Carp Edema Virus: Koi Sleepy Disease Reference

Overview and Taxonomy of Carp Edema Virus: Koi Sleepy Disease Reference

Introduction to Carp Edema Virus

Carp edema virus (CEV) is the etiological agent of koi sleepy disease (KSD), also referred to as carp edema virus disease (CEVD), a severe and increasingly prevalent viral affliction of both common carp (Cyprinus carpio) and its ornamental variety, koi [1, 5, 25]. The virus was first identified in Japan in 1974, where it was associated with a characteristic lethargic behavior in affected koi, giving rise to the colloquial name “koi sleepy disease” [6, 25, 29]. For several decades following its initial discovery, CEV was believed to be geographically restricted to Japan; however, the pathogen has since been documented across Europe, Asia, North America, and the Middle East, establishing itself as a globally significant threat to carp aquaculture and the ornamental fish trade [7, 18, 22, 27, 28]. The World Organisation for Animal Health (WOAH) recognizes CEV as a pathogen of substantial concern, and its emergence parallels that of other high-impact cyprinid viruses, including cyprinid herpesvirus 3 (CyHV-3), the causative agent of koi herpesvirus disease (KHVD) [1, 21, 37]. Unlike many viral pathogens of fish, CEV belongs to the family Poxviridae, a group of large, double-stranded DNA viruses traditionally associated with terrestrial vertebrate hosts, making its adaptation to an aquatic host an area of considerable virological interest [6, 25, 35].

Taxonomic Classification and Genomic Architecture

Carp edema virus is classified within the family Poxviridae, subfamily Chordopoxvirinae, but remains unassigned to a specific genus [27, 35]. This taxonomic placement is supported by the virus's large, enveloped, ovoid virion morphology, which is consistent with other poxviruses. Transmission electron microscopy (TEM) of branchial epithelial cells from infected fish reveals characteristic cytoplasmic virions with a complex internal core and lateral bodies, confirming the poxvirus-like architecture [17, 27]. The complete genome sequence of a Japanese CEV isolate (strain FTI2020) has been elucidated, revealing a double-stranded DNA genome of approximately 200–250 kilobase pairs, a size range typical of chordopoxviruses [16]. The genome encodes numerous open reading frames (ORFs) involved in viral replication, host immune modulation, and structural integrity, although the specific functions of many CEV genes remain to be fully characterized [13, 16]. Notably, the CEV genome exhibits a lower G+C content compared to many mammalian poxviruses, which may reflect adaptations to the physiological environment of its fish host [16, 35].

A critical feature of CEV taxonomy is the genetic diversity revealed through phylogenetic analyses, which has led to the delineation of distinct genogroups. The primary molecular marker used for classification is a fragment of the p4a gene, which encodes the major core protein of the virion [1, 5, 19, 26]. Early phylogenetic studies based on this locus identified two major genogroups: genogroup I (gI) and genogroup II (gII) [5, 34]. Subsequent, more detailed analyses have further subdivided gII into subgenogroups IIa and IIb, and have proposed the existence of additional clades, including genogroups IIIa and IIIb, based on isolates from Austria [26]. The epidemiological significance of these genogroups is pronounced: genogroup I is predominantly associated with common carp, particularly in European aquaculture settings, while genogroup II (especially IIa) is more frequently identified in koi and is often linked to international trade movements [5, 22, 34]. The complete genome sequence of the Japanese FTI2020 strain, which belongs to genogroup IIa, has provided a robust reference for comparative genomics and has facilitated the identification of other variable regions beyond the p4a gene, such as the cds46 locus, which may harbor markers for tracking haplotype diversity and viral spread [13, 16].

Virion Structure and Morphogenesis

The CEV virion is a complex, brick-shaped or ovoid particle, approximately 200–300 nm in diameter, consistent with the poxvirus morphology [25, 27]. The virus particle is enveloped, containing a double-stranded DNA genome encased within a core wall and flanked by lateral bodies. These lateral bodies are unique to poxviruses and contain enzymes involved in early viral transcription. The outer envelope is derived from host cell membranes during viral egress. In infected koi, TEM has documented the presence of these virions within the cytoplasm of gill epithelial cells, often in association with intracytoplasmic eosinophilic inclusion bodies, which are characteristic of poxvirus replication [17, 20]. The replication of CEV, like all poxviruses, occurs entirely within the cytoplasm of host cells, a feature that distinguishes them from most other DNA viruses, which replicate in the nucleus. This poxviral replication cycle involves the formation of viral factories, assembly of immature virions, and maturation into the infectious intracellular mature virion (IMV) form, followed by wrapping and egress as extracellular enveloped virions (EEV) [35]. The inability of CEV to propagate in conventional fish cell lines has been a major obstacle to detailed in vitro studies of its morphogenesis and entry mechanisms [5, 35]. However, limited replication has been achieved in primary gill explant cultures, confirming that gill tissue provides the necessary cellular environment for CEV replication [5].

Genogroup Diversity and Global Molecular Epidemiology

The molecular epidemiology of CEV has been extensively characterized using the p4a gene fragment, revealing a complex pattern of genetic diversity that correlates with host species and geographic origin. Genogroup I is the predominant lineage circulating in common carp populations across Europe. In a comprehensive survey of German carp populations, genogroup I variants were detected in 69% of cases from carp farms, with identical haplotypes appearing in geographically distant locations, suggesting a common source of virus spread through fish trade networks [22]. Similarly, wild common carp mortality events in the United States (New Jersey) and Italy involved genogroup I strains that clustered closely with European isolates, indicating transcontinental dissemination [27, 31]. Genogroup II, particularly subgenogroup IIa, is overwhelmingly associated with koi and has been identified in Asia (Japan, Korea, China, India), Europe (France, Germany, UK), and North America [6, 19, 22, 30, 33]. The detection of genetically identical genogroup IIa variants across different continents highlights the role of the international ornamental fish trade in the global dissemination of this pathogen [22, 34].

The genetic boundaries between these genogroups are defined by specific nucleotide substitutions and insertion/deletion (indel) events within the p4a amplicon. For instance, a diagnostic 121-base pair insertion is present in genogroup I haplotypes but absent in many genogroup II haplotypes, while the cds46 locus exhibits extensive variability, including complete absence in some lineages [13, 34]. This high degree of genetic plasticity suggests that CEV is evolving rapidly, potentially through mechanisms such as homologous recombination and point mutations [34]. Evidence of mixed-genotype infections within individual fish or fish batches has been documented at the cds46 locus, indicating that co-infection with multiple CEV strains can occur, which may facilitate recombination events and the emergence of novel variants [13]. The detection of a putative recombinant strain in France, possessing a chimeric p4a sequence combining elements of genogroup II and an unknown lineage, further underscores the evolutionary dynamism of this virus [34]. These findings have profound implications for vaccine development and diagnostic assay design, as assays targeting conserved regions (e.g., p4a) must reliably detect all genogroups, while monitoring efforts must account for the potential emergence of variant strains that may evade detection or exhibit altered virulence.

Host Range and Species Specificity

CEV exhibits a narrow host range, predominantly infecting species within the genus Cyprinus. The virus is highly pathogenic to common carp and koi (both Cyprinus carpio), but susceptibility varies among strains and age classes [5, 20]. Among carp strains, Amur wild carp (Cyprinus carpio haematopterus) and Amur sazan have demonstrated relative resistance to clinical disease following experimental infection, whereas koi and other domesticated carp strains (e.g., Ropsha scaly carp, Prerov scaly carp) are highly susceptible [5, 12]. This differential susceptibility is not fully explained by variations in type I interferon responses, suggesting that other genetic factors governing host-pathogen interactions are critical determinants of disease outcome [5, 12].

Importantly, CEV does not appear to cause disease in non-cyprinid fish species. Experimental challenges and field surveys have consistently failed to detect CEV replication or clinical signs in goldfish (Carassius auratus), Indian major carp (e.g., Labeo rohita, Catla catla), grass carp (Ctenopharyngodon idella), silver carp (Hypophthalmichthys molitrix), or native North American species such as bluegill (Lepomis macrochirus), even when these fish are cohabitating with heavily infected common carp [20, 32]. A notable exception is a hybrid of silver carp and bighead carp (Hypophthalmichthys molitrix × H. nobilis), which has been shown to develop clinical signs and mortality (up to 15–20%) following natural exposure in Ukrainian carp farms, suggesting that certain cyprinid hybrids may possess some level of susceptibility [14]. The host restriction of CEV is likely mediated by specific viral entry receptors or intracellular factors unique to Cyprinus species, a phenomenon that warrants further investigation. The ability of double-crested cormorants (Phalacrocorax auritus) to mechanically carry CEV DNA in fecal and regurgitant samples without developing infection indicates that piscivorous birds may act as vectors for the virus, transporting it between water bodies [32].

Transmission Dynamics and Environmental Persistence

CEV is transmitted horizontally through direct contact with infected fish or indirect exposure to contaminated water and fomites [5, 6]. The gills serve as the primary portal of entry and the major site of viral replication, with the highest viral loads consistently detected in gill tissue, followed by skin, brain, kidney, and intestine [5, 15, 19]. Cohabitation experiments have demonstrated that naïve fish become infected within 7 days of exposure to CEV-shedding fish, confirming the highly contagious nature of the virus [5, 15]. Viral shedding occurs via gill and skin secretions, as well as through feces and urine, contaminating the aquatic environment [6]. Notably, environmental samples such as shipping water and bag swabs have shown higher sensitivity for CEV detection than direct fish swabs, underscoring the substantial viral load released into the water column by infected fish [6]. This environmental contamination likely facilitates rapid spread within crowded aquaculture or ornamental holding facilities.

The virus can persist in water and on fomites for undetermined periods; however, its stability is influenced by temperature, UV radiation, and organic load. CEV is enveloped, rendering it relatively sensitive to desiccation and common disinfectants, although specific inactivation studies are lacking. The detection of CEV DNA in shipping water and bag swabs from imported koi, months after the initial outbreak, suggests that the virus can survive transport and remain infectious, highlighting the risk of introduction through asymptomatic carrier fish [6]. Once introduced into a facility, CEV can circulate endemically, with subclinically infected carriers acting as reservoirs for continuous transmission. The virus has been detected in all age groups of carp in full-system farms, including broodstock, larvae, and fry, indicating that vertical transmission cannot be ruled out and that the virus can persist across production cycles [14]. Importantly, CEV can go unnoticed for months after importation, with outbreaks often triggered by stressors such as temperature fluctuations, handling, or poor water quality [5, 8, 9, 25].

Pathogenesis and Organ Tropism

The pathogenesis of CEV infection is intrinsically linked to its pronounced tropism for branchial epithelium. The gills are the target organ, where the virus initiates a cascade of pathological events that disrupt respiratory, osmoregulatory, and excretory functions [3-5]. Following entry via the gill epithelium, CEV replicates in the cytoplasm of branchial epithelial cells, leading to cellular hypertrophy, necrosis, and the characteristic histological lesions of proliferative gill disease: severe epithelial hyperplasia, interlamellar fusion, and occlusion of the interlamellar spaces [3, 4, 17, 37]. This lamellar occlusion compromises the functional surface area of the gills, impairing gas exchange and leading to hypoxemia. The proteomic analysis of infected gills reveals a marked upregulation of interferon-stimulated genes (e.g., mx, mx2, gig1, trim21), heat shock proteins (indicative of cellular stress), and matrix metalloproteinases (e.g., mmp13), which are likely involved in the pathological tissue remodeling [3]. Concurrently, there is a suppression of antioxidant enzymes and cytoskeletal regulators, exacerbating cellular damage [3].

The functional consequences of this branchial pathology are profound. CEV-infected fish develop severe hyponatremia (plasma sodium levels as low as 71.65 mmol L⁻¹) and hyperammonemia (blood ammonia up to 1123.24 µmol L⁻¹), reflecting a catastrophic failure of osmoregulation and nitrogen excretion [4]. These electrolyte and metabolic disturbances are directly responsible for the clinical signs of lethargy, edema, and neurological dysfunction. The disease also triggers a pronounced stress response, with elevated plasma cortisol and glucose levels, which in turn suppresses adaptive immunity [9, 17]. While the gills are the primary site of infection, the virus disseminates to other organs. CEV DNA has been detected in the skin, brain, kidney, spleen, and intestine, although viral loads are typically 2–4 orders of magnitude lower than in gills [15, 19]. The kidney and spleen show evidence of immune activation, including upregulation of complement components (C3, B/C2, C9) and altered leukocyte populations, but these organs do not support robust viral replication [10, 15]. The invasion of the brain may contribute to the neurological signs (e.g., disorientation, loss of balance) observed in advanced KSD [15, 33].

Clinical Manifestations: From Acute Lethargy to Asymptomatic Carriage

The clinical hallmark of KSD is lethargy, from which the disease derives its name. Affected fish exhibit progressive apathy, reduced feeding, loss of escape response, and a tendency to rest on the pond bottom, often with flared gills and sunken eyes (enophthalmos) [1, 7, 17, 24, 25]. Other consistent signs include swollen, pale or necrotic gills, excessive mucus production, cutaneous edema (dropsy-like appearance), and skin ulcerations [7, 17, 24, 29]. Mortality rates can reach 80–100% in naive populations, particularly under conditions of high stocking density or suboptimal water quality [5, 33]. The onset and severity of disease are temperature-dependent. In koi, clinical outbreaks typically occur at water temperatures between 15°C and 25°C, with a peak around 18–22°C, whereas in common carp, disease is more frequently observed at lower temperatures (6–15°C) [5, 22, 25, 27]. This temperature differential may reflect adaptations of specific CEV genogroups (gII in koi, gI in carp) or host physiological factors.

A critical feature of CEV epidemiology is the occurrence of asymptomatic infections. Subclinically infected carrier fish harbor the virus in tissues, particularly in the gills, without exhibiting overt clinical signs [6, 8, 14]. These carriers serve as silent reservoirs, perpetuating the virus within populations and facilitating its spread through trade. Under stressful conditions, such as transportation, temperature shifts, poor water quality, or co-infections with parasites or bacteria, these latent infections can reactivate, leading to fulminant outbreaks [8, 23, 36]. The molecular mechanisms underlying latency and reactivation are not fully elucidated, but transcriptomic studies have identified specific differentially expressed genes (e.g., immune-related genes involved in viral sensing and apoptosis) that distinguish asymptomatic carriers from acutely infected fish, providing potential targets for biomarkers of reactivation risk [8]. The existence of a carrier state complicates disease control, as quarantine measures and diagnostic testing must be robust enough to detect low-level infections.

Immunological Paradox: Antiviral Activation Versus Adaptive Suppression

The immune response to CEV is characterized by a paradoxical dichotomy: robust activation of innate antiviral pathways coupled with profound suppression of adaptive immune components. Transcriptomic and proteomic analyses of infected gills consistently demonstrate marked upregulation of type I interferon-stimulated genes (ISGs), including mx, mx2, trim21, gig1, and viperin, as well as pro-inflammatory cytokines such as il-1β, tnf-α, il-6, and ifn-γ [2, 3, 8, 12]. This innate response is particularly pronounced in susceptible koi, suggesting that the host attempts to curb viral replication through interferon-mediated mechanisms. However, this response is not entirely effective, as viral loads remain high in susceptible strains [5, 12]. Concurrently, there is a striking downregulation of adaptive immune genes. Expression of cd4, cd8β, tcrα, and igm is significantly reduced in the gills of CEV-infected koi, especially at temperatures permissive for severe disease (18°C) [4, 9, 12]. This T-cell and B-cell anergy is accompanied by a four-fold drop in peripheral white blood cell counts, with a specific reduction in lymphocyte numbers and a relative increase in monocytes [4, 17]. Phagocytic activity, particularly the respiratory burst of phagocytes, is strongly enhanced in diseased fish, but this is attributed to an increased phagocyte count rather than enhanced per-cell activity [17].

The functional consequence of this immunosuppression is a failure to mount a sterilizing adaptive immune response. Fish that survive a primary CEV infection are not fully protected from reinfection; they may remain persistently infected and shed virus for extended periods [11]. Treatment of infected fish with salt (0.3–0.6% NaCl) ameliorates the osmoregulatory disturbance and reduces mortality, and

Molecular Pathogenesis of Carp Edema Virus: Genomic Structure, Replication, and Host Cellular Interactions

Carp edema virus (CEV), the etiological agent of koi sleepy disease (KSD) and carp edema virus disease (CEVD), is an enveloped, double-stranded DNA virus belonging to the family Poxviridae. Despite its designation as a poxvirus, CEV exhibits distinct genomic and biological features that set it apart from classical chordopoxviruses, reflecting its adaptation to an aquatic, poikilothermic host. The molecular pathogenesis of CEV is intrinsically linked to its genomic architecture, its restricted tropism for branchial epithelial cells, and a series of complex host–pathogen interactions that culminate in severe gill dysfunction, systemic metabolic collapse, and immunosuppression. Understanding these molecular underpinnings is critical for devising effective control strategies, especially given the virus’s widespread distribution in both farmed common carp and ornamental koi populations worldwide, as recognized by the World Organisation for Animal Health (WOAH) and the Food and Agriculture Organization (FAO) as a significant threat to carp aquaculture.

Genomic Structure and Genetic Diversity

The complete genome of CEV was first elucidated from the Japanese strain FTI2020, revealing a linear double-stranded DNA molecule of approximately 245 kb, with a G+C content of around 30% [16]. This genome size is consistent with other large poxviruses, yet CEV shares only limited sequence homology with members of the subfamily Chordopoxvirinae, notably lacking several conserved genes involved in nucleotide metabolism and immune modulation found in terrestrial poxviruses [16]. Whole-genome analyses indicate that CEV is a deeply divergent piscine poxvirus, likely representing an early-branching lineage within the family [16, 35].

The CEV genome encodes an estimated 140–150 open reading frames (ORFs), many of which are of unknown function. Among the few characterized genes, the p4a gene (encoding a major core protein homologue) and the 5′ untranslated region (5′UTR) are the most commonly used targets for molecular diagnostics and phylogenetic classification [19, 21]. Based on partial p4a sequencing, CEV isolates are divided into two major genogroups (I and II), with further subdivision into IIa and IIb [26]. Genogroup I is predominantly associated with common carp in Europe, while genogroup II is more frequently found in koi and has a global distribution [22, 25, 34]. However, recent work has identified additional putative genogroups (IIIa, IIIb) from Austrian isolates, suggesting that the genetic diversity of CEV is still poorly characterized [26].

A particularly intriguing genomic feature is the cds46 locus. This ORF encodes a predicted protein with weak homologies to endonucleases of iridoviruses, and AlphaFold modeling suggests a cellular endonuclease fold, possibly indicating horizontal gene transfer from an iridovirus ancestor [13]. Intriguingly, cds46 is entirely absent from genogroup I haplotypes and from some genogroup II variants, while in others it appears in two allelic forms defined by specific substitutions [13]. The presence of indels and putative recombination events at this locus further underscores the fluid nature of the CEV genome and suggests that cds46 may represent a genomic “hotspot” for diversification, potentially influencing host range or virulence [13]. Additionally, analyses of French isolates have revealed a microsatellite insertion-deletion marker that distinguishes genogroups I and II, as well as evidence of inter-genogroup recombination [34]. This genomic plasticity likely contributes to the virus’s ability to persist across diverse geographic regions and host populations.

Viral Replication and Cellular Tropism

CEV exhibits a strict tropism for epithelial cells of the gill, with viral loads consistently highest in gill tissue [5, 10, 15]. Experimental infections have demonstrated that CEV DNA can be detected in the skin, brain, kidney, and intestine as early as 7 days post-cohabitation, but the gills remain the primary site of replication [15]. Viral gene expression, assessed by quantitative PCR of mRNA transcripts, is most robust in the gills, confirming that this organ is the major replicative niche [5, 12]. Transmission electron microscopy has identified intracytoplasmic, brick-shaped poxvirus particles (approximately 200–300 nm in diameter) within branchial epithelial cells, often associated with inclusion bodies [27, 33]. These virions display the characteristic poxvirus morphology: an outer envelope, a lateral body, and a biconcave core containing the viral DNA.

The replication cycle of CEV is presumed to follow the general poxvirus model, entry via fusion with the host cell membrane or macropinocytosis, uncoating, transcription and replication in cytoplasmic viral factories, assembly of immature virions, and egress by cell lysis or exocytosis [35]. However, unlike many poxviruses, CEV has proven intractable to propagation in conventional fish cell lines, severely hampering studies of its replication kinetics [5, 35]. Limited replication has been achieved in gill explant cultures, but sustained passage in vitro remains elusive [5]. This inability to culture the virus has forced researchers to rely on in vivo infection models and transcriptomic/proteomic approaches to infer molecular events.

The temperature sensitivity of CEV replication is a critical component of its pathogenesis. In koi, clinical outbreaks typically occur at water temperatures between 15°C and 25°C, while in common carp, outbreaks are often observed at cooler temperatures (6–12°C) [22, 25]. Experimental infections at 18°C produce significantly higher viral loads and more severe histopathology than at 12°C, indicating that higher temperatures accelerate replication and disease progression [9]. This temperature dependence likely reflects the host’s immune competence as well as the intrinsic enzymatic requirements of viral DNA replication and transcription.

Host Cellular Interactions: Immune Modulation, Stress, and Tissue Pathology

Once CEV infects the gill epithelium, it triggers a cascade of host cellular responses that are both protective and pathological. Proteomic profiling of gills at the peak of disease has identified 91 differentially abundant proteins, with strong upregulation of interferon-stimulated genes (ISGs) such as Mx, Mx2, Gig1, and Trim21, as well as innate effector molecules like lysozyme C and apolipoprotein A1 [3]. These findings are corroborated by transcriptomic studies showing upregulation of type I interferon (IFN) pathway components, including IFN-γ, iNOS, and IL-10, in acutely infected koi [2, 8]. However, despite this robust antiviral response, the virus is not cleared, and the infection progresses.

A hallmark of CEV pathogenesis is the profound immunosuppression of adaptive immunity, particularly in susceptible koi strains. During acute infection, the expression of T-cell receptor (tcr a2), CD4, CD8β, and IgM is significantly downregulated in the gills [4, 12]. This suppression is accompanied by a four-fold drop in circulating white blood cell counts, specifically a reduction in lymphocytes and an increase in monocytes and phagocytes [4, 17]. The downregulation of adaptive immune genes correlates with viral load and is most pronounced at 18°C in freshwater conditions [9]. Salt treatment (0.3–0.6% NaCl) rescues fish from mortality by restoring osmotic balance and, notably, prevents this adaptive immunosuppression, allowing lymphocyte activation and antibody production during reinfection [11]. This suggests that the virus exploits the host’s osmoregulatory crisis to evade adaptive immunity, a unique mechanism among poxviruses.

In addition to immune modulation, CEV induces severe cellular stress. Heat shock proteins (HSP70, HSP90) and chaperonins are strongly upregulated in infected gills, along with matrix metalloproteinase 13 (Mmp13) and transglutaminase 1 (Tgm1) [3]. Mmp13 and Tgm1 are implicated in abnormal cell proliferation and tissue remodeling, likely contributing to the pronounced epithelial hyperplasia and interlamellar fusion that occludes the gill surface [3, 27]. Concurrently, antioxidant enzymes (e.g., superoxide dismutase, catalase) and metabolic enzymes involved in energy and xenobiotic metabolism are suppressed, indicating oxidative stress and metabolic shutdown [3]. The resulting gill dysfunction leads to severe hyponatremia (blood sodium as low as 71.6 mmol/L) and hyperammonemia (ammonia up to 1123 μmol/L), which in turn exacerbate immunosuppression and cause lethargy, the clinical hallmark of KSD [4, 9].

The virus also manipulates the host’s stress axis. Plasma cortisol and glucose levels are significantly elevated in CEV-infected koi at 18°C, correlating with upregulation of stress-related genes in the hypothalamus–pituitary–interrenal axis [9]. This stress response likely further suppresses immune function and promotes viral persistence.

Interestingly, the host cellular response differs markedly between susceptible (e.g., koi) and resistant (e.g., Amur wild carp) strains. In resistant Amur sazan, the magnitude of the type I IFN response in the gills is not higher than in susceptible koi, indicating that resistance is not purely interferon-driven [5, 12]. Instead, resistant strains show lower viral loads, less pronounced downregulation of adaptive immune genes, and a more balanced cytokine response, upregulating IL-6a and TNF-α2 but not the full panel of inflammatory cytokines seen in koi [12]. These strain-specific differences highlight the importance of host genetics in determining disease outcome.

Finally, CEV infection often occurs in concert with secondary pathogens, such as Flavobacterium branchiophilum, which colonize the damaged gill epithelium and exacerbate pathology [23]. The virus-induced immunosuppression and gill hyperplasia create a permissive environment for these co-infections, underscoring the multifactorial nature of KSD outbreaks [36]. The molecular basis of this synergy likely involves virus-mediated suppression of mucosal immunity, including downregulation of mucin genes and antimicrobial peptides in the gill [12, 23].

In summary, the molecular pathogenesis of CEV is a multifaceted process driven by a genetically diverse poxvirus that selectively replicates in gill epithelia, evades adaptive immunity by exploiting host osmoregulatory failure, and induces a tissue-destructive stress and inflammatory response. The interplay between viral genomic variability, host genetics, environmental temperature, and secondary pathogens shapes the clinical outcome of infection. Future research must prioritize the development of an in vitro culture system and reverse genetics tools to dissect the functions of individual CEV genes, particularly the unique cds46 and other hypothetical proteins, in order to fully understand the molecular determinants of virulence and host range.

Epidemiology of Carp Edema Virus: Global Distribution, Seasonal Patterns, and Risk Factors in Koi and Common Carp

The epidemiology of carp edema virus (CEV) presents a complex and rapidly evolving picture, reflecting the virus’s emergence from relative obscurity to a globally recognized threat to both ornamental koi and food-fish common carp production. Understanding the distribution, seasonal drivers, and multifactorial risk factors associated with CEV is paramount for designing effective surveillance, biosecurity, and control programs. Since its initial recognition in Japan in the 1970s, CEV has been detected across an expanding geographic range, and its prevalence within affected populations is often alarmingly high. This section provides a comprehensive analysis of the global distribution of CEV, the seasonal and temperature-dependent patterns of disease expression, and the intrinsic and extrinsic risk factors that govern infection, clinical manifestation, and viral dissemination.

Global Distribution and Emergence: A Pervasive Pathogen

CEV is now recognized as a pathogen with a truly global distribution, having been confirmed on every continent where common carp or koi are cultured or exist in the wild, with the notable exception of Australia and Antarctica, though its presence is considered likely in many unsampled regions [25]. The virus was first described in Japan in 1974, causing a lethargic, edematous condition in koi, but for decades outbreaks were thought to be confined to that nation [25, 29]. The first detections outside Japan occurred in the United Kingdom and the Netherlands in the early 2000s, followed by a rapid succession of reports from continental Europe, Asia, the Middle East, and North America [22, 29, 33]. This apparent global spread is likely a combination of true viral dissemination via the international ornamental fish trade and increased diagnostic awareness facilitated by the development of sensitive molecular tools such as polymerase chain reaction (PCR) and quantitative PCR (qPCR) [1, 22, 34].

Robust epidemiological surveys have revealed startling prevalence rates. In a landmark study of German carp and koi populations, CEV was detected in 69% of cases from common carp populations in major carp-producing areas and 41% of cases from koi populations across the country, with archival samples revealing its presence in carp as early as 2007 and in koi from 2009 [22]. This high prevalence is not an anomaly. Surveillance in Ukraine detected CEV in 19 out of 34 samples from 14 of 16 regions surveyed, demonstrating widespread circulation in Eastern European carp farming systems [14]. In Croatia, CEV was identified as a cause of mortality events in lakes following the introduction of infected fish, highlighting how anthropogenic movement fuels geographic expansion [18]. Similarly, Serbia has reported an increasing number of CEV outbreaks on carp farms since its first detection in 2017, indicating a pattern of establishment and subsequent spread within naïve populations [39]. A mass mortality event in wild common carp in New Jersey, USA, in the spring of 2017 marked the first confirmation of CEV-associated mortality in wild fish in North America, a watershed moment that demonstrated the virus's capability to move beyond the confines of aquaculture and ornamental ponds into natural ecosystems [27]. Subsequent mortality events in Minnesota and other US states have reinforced this concern, confirming that CEV is now enzootic in certain wild carp populations in North America [32].

The genetic diversity of CEV is organized into distinct genogroups that show a strong association with host species and geography. Phylogenetic analysis based on the partial p4a gene fragment has established three major genogroups: genogroup I (gI), and genogroup II (gII), which is further subdivided into IIa and IIb [22, 26]. Some researchers have proposed additional novel genogroups (IIIa and IIIb) based on Austrian isolates, though this classification is not universally adopted [26]. Critically, genogroup I is almost exclusively associated with infections in common carp, while genogroup II predominates in koi [22, 34]. This host segregation is not absolute but is strikingly consistent, suggesting a degree of host adaptation. For example, a study of German isolates found that all gI sequences originated from common carp, while all gII sequences originated from koi [22]. This pattern has been replicated globally, with gI sequences from European common carp showing high identity with the strain isolated from the wild carp kill in the USA, and gII sequences from koi in Asia, Europe, and North America clustering together [22, 27, 31, 33]. The genetic diversity is further complicated by evidence of recombination events, such as the identification of a putative recombinant in France, and the presence of mixed haplotype infections within single fish populations, as detailed by the analysis of the highly variable cds46 locus [13, 34]. This genetic fluidity has implications for virulence, host range, and the development of diagnostic and control strategies.

Seasonal Patterns and Water Temperature Dynamics

The clinical expression of CEV infection, koi sleepy disease (KSD), is profoundly influenced by water temperature, leading to characteristic seasonal patterns of outbreaks. However, the temperature ranges associated with disease differ markedly between common carp and koi, a phenomenon that reflects both the genetic background of the host and the prevailing genogroup. In common carp, clinical outbreaks of KSD occur predominantly during the spring and autumn, when water temperatures are relatively low, typically between 6 and 16°C, with the highest risk observed in the range of 8–12°C [22, 25, 27, 31]. The mass mortality event in wild common carp in New Jersey, USA, occurred at 15°C, while the Italian outbreak in Lake Santa Croce was recorded at approximately 15°C, and mortality in German carp populations was most common from April to June at 8–12°C, aligning perfectly with the spring warming period when temperatures enter this critical window [22, 27, 31]. This low-temperature association is so robust that CEV should be considered a primary differential diagnosis for unsolved gill disease or mortality in common carp during the spring [22, 31].

In contrast, KSD in koi is most frequently observed at higher water temperatures, generally between 15 and 25°C, with a peak risk from 18 to 22°C [22, 25, 29]. A study of German koi populations found that clinical KSD occurred primarily at water temperatures of 18–22°C, a stark contrast to the 8–12°C range for sympatric common carp [22]. This temperature-dependent dichotomy is not merely observational; controlled experimental infections have confirmed that high viral loads and severe histopathology in koi are exacerbated at 18°C compared to 12°C, directly linking temperature to disease virulence in this host-pathogen pair [9]. The biological basis for this difference may stem from the distinct genetic lineages (gII in koi vs. gI in carp) and their respective optimal replication temperatures, or from differing host physiological and immunological responses to the virus across thermal regimes [5, 22]. The practical implication is that surveillance and diagnostic efforts must be calibrated to the species and the season. In temperate regions, the spring and autumn peaks for common carp are predictable windows for clinical disease. For koi, the summer months pose the greatest risk, particularly during periods of rapid temperature increase or during transport and handling when fish are under thermal stress [25, 30].

Risk Factors for Disease Development and Viral Dissemination

The transition from CEV infection to overt KSD and mortality is governed by a complex web of interacting risk factors. Not all infected fish develop disease; asymptomatic infections are common and represent a critical epidemiological challenge [8, 15]. These asymptomatic carriers, often with low viral loads, can serve as reservoirs for viral transmission, shedding virus into the water without exhibiting any clinical signs [8, 24]. The reactivation of latent or persistent infections is believed to be triggered by stressors, a phenomenon that is likely central to the seasonal nature of outbreaks and the role of management practices.

Host Genetics and Strain Susceptibility: The most significant intrinsic risk factor is host genetics. Different strains of common carp exhibit profound differences in susceptibility to CEV. Experimental infection trials have consistently demonstrated that Amur wild carp (Amur sazan) and certain domesticated strains such as Ropsha scaly and Prerov scaly carp are significantly more resistant to developing clinical KSD compared to koi and other highly inbred ornamental varieties [5, 12]. In controlled cohabitation experiments, Amur wild carp exposed to CEV did not develop clinical signs, whereas koi and other susceptible strains suffered high morbidity and mortality [5]. The mechanistic basis for this resistance is not fully understood but does not appear to be linked simply to a more robust type I interferon response [5]. Transcriptomic and immunological studies suggest that resistant strains may mount a more balanced and less immunopathogenic response, avoiding the profound downregulation of adaptive immune genes seen in susceptible koi, such as the suppression of cd4, tcr a2, and igm expression in the gills [4, 12]. Susceptibility also extends to age and condition, with outbreaks often, though not exclusively, affecting larger, older fish or broodstock, possibly due to cumulative stress or age-related immunosenescence [14, 27].

Stress, Co-infections, and Immunosuppression: Stress is a powerful precipitating factor. The disease is notoriously linked to the stress of transport, handling, introduction to new ponds, and rapid temperature changes [25]. The physiological stress response, characterized by elevated cortisol and glucose, is intimately linked to CEV pathogenesis. Infected koi at permissive temperatures show a pronounced activation of the stress axis, which correlates with higher viral loads, more severe gill pathology, and a paradoxical downregulation of adaptive immune genes [4, 9]. This stress-induced immunosuppression creates a permissive environment for both CEV replication and secondary invaders. CEV infection is rarely a monopathogenic event. The virus causes severe branchial damage, including epithelial hyperplasia and lamellar fusion, which disrupts the physical and immunological barrier of the gills [3, 4, 27]. This insult provides a foothold for opportunistic pathogens. Co-infections with bacteria, particularly Flavobacterium branchiophilum, and parasites are extremely common and can dramatically exacerbate clinical disease [23, 36]. Flavobacterium species act as secondary pathogens, contributing to the proliferative gill disease pathology seen in many severe KSD outbreaks [23]. Indeed, in some cases, the mortality associated with a CEV outbreak may be driven as much by the secondary bacterial infection as by the virus itself [23, 36]. The immunosuppression triggered by CEV, including a significant reduction in white blood cell counts and a shift in leukocyte populations (increased monocytes, decreased lymphocytes), further predisposes fish to these co-infections [4, 17].

Trade, Transport, and Biosecurity: The international trade of live ornamental and food fish is the single greatest risk factor for the long-distance dissemination of CEV. Phylogenetic evidence linking identical or near-identical viral strains across continents, such as gIIa variants from France, the USA, and Korea, or gI variants from Germany and across Europe, points directly to the movement of infected, often asymptomatic, fish as the primary mechanism of spread [22, 33, 34, 38]. The introduction of newly purchased, untested koi into established populations is a classic epidemiological scenario for KSD outbreaks [29]. Critically, recent research has demonstrated that CEV can be detected in shipping water and on the inner surfaces of transport bags, with environmental sampling proving to be highly sensitive for detecting infected consignments before clinical signs appear [6]. This finding has profound implications for biosecurity. The virus can persist undetected in a facility for months after importation, with outbreaks occurring only when fish are subjected to a secondary stressor [6]. This "silent" introduction, followed by stress-triggered amplification, is a key reason why CEV has become so pervasive. In many regions, including Ukraine, Croatia, and Serbia, the virus is now considered endemic, circulating within and between farms and natural water bodies through fish movements, escapees, and potentially through piscivorous birds, which have been shown to carry CEV DNA in their regurgitant and feces [14, 18, 32, 39]. The absence of mandatory, routine testing for CEV in most countries before fish movement is a critical policy gap that allows the virus to remain under the radar, perpetuating its spread and the associated economic losses to the global carp and koi industries [18, 22, 39].

Clinical Manifestations and Pathological Features of Koi Sleepy Disease

Carp edema virus (CEV), a member of the family Poxviridae and the etiological agent of koi sleepy disease (KSD) and carp edema virus disease (CEVD), represents a globally emerging threat to both ornamental koi and food-production common carp (Cyprinus carpio). The World Organisation for Animal Health (WOAH) recognizes CEV as a significant pathogen warranting surveillance due to its substantial economic impact on aquaculture and the international ornamental fish trade. The clinical and pathological features of KSD are a direct consequence of the virus's pronounced tropism for branchial epithelial tissues, leading to a characteristic syndrome of respiratory distress, osmoregulatory failure, and metabolic derangement. Understanding these manifestations in exhaustive detail is critical for accurate field diagnosis, differentiation from other viral pathogens such as cyprinid herpesvirus-3 (CyHV-3), and implementation of effective biosecurity and therapeutic interventions.

Clinical Presentation: The Spectrum from Lethargy to Mortality

The hallmark clinical sign from which the disease derives its common name, "koi sleepy disease", is a profound, progressive lethargy or somnolence. Affected fish isolate themselves from the shoal, often retreating to the bottom or corners of the pond or tank, exhibiting a marked decrease in responsiveness to external stimuli [1, 7, 20]. This lethargic state is not merely a behavioral anomaly; it is a reflection of severe systemic physiological compromise. Fish typically display a loss of equilibrium, swimming with a listless or uncoordinated motion, and assuming a head-down or tail-down posture [17, 33]. In terminal stages, fish may lie on their sides on the substrate, exhibiting minimal opercular movement until death.

Concurrently, severe respiratory distress is universally observed. Affected koi and carp exhibit rapid, shallow opercular movements ("piping" or "flaring" of the gills) and may congregate at the water surface or near inflows in an attempt to access areas of higher oxygen concentration [24, 28, 29]. This dyspnea is a direct clinical correlate of the extensive branchial pathology. Cutaneous manifestations are also prominent. A subset of fish develops multifocal to coalescing skin lesions, which can range from subtle hyperemia and increased mucus production to overt ulcerative dermatitis and generalized edema (anasarca) [7, 24, 29]. Enophthalmos (sunken eyes), a consequence of severe dehydration and electrolyte imbalance, is a frequently noted and diagnostically useful sign, often combined with a pale or discolored integument [14, 29]. The intensity and combination of these signs can vary significantly based on water temperature, host genetics, and the CEV genogroup involved, but the core behavioral shift to profound lethargy remains the most consistent sentinel indicator of KSD.

Branchial Pathology: The Epicenter of Disease

The gills are the primary target organ for CEV replication and the locus of the most severe pathological changes. The disease process is fundamentally a proliferative and necrotizing branchitis. Grossly, the gills of affected fish present a striking appearance. They are typically pale, often described as "anemic" or "ash-grey," in stark contrast to the healthy bright red coloration [27, 39]. This pallor is multifactorial, resulting from edema, reduced blood flow due to lamellar occlusion, and necrosis. A thick, tenacious layer of mucus frequently coats the gill filaments, further impeding gas exchange and ion transport [39]. In more advanced or severe cases, the gill tissue becomes mottled with patchy hemorrhages and progresses to frank necrosis, with the edges of the filaments appearing ragged and eroded [14, 19, 29].

Histopathological examination reveals the underlying microscopic devastation. The earliest and most characteristic changes include severe epithelial hyperplasia and hypertrophy. The lamellar epithelium proliferates to the point of fusing adjacent secondary lamellae, a process described by Adamek et al. (2026) as "interlamellar occlusion" [3]. This obliteration of the interlamellar spaces effectively collapses the functional surface area of the gill, which is the primary anatomical driver of the observed respiratory compromise. Proteomic profiling of diseased gills implicates the upregulation of matrix metalloproteinase 13 (Mmp13) and transglutaminase 1 (Tgm1) as potential molecular drivers of this abnormal, occlusive cell proliferation [3].

Within the hyperplastic epithelium, a complex cellular infiltrate is observed. Inflammatory cells, including granulocytes and monocytes, migrate into the interlamellar spaces [17, 33]. This inflammation is accompanied by vacuolar degeneration and necrosis of respiratory epithelial cells [19]. A hallmark histopathological finding, confirmed by transmission electron microscopy (TEM), is the presence of intracytoplasmic eosinophilic inclusion bodies within the branchial epithelial cells. These structures represent viral factories (viroplasms) and mature virions and are pathognomonic for poxvirus infection [20, 27]. TEM further resolves the large, ovoid-to-pleomorphic poxvirus particles (typically 200–350 nm in diameter) within the cytoplasm of infected cells, providing definitive ultrastructural evidence of CEV replication [1, 27].

Systemic Pathophysiology: Hyponatremia, Hyperammonemia, and Immunosuppression

The localized branchial damage precipitates a cascade of systemic pathological consequences that define the clinical severity of KSD. The gill is a multifunctional organ responsible not only for respiration but also for osmoregulation and nitrogenous waste excretion. The hyperplastic and necrotic lesions severely compromise these functions. Adamek et al. (2021) demonstrated that KSD-affected carp suffer from a massive disturbance of osmotic balance, primarily manifesting as profound hyponatremia. Blood sodium levels in severely affected fish plummeted to as low as 71.65 mmol L⁻¹, compared to typical physiological levels [4]. This loss of ionic homeostasis is a direct consequence of the destruction of the ion-transporting chloride cells within the gill epithelium and the increased permeability of the damaged barrier.

Simultaneously, the failure of ammonia excretion across the compromised gill surface leads to a state of severe hyperammonemia, with plasma ammonia concentrations reaching as high as 1123.24 µmol L⁻¹ [4]. The combination of hyponatremia and hyperammonemia is a pathophysiological signature of KSD. Adamek and colleagues further established a crucial link between these metabolic derangements and immunosuppression. The metabolic stress triggered by the osmotic and excretory failure leads to a four-fold drop in circulating white blood cell counts [4]. This is correlated with a significant downregulation of adaptive immune gene expression in the gills, including cd4, tcr a2, and igm, indicating a state of local and systemic immunosuppression. This immunosuppressive environment facilitates secondary infections, particularly by flavobacteria such as Flavobacterium branchiophilum, which commonly act as opportunistic copathogens, exacerbating the gill pathology and mortality [23].

Subclinical and Carrier States: The Challenge of Asymptomatic Infection

A critical aspect of the clinical and pathological spectrum of KSD is the existence of asymptomatic or subclinical carrier states. It is now well-established that CEV can persist in fish populations without inducing overt clinical signs, particularly at low water temperatures or in fish with partial genetic resistance [5, 8]. Studies utilizing highly sensitive molecular diagnostics, such as real-time PCR and droplet digital PCR, have detected CEV DNA in gill swabs and environmental water samples from apparently healthy cohorts [6, 15]. For example, Montacq et al. (2025) demonstrated that shipping water from imported koi often contained high levels of CEV, indicating that asymptomatic shedders can be a major source of viral introduction into naïve populations [6]. These carrier fish are a profound challenge for disease control, as they can harbor the virus for months and precipitate an outbreak when stressed by handling, transport, or temperature fluctuations [8]. The pathological changes in these fish are minimal or absent, with histology often revealing only mild, focal hyperplasia. However, their role in viral transmission is epidemiologically paramount.

Evolutionary and Histopathological Patterns: Genogroup Associations and Complications

Phylogenetic analysis based on the p4a gene has delineated two major CEV genogroups (I and II), with further clades (IIa, IIb, and potentially III) proposed [5, 13, 26, 34]. These genogroups exhibit a degree of host and clinical association. Genogroup I (gI) isolates are predominantly recovered from common carp in Europe and North America and are often associated with outbreaks at lower water temperatures (8–15°C). In contrast, genogroup II (gII) isolates are more frequently found in koi and are linked to disease at higher temperatures (18–25°C) [5, 22, 34]. Pathologically, both genogroups induce the hallmark proliferative and necrotizing branchitis described above. However, studies using experimental infections suggest that viral virulence is influenced by the host-virus combination, with gI strains typically showing higher virulence towards common carp and gII strains towards koi [5]. Notably, infection with either genogroup in resistant strains like the Amur wild carp may result in minimal to no clinical signs or histopathology, despite detectable viral replication [5, 12].

Co-infections with other pathogens, particularly CyHV-3 (the agent of koi herpesvirus disease) and parasitic infestations (e.g., Trichodina, Dactylogyrus), are exceedingly common and can alter the clinical and pathological picture. KSD can be misdiagnosed as KHVD due to overlapping signs of lethargy and gill necrosis [1, 28]. However, a key point of differentiation is the absence of the severe systemic vasculitis and necrosis of the kidney and spleen that is characteristic of acute KHVD. In KSD, the pathology is more confined to the gills, and the systemic lesions (pancreatic necrosis, lymphopenia) are milder [17, 36]. The presence of parasitic or bacterial copathogens can dramatically increase disease severity, making it impossible to attribute mortality solely to CEV without quantitative diagnostics [36].

Diagnostics for Carp Edema Virus: Molecular Detection (LAMP, PCR), Pond-Side Testing, and Differential Diagnosis from CyHV-3

The accurate and timely diagnosis of carp edema virus (CEV) is paramount for the management and control of koi sleepy disease (KSD), a condition that poses a significant threat to global common carp and koi aquaculture [1, 2, 4]. The clinical presentation of KSD, characterized by lethargy, enophthalmos, gill necrosis, and skin lesions, overlaps substantially with that of koi herpesvirus disease (KHVD) caused by cyprinid herpesvirus-3 (CyHV-3) [1, 7, 18, 28, 30]. This clinical similarity, coupled with the frequent occurrence of co-infections, renders a definitive diagnosis based solely on gross pathology or histopathology unreliable [1, 23, 36]. Consequently, molecular diagnostic methods, particularly those targeting viral nucleic acid, have become the cornerstone of CEV detection and confirmation. The field has evolved from conventional endpoint PCR to more sophisticated real-time quantitative PCR (qPCR) and loop-mediated isothermal amplification (LAMP) assays, with a growing emphasis on pond-side applicability and non-lethal sampling strategies to facilitate rapid outbreak response and routine surveillance [1, 6, 14, 19].

Molecular Detection by PCR and Real-Time PCR

The primary molecular targets for CEV detection are specific regions of the viral genome, with the gene encoding the p4a major core protein being the most extensively validated and widely used [1, 14, 15, 26]. This gene has been instrumental in the phylogenetic classification of CEV isolates into distinct genogroups, primarily genogroup I (gI), found predominantly in common carp, and genogroup II (gII), further subdivided into IIa and IIb, which is more frequently associated with koi [5, 13, 22, 34]. The 5' untranslated region (5'UTR) of the CEV genome has also been successfully targeted, with quantitative PCR (qPCR) assays demonstrating a high analytical sensitivity with a reported detection limit of 4.0 fg/μL [19]. The nested PCR approach, employing two successive amplification rounds targeting the p4a gene, offers enhanced sensitivity, but higher risk for carryover product contamination compared to real-time methods and remains widely used for initial detection and subsequent sequencing for genotyping [14, 19, 29].

Real-time quantitative PCR (qPCR) represents the current gold standard for CEV diagnosis, providing the distinct advantages of quantification, reduced turnaround time, and lower contamination risk due to closed-tube formats [1, 6, 27, 39]. By quantifying the viral DNA copy number against a standard curve, qPCR allows researchers and diagnosticians to discriminate between high viral loads characteristic of acute KSD and the low-level viremia or residual nucleic acid potentially present in carrier or recovering fish [5, 8, 22]. This quantitative capacity is critical, as the mere presence of CEV DNA is not always indicative of active disease, especially in regions where the virus is enzootic [22, 36]. Studies have shown that clinical KSD is typically associated with high virus loads, often exceeding 10,000 copies of CEV-specific DNA per 250 ng of total gill DNA, whereas asymptomatic carriers or fish exposed to non-pathogenic levels of virus may harbor significantly fewer copies [22]. For surveillance and diagnostic accuracy, qPCR is most reliable when performed on gill tissue, as the gills are the primary target organ for CEV replication, consistently harboring the highest viral loads during active infection [5, 10, 15, 36]. Other tissues, including skin, kidney, brain, and spleen, can be positive, but viral loads are generally lower and detection less consistent, particularly in subclinical infections [15, 31].

The utility of real-time PCR extends beyond immediate outbreak diagnosis to include pathogenesis studies and monitoring of viral kinetics. For instance, the technique has been used to demonstrate that CEV loads peak in gills following experimental infection and correlate strongly with the severity of histopathological gill lesions, such as lamellar fusion and epithelial hyperplasia [3, 5, 9]. Furthermore, qPCR has been instrumental in confirming that while salt treatment (0.3–0.5% NaCl) can alleviate clinical signs and restore osmotic balance in CEV-infected koi, it does not eliminate the virus; persistently infected fish can remain PCR-positive for months, acting as potential reservoirs for future outbreaks [11, 24, 25]. The application of droplet digital PCR (ddPCR) has been explored for other aquatic pathogens and offers a potential future tool for CEV, providing absolute quantification without the need for a standard curve, which could improve reproducibility and sensitivity for low-copy-number detection [42].

Loop-Mediated Isothermal Amplification (LAMP) and Pond-Side Testing

A significant advancement in the diagnosis of CEV has been the development of real-time fluorescence LAMP assays designed for point-of-care or pond-side testing [1]. The LAMP method amplifies DNA with high specificity, efficiency, and rapidity under isothermal conditions, obviating the need for a thermocycler. This makes it ideally suited for field deployment, border inspection posts, and local testing by national authorities where laboratory infrastructure may be limited or where a rapid diagnosis is needed to inform immediate disease control measures [1]. Cano et al. [1] described a fluorescence real-time LAMP assay targeting the p4a gene of CEV genogroup I and the orf43 gene of CyHV-3, achieving a limit of detection of 10³ viral copies for CEV and 10² copies for CyHV-3 in under 25 minutes. When applied to clinical mucus swabs from common carp during disease investigations, the LAMP assay successfully detected CEV DNA in clinical samples within 4 to 13 minutes, with results that were in complete agreement with the reference laboratory qPCR analysis. This rapid detection time is a critical advantage, allowing for same-day decision-making regarding quarantine, movement restrictions, or therapeutic interventions like salt treatment [1, 24].

However, the current LAMP methods have limitations. The sensitivity of the CEV LAMP assay (10³ copies) is slightly lower than that of the most sensitive qPCR protocols [22], potentially missing low-level infections in asymptomatic carriers [1]. Furthermore, the incorporation of a reliable internal control (IC) to distinguish true negatives from test failures due to inhibitors or poor DNA extraction has proven challenging. Cano et al. [1] attempted to use a LAMP assay targeting the common carp ef1a gene as an IC, but it was only amplified in 61% of mucus swabs, highlighting the difficulty of developing a robust, host-derived IC for non-lethal, low-biomass samples like mucus. Despite these challenges, the LAMP approach represents a powerful tool for decentralized testing, particularly during the acute phase of an outbreak when viral loads are highest. Further optimization, including the incorporation of exogenous internal amplification controls and improved sample processing methods for mucus, will be critical to expanding its application for routine health assessments and pre-movement screening [1, 6].

Differential Diagnosis from CyHV-3 (Koi Herpesvirus)

The differential diagnosis between CEV and CyHV-3 is a persistent clinical challenge because both viruses cause severe gill disease and high mortality in koi and common carp, with overlapping clinical signs including lethargy, anorexia, and gill pathology [1, 7, 18, 30]. Both pathogens are also classified as emerging infectious diseases of ornamental fish with global distribution [18, 25, 41]. While some clinical and epidemiological features can provide clues, such as the tendency for CEV to cause more pronounced enophthalmos (sunken eyes) and skin edema, or CyHV-3 outbreaks often occurring at warmer temperatures (18–28°C) compared to the broader range seen with CEV (6–25°C) [7, 22, 25], these distinctions are insufficient for a definitive diagnosis. The neurotropic and branchiotropic nature of CEV can lead to a profound lethargy that is sometimes less pronounced in acute CyHV-3 infections, but this is highly variable [5, 15, 17].

The critical factor complicating differential diagnosis is the high prevalence of co-infections. Numerous studies have confirmed that CEV and CyHV-3 can simultaneously infect the same fish, sometimes comprising a mixed infection that exacerbates mortality [28, 30, 32, 36]. In a multifactorial gill disease outbreak, a fish might succumb to KHVD, KSD, or a synergistic co-infection, making it impossible to attribute the cause of death to a single agent based on clinical signs alone [36]. For instance, mass mortality events in Iraq [28], the Republic of Korea [30], and the USA [32] have all shown that CEV and CyHV-3 can be present in the same fish or within the same population during a mortality event. Therefore, molecular testing for both pathogens is mandatory in any diagnostic workup of carp with gill disease [40]. Quantitative real-time PCR is invaluable here, as it can not only confirm the presence of both viruses but also assess the relative viral load of each, helping to identify the primary etiological agent driving the current pathology [22, 36]. A fish with a CEV load of 10⁷ copies and a CyHV-3 load of 10² copies is far more likely to be suffering from KSD, whereas the reverse would implicate KHVD. This approach provides a scientific basis for treatment decisions, as salt therapy may benefit CEV-affected fish but has no effect on CyHV-3, for which no specific antiviral therapy exists [7, 24].

From an epidemiological and regulatory perspective, this differential diagnosis is critical. CyHV-3 is a notifiable pathogen to the World Organisation for Animal Health (WOAH), and its detection triggers official control and trade restrictions [1, 40]. While CEV is not currently listed by WOAH, its presence can still have profound economic consequences and should be a component of national surveillance programs for carp [18, 22, 39, 40]. The implementation of concurrent testing, using validated qPCR or nested PCR assays for both CEV (targeting p4a) and CyHV-3 (targeting orf43 or other conserved regions), should be the standard of care for any diagnostic laboratory investigating carp mortality [1, 14, 30, 36]. The use of non-lethal samples, such as gill swabs or mucus, for these molecular tests has been validated and is strongly recommended for routine surveillance and pre-import screening, facilitating the detection of both viruses without sacrificing high-value broodstock or ornamental koi [1, 6]. In summary, the molecular diagnostics for CEV have evolved from basic detection to sophisticated, quantitative, and field-deployable systems, but their greatest value lies not in isolation but in their systematic application alongside CyHV-3 testing, ensuring accurate etiological diagnosis of complex gill disease syndromes in carp.

Host Immune Responses to Carp Edema Virus Infection and Implications for Disease Susceptibility

The host immune response to carp edema virus (CEV) is a complex, multifaceted interplay between antiviral defence mechanisms, inflammatory cascades, and the physiological stress axis, ultimately determining the trajectory from asymptomatic carriage to fulminant koi sleepy disease (KSD). As CEV is a poxvirus with a tropism for branchial epithelium, the immune response is uniquely shaped by the organ’s dual roles in osmoregulation, respiration, and immunity, in addition to the virus’s capacity to modulate host defences [4, 35]. Understanding these interactions is critical for explaining stark differences in susceptibility between host strains and for informing intervention strategies, including salt therapy and potential vaccination approaches. The economic significance of CEVD, as recognised by global bodies such as the World Organisation for Animal Health (WOAH), underscores the need for a mechanistic understanding of host-pathogen dynamics in this emerging disease [18, 21, 37].

3.1 Innate Antiviral Responses: The Type I Interferon Axis and Interferon-Stimulated Genes

The initial recognition of CEV infection triggers a robust innate antiviral programme centred on the type I interferon (IFN) system. Transcriptomic and proteomic analyses of gill tissue from experimentally and naturally infected common carp consistently demonstrate a strong upregulation of interferon-stimulated genes (ISGs). In a comprehensive proteomic profiling study of gills at the peak of disease, several key effectors were identified as highly abundant, including Mx, Mx2, Gig1, and Trim21 [3]. Mx proteins, classical antiviral effectors, are induced by type I IFNs and function to inhibit viral replication by targeting nucleocapsids. The persistent upregulation of Mx2, observed in multiple studies, appears to be a hallmark of CEV infection [12]. Notably, in strains with differing susceptibility, Mx2 expression was induced in all groups but with distinct kinetics; in resistant Amur sazan (AS) and Ropsha scaly carp, Mx2 peaked at 6 days post-infection (dpi), whereas in susceptible koi, the expression remained elevated until 11 dpi, suggesting a failure to resolve the infection despite an active antiviral state [12].

Beyond the classic ISGs, the response includes innate effector proteins such as lysozyme C and apolipoprotein A1, both of which were strongly upregulated in the gill proteome [3]. Lysozymes, particularly type C and G, serve as critical components of mucosal immunity, directly degrading bacterial peptidoglycan. Their upregulation in gills and skin of infected fish likely represents a broad, non-specific defence response that is also a reaction to tissue damage and the risk of secondary bacterial invasion [10]. The complement system, a key humoral component of innate immunity, also shows a dynamic and tissue-specific response. During natural outbreaks, components of the complement cascade, including various C3 proteins, were upregulated in the head kidney and spleen but less so in the directly affected gills, whereas C9 showed upregulation in kidney, spleen, and gills [10]. This pattern suggests that while the systemic complement system is activated, local complement activity at the primary infection site, the gill, may be selectively modulated or suppressed, possibly a viral immune evasion strategy.

3.2 Inflammatory Cytokine Signalling and the Pro-Inflammatory-Inhibitory Balance

The innate response is accompanied by a marked inflammatory cytokine response, although its magnitude and character correlate strongly with clinical outcome. In susceptible koi undergoing acute CEV infection, a profound upregulation of pro-inflammatory cytokines occurs. mRNA expression analyses of gill tissue reveal significant increases in tumour necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β), and IL-6a [2, 12]. These cytokines are central to the orchestration of inflammation, recruiting leukocytes and inducing fever-like responses. Concurrently, the anti-inflammatory cytokine IL-10 is also strongly upregulated, representing a host attempt to regulate the potentially damaging inflammatory cascade [2, 12]. This simultaneous surge of both pro- and anti-inflammatory signals creates a delicate balance that, if tipped, can lead to excessive tissue pathology. The histopathological hallmark of KSD, severe epithelial hyperplasia leading to interlamellar occlusion, is directly linked to this dysregulated inflammatory environment [3, 4]. Indeed, ingenuity pathway analysis of the gill proteome identified networks associated with inflammation and abnormal cell proliferation, linking cytokine signalling to the activation of matrix metalloproteinase 13 (Mmp13) and transglutaminase 1 (Tgm1), enzymes involved in tissue remodelling and cell adhesion [3].

In striking contrast, more resistant carp strains, such as Amur wild carp and Amur sazan, exhibit a dampened and more controlled inflammatory profile. While they do show upregulation of IL-6a and TNF-α2, they do not mount the significant upregulation of IL-1β and IL-10 seen in koi [12]. This suggests that resistance is associated not with a stronger immune response, but with a more restrained and less pathological inflammatory reaction. The capacity to control inflammation without over-exuberant cytokine production appears to be a key determinant of survival, preventing the severe branchial hyperplasia and occlusion that leads to respiratory and osmoregulatory failure.

3.3 Dysregulation of Adaptive Immunity: T Cell and B Cell Suppression

A paradoxical and clinically crucial aspect of CEV pathogenesis is the pronounced suppression of adaptive immunity, particularly within the gill microenvironment. Despite a robust innate antiviral and inflammatory response, the adaptive arm of the immune system appears to be actively inhibited, contributing to viral persistence and disease progression. At the peak of clinical disease, fish with KSD exhibit a significant downregulation of key adaptive immune genes in the gills, including cd4, tcr a2, and igm [4]. This is accompanied by a profound drop in circulating white blood cell counts, specifically a fourfold reduction in total leukocytes [4]. Flow cytometric analysis further reveals a decrease in lymphocyte counts and a relative increase in monocytes, alongside enhanced phagocytic activity [17]. This shift indicates a reliance on innate effector mechanisms at the expense of a more targeted, long-lasting adaptive response.

The mechanism underlying this immunosuppression is intimately linked to the physiological dysfunction caused by the gill infection. The severe osmoregulatory failure resulting from branchial damage leads to hyponatraemia (as low as 71.65 mmol L⁻¹) and hyperammonaemia (up to 1123.24 µmol L⁻¹) [4]. These ionic and metabolic disturbances directly impact lymphocyte function. Importantly, when the hydro-mineral balance is restored by increasing ambient salt concentration (0.3–0.6% NaCl), the downregulation of cd4 and igm is abrogated, and lymphocyte numbers are partially restored [4, 9]. This suggests that the immunosuppression is not a direct viral effect but a secondary consequence of the loss of gill osmoregulatory function.

Further evidence of adaptive immune impairment comes from studies on cytotoxic cell responses. The expression of GzmA (granzyme A) and CD8b1 is significantly downregulated in the gills of both susceptible koi and resistant AS strains during CEV infection [12]. This suppression of cytotoxic T lymphocyte activity would theoretically impair the ability to clear infected cells. Additionally, the failure of B lymphocytes to respond effectively is highlighted by the persistent viral shedding observed in immunised fish. In a salt rescue model, where primary CEV infection was followed by salt treatment to prevent mortality, immunized fish showed lymphocyte activation and antibody production upon reinfection, yet they failed to fully clear the virus and continued to shed infectious particles for a prolonged period [11]. This indicates that while a secondary adaptive response can be mounted, it is insufficient for sterile immunity, and CEV can establish a persistent, low-grade infection even in the presence of an active antibody response.

3.4 Stress Axis Activation and its Immunomodulatory Consequences

CEV infection imposes a significant physiological stress on the host, leading to activation of the hypothalamic-pituitary-interrenal (HPI) axis. This stress response has profound immunomodulatory consequences that further shape disease susceptibility. In experimentally infected koi, CEV infection resulted in a marked increase in plasma cortisol and glucose levels, particularly in fish kept at higher temperatures (18°C) where viral replication was most intense [9]. The upregulation of stress-related genes was detected in the head kidney, the primary stress axis organ in fish. Importantly, the magnitude of this stress response correlated directly with viral load and disease severity [9].

Cortisol is a potent immunosuppressive hormone in fish, known to inhibit lymphocyte proliferation, suppress antibody production, and skew the immune response towards a Th2-type or regulatory phenotype. Therefore, the CEV-induced stress response likely contributes to the observed suppression of T and B cell responses, creating a feedback loop where viral replication causes physiological damage, which in turn induces cortisol release that further impairs antiviral immunity. This mechanism may explain why asymptomatic carriers, when subjected to stressors such as handling, transport, or temperature fluctuations, can experience viral reactivation and proceed to clinical disease [8]. The ability of resistant carp strains, such as Amur sazan, to better maintain physiological homeostasis under infection pressure, as evidenced by less severe electrolyte disturbances, may be a key factor in their resilience [9].

3.5 Strain-Specific Susceptibility, Viral Persistence, and Implications for Disease Control

The differential susceptibility between common carp strains to CEV is a subject of intense investigation, with implications for selective breeding programmes. Amur wild carp (Cyprinus carpio haematopterus) and Amur sazan have been identified as relatively resistant to KSD, failing to develop clinical signs even when exposed to the virus, while koi and other domesticated lines are highly susceptible [5, 12]. Crucially, this resistance cannot be attributed simply to a higher magnitude of type I IFN response. In fact, Amur carp often show lower expression of antiviral genes compared to koi under infection pressure [5]. Instead, resistance appears to be a complex trait governed by the ability to mount a controlled inflammatory response, avoid excessive tissue occlusion, maintain osmoregulatory function, and limit the magnitude of the stress-induced immunosuppression [9, 12].

The inability of the host immune system to achieve sterile immunity has major implications for disease epidemiology and biosecurity. Asymptomatic carriers, or fish that recover from an acute episode with the aid of salt therapy, can harbour CEV for months and continue to shed the virus [6, 11, 24]. The virus is shed into the water and can be detected in shipping water and tank environments, representing a significant route of transmission within the fish trade [6]. The detection of CEV DNA in faecal and regurgitant samples from double-crested cormorants also raises the possibility of avian vectors in the spread of the virus to wild carp populations [32]. These findings, combined with the virus’s broad distribution across Europe, Asia, and North America, underscore the need for robust surveillance programmes, as advocated by WOAH for emerging fish pathogens, and highlight the limitations of current mitigation strategies that rely solely on non-sterilizing treatments like salt baths [1, 18, 22, 24, 27, 32, 39].

Prevention, Control, and Biosecurity Strategies for Koi Sleepy Disease in Aquaculture and Ornamental Fish Trade

The management of carp edema virus (CEV) and the consequent koi sleepy disease (KSD) presents a formidable challenge to both the ornamental koi trade and the global common carp aquaculture industry. Unlike many other viral pathogens of cyprinids, CEV is a poxvirus that does not replicate in conventional cell culture systems, complicating vaccine development and in vitro neutralization assays [35, 41]. Furthermore, the virus induces a complex pathophysiological state characterized by severe branchial dysfunction, osmoregulatory collapse, and a paradoxical immunosuppression that leaves fish vulnerable to secondary invaders [4, 23]. Consequently, effective prevention and control cannot rely on a single intervention but must be built upon a multi-layered, integrated strategy encompassing rigorous biosecurity, advanced surveillance technologies, environmental management, and a nuanced understanding of host-pathogen interactions. The following sections delineate the current state of knowledge and best practices for mitigating the impact of this devastating pathogen.

Surveillance, Early Detection, and Diagnostic Strategies

The cornerstone of any effective control program is the capacity for rapid, sensitive, and specific detection of the pathogen, particularly in the absence of clinical signs. CEV is notoriously capable of establishing subclinical or asymptomatic infections, which serve as cryptic reservoirs for viral dissemination, especially within the high-value, globally traded ornamental koi sector [6, 8]. Traditional diagnostic methods relying on lethal gill sampling are impractical for routine surveillance of valuable broodstock or for screening live shipments at border inspection posts. This has driven the development and validation of non-lethal sampling strategies.

Recent work has demonstrated the remarkable utility of environmental DNA (eDNA) sampling from shipping water and bag swabs as a highly sensitive, non-invasive surveillance tool. In a comprehensive study of the koi trade, environmental samples, particularly shipping water, exhibited a sensitivity exceeding 89% for detecting CEV, often outperforming direct gill swabs from the same fish [6]. This approach allows for the screening of entire batches of fish prior to import or movement, identifying potentially infected cohorts without the stress and expense of handling individual animals. The detection of viral DNA in shipping environments that genetically matched subsequent outbreak strains strongly suggests that this method detects viable, infectious particles rather than mere environmental contamination, making it a powerful biosecurity tool [6].

For point-of-care or on-site diagnostics, particularly during active disease investigations, loop-mediated isothermal amplification (LAMP) assays have been developed and field-tested. These assays target the p4a gene of CEV and can amplify viral DNA from non-lethal mucus swabs in under 20 minutes, with a limit of detection of approximately 10³ viral copies [1]. While the sensitivity is slightly lower than laboratory-based quantitative PCR (qPCR), the speed, ease of use, and cost-effectiveness make LAMP assays ideal for decentralizing testing during outbreaks, enabling rapid confirmation of KSD at the farm or retail level and facilitating immediate implementation of quarantine measures [1]. However, the development of robust internal controls for these field assays remains an area requiring further optimization to prevent false-negative results from poor sample quality [1].

In the laboratory setting, qPCR targeting the 4a or 5'UTR genes remains the gold standard for confirmation and viral load quantification [15, 19, 36]. The ability to quantify viral load is critical, as it helps differentiate between a primary CEV infection and a subclinical carrier state, and is essential for understanding the dynamics of co-infections with other pathogens like cyprinid herpesvirus-3 (CyHV-3) or Flavobacterium spp. [23, 36]. The World Organisation for Animal Health (WOAH) recognizes the importance of such molecular diagnostics for notifiable pathogens, and the principles of test validation, including repeatability and reproducibility, are paramount for ensuring reliable results across different laboratories and jurisdictions [43]. The integration of these advanced molecular tools into national surveillance programs is no longer optional but a necessity for any country with a significant carp aquaculture or ornamental fish trade sector [18, 22, 39].

Biosecurity Protocols and Movement Control

Given the high prevalence of CEV in many major carp-producing regions, with studies from Germany detecting the virus in 69% of carp populations and 41% of koi populations [22], and its ability to persist asymptomatically, strict biosecurity is the most critical line of defense. The primary route of introduction into naive populations is almost invariably the movement of live fish, often from a single infected source [29, 31]. Epidemiological investigations have repeatedly traced outbreaks back to the introduction of new, untested fish from retailers or farms with unknown health status [18, 29].

Therefore, the first and most effective biosecurity measure is the implementation of a robust quarantine protocol for all incoming fish. This should be coupled with mandatory pre-movement testing for CEV using the sensitive non-lethal methods described above [6, 18]. The quarantine period should be of sufficient duration, at least 4-6 weeks, and conducted at a temperature that may favor viral replication (e.g., 15-20°C for genogroup II strains affecting koi) to allow any latent infections to become detectable [5, 22]. During quarantine, fish should be monitored daily for clinical signs, and sentinel fish can be used to increase the sensitivity of detection.

For facilities that have experienced an outbreak, depopulation, disinfection, and fallowing are the most reliable methods for eradication. CEV, like other poxviruses, is likely susceptible to common disinfectants such as sodium hypochlorite (bleach), potassium peroxymonosulfate (e.g., Virkon®), and iodophors, though specific inactivation studies for CEV are limited. The virus is enveloped, which generally confers susceptibility to lipid solvents and detergents. All equipment, tanks, and water systems must be thoroughly cleaned and disinfected. Following disinfection, a dry fallowing period of several weeks is recommended to ensure complete inactivation of any residual virus. It is crucial to recognize that salt-treated, clinically recovered fish can remain long-term carriers of CEV and continue to shed infectious particles, posing a significant risk to naive populations if introduced into a clean system [11, 24]. Therefore, "rescued" fish from a KSD outbreak should never be considered safe for introduction into a CEV-free facility.

Therapeutic and Environmental Control: The Salt Rescue Model and Its Implications

A unique and highly effective therapeutic intervention for managing clinical KSD is the addition of sodium chloride (salt) to the water. The pathophysiological basis for this treatment is now well understood. CEV infection causes severe branchial hyperplasia and lamellar fusion, which disrupts the gill's essential functions in ionoregulation and ammonia excretion. This leads to profound hyponatremia (plasma sodium levels can drop below 72 mmol/L) and life-threatening hyperammonemia (plasma ammonia exceeding 1100 µmol/L) [4]. The addition of salt to the water, typically at concentrations of 0.3% to 0.5% (3-5 g/L), re-establishes the osmotic gradient, allowing fish to restore their plasma sodium levels and facilitating the excretion of ammonia, thereby preventing mortality [4, 24, 25].

While salt treatment is a life-saving intervention, it is not a cure. Critically, it does not eliminate the virus. Fish that recover under salt therapy remain infected and can become long-term carriers, shedding virus into the environment [11, 24]. This has profound implications for control strategies. The "salt rescue model" has been instrumental in revealing the immunological consequences of CEV infection. The severe hyponatremia and hyperammonemia are directly linked to a state of profound local and systemic immunosuppression, characterized by a significant drop in white blood cell counts and downregulation of key adaptive immune genes such as cd4, tcr a2, and igm in the gills [4]. By correcting the ionic imbalance, salt treatment not only saves the fish physiologically but also restores immune competence, allowing for the development of an adaptive immune response [9, 11].

This immunological restoration has a dual edge. On one hand, it suggests that a controlled primary infection followed by salt treatment could serve as a form of immunization. Indeed, studies have shown that immunized fish, when re-exposed to CEV, mount a robust adaptive immune response with lymphocyte activation and antibody production, and they do not develop clinical disease [11]. On the other hand, these immunized fish are unable to clear the virus completely and remain persistently infected for extended periods, acting as shedders [11]. This precludes the use of such "vaccination" strategies in facilities aiming for CEV-free status. The practical takeaway is that salt is an invaluable tool for reducing mortality during an acute outbreak, but its use must be accompanied by strict quarantine and biosecurity measures to prevent the spread of the virus from recovered carrier fish.

Genetic Resistance and Immunological Considerations for Long-Term Control

The development of genetically resistant strains of common carp represents the most sustainable long-term strategy for controlling KSD in aquaculture. Experimental infections have revealed striking differences in susceptibility among carp strains. The Amur wild carp (Cyprinus carpio haematopterus) and its derivative, the Amur sazan (AS), have consistently demonstrated a remarkable resistance to CEV infection, failing to develop clinical signs of KSD even when cohabitated with heavily infected koi [5, 12]. This resistance is not absolute, the virus can replicate in these fish, but it does not trigger the severe branchial pathology and systemic collapse seen in susceptible koi and common carp strains [5].

The mechanistic basis for this resistance is complex and not simply a matter of a more robust interferon response [5]. Transcriptomic and proteomic studies have shown that resistant strains like AS mount a controlled and effective antiviral response, with upregulation of key interferon-stimulated genes (ISGs) like Mx2 and a balanced inflammatory response, without the overwhelming and dysregulated cytokine storm seen in susceptible koi [12, 44]. In contrast, susceptible koi exhibit a profound and ultimately maladaptive immune response characterized by a strong upregulation of pro-inflammatory cytokines (IL-1β, TNF-α2) and the immunosuppressive cytokine IL-10, coupled with a failure to mount an effective T-cell response, as evidenced by the downregulation of CD4 and CD8 [12, 44]. This suggests that the pathology of KSD is not solely due to viral cytopathology but is significantly driven by an aberrant host immune response.

These findings strongly advocate for the inclusion of resistant strains like Amur wild carp in selective breeding programs for aquaculture [5]. By introgressing resistance alleles into commercial common carp strains, it may be possible to produce fish that are less susceptible to KSD, reducing the reliance on therapeutic interventions and the risk of outbreaks. Furthermore, understanding the specific immune pathways that confer resistance, such as the balance between type I interferon responses and T-cell activation, provides clear targets for future immunomodulatory therapies or the rational design of effective vaccines [2, 10, 21]. While a commercial vaccine remains elusive due to the inability to culture the virus in vitro, the demonstration that salt-mediated recovery can lead to protective immunity offers a proof-of-concept that vaccination is a biologically achievable goal [11]. Future vaccine efforts should focus on delivering key viral antigens in a way that elicits the protective, cell-mediated immune response observed in resistant strains, rather than the dysfunctional, inflammatory response seen in susceptible ones.

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