression and neoplasia in poultry. Molecular analyses have demonstrated that PPV field isolates from China, Egypt, and other regions often harbour integrated REV sequences of approximately 7.9–8.0 kbp between ORF201 and ORF203 of the PPV genome [5]. In the Taiyuan isolate from Shanxi, China, the integrated REV sequence was 100% identical to that found in co-circulating FWPV strains, suggesting horizontal gene transfer between avipoxvirus species or common ancestral integration [5]. Remarkably, this integration appears to be stable and can enhance the virulence and transmissibility of the recombinant virus. In FWPV, integration of REV has been associated with increased pathogenic potential and vaccine breakthroughs, and a similar scenario is plausible for PPV [6].

Interestingly, not all PPV strains carry REV inserts. A comprehensive survey in Egypt using PCR targeting the REV-5′ long terminal repeat (LTR) found that while FWPV, turkeypox, and canarypox isolates from the same region were positive for REV integration, all pigeonpox isolates tested were negative [11]. This discrepancy may reflect ecological or host-specific barriers to retroviral integration, or simply a lack of exposure to REV-infected bird populations. The presence or absence of REV sequences has profound implications for the biology of PPV: strains with integrated REV may exhibit altered tissue tropism, enhanced suppression of the host immune response, and even increased oncogenic potential due to the expression of REV env-related proteins. Furthermore, REV integration can act as a molecular marker for tracking the evolution and movement of PPV lineages, as demonstrated by identity analysis showing that the REV sequences in Chinese PPV isolates cluster with those from northeastern China, Guangdong, and Guangxi, pointing to a common source of retroviral insertion [5].

Phylogenetic Diversity and Molecular Epidemiology

Phylogenetic analysis based on the p4b gene has consistently placed PPV within clade A, subclade A2 of the avipoxvirus tree, distinct from FWPV (subclade A1) and canarypox virus (subclade B) [7, 8]. This classification is robust and has been corroborated by concatenated amino acid phylogenies of whole genomes, which show that Indian and Chinese isolates are most closely related to feral pigeonpox viruses from South Africa and to other subclade A2 members from Egypt, India, and Tanzania [4, 8]. The Tanzanian PGPV isolate, for example, exhibited 91% identity to local fowlpox viruses and 99% identity to three global PGPV sequences from India (DQ873811), Egypt (JQ665840), and an unknown origin (AY530303) [8]. Such high sequence conservation across continents suggests a long evolutionary history with limited divergence, likely due to the strong purifying selection acting on essential structural genes.

However, whole-genome sequencing has revealed greater heterogeneity in non-conserved regions, especially in the ITRs and central regions encoding host-range factors. Microsatellite repeats were found to be ubiquitously distributed across the Indian PPV genome, predominantly within functional genes, which may contribute to rapid adaptation through replication slippage [4]. While recombination events were not detected in the Indian strain, the use of recombination detection programs such as RDP4 (which can distinguish recombination from reassortment) remains essential for future surveillance, as poxviruses are known to engage in homologous recombination under selective pressure [12].

Concluding Remarks for the Section (without summary)

The molecular characterization of pigeonpox virus has advanced considerably through complete genome sequencing, functional gene annotation, and the identification of REV integrations. These data not only clarify the evolutionary relationships within the avipoxvirus genus but also provide a foundation for understanding the mechanisms that underpin the distinct cutaneous and diphtheritic disease forms. The development of species-specific multiplex PCR assays now enables rapid differentiation from fowlpox virus, a critical step for outbreak response. As highlighted by the World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO), avian pox diseases, although not always notifiable, pose significant economic threats to the pigeon industry and to bird conservation efforts globally. Continued genomic surveillance, particularly in wild bird reservoirs, will be essential to monitor the emergence of recombinant or REV-integrated PPV strains with altered pathogenic potential.

Epidemiology and Global Distribution of Pigeonpox Virus

Pigeonpox virus (PPV), a distinct species within the genus Avipoxvirus, exhibits a global distribution that is far more heterogeneous and undercharacterized than its well-studied counterpart, fowlpox virus (FWPV). Unlike the ubiquitous FWPV, which has been documented extensively across commercial poultry operations worldwide, PPV epidemiology remains critically underreported, with the majority of confirmed cases emerging only in the past decade as molecular diagnostic tools have become more accessible in non-industrialized settings. This epidemiological disparity is not merely a function of sampling bias but reflects fundamental differences in host ecology, transmission dynamics, and the complex interplay between viral evolution and host population immunity. The true global burden of pigeonpox is almost certainly underestimated, as the disease often presents in its milder cutaneous form in domestic pigeons, rarely triggering the mortality events that would prompt formal veterinary investigation in many regions.

Geographic Distribution and Host Range

The documented distribution of PPV spans multiple continents, though the available evidence suggests a patchwork of endemic foci rather than a continuous global presence. In Africa, confirmed PPV circulation has been reported in Ghana, Tanzania, Egypt, and South Africa, with molecular characterization providing critical insights into strain relatedness across the continent [1, 8, 11]. The first formal report of PPV in West Africa, originating from Ghana in 2025, documented a dual-form (cutaneous and diphtheritic) infection in a 5-month-old female pigeon, with phylogenetic analysis revealing 100% identity to the reference isolate FeP2 [1]. This finding is particularly significant given that Ghana had no prior record of PPV circulation, raising urgent questions about whether the virus has been circulating undetected or represents a recent incursion. In East Africa, Tanzanian isolates from the Morogoro region demonstrated 90–99% sequence identity to diverse avipoxviruses from multiple bird species, with the Tanzanian PPV isolate showing 99% homology to isolates from India, Egypt, and an unspecified origin [8]. This high degree of conservation across vast geographic distances suggests either recent viral spread or the existence of a highly conserved, globally distributed PPV lineage.

Asia represents another major endemic zone, with confirmed reports from China, India, Bangladesh, and Taiwan. The epidemiological situation in China is particularly complex. A comprehensive investigation described the distribution of PPV across multiple provinces in mainland China, successfully isolating viral strains and performing the first complete genome sequencing of a Chinese PPV strain [2]. Concurrent work from Taiyuan, Shanxi Province, identified a PPV isolate belonging to type A2, exhibiting 100% identity with strains PPLH and ROPI/W370/ON/2012, indicating that genetically similar strains circulate across both China and South Africa [5]. The Indian subcontinent has yielded the first complete genome of an Indian PPV isolate (PPV/Pur-Od-4b/01/Ind), a 280,058 bp genome with 270 open reading frames and a GC content of 29.51%, which phylogenetic analysis placed in close relation to a South African feral pigeonpox virus [4]. This finding challenges simplistic geographic partitioning of PPV lineages, suggesting historical or contemporary viral exchange between disparate regions. In Bangladesh, a prevalence study of avipoxvirus infections in the Mymensingh division found that 40% of suspected pigeonpox cases (8 out of 20 samples) were confirmed positive by PCR, an overall avipox prevalence of 41.67% across turkeys, chickens, and pigeons [9]. This epidemiological snapshot, while geographically limited, underscores the endemic nature of PPV in South Asian pigeon populations.

The Middle East has also yielded important data. Egyptian surveillance efforts using pooled cutaneous nodular samples from Dakahlia Governorate identified a single PPV isolate among 40 pooled avipoxvirus samples, which was grouped into clade 2 alongside reference PGPVs retrieved from GenBank [11]. Notably, this Egyptian PPV isolate, unlike the co-circulating FWPV, TKPV, and CNPV strains, lacked reticuloendotheliosis virus (REV) 5′ long terminal repeat (LTR) integration, a feature discussed in depth below. This finding suggests that REV integration may be less common in PPV than in FWPV, at least in certain geographic regions, with potential implications for viral pathogenesis and vaccine efficacy.

Molecular Epidemiology and Phylogenetic Structure

The molecular epidemiology of PPV has been revolutionized by the application of PCR-based diagnostics targeting the highly conserved P4b gene, which encodes the 4b core protein and serves as the gold standard for avipoxvirus detection and differentiation [1, 7-9]. The P4b gene amplicon of 578 bp has enabled phylogenetic placement of PPV isolates into subclade A2, consistently demonstrating that pigeonpox viruses form a monophyletic group distinct from FWPV (clade A1) and other avipoxviruses [7, 8]. This phylogenetic separation is robust across geographic origins; Tanzanian, Indian, Egyptian, and Chinese isolates all cluster within subclade A2, sharing a recent common ancestor with members of subclade A3 [8]. However, the resolution afforded by single-gene phylogenies is limited. The recent availability of complete PPV genomes from China and India has provided unprecedented insights into genomic architecture and evolutionary relationships. The Chinese PPV genome revealed a typical avipoxvirus organization with inverted terminal repeats (ITRs) flanking the core region, while the Indian genome confirmed the presence of ITRs of 4,689 bp at each terminus [2, 4]. Comparative genomics has identified that the Indian isolate shares a concatenated amino acid phylogeny most closely related to the South African feral pigeonpox virus, suggesting that PPV may have been disseminated globally through anthropogenic movement of pigeons for racing, exhibition, or religious purposes [4].

The molecular epidemiology of PPV is further complicated by the detection of REV integration within the PPV genome. While REV integration is well-documented in FWPV, where it has been associated with enhanced pathogenicity and vaccine failure, its occurrence in PPV has only recently been confirmed. In the Taiyuan study, the PPV isolate was found to harbor a complete REV sequence of approximately 7,942–8,005 bp, with the integrated REV belonging to type III based on gp90 protein gene analysis [5]. This integration occurred between ORF201 and ORF203, mirroring the integration site seen in FWPV. The identity analysis revealed that the REV sequence integrated into PPV had the highest homology with integrated REV sequences from FWPV strains circulating in Northeast China, Guangdong, and Guangxi regions [5]. This finding suggests that REV may be capable of horizontal transfer between different avipoxvirus species, potentially through co-infection of the same avian host. In stark contrast, the Egyptian study found that all PPV isolates were free from REV-5′LTR integration, while 30 of the 40 pooled samples from chickens, turkeys, and canaries were positive [11]. This geographic dichotomy in REV integration prevalence raises intriguing questions about the ecological and evolutionary factors that govern the acquisition of retroviral sequences by avipoxviruses. It is plausible that the integration event is recent in evolutionary time and has not yet spread to all PPV populations, or that host-specific factors in different pigeon populations modulate susceptibility to REV integration.

A critical development in PPV molecular epidemiology has been the creation of a multiplex PCR assay capable of differentiating FWPV and PPV without sequencing. This assay targets unique DNA fragments from the fpv122 gene of FWPV and the HM89_gp120 gene of PPV, combined with genus-specific PanAPV primers [3]. Validation using nine samples from unvaccinated chickens, pigeons, and a turkey demonstrated perfect concordance with phylogenetic outputs, establishing a rapid, cost-effective tool for species-level identification [3]. The diagnostic utility of this assay in epidemiological surveillance cannot be overstated; prior to its development, differentiation between FWPV and PPV required sequencing of the P4b gene, a resource-intensive process that limited large-scale surveillance in low-resource settings.

Vectors, Transmission Dynamics, and Environmental Drivers

The epidemiology of pigeonpox is inextricably linked to the biology of its arthropod vectors. While FWPV is predominantly transmitted via aerosol or biting insects, the relative contribution of mechanical vector transmission versus direct contact for PPV remains poorly quantified. Drawing from the broader avipoxvirus literature and the known ecology of pigeon-associated arthropods, the primary vectors for PPV are likely to include Culex and Aedes mosquitoes, as well as blood-feeding dipterans such as Stomoxys calcitrans (stable flies) and Culicoides midges. These vectors acquire the virus by feeding on an infected pigeon with viremia or cutaneous lesions, then transmit the virus mechanically to naive hosts during subsequent blood meals. The importance of vector transmission is underscored by seasonal patterns observed in many endemic regions; outbreaks tend to peak during warm, wet months when mosquito populations are at their maximum. This seasonality has been documented anecdotally across multiple studies but has not been subjected to rigorous epidemiological analysis.

The role of direct contact transmission, while undoubtedly important in high-density loft conditions, is likely secondary to vector-borne spread in free-ranging pigeon populations. Pigeons, particularly feral populations in urban environments, exhibit high population densities and frequent inter-group contact at feeding sites, roosts, and water sources. These conditions facilitate both direct transmission (via ingestion of desquamated scab material or inhalation of aerosolized virus) and indirect transmission (via contaminated fomites such as shared perches, nesting material, and feeding troughs). The diphtheritic form of the disease, characterized by fibronecrotic lesions in the oral cavity, pharynx, and upper respiratory tract, results in copious viral shedding in saliva and respiratory secretions, amplifying the potential for direct transmission [1]. The coexistence of both cutaneous and diphtheritic forms in a single bird, as documented in the Ghanaian index case, suggests that the viral load and route of infection may influence clinical manifestation, with implications for transmission efficiency [1].

Environmental persistence of PPV contributes significantly to its epidemiological maintenance. Avipoxviruses are among the most environmentally resistant viruses known, capable of surviving for months to years in dried scabs, feathers, and organic debris, particularly in shaded, cool environments. This environmental stability allows the virus to persist in contaminated lofts, abandoned buildings, and urban structures even in the absence of an active infected host population. The World Organisation for Animal Health (WOAH) recognizes the importance of biosecurity in preventing avipoxvirus introduction, recommending thorough cleaning and disinfection of contaminated premises, with particular attention to the removal of organic material that can protect the virus from chemical inactivation.

Epidemiological Significance and Surveillance Gaps

The epidemiological significance of PPV extends beyond its immediate impact on domestic pigeon health and productivity. Pigeons serve as important sentinel species for monitoring the circulation of avipoxviruses in the urban-wildlife interface, and their high population densities in cities worldwide make them potential reservoirs for viral spillover into other avian species. The susceptibility of multiple bird species to PPV infection, including chickens, turkeys, and various wild birds, raises the specter of cross-species transmission events. While experimental evidence is limited, phylogenetic analyses have demonstrated that PPV isolates from pigeons can cluster with isolates from other species, suggesting that host range may be broader than currently appreciated [7, 8].

Surveillance for PPV remains grossly inadequate on a global scale. The majority of published reports originate from Africa and Asia, regions where diagnostic capacity has improved in recent years but where systematic surveillance programs are largely absent. Europe and North America, despite having large pigeon populations (both domestic and feral), have remarkably few published reports of PPV in the last two decades, likely reflecting a combination of underdiagnosis, lack of reporting requirements, and the perception that pigeon pox is a minor disease of limited economic consequence. However, the emergence of PPV in previously unreported regions, such as Ghana [1], and the genetic characterization of novel strains from India [4] and China [2], underscores the need for a coordinated international surveillance effort. The Food and Agriculture Organization of the United Nations (FAO) and WOAH have established frameworks for the surveillance of transboundary animal diseases, but pigeonpox is not currently classified as a notifiable disease, limiting the incentive for reporting.

The economic impact of PPV is difficult to quantify but is likely substantial in regions where pigeon farming is an important livelihood activity. Pigeon breeding for meat (squab production), racing, and exhibition represents a significant agricultural sector in parts of Asia, the Middle East, and North Africa. Outbreaks of pigeonpox can result in mortality rates of 10–30% in naive flocks, with higher losses in young birds, particularly when the diphtheritic form compromises feeding and respiration. Reduced growth rates, decreased egg production, and increased susceptibility to secondary bacterial infections contribute to economic losses that may be considerably greater than direct mortality figures suggest. The development of a specific, attenuated PPV vaccine for use in China, which demonstrated 100% protection in experimental trials, offers a promising avenue for disease control [2]. However, the widespread use of heterologous FWPV vaccines for protection against PPV in many regions, while documented to provide some cross-protection, may be suboptimal and may contribute to the emergence of vaccine-resistant strains, as has been observed with FWPV [6].

In conclusion, the epidemiology and global distribution of pigeonpox virus reflect a complex interplay of viral evolution, host ecology, vector dynamics, and anthropogenic factors. The virus is endemic in multiple regions of Africa and Asia, with evidence of widespread circulation in domestic and feral pigeon populations. The recent emergence of complete genome sequencing from diverse geographic origins, coupled with the development of species-specific molecular diagnostics, has provided critical tools for understanding the global diversity and evolutionary history of PPV. However, significant gaps remain, particularly regarding the prevalence of PPV in Europe and the Americas, the role of wildlife reservoirs, and the impact of REV integration on viral pathogenesis and vaccine efficacy. Future surveillance efforts must prioritize systematic, risk-based sampling across geographic regions and host species, with an emphasis on integrating molecular epidemiological data with ecological and demographic information to develop predictive models of PPV emergence and spread.

Clinical Manifestations and Pathological Features of Pigeonpox Virus (Cutaneous and Diphtheritic Forms)

Pigeonpox virus (PPV), a member of the genus Avipoxvirus within the family Poxviridae, induces a disease characterized by a spectrum of clinical presentations that are broadly categorized into two principal forms: the cutaneous (dry) form and the diphtheritic (wet) form. These manifestations are not mutually exclusive, as concurrent infections, termed the mixed form, are well-documented and often signify a more severe systemic involvement [1]. The clinical trajectory and pathological landscape of PPV infection are dictated by a complex interplay of viral virulence factors, host immune status, age, and environmental stressors, with the virus exhibiting a pronounced tropism for epithelial tissues of the skin and mucous membranes. Understanding the nuanced clinical and pathological features of these forms is paramount for accurate diagnosis, effective disease management, and the implementation of robust biosecurity protocols in both commercial and aviary settings.

The Cutaneous (Dry) Form: Pathogenesis and Clinical Presentation

The cutaneous form is the most frequently observed manifestation of pigeonpox and is typically associated with a more favorable prognosis compared to its diphtheritic counterpart. The clinical hallmark of this form is the development of proliferative, nodular lesions on unfeathered or sparsely feathered areas of the body. These lesions are the direct result of viral replication within the cytoplasm of epidermal keratinocytes, leading to cellular hypertrophy, hyperplasia, and subsequent degeneration [6]. The incubation period following natural exposure, often via mechanical transmission by arthropod vectors such as mosquitoes (Culex and Aedes spp.) or through direct contact with contaminated fomites, ranges from 4 to 10 days [6, 10].

Lesion Progression and Morphology: The initial stage of cutaneous pox is characterized by the appearance of small, raised, erythematous papules, often first observed on the eyelids, cere (the fleshy, white structure at the base of the beak), the corners of the beak, and the legs. These papules rapidly evolve into vesicles, though the vesicular stage is transient and often overlooked in birds due to the fragility of the epithelial covering. The vesicles subsequently pustulate and rupture, leading to the formation of characteristic dry, crusty, yellowish-brown to dark scabs. These scabs are composed of necrotic cellular debris, fibrin, and inflammatory cells. Over a period of 1 to 3 weeks, the scabs desiccate and eventually slough off, typically leaving a smooth, scar-free surface, although secondary bacterial infections can lead to more severe tissue damage and scarring [1, 7, 8].

Histopathological Hallmarks: Histological examination of cutaneous lesions reveals a pathognomonic feature of avipoxvirus infection: the presence of large, eosinophilic, intracytoplasmic inclusion bodies known as Bollinger bodies [1]. These inclusion bodies represent aggregates of mature virions (elementary bodies, or Borrel bodies) embedded within a proteinaceous matrix. The epidermis exhibits marked acanthosis (thickening of the stratum spinosum) due to hyperplasia of prickle cells, accompanied by ballooning degeneration of keratinocytes. The dermis is typically infiltrated by a mixed population of inflammatory cells, including heterophils (the avian equivalent of neutrophils), lymphocytes, and macrophages, indicative of a robust local inflammatory response. The scab itself is composed of a laminated mass of necrotic epithelium, fibrin, and heterophilic debris [1, 6]. The detection of Bollinger bodies on routine hematoxylin and eosin (H&E) staining is considered confirmatory for poxvirus infection, though molecular techniques such as PCR targeting the P4b core protein gene are now the gold standard for definitive diagnosis and species differentiation [1, 3, 7].

Clinical Impact and Sequelae: While the cutaneous form is rarely fatal in immunocompetent adult pigeons, significant morbidity can ensue. Lesions on the eyelids can lead to blepharitis, conjunctivitis, and, in severe cases, partial or complete closure of the palpebral fissure, resulting in temporary blindness and subsequent inability to locate food and water [1, 8]. Lesions on the beak and cere can interfere with feeding behavior. Secondary bacterial infections, particularly with Staphylococcus spp. or Streptococcus spp., are a common complication, leading to suppurative dermatitis and delayed healing. In young squabs, extensive cutaneous involvement can be debilitating, leading to weight loss, stunted growth, and increased susceptibility to other pathogens [9].

The Diphtheritic (Wet) Form: Pathogenesis and Clinical Presentation

The diphtheritic form, also known as the "wet" form, is a more severe and often life-threatening manifestation of PPV infection. It is characterized by the formation of fibronecrotic plaques on the mucous membranes of the upper respiratory tract, oral cavity, and esophagus. This form carries a significantly higher case-fatality rate than the cutaneous form, primarily due to mechanical obstruction of the airways and the inability to ingest food and water [1, 6]. The diphtheritic form can occur independently or concurrently with the cutaneous form, a scenario that dramatically worsens the clinical outcome [1].

Lesion Development and Localization: The diphtheritic form begins with the development of small, yellowish-white nodules on the mucous membranes of the tongue, palate, pharynx, larynx, and trachea. These nodules rapidly coalesce and undergo necrosis, forming thick, adherent, diphtheritic membranes. These membranes are composed of a fibrinous exudate, necrotic epithelial cells, and a dense population of proliferating virus particles. The membranes are firmly attached to the underlying mucosa and, when forcibly removed, leave a raw, hemorrhagic, ulcerated surface [1, 6]. In severe cases, the lesions can extend into the sinuses, causing sinusitis, and down the trachea, leading to tracheitis and severe respiratory distress. Lesions may also be observed in the esophagus and crop, contributing to regurgitation and anorexia.

Histopathological Features: Histologically, the diphtheritic form is characterized by extensive necrosis and ulceration of the mucosal epithelium. The underlying lamina propria is heavily infiltrated by inflammatory cells, predominantly heterophils and macrophages. The hallmark Bollinger bodies are readily identifiable within the cytoplasm of intact epithelial cells at the periphery of the necrotic lesions, as well as within macrophages that have phagocytosed viral debris [1]. The diphtheritic membrane itself is a layered structure: the superficial layer consists of fibrin, necrotic cellular debris, and bacterial colonies; the middle layer contains a dense accumulation of inflammatory cells; and the deep layer is composed of viable, hyperplastic epithelial cells containing numerous intracytoplasmic inclusion bodies. This deep layer is the site of active viral replication and is critical for the propagation of the lesion.

Clinical Impact and Morbidity: The clinical consequences of the diphtheritic form are profound. The formation of obstructive plaques in the glottis and trachea leads to dyspnea, open-mouth breathing, and characteristic respiratory sounds (rales). Affected birds often exhibit marked depression, anorexia, and weight loss due to the pain and mechanical obstruction associated with swallowing. The inability to effectively clear the airway predisposes birds to secondary bacterial pneumonia and aspiration pneumonia, which are common terminal events [1, 6]. In young pigeons, the diphtheritic form is particularly devastating, with mortality rates often exceeding 50% in untreated outbreaks. The systemic effects are compounded by the release of viral toxins and the profound inflammatory response, leading to septicemia in advanced cases.

The Mixed Form and Systemic Pathology

The simultaneous occurrence of both cutaneous and diphtheritic lesions, termed the mixed form, represents the most severe clinical presentation of pigeonpox. This form is indicative of a high viral load and a compromised or overwhelmed host immune system. The index case of PPV in Ghana, as reported by Abbiw et al. (2025), exemplifies this presentation in a 5-month-old female pigeon that exhibited both nodular scabs on the unfeathered skin and diphtheritic plaques in the oral cavity [1]. The presence of the mixed form is a poor prognostic indicator, often leading to rapid debilitation and death.

Beyond the primary epithelial lesions, systemic pathological changes are observed in severe or chronic cases. The virus can disseminate via the bloodstream, leading to viremia and infection of internal organs. Gross necropsy findings may include hepatomegaly, splenomegaly, and congestion of the lungs and kidneys. Microscopically, focal areas of necrosis and inflammation can be found in the liver, spleen, and bone marrow, often accompanied by the presence of intracytoplasmic inclusion bodies in reticuloendothelial cells [1]. The integration of reticuloendotheliosis virus (REV) sequences into the PPV genome, a phenomenon increasingly reported in field isolates, may further exacerbate pathogenicity by inducing immunosuppression, thereby predisposing the bird to more severe and disseminated poxvirus lesions [5, 11]. This genomic integration, particularly of the REV 5' long terminal repeat (LTR), has been linked to enhanced virulence and vaccine failures in fowlpox, and its role in PPV pathogenesis is an area of active investigation [5, 6, 11].

Differential Diagnosis and Diagnostic Confirmation

The clinical presentation of pigeonpox, particularly the diphtheritic form, can be confused with other respiratory and mucosal diseases. Key differential diagnoses include:

• Trichomoniasis (Canker): Caused by Trichomonas gallinae, this infection produces caseous, yellowish lesions in the oral cavity and crop. However, trichomonad lesions are typically cheesy and can be easily removed, unlike the adherent diphtheritic membranes of pox. Microscopic examination of wet mounts reveals motile protozoa.
• Avian Herpesvirus (Pigeon Herpesvirus 1): Can cause conjunctivitis and diphtheritic lesions, but is often associated with neurological signs and hepatic necrosis.
• Vitamin A Deficiency: Leads to squamous metaplasia of mucous membranes, which can appear as pustules and plaques, but is not associated with the systemic signs or inclusion bodies of poxvirus.
• Candidiasis (Yeast Infection): Produces white, plaque-like lesions in the oral cavity, but these are typically more superficial and can be easily scraped off. Microscopic examination reveals budding yeast cells.

Definitive diagnosis relies on laboratory confirmation. Histopathological demonstration of Bollinger bodies in tissue sections remains a valuable and rapid diagnostic tool [1]. However, molecular methods, particularly polymerase chain reaction (PCR) targeting the highly conserved P4b core protein gene, provide superior sensitivity and specificity [1, 3, 7, 8]. Furthermore, species-specific multiplex PCR assays, such as those differentiating PPV from Fowlpox virus (FWPV) by targeting the HM89_gp120 gene, are now available, enabling rapid and accurate identification of the causative agent without the need for sequencing [3]. Virus isolation on the chorioallantoic membrane (CAM) of embryonated chicken eggs, which produces characteristic pock lesions, remains a classical virological technique for confirmation and strain characterization [7, 9]. The integration of these diagnostic modalities is essential for confirming PPV as the etiological agent, differentiating it from other avipoxviruses, and guiding appropriate control and vaccination strategies.

Diagnostic Approaches: Histopathology, PCR, and Multiplex Assays for Pigeonpox Virus

The accurate and definitive diagnosis of Pigeonpox virus (PGPV) is a cornerstone of effective disease surveillance, outbreak management, and epidemiological investigation. Given the clinical similarity between PGPV and other avian poxviruses (APVs), such as Fowlpox virus (FWPV), as well as the potential for concurrent infections and the integration of exogenous viral elements like Reticuloendotheliosis virus (REV), a multi-faceted diagnostic approach is essential. This section provides an exhaustive analysis of the three primary diagnostic pillars, histopathology, polymerase chain reaction (PCR), and advanced multiplex assays, detailing their underlying principles, applications, limitations, and interpretive frameworks within the context of PGPV infection.

Histopathological Examination: The Gold Standard for Cellular Pathology

Histopathology remains an indispensable, albeit traditional, component of PGPV diagnosis, offering direct visualization of virus-induced cytopathology. The hallmark of any avipoxvirus infection, including PGPV, is the presence of Bollinger bodies, which are large, intracytoplasmic, eosinophilic inclusion bodies [1]. These structures represent aggregates of mature virions (the B-type inclusion body, or “virus factory”) and are pathognomonic for poxvirus infection [1, 6]. In cases presenting with the cutaneous (dry) form of pigeon pox, histological examination of biopsied nodular lesions typically reveals epithelial hyperplasia, ballooning degeneration of keratinocytes, and prominent acanthosis [1]. The overlying epidermis often exhibits severe hyperkeratosis and parakeratosis. The diphtheritic (wet) form, which affects the mucous membranes of the respiratory and digestive tracts, presents with a more severe necrotic and fibrino-purulent inflammation [1]. Here, histopathology demonstrates extensive ulceration of the mucosal epithelium, with a thick pseudomembrane composed of fibrin, necrotic cellular debris, and heterophils. Bollinger bodies are readily identifiable within the cytoplasm of intact epithelial cells bordering these necrotic foci [1].

The biological mechanism driving these changes is the virus's direct lytic and proliferative effect on epithelial cells. The virus replicates in the cytoplasm, leading to cellular hypertrophy and eventual rupture, which triggers a robust inflammatory response. While the observation of Bollinger bodies is confirmatory of a poxvirus, it is not definitively diagnostic for PGPV versus other species like FWPV or canarypox virus. The morphology of the inclusion bodies and the specific tissue tropism can offer clues, but definitive species-level identification requires molecular techniques to differentiate the genetic determinants of pathogenicity and host range [1, 3]. Furthermore, histopathology can reveal secondary bacterial infections, which are common complications, particularly in the diphtheritic form, where the necrotic tissue provides a rich substrate for opportunistic pathogens. Despite the advent of molecular diagnostics, histopathology retains value for its rapidity (within 24-48 hours of sample collection) and its ability to provide a spatial context of infection, confirming that the virus is indeed replicating within the lesion and not merely a surface contaminant.

Molecular Detection via Polymerase Chain Reaction: The Pan-APV and Species-Specific Approach

The advent of PCR has revolutionized the diagnosis of PGPV, enabling highly sensitive and specific detection of viral nucleic acids directly from clinical samples. The most widely employed PCR target is the P4b gene (also known as the 4b core protein gene), a highly conserved region within the avipoxvirus genome that encodes a core structural protein [1, 7-9]. These "Pan-APV" primers are designed to amplify a fragment (commonly 578 bp in size) from a broad spectrum of avipoxviruses, serving as an excellent initial screening tool [7]. The amplification and sequencing of this gene allow for phylogenetic placement of the isolate within the larger APV clades. PGPV isolates are consistently grouped within Subclade A2, distinct from FWPV (Subclade A1) and canarypox virus (Subclade B1) [1, 7, 8]. For instance, the Tanzanian and Ghanaian PGPV isolates, when sequenced, showed 99% identity to reference PGPV strains and clustered robustly within Subclade A2, confirming their identity [1, 8]. This phylogenetic positioning is crucial for epidemiological tracing and understanding the evolutionary relationships between global isolates. The P4b PCR, therefore, provides a genus-level identification and initial species-level typing via sequence analysis.

However, reliance solely on P4b sequencing can be time-consuming and labor-intensive for large-scale surveillance. To address this, researchers have developed species-specific PCR assays. A more targeted approach involves the application of primers designed to amplify unique regions of the PGPV genome. The HM89_gp120 gene has been identified as a specific target for PGPV, analogous to the fpv122 gene for FWPV [3]. These genes are unique to their respective genomes, allowing for direct, sequencing-free differentiation between the two major APV species in a conventional PCR [3]. This is a significant advancement, as it bypasses the need for expensive and time-consuming sequencing for routine diagnostics. The diagnostic workflow thus often involves a two-tiered system: initial screening with Pan-APV P4b primers, followed by species-specific PCR (e.g., targeting HM89_gp120) on P4b-positive samples to rapidly confirm PGPV versus FWPV.

Advanced Diagnostic Strategies: Multiplex Assays and Detection of Co-Integrating Elements

The complexity of avipoxvirus infections demands diagnostic tools capable of resolving multiple targets simultaneously. The development of a multiplex PCR assay represents a critical leap forward. This novel assay, validated in silico and with clinical samples, combines three primer sets in a single reaction: the pan-APV (genus-specific) primers, the FWPV-specific primers (fpv122 gene), and the PGPV-specific primers (HM89_gp120 gene) [3]. This elegant design provides a single-tube solution that not only confirms the presence of an avipoxvirus (via the pan-APV band) but also simultaneously identifies the specific species (FWPV or PGPV) based on the presence and size of the amplicons [3]. The assay was successfully tested on samples from chickens, pigeons, and turkeys, correctly differentiating the causative agents in each case without the need for subsequent sequencing [3]. This capability is particularly valuable in mixed-poultry operations or in wild bird populations where multiple avipoxvirus species may circulate. The ability to rapidly and accurately differentiate PGPV from FWPV has profound implications for control measures, as vaccination strategies differ (e.g., use of heterologous fowlpox or pigeonpox vaccines) [6].

Furthermore, the diagnostic landscape has expanded to include the detection of Reticuloendotheliosis virus (REV) integration within the PGPV genome. It is now well-documented that field strains of APVs, including PGPV, frequently harbor integrated full-length or partial REV proviral sequences [5, 11]. This integration, often involving the 5' Long Terminal Repeat (LTR) and the env gene, is not merely a biological curiosity; it has significant implications. The presence of REV sequences can enhance the pathogenicity and virulence of the APV strain and is a known cause of inadequate vaccine protection [5, 6, 11]. Therefore, a comprehensive diagnostic approach must screen for REV integration. PCR assays targeting the REV gp90 gene or the REV LTR region are used adjunctively to characterize the integrated elements [5, 11]. For example, a study on Egyptian avipoxviruses found that while other APVs (FWPV, turkeypox) harbored the REV-5'LTR, the PGPV isolates were notably free from this integration [11]. In contrast, a study from China detected a full REV sequence within a PGPV isolate, demonstrating the variability in integration patterns across geographic regions [5]. A complete molecular characterization, therefore, involves not only typing the APV itself but also determining its REV integration status.

Interpreting Diagnostic Outcomes and Epidemiological Context

The selection and interpretation of diagnostic tests must be grounded in an understanding of the disease's pathogenesis and epidemiology. The cutaneous or diphtheritic form can influence sample selection. For PCR, dried scabs and nodular lesions from the cutaneous form are excellent samples [7, 11]. For the diphtheritic form, swabs or tissue biopsies from the oral cavity or trachea are preferred [1]. Virus isolation in embryonated chicken eggs, while more laborious, remains a valuable confirmatory and research tool. Inoculation of suspect material onto the chorio-allantoic membrane (CAM) of 11-day-old embryonated eggs produces characteristic pock lesions after 5-7 days, with PGPV typically causing smaller, less hemorrhagic pocks compared to FWPV [7, 9]. This biological assay can be integrated with PCR, wherein the CAM tissue is directly used for nucleic acid extraction and amplification [7].

Ultimately, a definitive diagnosis of PGPV is best achieved through a combination of these techniques. Histopathology provides rapid, preliminary confirmation of a poxvirus infection. A positive pan-APV PCR confirms the genus. Subsequent species-specific PCR or sequencing confirms PGPV. Finally, screening for REV integration provides a complete picture of the viral pathogen's genomic landscape. This integrated approach is vital for informing control strategies, such as the selection of appropriate autogenous or commercial vaccines [2, 6], and for understanding the complex evolutionary dynamics driving the emergence of novel, potentially more virulent strains [13]. The World Organisation for Animal Health (WOAH) recognizes the importance of such comprehensive molecular diagnostics in the surveillance of economically significant avian diseases, underscoring the need for rapid, specific, and multiplexable assays to track the global spread of PGPV and its recombinant variants.

Phylogenetic Analysis and Strain Diversity of Pigeonpox Virus

The genus Avipoxvirus, a member of the subfamily Chordopoxvirinae within the family Poxviridae, encompasses a diverse array of viruses infecting over 300 avian species across numerous orders [6]. Among these, pigeonpox virus (PPV), formally recognized as an independent species within the genus, stands as the primary etiological agent of pigeonpox, an economically significant disease of domestic pigeons (Columba livia domestica) and a growing concern for aviculture and conservation [1, 2, 4]. The phylogenetic architecture and strain diversity of PPV are critical to understanding its evolutionary biology, host range dynamics, vaccine cross-protection, and the molecular epidemiology of outbreaks. Advances in molecular diagnostics and whole-genome sequencing have substantially refined the taxonomic placement of PPV and revealed a complex landscape of genetic variation, including the presence of integrated reticuloendotheliosis virus (REV) elements that can influence pathogenicity and diagnostic accuracy.

Taxonomic Framework and Clade Delineation

Early phylogenetic studies of avipoxviruses relied heavily on amplification and sequencing of the p4b core protein gene, a highly conserved but discriminative locus that enables genus- and species-level classification [1, 7, 8]. Using this marker, PPV isolates consistently segregate within a well-supported monophyletic clade designated as subclade A2, which is distinct from the fowlpox virus (FWPV)-dominated subclade A1 and the canarypox-like subclade A3 [7, 8]. This tripartite structure has been corroborated by multiple independent analyses spanning isolates from Africa, Asia, and Europe [1, 7, 9]. For instance, Rajasekaran et al. (2018) amplified the p4b gene from an Indian PPV isolate and placed it definitively within subclade A2, noting its clustering with other pigeonpox viruses from diverse geographic origins [7]. Similarly, Masola et al. (2015) reported that a Tanzanian PPV isolate belonged to subclade A2, sharing a recent common ancestor with members of subclade A3 [8]. Abbiw et al. (2025) provided the first molecular characterization of PPV in Ghana, demonstrating 100% nucleotide identity of the p4b gene to the reference PPV strain FeP2, and confirming its placement within subclade A2 [1].

The utility of the p4b locus, however, is limited by its relatively low resolution for delineating fine-scale intra-clade diversity. As such, more recent investigations have expanded phylogenetic inference to include concatenated amino acid sequences derived from complete or nearly complete genomes. The landmark first complete genome sequencing of an Indian PPV isolate (PPV/Pur-Od-4b/01/Ind) by Sahu et al. (2025) afforded a comprehensive assessment of its evolutionary relationships [4]. In a concatenated amino acid phylogenetic tree, this isolate demonstrated a close relationship with a feral pigeonpox virus from South Africa, underscoring the genetic connectivity between domestic and wild pigeon populations and suggesting a potential reservoir role for feral birds [4]. The genome of the Indian isolate comprised 280,058 bp, encoding 270 predicted open reading frames (ORFs) flanked by inverted terminal repeats (ITRs) of 4,689 bp each [4]. This genomic architecture, while typical of avipoxviruses, provides a robust foundation for resolving phylogenetic ambiguities that cannot be addressed by single-gene analyses.

Global Strain Diversity and Geographic Distribution

The strain diversity of PPV reflects a global distribution, with documented outbreaks across Africa, Asia, Europe, and the Middle East [2, 4, 8]. Despite this wide geographic footprint, the genomic heterogeneity among PPV isolates appears to be relatively constrained compared to the broader diversity observed within the genus Avipoxvirus. Zhu et al. (2026) conducted an epidemiological survey of pigeonpox across multiple provinces in mainland China, successfully isolating several PPV strains from infected pigeons and performing whole-genome sequencing on a representative isolate [2]. This study noted that the Chinese PPV strain clustered closely with other pigeonpox viruses from Asia, but also exhibited unique genomic features, including the absence of certain genes associated with host range and immune evasion, which may be relevant for future live-attenuated vaccine development [2]. The genetic connectivity among PPV isolates from different continents is highlighted by the observation that the Tanzanian isolate was 99% identical to PPV isolates from India, Egypt, and an isolate of unknown origin, all of which grouped within subclade A2 [8]. This high degree of sequence conservation at the p4b locus suggests that PPV may have undergone a relatively recent global expansion, possibly facilitated by the international trade of pigeons and the migratory movements of feral populations [1, 8].

Notably, molecular differentiation between PPV and FWPV has been a persistent diagnostic challenge. Özgünlük et al. (2024) addressed this by developing a multiplex PCR assay targeting species-specific genetic markers: the fpv122 gene for FWPV and the HM89_gp120 gene for PPV [3]. These primers were validated against field samples from chickens, pigeons, and a turkey, and the multiplex assay successfully differentiated the two species, with results consistent with phylogenetic analysis [3]. This advancement not only streamlines differential diagnosis but also confirms that PPV and FWPV, despite their antigenic cross-reactivity and historical use of FWPV as a heterologous vaccine for pigeons, represent genetically distinct lineages with unique gene content [3, 6].

Genomic Architecture and Molecular Markers

Beyond the core genes used for phylogenetic classification, the genomic diversity of PPV is marked by the presence of microsatellite repeats and integrated viral sequences. Sahu et al. (2025) identified that microsatellites are ubiquitously distributed across the PPV genome and are particularly prevalent within functional genes, suggesting a potential role in modulating gene expression and contributing to strain-specific phenotypic variation [4]. The existence of such molecular markers may prove valuable for high-resolution strain typing, molecular epidemiology, and tracking transmission networks during outbreaks.

A particularly salient feature of PPV strain diversity is the integration of retroviral sequences, specifically those of reticuloendotheliosis virus (REV), into the avipoxvirus genome. REV is a gammaretrovirus that causes immunosuppression and neoplasia in chickens and other avian species. Liu et al. (2024) demonstrated that a PPV isolate collected in Taiyuan, Shanxi Province, China, had integrated the complete REV genome (7942–8005 bp) between ORF201 and ORF203 [5]. This integrated REV sequence showed high identity to REV sequences found in Chinese FWPV isolates, belonging to type III based on gp90 protein gene analysis [5]. The discovery of REV integration in PPV is epidemiologically significant because it has been associated with increased virulence and reduced vaccine efficacy in FWPV. Mosad et al. (2020) screened forty APV field strains from Egypt and detected REV-5'LTR integration in FWPV, turkeypox, and canarypox isolates, but notably, their pigeonpox virus isolate was free from REV integration [11]. This discrepancy may reflect regional variation in REV circulation or differences in sampling strategies, but it also raises important questions about whether PPV isolates from different geographic regions exhibit differential susceptibility to REV integration and what the biological consequences of such integration might be. Given that REV integration in FWPV has been linked to enhanced pathogenicity, the presence of REV sequences in Chinese PPV isolates could represent an emerging threat to pigeon health and a complicating factor for disease control [5, 6].

Implications for Diagnostics, Vaccinology, and Surveillance

The phylogenetic distinctiveness of PPV within subclade A2 has direct implications for vaccine development and diagnostic accuracy. Historically, pigeonpox prophylaxis has relied on heterologous vaccination with live-attenuated FWPV vaccines, which are antigenically related but not fully protective due to genomic differences [2, 6]. The complete genome sequencing efforts by Zhu et al. (2026) and Sahu et al. (2025) have laid the groundwork for the rational design of homologous PPV vaccines, potentially offering superior immunogenicity and safety [2, 4]. Indeed, the Chinese group demonstrated that serial passage of a PPV strain in embryonated eggs led to a safe, attenuated vaccine that provided 100% protection in pigeons [2].

Furthermore, the strain diversity observed among PPV isolates emphasizes the need for geographically targeted surveillance. The detection of PPV in Ghana [1], combined with its presence across Tanzania [8], China [2, 5], and India [4, 7], indicates that the virus is likely underreported, particularly in regions where diagnostic capacity is limited and clinical signs may be confused with other diseases such as trichomoniasis or vitamin A deficiency. The use of PanAPV primers, as validated by Özgünlük et al. (2024), offers a genus-specific screening tool that can be paired with species-specific primers to simultaneously detect and differentiate PPV from other avipoxviruses in a single reaction [3]. This approach is highly suitable for large-scale epidemiological surveys and for monitoring the emergence of novel variants.

In conclusion, the phylogenetic analysis and characterization of strain diversity among pigeonpox viruses reveal a globally distributed pathogen with a relatively conserved core genome, yet with significant variation driven by microsatellite polymorphisms and the sporadic integration of REV sequences. The consistent placement of PPV within subclade A2 of the Avipoxvirus genus provides a robust taxonomic framework for future studies. As complete genome sequencing becomes more routine, the field will be better equipped to resolve the evolutionary history of PPV, identify genetic determinants of host range and virulence, and develop rational control strategies including refined diagnostics and homologous vaccines. The expanding geographic range and the potential for REV-mediated intensification of disease underscore the need for sustained genomic surveillance, particularly in regions where pigeon production is intensifying and where feral pigeon populations may act as a genetic reservoir for viral diversity.

Prevention and Control Strategies: Vaccination and Attenuated Vaccine Development for Pigeonpox Virus

The control of pigeonpox virus (PPV) infection in domestic and feral pigeon populations has historically relied upon a combination of biosecurity measures, vector control, and, most critically, prophylactic vaccination. However, the landscape of PPV prevention is undergoing a paradigm shift, driven by the emergence of novel viral strains, the documented integration of immunosuppressive retroviruses into avipoxvirus genomes, and the growing recognition that heterologous vaccines, primarily those derived from fowlpox virus (FWPV), offer suboptimal protection against contemporary PPV field isolates. This section provides an exhaustive analysis of current vaccination strategies, the biological rationale for homologous attenuated vaccine development, and the molecular and epidemiological considerations that must underpin next-generation control measures.

The Historical Precedent and Limitations of Heterologous Vaccination

For decades, the cornerstone of pigeonpox prophylaxis has been the use of live, attenuated vaccines derived from either FWPV or, less commonly, from PPV strains themselves. The rationale for using FWPV-based vaccines in pigeons is rooted in antigenic cross-reactivity within the Avipoxvirus genus; both viruses belong to the same genus and share conserved immunogenic epitopes, particularly within the core protein genes such as p4b [6, 7]. Indeed, early studies demonstrated that vaccination with FWPV could induce a degree of protective immunity in pigeons, and this practice became widespread due to the commercial availability and established safety profile of FWPV vaccines in poultry.

However, a growing body of evidence from molecular epidemiological studies has exposed the inadequacy of this approach. Phylogenetic analyses consistently demonstrate that PPV isolates cluster within a distinct subclade (A2) that is separate from the FWPV subclades (A1 and B) [7, 8]. This genetic divergence is not merely taxonomic; it translates into significant antigenic variation. The core protein P4b, while useful for genus-level diagnosis, is not the primary target of neutralizing antibodies. The surface proteins responsible for inducing a protective humoral response, such as the homologues of the FWPV envelope proteins, are likely more variable. Consequently, immunity elicited by FWPV vaccines may fail to recognize critical epitopes on circulating PPV strains, leading to vaccine breakthrough. This is reflected in recurrent outbreaks of pigeonpox in previously vaccinated flocks, a phenomenon increasingly reported in regions where FWPV vaccination is standard practice [6]. The situation is further complicated by the fact that many commercial "pigeonpox" vaccines are, in reality, FWPV strains that have been passaged in pigeons to partially adapt them, a process that does not fully address the fundamental antigenic mismatch.

The Imperative for Homologous Attenuated Vaccine Development

The limitations of heterologous vaccination have created an urgent, industry-driven demand for a species-specific, homologous PPV vaccine. The development of such a vaccine requires a systematic approach encompassing virus isolation, whole-genome characterization, rational attenuation, and rigorous safety and efficacy testing. A landmark study by Zhu et al. (2026) has provided the first complete genome sequence of a PPV strain from mainland China and, critically, has demonstrated the feasibility of developing a safe and effective attenuated vaccine from a wild-type isolate [2]. This work serves as a template for future vaccine development efforts globally.

Isolation and Genomic Characterization: The first step in developing an attenuated vaccine is the isolation of a virulent field strain. This is typically achieved by inoculating the chorioallantoic membrane (CAM) of embryonated chicken eggs with homogenized scab material or tissue from cutaneous or diphtheritic lesions [7, 9]. Successful isolation is confirmed by the development of characteristic pock lesions on the CAM, followed by molecular confirmation via PCR amplification of the p4b gene [1, 7, 9]. The isolated virus must then be subjected to comprehensive genomic sequencing. The complete genome of PPV is approximately 280 kbp in size, with a GC content of around 29.5%, and contains roughly 270 open reading frames (ORFs) flanked by inverted terminal repeats (ITRs) [4]. Whole-genome sequencing is not merely an academic exercise; it is essential for identifying virulence genes, understanding the genetic basis of host range, and, most importantly, for detecting the presence of integrated sequences from other viruses, such as Reticuloendotheliosis virus (REV).

The REV Integration Problem: A major hurdle in avipoxvirus vaccine development is the high prevalence of REV proviral DNA integrated into the genomes of field strains of both FWPV and PPV [5, 11]. REV is an immunosuppressive retrovirus that can cause runting, immunosuppression, and neoplasia in birds. The integration of full-length or partial REV sequences into the poxvirus genome has been shown to enhance the pathogenicity of the recombinant virus and can lead to vaccine-induced disease if such strains are used as live vaccines. Liu et al. (2024) demonstrated that a PPV isolate from China contained a complete REV genome (approximately 7.9–8.0 kbp) integrated between ORF201 and ORF203, with the REV strain belonging to type III [5]. Similarly, Mosad et al. (2020) screened Egyptian PPV isolates and found that, unlike FWPV isolates, the PPV strains were free of REV-5'LTR integration, suggesting geographic variation in this phenomenon [11]. For a vaccine candidate to be safe, it must be confirmed to be free of REV integration, or the integrated sequences must be deleted during the attenuation process. The presence of REV not only poses a direct safety risk but also complicates the interpretation of vaccine efficacy, as REV-induced immunosuppression could negate the protective effect of the poxvirus antigens.

Rational Attenuation Strategies: The classical method of attenuation involves serial passage of the virus in a heterologous host system, typically embryonated chicken eggs or cell cultures derived from a non-target species. Zhu et al. (2026) employed this approach, passaging a wild-type PPV strain multiple times in chicken embryo fibroblasts (CEFs) until it lost its virulence for pigeons while retaining its immunogenicity [2]. The biological basis of this attenuation is the accumulation of mutations in genes essential for replication and pathogenesis in the natural host but dispensable for growth in the permissive cell line. These mutations often occur in genes encoding host range factors, immunomodulatory proteins (e.g., interferon inhibitors), and proteins involved in viral dissemination. The resulting attenuated strain is replication-competent in the vaccinee but causes only a localized, self-limiting infection that stimulates a robust and durable immune response.

The safety of such an attenuated vaccine is paramount. In the study by Zhu et al. (2026), the attenuated PPV strain was administered to pigeons via the standard wing-web stab method. The vaccinated birds showed only mild, localized swelling at the inoculation site, with no signs of systemic disease, respiratory distress, or mortality [2]. This is in stark contrast to the severe, disseminated disease seen in unvaccinated controls challenged with the virulent parental strain. The efficacy data were equally compelling: vaccinated pigeons were completely protected (100% survival and no clinical signs) against a lethal challenge, whereas all unvaccinated controls developed severe diphtheritic and cutaneous lesions and succumbed to the infection [2]. This level of protection is the gold standard for any veterinary vaccine.

Diagnostic Differentiation and Surveillance as a Control Strategy

Effective vaccination programs cannot exist in a vacuum; they must be integrated with robust diagnostic surveillance to monitor circulating strains and detect vaccine breakthroughs. The inability to rapidly differentiate between FWPV and PPV has been a significant impediment to targeted control. Traditional PCR methods targeting the p4b gene can confirm the presence of an avipoxvirus but cannot distinguish between species without sequencing [1, 7]. To address this, Özgünlük et al. (2024) developed a novel multiplex PCR assay that simultaneously amplifies a pan-APV genus-specific fragment and species-specific fragments from the fpv122 gene of FWPV and the HM89_gp120 gene of PPV [3]. This assay allows for the rapid, cost-effective, and high-throughput differentiation of the two viruses directly from clinical samples, eliminating the need for sequencing. The deployment of such diagnostic tools is critical for determining whether an outbreak is due to vaccine failure (e.g., a FWPV vaccine failing to protect against a PPV challenge) or a failure of vaccine coverage. Furthermore, this assay can be used to screen wild bird populations, as the discovery of PPV in Ghana highlights the need for surveillance of both wild and domestic avian populations to prepare for future outbreaks [1].

The Role of Genomic Data in Future Vaccine Design

The availability of complete PPV genome sequences, such as those from India and China, is revolutionizing vaccine development [2, 4]. Comparative genomics allows for the identification of conserved, essential genes that could serve as targets for next-generation vaccines, such as recombinant vectored vaccines or virus-like particles. The large genome of PPV (280 kbp) provides ample capacity for the insertion of foreign genes, making it an attractive vector for vaccinating against other avian pathogens, a strategy already well-established for FWPV [6]. By deciphering the genetic composition of PPV, researchers can identify the specific immunogens responsible for protective immunity and engineer them into safer, non-replicating platforms. For instance, the deletion of specific virulence genes (e.g., those encoding interferon antagonists) could yield a more defined, genetically stable attenuated vaccine than those produced by serial passage alone. This rational approach, guided by the genomic insights from sources [2] and [4], represents the future of PPV control, moving away from empirical attenuation toward precision vaccinology.

References

[1] Abbiw R, Amoako KK, Enyetornye B, Odoom T, Boateng PA, Abbang SM, et al.. Isolation and Molecular Detection of Pigeonpox Virus in a Pigeon With Both Cutaneous and Diphtheritic Forms of Pigeon Pox Disease in Ghana. Veterinary Medicine International. 2025. DOI: https://doi.org/10.1155/vmi/7523480

[2] Zhu Y, Zhang B, Ji Z, Su J, Chang J, Fan K. First Complete Genome Sequencing of a Pigeonpox Virus Strain from Mainland China and Preliminary Evaluation of Its Attenuated Potential. Veterinary Sciences. 2026. DOI: https://doi.org/10.3390/vetsci13040393

[3] Özgünlük İ, Yücetepe AG, Çetiner B, Keskin O, Özyörük F. Development of a Multiplex PCR Assay for Rapid Differentiation of Fowlpox and Pigeonpox Viruses. Avian diseases. 2024. DOI: https://doi.org/10.1637/aviandiseases-D-23-00042

[4] Sahu B, Panda S, Singh R, Swain S, Sahoo N, Sahoo AK, et al.. First complete genome characterisation of an Indian pigeon pox virus directly from a clinical sample.. Virology. 2025. DOI: https://doi.org/10.1016/j.virol.2025.110663

[5] Liu H, Li T, Tang J, Ding S, Wang D. Complete genome sequence analysis of Reticuloendotheliosis virus integrated in nonhomologous Avipoxvirus.. Microbial Pathogenesis. 2024. DOI: https://doi.org/10.1016/j.micpath.2024.106827

[6] Umar B, Adamu J, Ahmad M, Ahmad KH, Sada A, Orakpoghenor O. Fowlpox virus: an overview of its classification, morphology and genome, replication mechanisms, uses as vaccine vector and disease dynamics. Worlds Poultry Science Journal. 2021. DOI: https://doi.org/10.1080/00439339.2021.1959278

[7] Rajasekaran R, Kirubaharan J, Shilpa P. Isolation, Molecular Detection and Phylogenetic analysis of Avipox Virus obtained from Pigeon. THE INDIAN JOURNAL OF VETERINARY SCIENCES AND BIOTECHNOLOGY. 2018. DOI: https://doi.org/10.21887/IJVSBT.14.3.10

[8] Masola S, Mzula A, Mwega E, Kasanga C, Wambura P. Detection and Genetic Characterization of an Avipox Virus Isolate from Domestic Pigeon (Columba livia domestica) in Morogoro Region, Eastern Tanzania. Artificial Intelligence Review. 2015. DOI: https://doi.org/10.9734/AIR/2015/13382

[9] Rahman S, Islam MA, Islam M, Nazir K, Khan MS. Isolation and molecular detection of Avipoxvirus from field outbreaks in Mymensingh, Bangladesh. Journal of Advanced Veterinary and Animal Research. 2019. DOI: https://doi.org/10.5455/javar.2019.f314

[10] Sprygin A, Pestova Y, Wallace DB, Tuppurainen E, Kononov A. Transmission of lumpy skin disease virus: A short review.. Virus Research. 2019. DOI: https://doi.org/10.1016/j.virusres.2019.05.015

[11] Mosad SM, El-Tholoth M, El-Kenawy A, Abdel-Hafez LJM, El-Gohary F, El-Sharkawy H, et al.. Molecular Detection of Reticuloendotheliosis Virus 5′ Long Terminal Repeat Integration in the Genome of Avipoxvirus Field Strains from Different Avian Species in Egypt. Biology. 2020. DOI: https://doi.org/10.3390/biology9090257

[12] Martin D, Murrell B, Golden M, Khoosal A, Muhire B. RDP4: Detection and analysis of recombination patterns in virus genomes. Virus Evolution. 2015. DOI: https://doi.org/10.1093/ve/vev003

[13] Fusaro A, Zecchin B, Giussani E, Palumbo E, Agüero-García M, Bachofen C, et al.. High pathogenic avian influenza A(H5) viruses of clade 2.3.4.4b in Europe, Why trends of virus evolution are more difficult to predict. Virus Evolution. 2024. DOI: https://doi.org/10.1093/ve/veae027