Koi Herpesvirus

Overview and Taxonomy of Koi Herpesvirus

Introduction and Significance

Koi herpesvirus (KHV), formally designated as cyprinid herpesvirus 3 (CyHV-3), represents one of the most formidable pathogens confronting global cyprinid aquaculture and ornamental fish industries. The virus is the etiological agent of koi herpesvirus disease (KHVD), a highly contagious and often fatal condition affecting common carp (Cyprinus carpio) and its ornamental variety, koi carp (Cyprinus rubrofuscus). Since its initial emergence in the late 1990s, KHV has caused devastating economic losses worldwide, prompting its classification as a notifiable disease by the World Organisation for Animal Health (WOAH) and inclusion on the European Commission's list of significant aquatic pathogens [1, 4, 9]. The disease is characterized by mass mortality events that can exceed 80–100% in naive populations, with all age classes of fish susceptible, thereby threatening both food security derived from carp aquaculture and the multi-billion-dollar ornamental koi trade [4, 17]. The global impact of KHV is compounded by the fish's wide geographic distribution, with carp being one of the most extensively cultured freshwater species across Asia, Europe, and increasingly in other regions. Surveillance data from North Macedonia between 2015 and 2023, for example, involved testing 2,760 samples for KHVD, underscoring the rigorous monitoring efforts nations undertake to prevent incursion of this pathogen [1]. Despite such efforts, the virus continues to spread to new territories, with first detections reported in recent years from Iraq, India, and other regions, highlighting its relentless expansion [2, 3, 23].

Taxonomic Classification and Nomenclature

KHV is a member of the family Alloherpesviridae, a distinct lineage within the order Herpesvirales that comprises herpesviruses infecting fish and amphibians. This family is fundamentally different from the Herpesviridae family that infects mammals, birds, and reptiles, reflecting a deep evolutionary divergence among herpesviruses [9]. Within the Alloherpesviridae family, KHV is classified under the genus Cyprinivirus, which includes other significant cyprinid pathogens such as cyprinid herpesvirus 1 (CyHV-1, the agent of carp pox) and cyprinid herpesvirus 2 (CyHV-2, causing herpesviral hematopoietic necrosis in goldfish) [5, 9]. The genus Cyprinivirus is characterized by large double-stranded DNA genomes, with KHV possessing the largest genome among all known herpesviruses, a feature that contributes to its genetic complexity and evolutionary adaptability [6, 9]. Historically, the virus has been referred to by several synonyms, including Cyprinid herpesvirus 3 (CyHV-3), Koi herpesvirus (KHV), and, in earlier literature, carp nephritis and gill necrosis virus (CNGV). The official designation by the International Committee on Taxonomy of Viruses (ICTV) is Cyprinid herpesvirus 3, though "koi herpesvirus" remains the most commonly used vernacular term in both scientific and industry contexts [9, 13]. This nomenclatural history reflects the virus's initial discovery through its distinct pathological presentation in koi and common carp during concurrent outbreaks in Israel and the United States in 1998 [13].

Morphological and Genomic Characteristics

Electron microscopic examination of KHV-infected cells reveals virions with typical herpesvirus morphology, comprising an icosahedral capsid approximately 100–110 nm in diameter, surrounded by a tegument layer and an envelope derived from host cell membranes. The complete virion measures approximately 150–170 nm, consistent with observations from various isolates [13, 19, 20]. The genome is a linear double-stranded DNA molecule, ranging from approximately 295 to 310 kilobase pairs, making it the largest among all herpesviruses characterized to date [6]. This expansive genome encodes over 150 open reading frames (ORFs), many of which are involved in immune evasion, viral replication, and structural integrity. The thymidine kinase (TK) gene, encoded by ORF55, has been extensively utilized for molecular diagnostics and phylogenetic studies due to its conservation and utility in distinguishing viral lineages [2, 11]. Additionally, the major capsid protein gene and the ORF72 capsid protein are critical targets for both diagnostic assay development and vaccine design [3, 18]. The genome organization of KHV shows high identity (99.1%) among sequenced strains, yet subtle variations, particularly in repetitive regions and ORF150, are associated with attenuation and virulence [6, 19]. Importantly, the virus exhibits significant structural variation (SV) dynamics, including deletions, inversions, and insertions, which occur at high frequency during in vitro propagation and likely contribute to the emergence of defective genomes and attenuated variants [6].

Phylogenetic Lineages and Global Genetic Diversity

Phylogenetic analyses based on various genetic markers, including the TK gene, ORF136, and markers I and II, have consistently resolved KHV into three major genogroups: the Asian genotype, the European genotype, and the North American (US) genotype [2, 5, 10]. The Asian lineage encompasses isolates from Japan, China, Indonesia, Iran, Taiwan, and other East Asian nations, and is often subdivided further into distinct clades. For instance, a recent study from Iraq identified three clades among KHV strains, with the Iraqi isolates clustering within the Asian genotype and showing closest affinity to Iranian and Indonesian strains [2]. Similarly, a Korean isolate was characterized as Asian type, closely related to isolates from Japan, Indonesia, Belgium, Taiwan, and China [5]. The European lineage comprises isolates from the United Kingdom, continental Europe, and some Middle Eastern regions, while the North American genotype includes isolates from the USA and Mexico [2]. Notably, the Japanese strain (J strain), also designated as TUSMT1 with the I++II+ allele, has been identified in outbreaks across Asia and the Middle East, including Iraq [10]. This phylogenetic diversity has practical implications for disease control, as different genotypes may vary in virulence, environmental persistence, and antigenic properties. The development of variable number of tandem repeats (VNTR) typing methods has further refined the ability to trace KHV transmission pathways and distinguish closely related strains, providing a molecular epidemiological tool essential for outbreak investigations [21]. The emergence of lineage-specific markers, such as the I++II+ allele associated with Asian isolates, facilitates rapid genotyping and informs the selection of appropriate vaccine strains for regional application [10].

Host Range and Tropism

The primary and most severely affected hosts for KHV are common carp and koi carp, both belonging to the species Cyprinus carpio (and its recently reclassified Asian lineage Cyprinus rubrofuscus) [4, 13]. However, a substantial body of evidence demonstrates that the host range of KHV extends beyond cyprinids. The virus has been detected in several non-carp species that may serve as mechanical vectors or reservoir hosts, including tench (Tinca tinca), roach (Rutilus rutilus), crucian carp (Carassius carassius), grass carp, Prussian carp, and brown bullhead [15, 22]. Experimental infections have confirmed that some of these species, such as tench and roach, can harbor viral DNA for up to 49 days post-infection and transmit the virus to naive common carp via cohabitation [15]. More alarmingly, KHV has been identified in rainbow trout (Oncorhynchus mykiss), a non-cyprinid salmonid, which not only becomes infected but also mounts a humoral immune response and can transmit the virus to carp at permissive temperatures [8]. Additionally, invasive species such as the round goby (Neogobius melanostomus) have been found to carry KHV DNA in European waters, raising concerns about their role in disseminating the virus across ecosystems [12]. Remarkably, KHV DNA has even been amplified from the intestinal contents of migratory wild ducks in North America, suggesting that avian vectors may contribute to long-distance dispersal [16]. In contrast, species such as fathead minnow (Pimephales promelas) and goldfish (Carassius auratus) appear refractory to productive infection, as evidenced by the absence of viral mRNA and inability to isolate infectious virus, despite occasional detection of KHV DNA [7]. Similarly, silver crucian carp (Carassius auratus langsdorfii) show no clinical signs or viral replication following experimental challenge, confirming their lack of susceptibility [14]. These findings underscore the complexity of KHV epidemiology and the importance of understanding potential reservoir species for effective disease management. The concept of "KHVD" versus "KHV infection" is critical: while many species can be infected and harbor the virus, only carp manifest the clinical disease, with other species acting as asymptomatic carriers [9, 15].

Molecular Pathogenesis and Viral Replication Mechanisms

Koi herpesvirus (KHV), formally designated as Cyprinid herpesvirus 3 (CyHV-3), represents the most extensively characterized member of the Alloherpesviridae family, a lineage of large double-stranded DNA (dsDNA) viruses that infect fish and amphibians. With a genome of approximately 295 kilobase pairs, KHV possesses the largest known genome among all herpesviruses, encoding over 150 open reading frames (ORFs) [6, 19]. This genomic complexity underpins a sophisticated replication strategy, a formidable capacity for host immune subversion, and a propensity for establishing lifelong latent infections. Understanding the molecular pathogenesis and replication mechanisms of KHV is not merely an academic exercise; it is critical for elucidating the drivers of the acute, often devastating, disease known as KHV disease (KHVD), a condition listed as notifiable by the World Organisation for Animal Health (WOAH) due to its profound economic impact on global carp aquaculture [13, 32, 36]. The virus’s life cycle, from host cell entry to virion egress, is intimately linked to host cell machinery and temperature-dependent processes, while its pathogenesis is defined by a dynamic interplay between viral immune evasion strategies and the host’s innate and adaptive immune responses.

Viral Entry, Tropism, and Cellular Replication Cycle

The replication cycle of KHV begins with the attachment of the virion to the host cell surface, a process mediated by envelope glycoproteins. The ORF25 family, which includes the immunogenic envelope proteins pORF25, pORF65, pORF148, and pORF149, plays a crucial role in this initial step. Deletion of ORF25 or ORF149 results in reduced plaque sizes and lower virus titers, attributable to a delayed entry into host cells, underscoring their functional importance in the entry process [37]. Following attachment, the viral envelope fuses with the host cell membrane, and the nucleocapsid is transported to the nucleus. The virus exhibits a broad but specific tropism for cyprinid fish, with common carp (Cyprinus carpio) and koi carp being the primary susceptible hosts [4, 29]. Experimental studies have shown that while viral DNA can be detected transiently in species like silver crucian carp and fathead minnows, the absence of viral mRNA transcription indicates a block at the entry or early replication stage, confirming that these species are not permissive for productive infection [7, 14]. Interestingly, KHV can also infect and replicate in non-cyprinid species such as Nile tilapia and rainbow trout, albeit with varying outcomes and the potential to act as reservoir hosts [8, 20].

Once inside the nucleus, the viral genome undergoes a temporally regulated cascade of gene expression, typical of herpesviruses. Immediate-early (IE) genes, encoding regulatory proteins, are transcribed first, followed by early (E) genes that produce enzymes necessary for viral DNA replication, and finally late (L) genes that encode structural proteins for the assembly of progeny virions. The virus utilizes its own DNA replication machinery, including a viral thymidine kinase (TK) encoded by ORF55 and a deoxyuridine-triphosphatase (dUTPase) encoded by ORF123, to ensure a sufficient nucleotide pool for genome synthesis. These enzymes are non-essential for replication in cell culture but are critical for virulence in vivo; deletion of both TK and DUT results in a highly attenuated virus that can serve as a live vaccine [11]. Genome replication occurs in the nucleus, where the formation of concatemeric DNA is followed by cleavage and packaging into preformed capsids. The capsid protein pORF72, essential for capsid assembly, contains a nuclear export signal (NES) and is associated with mitochondrial localization, suggesting a role in modulating cellular metabolism during infection [18].

Mature nucleocapsids then bud through the inner nuclear membrane, acquiring a primary envelope, which is subsequently lost upon fusion with the outer nuclear membrane. The final envelopment occurs in the cytoplasm, where tegument proteins and envelope glycoproteins are acquired at trans-Golgi network vesicles before the infectious virion is released from the cell. Ultrastructural analysis of infected cells reveals the presence of viral particles at various stages of morphogenesis within the nucleus and cytoplasm, alongside characteristic cytopathic effects including nuclear membrane disintegration, chromatin margination, and the formation of intranuclear inclusion bodies [19, 30]. The entire replicative cycle is exquisitely sensitive to temperature; optimal replication and disease manifestation occur between 18-28°C, while temperatures below 13°C or above 30°C severely restrict viral replication, leading to an asymptomatic carrier state [9, 13].

Latency and Molecular Mechanisms of Persistence

A defining feature of KHV pathogenesis is its ability to establish a latent infection, allowing the virus to persist within the host for extended periods, even at non-permissive temperatures. Latency is a major challenge for disease control, as latently infected fish can serve as asymptomatic carriers, reactivating and shedding virus when conditions become favorable, such as upon a rise in water temperature. The primary reservoir for latent KHV is peripheral blood leukocytes. Transcriptomic profiling of these cells from latently infected common carp has identified a specific transcriptional signature that deviates from the lytic cycle, characterized by low production of reactive oxygen species, high iron export, and a shift towards an anti-inflammatory M2-like macrophage polarization [25]. This state of immune dampening is thought to be actively maintained by the virus, possibly through the action of viral microRNAs or long non-coding RNAs (lncRNAs), which can modulate host gene expression to silence lytic genes and evade immune surveillance.

Deep sequencing analyses of KHV genomes have revealed that structural variations (SVs), including deletions, inversions, and duplications, are a major source of genetic diversity and a key driver of viral evolution in vitro and likely in vivo. These SVs can generate defective genomes that interfere with replication and contribute to viral persistence [6]. The ORF150 region, in particular, has been identified as a hotspot for mutations and deletions during serial passage in cell culture, and a partial deletion in this region is responsible for the attenuation of a live vaccine candidate, highlighting the plasticity of the KHV genome [6, 26]. This genetic variability enables rapid adaptation to different host environments and selection pressures, facilitating the emergence of new genotypes and complicating the development of universally effective vaccines.

Host Immune Subversion and Antagonism

The molecular pathogenesis of KHV is inextricably linked to its sophisticated arsenal of immune evasion strategies. The virus actively subverts both the innate and adaptive arms of the host immune response. A key target is the type I interferon (IFN) system, a frontline antiviral defense. Unlike infection with spring viremia of carp virus (SVCV), which elicits a robust type I IFN response, KHV infection induces only a weak IFN response, suggesting a potent viral antagonism of this pathway [28, 33]. KHV achieves this by interfering with the sensing of viral nucleic acids by pattern recognition receptors (PRRs) such as the RIG-I-like receptors. It has been demonstrated that the host tetraspanin protein CD63 is upregulated during KHV infection and plays a crucial role in the RIG-I/MAVS/TRAF3/TBK1/IRF3 signaling axis, promoting an antiviral state. Knockdown of CD63 enhances KHV replication, indicating that the virus may actively target this or similar pathways [31]. Furthermore, transcriptomic studies have identified a massive downregulation of host lncRNAs and their target mRNAs in immune-related pathways, including the Jak-STAT signaling pathway and the TNF signaling pathway, suggesting that KHV utilizes lncRNAs to orchestrate a broad suppression of the host immune response [27].

Beyond the intracellular environment, KHV also subverts humoral and mucosal immune responses. At the mucosal surface, KHV infection leads to the downregulation of complement component 3 (C3) and secretory IgM (IgMsec) in the skin mucus, while simultaneously upregulating lysozyme G. This subversion of the complement system and poor antibody secretion at the mucosal site contributes to a prolonged window of viral shedding, facilitating transmission to naïve fish [35]. The virus also modulates the serological response; a study on rainbow trout showed that KHV-infected non-cyprinid hosts can produce antibodies with different reactivity profiles compared to those from carp, suggesting that the virus can alter its antigenic presentation in different host species [8]. This capacity for immune subversion is a cornerstone of KHV pathogenesis, enabling the virus to replicate to high titers, cause extensive tissue damage, and establish a persistent infection even in the face of an activated host immune system.

Genetic Determinants of Virulence and Cellular Pathology

The outcome of KHV infection is governed by a complex interplay between viral genotype and host genetics. Several viral genes have been identified as critical determinants of virulence. In addition to the nucleotide metabolism enzymes encoded by TK and DUT [11], the envelope glycoproteins of the ORF25 family, while immunogenic, are not essential for virulence. Deletion mutants of pORF148 and pORF149 are insufficiently attenuated for use as live vaccines, but they still confer protection, indicating that these proteins are important for a robust immune response but are not the sole drivers of pathogenicity [37]. The ORF150 gene product remains a key focus of virulence research, as its deletion consistently leads to attenuation [26].

At the host level, resistance to KHV is a heritable trait with a strong genetic component, as evidenced by genome-wide association studies (GWAS) and quantitative trait loci (QTL) mapping. A major QTL on linkage group 44, which explains approximately 7% of the additive genetic variance for resistance, contains the gene for TRIM25, a protein involved in the RIG-I antiviral signaling pathway. A premature stop mutation in TRIM25 has been identified, highlighting a potential mechanism for genetic susceptibility [34, 36]. Heritability estimates for KHVD resistance are high (0.43 on the observed scale), indicating that selective breeding is a viable strategy for improving resistance. Resistant carp strains, such as the Amur wild carp, exhibit an earlier and more robust type I IFN response and complement activation compared to susceptible strains like koi, underscoring the importance of the timing and magnitude of the innate immune response in determining disease outcome [28, 33].

The pathological consequences of viral replication are most severe in the gills, skin, and internal organs. In infected fish, histopathological examination reveals severe gill pathology, including necrosis and sloughing of secondary lamellae, fusion of lamellae, and infiltration of inflammatory cells with the presence of intranuclear inclusion bodies [30, 38]. Systemic effects include necrosis in the liver, spleen, and kidney, accompanied by hemorrhage and inflammation [38]. The viral load in these tissues can reach extremely high levels; real-time quantitative PCR has detected up to 3.4 × 10⁷ viral copies per sample in the gills of acutely infected fish [40]. This high viral burden, coupled with the virus’s ability to suppress the host immune system, ultimately leads to multi-organ failure and mortality. The precise mechanisms by which viral proteins induce apoptosis or necrosis in host cells are an area of active investigation, with pathways such as NF-κB and Nrf2/Keap1-ARE being implicated in the cellular damage and oxidative stress observed during infection [39]. The synergistic effect of environmental pollutants, such as nanoplastic particles, further exacerbates this pathology by enhancing viral replication 3- to 10-fold and intensifying oxidative stress and immune dysregulation, demonstrating that external factors can profoundly influence the molecular trajectory of KHV pathogenesis [24].

Epidemiology and Global Surveillance of Koi Herpesvirus Disease

The global epidemiology of Koi Herpesvirus Disease (KHVD) represents a complex and dynamic tapestry of viral emergence, dissemination, and establishment, driven by a confluence of anthropogenic factors, viral evolutionary plasticity, and ecological interactions. Since its first recognized outbreaks in 1998 in Israel and the United States [13], KHV (Cyprinid herpesvirus 3, CyHV-3) has rapidly transitioned from an emergent pathogen to a pervasive, enzootic threat to both wild and cultured populations of common carp (Cyprinus carpio) and koi carp (Cyprinus rubrofuscus) across virtually all continents where cyprinid aquaculture exists [4, 9]. The disease is listed as notifiable by the World Organisation for Animal Health (WOAH) and the European Union, underscoring its profound economic impact on global aquaculture, an industry already grappling with the challenges of food security and sustainability [32, 34]. Understanding the intricate epidemiological patterns of KHVD is not merely an academic exercise; it is a prerequisite for designing effective surveillance programs, implementing robust biosecurity protocols, and developing sustainable control strategies, including vaccination and genetic selection for resistance.

Global Distribution and Emergence Patterns

The initial emergence of KHVD was characterized by a series of explosive, high-mortality outbreaks in major carp-producing regions. The virus was first isolated in 1998 from die-offs in Israel and the USA, and subsequently confirmed in Germany in 1997, indicating a cryptic circulation prior to its formal identification [13]. From these epicenters, KHV spread with alarming speed, facilitated by the global trade in live ornamental koi and the movement of infected fingerlings for aquaculture. By the early 2000s, the virus had been reported across Europe, Asia, and North America, with subsequent incursions into South America, Africa, and Australia [29, 51]. The epidemiological landscape is now characterized by a mosaic of endemic regions, sporadic outbreak zones, and a few areas that remain free of the virus.

In Asia, where the majority of the world’s carp is produced, KHV is endemic in many nations, including Indonesia, Japan, China, Taiwan, and Korea [5, 53]. The virus has been a persistent challenge in Indonesia since its introduction in 2002, causing significant losses in both common carp and koi production [40, 54]. In Korea, surveillance efforts have identified both Asian and European genotypes of KHV, often in co-infections with other pathogens like Carp Edema Virus (CEV), highlighting the complexity of disease diagnosis and management in endemic settings [5]. The genetic characterization of KHV isolates from Sulaymaniyah, Iraq, has revealed a close relationship with Iranian and Indonesian strains, suggesting a potential transmission pathway through the live fish trade across the Middle East and South Asia [2]. The first detection of CyHV-3 in India in 2022, from ornamental koi in Chennai, further underscores the ongoing risk of introduction into new territories via the ornamental fish trade [23].

Europe presents a similarly complex epidemiological picture. While some regions, such as the Monticolo lakes in South Tyrol, Italy, have achieved KHV-free status through rigorous surveillance and eradication programs [49], other areas remain endemic. Croatia has experienced significant outbreaks, with a successful eradication in one isolated area in 2016, but subsequent detections of CEV and KHV in other lakes highlight the constant threat of re-introduction [45]. In the United Kingdom, seasonal surveillance using pond-side LAMP assays has demonstrated that KHV outbreaks are often linked to specific environmental triggers, such as water temperature, and can involve co-infections with CEV [44]. The Czech Republic, a major carp producer, has documented the presence of KHV in multiple locations, necessitating ongoing national monitoring programs [52]. A comprehensive nine-year surveillance study in North Macedonia (2015-2023) tested over 2,760 samples for KHVD and found no positive cases, demonstrating that sustained, rigorous surveillance can confirm freedom from disease, a critical status for international trade [1].

Reservoir Hosts, Vectors, and Transmission Dynamics

A critical epidemiological feature of KHVD is the ability of KHV to establish latent, persistent infections in surviving fish, which then act as asymptomatic carriers and lifelong reservoirs of the virus [9, 35]. This carrier state is the single most important factor complicating disease control. Fish that survive an initial outbreak at permissive temperatures (16-28°C) can harbor the virus in a latent form, primarily in leukocytes and other tissues, with reactivation possible during periods of stress or when water temperatures fluctuate [25, 48]. The serological detection of anti-KHV antibodies, particularly through ELISA, has proven superior to PCR for identifying these carrier fish, especially during non-permissive temperature periods when viral replication is minimal [48]. This has profound implications for surveillance, as reliance solely on molecular detection of viral DNA may underestimate the true prevalence of infection in a population.

The host range of KHV, while primarily restricted to cyprinids, is broader than initially thought. While clinical disease and high mortality are largely confined to common carp and koi, a growing body of evidence identifies numerous other fish species as potential mechanical vectors or subclinical carriers. Experimental infections have demonstrated that species such as tench (Tinca tinca), roach (Rutilus rutilus), crucian carp (Carassius carassius), grass carp, Prussian carp, and brown bullhead can harbor KHV DNA for extended periods (up to 49 days in tench and roach) and, critically, can transmit the virus to naïve common carp through cohabitation [15, 22]. The invasive round goby (Neogobius melanostomus), which is rapidly spreading through European waterways, has also been found to carry KHV DNA, raising concerns about its role as a vector in wild ecosystems [12]. Even more surprisingly, non-cyprinid species like rainbow trout (Oncorhynchus mykiss) have been shown to be experimentally infected, develop a humoral immune response, and transmit KHV to naïve carp, although they do not develop clinical disease themselves [8]. This demonstrates that the concept of host specificity for KHV is more accurately described as disease specificity, with many species capable of acting as Trojan horses for the virus.

The role of environmental DNA (eDNA) and fomites in transmission is also significant. KHV can be shed in high quantities in the mucus, feces, and urine of infected fish, and viral DNA can be detected in tank biofilters and water samples [7, 35]. The development of methods to concentrate and detect KHV from water samples, such as filtration-elution, offers a non-lethal, population-level surveillance tool that could be invaluable for early warning systems in aquaculture facilities [43]. Furthermore, the detection of KHV DNA in the intestinal contents of migratory wild ducks in North America suggests that avian vectors may play a role in the long-distance dispersal of the virus between water bodies, a previously underappreciated transmission pathway [16].

Genetic Diversity, Lineages, and Evolutionary Drivers

The global spread of KHV has been accompanied by the emergence of distinct genetic lineages, which have been classified into three major genotypes: Asian, European, and North American [2, 9]. The Asian genotype is further subdivided into clades that include isolates from Japan, Indonesia, China, and Iran, while the European genotype is prevalent across the continent. The North American genotype, first characterized from a wild common carp in Minnesota, is closely related to European strains, suggesting a relatively recent introduction [7]. The genetic basis for these differences is increasingly understood through whole-genome sequencing and phylogenetic analyses. For instance, the Iraqi isolate from Sulaymaniyah was found to be closely related to the Asian genotype, specifically strains from Iran and Indonesia, confirming a transmission link from the Asian continent [2]. Similarly, the first KHV isolate from wild carp in North America (strain US-01-WC) was sequenced and found to be a distinct type strain, providing a crucial reference for future epidemiological tracing in that region [7].

The evolution of KHV is not a slow, gradual process. Research using ultra-deep sequencing during serial in vitro passages has revealed that KHV populations are highly dynamic, characterized by rapid turnovers of structural variations (SVs), including deletions, inversions, and insertions [6]. These SVs, particularly in pathogenesis-associated regions like ORF150, can lead to the emergence of defective genomes and attenuated variants, a phenomenon that has been exploited for the development of live-attenuated vaccines [6, 26]. The presence of variable number of tandem repeats (VNTRs) across the genome provides a powerful molecular tool for tracing the origin and spread of specific virus strains in the field, enabling high-resolution epidemiological investigations [21]. This genetic plasticity means that KHV is capable of adapting to new hosts, environmental conditions, and potentially even vaccine-induced selective pressures, making continuous genomic surveillance a necessity.

Environmental and Host Factors Influencing Outbreaks

The epidemiology of KHVD is exquisitely sensitive to environmental temperature. The disease is most severe at water temperatures between 16°C and 25°C, with optimal replication and mortality occurring at 22-24°C [13, 17]. At temperatures below 13°C or above 30°C, clinical disease is suppressed, but the virus can persist in a latent state, and survivors become carriers [35, 48]. This temperature dependence creates a distinct seasonal pattern of outbreaks in temperate regions, typically occurring in late spring and early autumn when water temperatures are within the permissive range. The phenomenon of temperature-induced reactivation is a critical trigger for outbreaks; a rapid drop in temperature can stress carrier fish and lead to viral recrudescence, followed by a spike in mortality when temperatures rise again [17].

Host genetics play a profound role in determining the outcome of KHV exposure. There is a well-documented gradient of resistance among different carp strains and breeds. Amur wild carp (Cyprinus rubrofuscus) and their crosses (e.g., Amur mirror carp) are consistently more resistant to KHVD than European common carp strains or highly inbred ornamental koi [28, 33]. This resistance is heritable, with estimates of heritability for KHVD resistance ranging from 0.43 to 0.72, indicating a strong genetic component [32, 47]. Genomic studies have identified a significant quantitative trait locus (QTL) on linkage group 44, with the TRIM25 gene emerging as a promising candidate for resistance [36]. Furthermore, transcriptomic analyses comparing resistant Amur wild carp (AS) and susceptible koi (KOI) have revealed that resistant fish mount a more rapid and robust early innate immune response, particularly involving class I interferon signaling and the complement cascade [28]. The ability to breed for resistance is a powerful, long-term strategy for disease management, and genomic selection (GS) has been shown to be 8-18% more accurate than traditional pedigree-based selection for identifying resistant individuals [32]. Importantly, genetic correlations between KHVD resistance and most production traits (e.g., growth, fillet yield) are generally favorable or non-existent, suggesting that selective breeding for disease resistance can be implemented without compromising productivity [47].

Surveillance Methodologies and Diagnostic Challenges

Effective global surveillance for KHVD relies on a multi-tiered diagnostic approach, combining clinical observation, molecular detection, serology, and, where possible, virus isolation. The WOAH-recommended gold standard for active infection is the detection of viral DNA by PCR, with real-time quantitative PCR (qPCR) offering the highest sensitivity and specificity, capable of detecting as few as 8.4 copies/μL [40, 41]. However, the sensitivity of PCR is highly dependent on the sample type, timing of collection, and the viral load. Gill biopsies and mucus swabs are commonly used for non-lethal sampling, but their sensitivity can be lower than that of internal organs like kidney and spleen, particularly during latent infections [48].

The development of rapid, point-of-care (POC) diagnostics has been a major advance for field surveillance and outbreak response. Cross-priming amplification combined with lateral flow assays (CPA-LFA) can detect KHV with 93.67% sensitivity and 100% specificity in under an hour, making it suitable for on-site testing at farms or border inspection posts [41]. Similarly, fluorescence real-time loop-mediated isothermal amplification (LAMP) assays targeting the orf43 gene of the European genotype can amplify viral DNA from clinical samples in less than 20 minutes, with a limit of detection of 10² copies [44]. These POC tools are invaluable for rapid decision-making during outbreaks, but they require careful validation and the incorporation of internal controls to avoid false negatives, particularly when testing asymptomatic carriers [44].

Serological surveillance, primarily using enzyme-linked immunosorbent assays (ELISA) to detect anti-KHV antibodies, is the method of choice for identifying past exposure and latent carriers. A validated antibody ELISA has been shown to be superior to gill or blood qPCR for detecting prior KHV exposure, especially in fish that have cleared the acute infection [48]. This is crucial for epidemiological surveys aimed at determining the true prevalence of infection in a population, as well as for certifying fish as free of infection before movement. The development of recombinant single-chain variable fragments (scFvs) that bind specifically to KHV particles offers a new avenue for direct antigen detection, potentially filling a gap in diagnostics for cases where virus titers are low [42].

Coinfections and the Changing Disease Landscape

A critical emerging theme in KHVD epidemiology is the high frequency of coinfections with other viral and bacterial pathogens, which can dramatically alter disease presentation, severity, and diagnosis. The most commonly reported co-infection is with Carp Edema Virus (CEV), the causative agent of Koi Sleepy Disease (KSD). Coinfections of KHV and CEV have been documented in wild carp populations in the United States, in farmed carp in Croatia, and in koi in Korea, often during mass mortality events [5, 45, 46]. The clinical signs of KHV and CEV can be very similar (e.g., gill necrosis, lethargy), making differential diagnosis based on gross pathology unreliable [44]. The simultaneous detection of both viruses is essential for accurate diagnosis and appropriate management. Furthermore, KHV infection often predisposes fish to secondary bacterial infections, particularly with Aeromonas hydrophila, which can exacerbate mortality and complicate clinical presentations [50].

The interaction between KHV and emerging environmental pollutants, such as nanoplastics, represents a new and concerning dimension to the disease’s epidemiology. Experimental evidence demonstrates that exposure to polystyrene nanoparticles (PS-NPs) can synergistically enhance KHV replication by 8-10-fold, while simultaneously suppressing type-I interferon responses and disrupting the gut microbiota [24]. This suggests that environmental pollution could be an unrecognized co-factor that increases the susceptibility of carp populations to KHVD and amplifies the severity of outbreaks. The implications for surveillance and risk assessment are profound, as they imply that the health status of a water body, including its chemical and particulate load, may be as important as the presence of the virus itself.

In conclusion, the epidemiology of KHV is a multifaceted and rapidly evolving field. The virus has successfully exploited global trade networks, a broad and expanding host range, and a sophisticated latency mechanism to become a permanent fixture in the global carp landscape. Effective surveillance must therefore be equally sophisticated, integrating molecular, serological, and environmental monitoring, while accounting for host genetics, coinfections, and anthropogenic stressors. The continued emergence of new genotypes and the potential for environmental co-factors to modulate disease severity underscore the need for sustained, international collaborative surveillance efforts to protect both the economic viability of carp aquaculture and the ecological integrity of wild fish populations.

Diagnostic Approaches: Molecular and Serological Techniques

The accurate and timely diagnosis of Koi Herpesvirus (KHV, CyHV-3) infection is the cornerstone of effective disease management, surveillance, and control programs. Given the virus's status as a notifiable pathogen to the World Organisation for Animal Health (WOAH) and its capacity to cause devastating economic losses in global carp aquaculture, diagnostic approaches must be robust, sensitive, and capable of detecting virus in both clinical and subclinical cases [13, 29]. The diagnostic landscape for KHV has evolved significantly, moving from traditional virus isolation and histopathology to a sophisticated arsenal of molecular and serological techniques. These methods are not merely interchangeable; they serve distinct purposes, from confirming acute infections and quantifying viral load to identifying latent carriers and assessing population-level exposure. A comprehensive diagnostic strategy integrates these tools, leveraging the unparalleled sensitivity of nucleic acid amplification for active infections and the retrospective power of serology for exposure history.

Molecular Detection: The Gold Standard for Active Infection

Molecular techniques, primarily polymerase chain reaction (PCR) and its derivatives, have become the gold standard for the direct detection of KHV DNA, offering exceptional sensitivity, specificity, and speed. The selection of a specific molecular method depends on the diagnostic objective, whether it be rapid point-of-care testing, high-throughput surveillance, or precise viral quantification.

Conventional and Nested PCR: Conventional PCR, targeting genes such as the thymidine kinase (TK) gene, the major capsid protein (MCP) gene, or the SphI fragment, remains a widely used and reliable method for initial detection and confirmation of KHV in clinical samples [3, 10, 23, 30, 50]. Its utility is particularly evident in resource-limited settings and for confirming outbreaks where clinical signs are suggestive. The development of nested PCR assays, which involve two successive amplification rounds, has significantly enhanced analytical sensitivity, allowing for the detection of very low viral loads that might be missed by single-round PCR. This approach has been instrumental in confirming the presence of KHV in carrier fish and environmental samples where viral titers are minimal [3, 56]. For instance, semi-nested PCR assays have been successfully employed to confirm the presence of KHV in mass mortality events in Iraq, providing definitive molecular evidence alongside pathological findings [10]. Furthermore, duplex PCR methods have been developed to simultaneously detect KHV and common bacterial co-pathogens like Aeromonas hydrophila, which is critical for understanding complex disease presentations and guiding appropriate therapeutic interventions [50].

Quantitative Real-Time PCR (qPCR): The advent of quantitative real-time PCR (qPCR) has revolutionized KHV diagnostics by providing not only detection but also precise quantification of viral DNA. This technique, often employing TaqMan probes or SYBR Green, offers superior sensitivity, with detection limits as low as 8.384 copies/μL, and eliminates the need for post-amplification processing, reducing the risk of cross-contamination [40, 41]. The ability to quantify viral load is invaluable for several applications. It allows for the monitoring of viral kinetics during experimental infections, assessing the efficacy of antiviral compounds or vaccines by measuring reductions in viral burden [59]. In epidemiological studies, qPCR is used to determine the intensity of infection in different tissues, revealing tissue tropism and the dynamics of viral shedding. For example, studies have shown that KHV DNA can be detected in gill biopsies, blood, and mucus, with qPCR providing a clear quantitative measure of infection severity [35, 48]. The technique is also critical for differentiating between active replication and residual nucleic acid from non-viable virus, particularly in vaccine efficacy trials where a DIVA (Differentiating Infected from Vaccinated Animals) strategy is employed. A triplex real-time PCR has been specifically developed to differentiate between wild-type KHV and TK-deleted vaccine strains, a crucial capability for field deployment of live attenuated vaccines [11]. The high sensitivity of qPCR has also been harnessed for detecting KHV in environmental water samples, a non-invasive approach for surveillance. The filtration-elution method, followed by qPCR, has proven to be the most effective for concentrating viral particles from water, enabling the detection of the virus even when fish are asymptomatic [43].

Isothermal Amplification Assays: For point-of-care (POC) or field-deployable diagnostics, isothermal amplification methods offer a compelling alternative to PCR, as they do not require a thermocycler. Two prominent examples are Loop-Mediated Isothermal Amplification (LAMP) and Cross-Priming Amplification (CPA).

• Real-Time LAMP: Fluorescence real-time LAMP assays have been designed for the rapid detection of KHV, targeting genes such as ORF43. These assays can amplify viral DNA to detectable levels in under 20-25 minutes, with a limit of detection of approximately 10² viral copies [44]. The speed and simplicity of LAMP make it ideal for use at border inspection posts, quarantine facilities, and during on-farm outbreak investigations. A study evaluating a CyHV-3 LAMP assay demonstrated 95.6% specificity against a panel of 72 fish herpesviruses and successfully detected KHV in clinical mucus swabs from 13 out of 16 disease investigation sites in the UK, with results validated by reference laboratory qPCR [44]. However, challenges remain, including the need for robust internal controls to prevent false negatives, as the host ef1a gene was not consistently amplified in all mucus samples [44].

• CPA-Lateral Flow Assay (CPA-LFA): The CPA-LFA represents a further simplification, combining isothermal amplification with a lateral flow readout, similar to a pregnancy test. This method eliminates the need for specialized fluorescence detection equipment. A recently developed CPA-LFA targeting the TK gene demonstrated a diagnostic sensitivity of 93.67% and a diagnostic specificity of 100% when validated against a panel of 179 fish samples [41]. While its detection limit (675.69 copies/μL) is higher than that of qPCR, its simplicity, rapid turnaround time, and lack of need for sophisticated instrumentation make it exceptionally well-suited for POC diagnosis in the field, enabling rapid decision-making for quarantine and biosecurity measures [41].

Serological Techniques: Unveiling Past Exposure and Latent Carriers

While molecular methods excel at detecting active viral replication, they are often inadequate for identifying latent or past infections, a critical aspect of KHV epidemiology. During latency, the virus persists in tissues such as blood leukocytes and the central nervous system at extremely low copy numbers, often below the detection limit of even the most sensitive PCR assays [9, 25]. In such cases, serological techniques, which detect the host's antibody response to the virus, become indispensable.

Enzyme-Linked Immunosorbent Assay (ELISA): The ELISA is the most widely validated serological tool for KHV. It detects anti-KHV antibodies (primarily IgM) in the serum or plasma of infected fish. The WOAH-recommended ELISA has been extensively validated for use in common carp and koi, demonstrating high sensitivity and specificity for identifying fish that have been exposed to the virus [8, 48]. The diagnostic performance of ELISA has been shown to be superior to qPCR of gill or blood samples for detecting prior exposure, particularly in fish that have cleared the active infection and entered a latent state [48]. This makes ELISA an essential tool for surveillance programs aimed at certifying populations as KHV-free, as required by international trade regulations. For instance, in a study monitoring koi over an 11-month period following experimental infection, ELISA consistently identified exposed fish long after qPCR results from gills and blood had become negative [48]. The test has also been adapted to detect antibodies in non-cyprinid species, such as rainbow trout, which can act as reservoir hosts, though the antibody response in these species may differ in its reactivity pattern compared to that in carp [8].

Virus Neutralization Test (VNT): The VNT is a functional serological assay that measures the ability of serum antibodies to neutralize the infectivity of KHV in cell culture. While more labor-intensive and time-consuming than ELISA, the VNT provides a direct measure of protective humoral immunity. It is particularly useful for assessing the efficacy of vaccination, as neutralizing antibodies are a key correlate of protection [11, 26, 55]. Studies have shown that vaccination with live attenuated or DNA vaccines induces neutralizing antibodies that correlate with survival upon challenge [26, 55]. The VNT can also reveal differences in the quality of the antibody response; for example, rainbow trout infected with KHV at 15°C produced neutralizing antibodies, whereas those infected at 20°C did not, despite both groups being seropositive by ELISA [8]. This highlights the nuanced information that can be gleaned from functional serological assays.

Immunohistochemistry (IHC) and Immunocytochemistry (ICC): These techniques provide spatial resolution, allowing for the direct visualization of KHV antigens within tissues or cells. IHC, using monoclonal or polyclonal antibodies against viral proteins such as ORF72, can confirm the presence of the virus in histopathological lesions, linking the molecular detection to tissue damage [18, 38]. This is particularly valuable for confirming the etiology of lesions observed during necropsy. For example, IHC has been used to demonstrate the presence of KHV protein in the cytoplasm of infected kidney cells and splenocytes, with a stronger signal observed in susceptible koi compared to more resistant hybrid crosses [38, 57]. Similarly, ICC using streptavidin-biotin methods on blood smears has been proposed as a simple, practical, and accurate method for early identification of KHV infection, with positive reactions indicated by a characteristic golden-brown color [53, 58]. While less sensitive than PCR, these methods are powerful for confirming infection in tissues with characteristic pathology and for studying viral pathogenesis at the cellular level.

Strategic Integration of Diagnostic Approaches

No single diagnostic test is sufficient for all scenarios. A robust diagnostic strategy for KHV must integrate multiple techniques, each selected based on the specific question being asked. For an acute outbreak with high mortality, qPCR on gill or kidney tissue is the fastest and most sensitive method to confirm the diagnosis [40, 41]. For pre-movement screening or certification of a population as KHV-free, a combination of qPCR (to detect active shedders) and ELISA (to detect latently infected, seropositive fish) is recommended [48]. This dual approach is critical because a fish can be a latent carrier with a negative PCR result but a positive serological result, posing a risk of virus reactivation under stress [9, 48]. For vaccine efficacy trials, qPCR is essential for quantifying viral load reduction, while VNT and ELISA are used to assess the humoral immune response [11, 26]. The development of DIVA-compatible molecular tests, such as the triplex qPCR for wild-type versus TK-deleted vaccine strains, further refines our ability to manage vaccination programs in the field [11]. The continued evolution of diagnostic technologies, including the refinement of POC tests like CPA-LFA and the application of high-throughput sequencing for genomic epidemiology, promises to further enhance our capacity to detect, monitor, and ultimately control this devastating pathogen [6, 41].

Clinical Disease and Histopathological Manifestations in Cyprinus carpio

The clinical presentation and pathological hallmarks of koi herpesvirus disease (KHVD) in Cyprinus carpio (common carp and koi carp) represent a complex, multifactorial syndrome driven by the cytolytic replication of cyprinid herpesvirus 3 (CyHV-3). The disease trajectory is profoundly influenced by water temperature, host genetics, viral strain virulence, and the presence of co-infecting agents. Understanding these manifestations is critical for field diagnosis, differential diagnosis from other carp viruses such as carp edema virus (CEV) and spring viremia of carp virus (SVCV), and for informing biosecurity measures that align with World Organisation for Animal Health (WOAH) standards. The disease is notifiable to WOAH due to its devastating economic impact on global carp aquaculture, and no effective curative treatments exist, making early recognition of clinical signs paramount [4, 9, 13].

Clinical Manifestations: From Behavioral Disruption to External Lesions

The clinical course of KHVD is acutely dependent on ambient water temperature, with disease expression typically occurring between 16°C and 28°C, while temperatures below 13°C or above 30°C suppress clinical signs but may permit a latent carrier state [13, 17]. The incubation period following exposure via waterborne virus (bath or cohabitation) is typically 7–14 days, though mortalities can begin as early as 3 days post-infection under optimal thermal conditions [23, 60]. The first observable signs are invariably behavioral. Affected carp exhibit profound lethargy, disorientation, and erratic swimming patterns, including spiraling or listing to one side at the tank bottom [13, 61]. Fish frequently congregate at the water surface or near aeration points, displaying an increased breathing frequency (pumping) and rapid opercular movements indicative of severe respiratory distress [56, 61]. Anorexia is a consistent and early feature, with affected fish ceasing to feed as the disease progresses [61].

Externally, the pathognomonic gross lesions are centered on the gills and skin, reflecting the primary sites of viral entry and replication. Gill pathology is the most consistent and striking feature. The gills undergo a characteristic progression from diffuse pallor or mottling to severe multifocal necrosis, often described as "gill rot" [3, 23, 30]. The affected gill tissue appears eroded, with white or grey necrotic patches and an excessive production of thick, tenacious mucus that often clogs the interlamellar spaces [10, 17, 30]. This mucus hypersecretion, combined with lamellar destruction, critically impairs gas exchange, directly contributing to the observed respiratory distress and mortality. Skin lesions manifest variably, ranging from generalized hyperemia and hemorrhage at the operculum and fin bases to discrete, shallow ulcers and erosions on the trunk and head [3, 10, 23, 61]. The caudal fin often appears "frayed" or necrotic at its margins [61]. Enophthalmos (sunken eyes) is a frequently reported sign, often accompanied by a pale or whitish discoloration of the cornea [3, 17, 61]. In some cases, particularly in koi varieties, a loss of skin pigmentation or a "sandpaper-like" texture due to epithelial hyperplasia may be observed [17]. It is critical to note that these clinical signs, while highly suggestive, are not entirely specific to KHVD; co-infections with CEV (koi sleepy disease) and secondary bacterial invaders such as Aeromonas hydrophila can produce overlapping presentations, necessitating confirmatory molecular diagnostics [5, 44, 46, 50].

Gross Pathological and Histopathological Manifestations

At necropsy, the gross pathological changes correlate with the external findings. The gills are universally affected, displaying the focal to diffuse necrosis described above. Internally, the viscera may show nonspecific congestion and petechial hemorrhages, but the most consistent and diagnostically useful histopathological changes are found in the gills, skin, kidney, and spleen [10, 17].

Gill histopathology is the cornerstone of microscopic diagnosis. The earliest lesions involve hyperplasia and hypertrophy of the lamellar epithelium, leading to partial or complete fusion of adjacent secondary lamellae, effectively reducing the respiratory surface area [30, 38, 62]. As infection progresses, this is followed by widespread necrosis and sloughing of the lamellar epithelium, with exposure of the underlying cartilaginous support structures. The lamellar capillaries become severely congested, and the interlamellar spaces are infiltrated with a mixed population of inflammatory cells [10, 30]. A hallmark feature, consistently reported across diverse geographic outbreaks, is the presence of intranuclear inclusion bodies within infected epithelial cells of the gill and skin [10, 20, 30]. These inclusions are typically eosinophilic (Cowdry Type A), marginating the host cell chromatin. Their presence is highly indicative of active herpesvirus replication [10, 20]. Additionally, increased numbers of goblet cells and mucus secretion are observed on the gill surface, contributing to the clinical mucoid discharge [30].

Systemic histopathological changes extend beyond the gill. In the skin, the epidermis shows similar changes: hyperplasia, necrosis, and intraepithelial vesicle formation, often with the presence of intranuclear inclusions [10]. In the kidney, the principal lesion is a severe interstitial nephritis, with focal necrosis of hematopoietic tissue and renal tubular epithelium [17, 38, 57]. In the spleen, there is lymphoid depletion with multifocal necrosis and infiltration of macrophages and lymphocytes [57]. In the liver, focal areas of coagulation necrosis and hepatocellular degeneration are frequently observed, often accompanied by a mild inflammatory response [17, 57]. The gastrointestinal tract may show enteritis with necrosis of the lamina propria [17]. Advanced molecular techniques, such as in situ hybridization, have confirmed the presence of viral nucleic acid within these affected tissues, with the strongest signals in the gills, skin, and kidney of susceptible carp, providing a direct link between viral replication and tissue destruction [22]. Ultrastructural examination via transmission electron microscopy (TEM) of infected cells reveals the classical morphology of a herpesvirus: intranuclear capsids (approximately 100–110 nm in diameter) and enveloped virions (150–170 nm) in the cytoplasm and extracellular space, alongside disrupted nuclear membranes and marginalized chromatin [19, 20].

Factors Modulating Histopathological Severity and Clinical Outcome

The severity and outcome of infection are not uniform across all Cyprinus carpio populations. There are profound genetic differences in susceptibility between carp strains. Asian-origin breeds, such as Amur wild carp (Cyprinus rubrofuscus), exhibit significantly higher resistance to KHVD than European common carp strains, such as Prerov scaly carp or the highly susceptible koi varieties [28, 33]. This resistance is correlated with a more rapid and robust innate immune response, including earlier activation of type I interferon signaling and complement cascades [28, 33]. Conversely, highly susceptible strains show a delayed and subdued antiviral response, allowing for higher viral loads and more extensive tissue destruction. Quantitative trait locus (QTL) mapping has identified genomic regions associated with resistance, including the TRIM25 gene, which is involved in the innate antiviral response [34, 36]. This genetic variation is directly reflected in histopathological outcomes: resistant strains display milder gill lesions and lower viral antigen loads in tissues compared to susceptible strains, which suffer from severe, diffuse necrosis [28, 38].

Co-infections dramatically exacerbate the clinical and histopathological picture. Co-infection with CEV is common and produces a more severe disease syndrome, with overlapping but distinct pathological features [5, 46, 56]. Bacterial co-infections, particularly with Aeromonas hydrophila, are frequent secondary invaders that exploit the immunocompromised and damaged epithelia, leading to profound septicemia and extensive hemorrhagic ulceration that may obscure the primary viral lesions [50]. Furthermore, environmental stressors such as exposure to nanoplastics (e.g., polystyrene nanoparticles) have been shown to synergistically amplify KHV pathogenesis. In these scenarios, nanoplastics enhance viral replication (by 8–10 fold), exacerbate oxidative stress, and disrupt the gut microbiota, leading to a more severe inflammatory response and greater histopathological damage in the gills and other organs [24]. Ultimately, the histopathological picture of KHVD is that of a severe, multifocal, necrotizing infection of the gill and skin epithelia, with systemic dissemination to the kidney, spleen, and liver, the extent of which is determined by a complex interplay of viral virulence, host genetics, and environmental context.

Host Immune Response and Immunomodulation by Koi Herpesvirus

The interaction between Koi Herpesvirus (KHV, CyHV-3) and the immune system of its cyprinid hosts represents a complex and dynamic battlefield, characterized by a robust but often insufficient host response and a sophisticated array of viral immunomodulatory strategies. Understanding this interplay is critical for developing effective vaccines, therapeutics, and management strategies for Koi Herpesvirus Disease (KHVD), a condition listed as notifiable by the World Organisation for Animal Health (WOAH) due to its devastating economic impact on global carp aquaculture [13, 32, 34, 36]. The host response is a multi-layered process, beginning with immediate innate defenses and progressing to adaptive immunity, while the virus has evolved mechanisms to subvert these defenses, establish latency, and ensure its transmission.

The Innate Immune Response: A First Line of Defense and a Target for Subversion

The initial host response to KHV infection is orchestrated by the innate immune system, which relies on pattern recognition receptors (PRRs) to detect pathogen-associated molecular patterns (PAMPs). Transcriptomic analysis of peripheral blood leukocytes from KHV-infected common carp has revealed a robust activation of several toll-like receptors (TLRs), including tlr2, tlr5, tlr7, and tlr13, alongside other PRRs and signaling mediators like the urokinase plasminogen activator surface receptor-like and galectin proteins [25]. This PRR engagement triggers downstream signaling cascades, most notably the mitogen-activated protein kinase (MAPK) pathway and the vascular endothelial growth factor (VEGF) pathway, which are central to the inflammatory response [25]. However, the virus appears to actively manipulate these pathways. A key finding is that KHV infection leads to a suppression of type I interferon (IFN-I) expression [24]. This is a critical immunomodulatory tactic, as type I IFNs are the cornerstone of the antiviral state, inducing hundreds of interferon-stimulated genes (ISGs) that inhibit viral replication. The ability of KHV to dampen this response, potentially by interfering with the cellular sensing of foreign nucleic acids, is a major factor in its virulence [33].

The role of type I IFN signaling in resistance is nuanced and breed-dependent. Studies comparing the highly resistant Amur wild carp (AS) with the highly susceptible koi carp (KOI) have shown that while both breeds mount an IFN response, the kinetics and magnitude differ. Resistant AS carp exhibit an earlier and more pronounced activation of class I interferon signaling and the complement cascade compared to susceptible KOI carp [28]. This suggests that the speed and efficiency of the innate response, rather than its mere presence, are crucial for controlling viral replication. Paradoxically, while a robust IFN response is beneficial, KHV infection generally induces a low type I IFN response compared to other viruses like Spring Viremia of Carp Virus (SVCV), further indicating a potent viral mechanism for suppressing this pathway [33]. This suppression is linked to the virus's ability to interfere with cellular sensing, a hallmark of many herpesviruses.

Cellular Immunity and the Oxidative Stress-Inflammation Axis

The innate immune response also involves a complex interplay of cellular metabolism, oxidative stress, and inflammation. KHV infection triggers significant changes in the oxidative state of the host. Studies have demonstrated that infection leads to increased activity of superoxide dismutase (SOD) and elevated levels of interleukin-1β (IL-1β), a key pro-inflammatory cytokine, while simultaneously decreasing acid phosphatase (ACP) activity and increasing alkaline phosphatase (AKP) activity [24]. This indicates a state of heightened oxidative stress and inflammation. The virus also manipulates cellular processes within leukocytes. Transcriptomic data from KHV-infected leukocytes show a low production of reactive oxygen species (ROS) and altered glutathione metabolism, alongside high iron export and phagocytosis activity, but low autophagy [25]. This specific metabolic profile suggests the virus is actively modulating the host cell environment to favor its own replication and persistence, potentially by reducing ROS-mediated damage and limiting autophagic degradation of viral components.

The polarization of macrophages is another critical aspect of the immune response. KHV infection appears to drive macrophages towards an M2-like, anti-inflammatory phenotype, as evidenced by the up-regulation of genes such as arginase non-hepatic 1-like, macrophage mannose receptor-1, crem, il-10, and il-13 receptors [25]. This shift away from a classical M1, pro-inflammatory, and antiviral state is a sophisticated viral strategy to dampen the host's ability to clear the infection. Concurrently, markers for cytotoxic T cells are down-regulated, further crippling the cell-mediated adaptive immune response [25]. This multifaceted suppression of cellular immunity is a key feature of KHV pathogenesis.

The Adaptive Immune Response and the Challenge of Latency

The adaptive immune response, particularly the humoral arm, is crucial for controlling and clearing KHV infection, especially during the transition from acute disease to latency. The production of neutralizing antibodies is a hallmark of a protective immune response. Vaccination studies have consistently shown that the induction of KHV-specific IgM antibodies correlates with protection. For instance, an oral probiotic vaccine expressing the KHV ORF81 protein elicited a significant level of antigen-specific IgM with neutralizing activity, providing approximately 85% protection against viral challenge [55]. Similarly, DNA vaccines encoding immunogenic proteins like ORF149, delivered via carbon nanotubes, have been shown to induce strong serum antibody production and confer high levels of protection (up to 81.9%) [63, 64]. The development of these antibodies is a T-cell-dependent process, and the presence of memory B cells is essential for long-term immunity.

However, the adaptive immune system faces a formidable challenge in the form of viral latency. KHV is known to establish a persistent, latent infection in blood leukocytes and other tissues, which is a major obstacle to eradication [25]. During latency, viral gene expression is severely restricted, and virus production is minimal, allowing the virus to evade immune surveillance. The mechanisms governing latency are complex and involve both viral and host factors. The host's serological response is a key indicator of prior exposure, and ELISA-based antibody detection is a powerful tool for identifying carrier fish, particularly when viral DNA is undetectable by PCR [48]. The interplay between the virus and the host's mucosal immune system is also critical. During viral shedding, which can last for weeks, there is a down-regulation of complement component 3 (c3) and secretory IgM (IgMsec) transcripts at the mucosal surface, coupled with poor immunoglobulin secretion [35]. This subversion of the mucosal immune response facilitates a prolonged window of viral shedding, enabling transmission to new hosts.

Immunomodulation by Viral Proteins and Host Genetics

KHV employs a diverse arsenal of viral proteins to modulate the host immune response. The envelope glycoproteins encoded by the ORF25 gene family (ORF25, ORF65, ORF148, ORF149) are immunogenic, but their deletion does not prevent the induction of a protective immune response, indicating that they are not essential for immunity [37]. Other viral proteins, such as the ORF72 capsid protein, are targeted by the host immune system and are considered candidates for diagnostic reagents and vaccines [18]. The virus also manipulates host cell signaling pathways. For example, the host protein CD63, a tetraspanin, is up-regulated in response to KHV infection and appears to play an antiviral role by modulating the RIG-I/MAVS/TRAF3/TBK1/IRF3 signaling pathway [31]. KHV, in turn, may attempt to counteract this by down-regulating CD63 expression to facilitate its replication [31]. Furthermore, the virus can induce the expression of long non-coding RNAs (lncRNAs) that negatively regulate key immune pathways, including apoptosis, NOD-like receptor signaling, Jak-STAT signaling, and NF-κB signaling, providing a mechanism for immune escape at the transcriptional level [27].

The genetic background of the host is a major determinant of the outcome of KHV infection. There are well-documented differences in resistance between carp strains. Asian-origin strains like Amur wild carp (AS) are significantly more resistant to KHVD than European strains like koi or Prerov scale carp (PS) [28, 33]. This resistance is heritable, with genomic selection studies estimating a heritability of 0.43-0.72 for KHVD resistance [32, 47]. Genome-wide association studies (GWAS) have identified a significant quantitative trait locus (QTL) on linkage group 44, which contains the TRIM25 gene, a known antiviral factor [36]. A putative premature stop mutation in this gene was discovered, suggesting that functional variants in TRIM25 could be a key determinant of resistance [36]. Other candidate genes identified through QTL mapping and GWAS include tnfa, hif1a, galectin-8, and genes involved in the mTOR signaling pathway and the Herpes simplex infection pathway [34]. The ability to breed for resistance is further supported by the negative genetic correlation between KHVD resistance and a more elongated body shape, indicating that selection for resistance is possible without negatively impacting all production traits [47]. The development of synthetic populations, such as the Indonesian common carp strain, which showed a 62% survival rate compared to 20-26.7% in other strains, demonstrates the practical potential of genetic selection for enhancing resistance [60].

Environmental Cofactors and Synergistic Interactions (e.g., Nanoplastic Exposure)

The pathogenesis and epizootiology of Koi Herpesvirus (KHV, CyHV-3) disease (KHVD) are not solely determined by the intrinsic virulence of the virus or the genetic susceptibility of the host. A growing body of evidence indicates that environmental cofactors, ranging from water temperature and chemical pollutants to the presence of other pathogens, profoundly modulate the host-virus interface, often exacerbating disease severity and facilitating viral transmission. Among these, the emergence of nanoplastics as ubiquitous freshwater pollutants represents a paradigm-shifting environmental stressor that synergistically amplifies KHV pathogenicity through mechanisms involving oxidative stress, immune dysregulation, and gut microbiota disruption. This section provides an exhaustive analysis of these environmental cofactors, with a particular focus on the mechanistic basis of nanoplastic–KHV synergism, while contextualizing these interactions within the broader framework of water quality parameters, temperature dynamics, and polymicrobial coinfections.

Water Temperature as a Master Regulator of KHV Pathogenesis

Water temperature remains the most well-established environmental determinant of KHVD expression. The virus exhibits a defined permissive temperature range, with clinical disease and mass mortality typically occurring between 16°C and 25°C, while temperatures at or below 13°C and above 30°C effectively suppress disease manifestation [13, 17]. This temperature dependency is not merely a matter of viral replication kinetics; it fundamentally alters the host's capacity to mount an effective antiviral response. At permissive temperatures (17–22°C), KHV replicates rapidly, triggering a robust but ultimately dysregulated immune response characterized by mitochondrial oxidative phosphorylation, MAPK signaling, and VEGF pathway activation in peripheral blood leukocytes [25]. Conversely, at non-permissive temperatures (12°C), the virus can establish a subclinical carrier state with prolonged viral shedding lasting up to 57 days (684 degree days), as demonstrated by Cano et al. (2020), who observed that asymptomatic fish held at 12°C continued to shed virus even after temperature increase to 22°C [35].

The mechanistic basis for temperature-mediated pathogenesis involves differential regulation of mucosal immune parameters. At permissive temperatures, KHV subverts the complement system at mucosal sites while suppressing immunoglobulin secretion, creating a prolonged window for viral shedding that facilitates transmission [35]. Critically, the temperature at which initial exposure occurs dictates subsequent disease outcomes: fish exposed at 12°C and then subjected to temperature elevation to 22°C exhibited 100% survival, whereas those exposed at 17°C or 22°C experienced 20% and 0% survival, respectively [35]. This phenomenon has profound implications for aquaculture management, as seasonal temperature fluctuations can trigger KHVD outbreaks in carrier populations. The World Organisation for Animal Health (WOAH) recognizes temperature as a critical factor in KHVD surveillance, recommending that diagnostic sampling account for temperature-mediated latency [9, 13].

Nanoplastic Exposure: A Novel Synergistic Stressor

The recent study by Zhao et al. (2025) represents a landmark investigation into the synergistic effects of polystyrene nanoparticles (PS-NPs) and KHV co-exposure, revealing a multifaceted mechanism by which environmental nanoplastics amplify viral pathogenesis [24]. This work is particularly timely given the increasing detection of nanoplastics in freshwater ecosystems globally, where they originate from the degradation of plastic waste, industrial effluents, and cosmetic microbeads.

Dose-Dependent Ingestion and Viral Enhancement

Zhao et al. demonstrated that koi carp ingest PS-NPs in a dose-dependent manner, with subsequent accumulation in gastrointestinal tissues. The critical finding is that PS-NPs significantly enhanced KHV replication in both in vitro (Cyprinid carp brain cells) and in vivo systems. Viral protein levels increased by 3- to 5-fold, while viral gene expression was upregulated by 8- to 10-fold compared to KHV infection alone [24]. This magnitude of enhancement is unprecedented among environmental cofactors and suggests that nanoplastic exposure fundamentally alters the host's permissiveness to viral replication. The implications for aquaculture facilities located near urban or industrial runoff, where nanoplastic concentrations are highest, are substantial, as even subclinical KHV infections could be amplified to epizootic levels.

Oxidative Stress and Antioxidant Dysregulation

The synergistic interaction between PS-NPs and KHV is mediated, in part, through the induction of oxidative stress. Zhao et al. observed that co-exposure led to increased superoxide dismutase (SOD) activity, a marker of cellular oxidative stress response, alongside elevated reactive oxygen species (ROS)-related markers [24]. This is consistent with the known capacity of nanoplastics to generate ROS through their high surface area-to-volume ratio and surface functional groups. The resulting oxidative milieu creates a permissive environment for viral replication, as ROS can activate redox-sensitive transcription factors such as NF-κB, which in turn promotes viral gene expression. Furthermore, the Nrf2/Keap1-ARE pathway, which normally serves as a cellular antioxidant defense mechanism, may be overwhelmed under conditions of combined nanoplastic and viral stress. This is particularly relevant given that emodin, a natural compound that activates Nrf2 signaling, has been shown to suppress KHV replication and improve survival in koi [39], suggesting that antioxidant capacity is a critical determinant of disease outcome.

Immune Dysregulation: Inflammation and Interferon Suppression

Perhaps the most striking finding from Zhao et al. is the differential modulation of immune parameters under combined exposure. Interleukin-1β (IL-1β) gene expression was significantly increased, indicating a heightened inflammatory state, while type-I interferon (IFN-I) expression was markedly decreased [24]. This dichotomy is mechanistically significant: type-I interferons constitute the first line of antiviral defense, and their suppression directly facilitates viral replication. The concurrent elevation of IL-1β suggests that nanoplastic exposure skews the immune response toward a pro-inflammatory, but ineffective, antiviral state. This pattern mirrors observations in other viral systems where environmental pollutants suppress interferon signaling while promoting inflammation, creating a "cytokine storm" phenotype that exacerbates tissue damage.

Additionally, Zhao et al. documented alterations in innate immune enzymes: acid phosphatase (ACP) activity declined, while alkaline phosphatase (AKP) activity increased [24]. ACP is a key lysosomal enzyme involved in pathogen degradation, and its suppression may impair the host's ability to clear viral particles. Conversely, elevated AKP is often associated with tissue damage and inflammation. These enzymatic changes, combined with the ROS-related alterations, paint a picture of broad immune dysregulation that extends beyond the classical antiviral pathways.

Gut Microbiota Dysbiosis and the Gut-Immune Axis

A particularly novel aspect of the Zhao et al. study is the demonstration that PS-NPs disrupt gut microbiota composition, leading to dysbiosis that further weakens host immunity and facilitates viral infection [24]. The gut microbiota plays a crucial role in shaping systemic immune responses in fish, including the regulation of mucosal immunity and the production of antimicrobial peptides. Nanoplastic-induced dysbiosis likely reduces the abundance of beneficial commensal bacteria while promoting pathobiont expansion, thereby compromising the intestinal barrier and creating a portal for viral entry. This finding aligns with the growing recognition that the gut-immune axis is a critical determinant of disease resistance in teleosts. The potential for probiotics to mitigate these effects is supported by studies showing that lactic acid bacteria postmetabolites can inhibit KHV replication and block viral attachment to host cells [65], and that oral probiotic vaccines expressing KHV antigens can provide up to 85% protection [55]. However, the efficacy of such interventions under conditions of concurrent nanoplastic exposure remains to be determined.

Coinfections and Polymicrobial Synergism

Beyond abiotic environmental factors, the presence of other pathogens represents a critical biotic cofactor that can synergistically exacerbate KHVD. Coinfections with carp edema virus (CEV), the causative agent of koi sleepy disease, have been documented in multiple geographic regions, including the United States [46], Croatia [45], and Iraq [56]. In the US, Padhi et al. (2019) reported KHV/CEV coinfections in 4 of 22 mortality events affecting wild common carp across Minnesota, Iowa, Pennsylvania, and Wisconsin [46]. The clinical presentation of coinfected fish often includes overlapping signs such as gill necrosis, lethargy, and erratic swimming, complicating diagnosis based on gross pathology alone [44]. The synergistic mechanism likely involves immunosuppression induced by one virus facilitating replication of the other, although the precise molecular interactions remain poorly characterized.

Bacterial coinfections, particularly with Aeromonas hydrophila, are also common and can dramatically increase mortality. Novita et al. (2020) developed a duplex PCR method for simultaneous detection of KHV and A. hydrophila, recognizing that these pathogens frequently co-occur in common carp [50]. The immunosuppressive effects of KHV infection, including downregulation of complement components and impaired phagocytosis [25], create an opportunistic window for bacterial invasion, leading to hemorrhagic septicemia and accelerated mortality. Similarly, parasitic infestations with Dactylogyrus spp. and metacercariae have been reported in KHV outbreaks in Iraq, where gill damage from parasites may facilitate viral entry and exacerbate respiratory distress [56].

Water Quality Parameters and Chemical Stressors

Water quality parameters beyond temperature can modulate KHV pathogenesis. The Babylon River outbreak in Iraq occurred at a pH of 6.85 and salinity of 760 ppt, conditions that may have stressed the fish and increased susceptibility [30]. While KHV can survive in a range of pH conditions, extreme values can compromise the mucosal barrier and gill epithelium, facilitating viral entry. Chemical pollutants, including heavy metals and pesticides, have not been systematically studied in the context of KHV, but their known immunosuppressive effects in fish suggest they could act as cofactors. The detection of KHV DNA in water samples using filtration-elution methods [43] underscores the importance of environmental surveillance, as the virus can persist in the water column and be influenced by chemical constituents that affect viral stability.

Implications for Disease Management and Biosecurity

The recognition that environmental cofactors, particularly nanoplastics, can synergistically amplify KHV pathogenesis has profound implications for disease management. Traditional biosecurity measures focused on temperature control, quarantine, and disinfection may be insufficient in environments contaminated with nanoplastics. The virucidal effects of agents such as protease (Neutrase®), peracetic acid, and quicklime have been demonstrated against KHV [66], but their efficacy in the presence of organic matter and nanoplastics requires further investigation. The development of point-of-care diagnostic tools, such as cross-priming amplification-based lateral flow assays (CPA-LFA) and real-time PCR [41], as well as fluorescence LAMP assays for field use [44], will be essential for rapid detection of KHV in environments where cofactors may be present.

The potential for nanoplastic exposure to convert subclinical KHV infections into overt disease outbreaks highlights the need for integrated environmental monitoring programs that assess both viral presence and pollutant levels. The WOAH and FAO have emphasized the importance of surveillance for emerging diseases, but the role of environmental cofactors is not yet incorporated into standard risk assessment frameworks. Future research should prioritize dose-response studies to establish threshold concentrations of nanoplastics that potentiate KHV infection, as well as field surveys to correlate nanoplastic levels with KHVD incidence in aquaculture facilities. The synergistic effects observed by Zhao et al. [24] represent a sentinel warning that the intersection of plastic pollution and viral disease may pose an escalating threat to global carp aquaculture, necessitating a paradigm shift in how we conceptualize and manage KHVD risk.

Biosecurity, Control Strategies, and Disease Management in Aquaculture

The management of Koi Herpesvirus disease (KHVD) necessitates a multi-faceted, strategically layered approach that integrates rigorous biosecurity protocols, advanced diagnostics, host genetic improvement, and immunoprophylaxis. The virus, classified as a notifiable pathogen by the World Organisation for Animal Health (WOAH) and the European Union, imposes severe constraints on global carp aquaculture, demanding that control strategies evolve from reactive outbreak responses to proactive, science-based prevention frameworks. The complexity of KHV biology, including its capacity for latency, its variable host range, and its environmental resilience, dictates that no single intervention is sufficient; rather, an integrated health management paradigm is essential for sustainable control.

Foundational Biosecurity and Risk Mitigation

The cornerstone of KHVD prevention is the establishment and rigorous enforcement of compartmentalized biosecurity. This begins with sourcing fish from verified KHV-free stocks, a practice validated by long-term surveillance programs such as the one implemented in the Monticolo lakes of South Tyrol, Italy, which successfully achieved WOAH-compliant Category I health status through two years of systematic negative testing for KHV, SVCV, and CEV [49]. This demonstrates that defined geographical compartments can be maintained as disease-free zones even amidst regional outbreaks, provided that movement restrictions and quarantine protocols are strictly observed.

However, the epidemiological landscape is complicated by the existence of multiple reservoir and vector species. Experimental evidence has definitively shown that non-cyprinid fish such as rainbow trout (Oncorhynchus mykiss) can become subclinically infected, seroconvert, and transmit infectious KHV to naïve common carp at permissive temperatures (20°C), even neutralizing antibodies are produced without clinical disease in the trout themselves [8]. Similarly, invasive species like the round goby (Neogobius melanostomus) have been found to harbor KHV DNA in European waterways, posing a silent transmission risk when these fish are introduced during pond refilling operations [12]. The implications for farm-level biosecurity are profound: disinfection protocols must extend beyond the target species. Water sources must be treated, and any cohabiting or invasive fish must be considered potential vectors. The finding that migratory wild ducks in North America can carry CyHV-3 DNA in their intestinal contents further expands the risk perimeter, suggesting that avian vectors may facilitate long-distance dissemination between water bodies [16].

Physical and chemical disinfection remains a critical barrier. Virucidal efficacy studies have demonstrated that a commercially available protease (Neutrase®) can inactivate KHV to below detectable limits, offering an environmentally friendly alternative to traditional agents like peracetic acid and quicklime [66]. The practicality of such agents is enhanced when they remain effective in the presence of organic matter, such as quartz sand, which simulates the conditions of pond sediment. Furthermore, the use of 0.6 M NaCl has been identified as a simple, cheap inactivation step for KHV in laboratory settings, useful for downstream processing but with limited applicability in large-scale aquaculture [68].

Surveillance, Diagnostics, and Early Detection

Effective disease management is inextricably linked to the availability of rapid, sensitive, and field-deployable diagnostic tools. Polymerase chain reaction (PCR)-based methods remain the gold standard for confirmatory diagnosis, with TaqMan probe-based real-time PCR achieving detection limits as low as 8.384 copies/μL and perfect diagnostic sensitivity/specificity (100%) [41]. However, the need for centralized laboratory infrastructure limits its utility for real-time decision-making. Consequently, point-of-care (POC) technologies have been developed to bridge this gap.

The cross-priming amplification-based lateral flow assay (CPA-LFA) represents a significant advance, offering a detection limit of 675.69 copies/μL with 93.67% diagnostic sensitivity and 100% specificity [41]. Its simple visual readout makes it ideal for on-farm triage. Similarly, fluorescence real-time loop-mediated isothermal amplification (LAMP) assays targeting the CyHV-3 ORF43 gene can amplify viral DNA from mucus swabs within 4-13 minutes, with a limit of detection of 102 viral copies [44]. These POC tests empower farmers and field veterinarians to make immediate biosecurity decisions, such as isolating suspected stock or delaying harvesting.

The choice of sampling matrix is equally critical for non-lethal surveillance. Comparative studies have shown that ELISA for anti-KHV antibodies is superior to gill or blood qPCR for detecting prior exposure or latent infections, especially at non-permissive temperatures or during persistent infection states [48]. A combined diagnostic approach, using qPCR for acute viral detection (particularly on gill biopsies) and ELISA for serological screening, provides the most comprehensive picture of herd health status [48]. For environmental surveillance, the filtration-elution method has proven more reliable than PEG precipitation or antibody flocculation for recovering KHV DNA from water samples, enabling passive monitoring of pond water as a risk assessment tool [43].

Host Genetic Resistance and Selective Breeding

A paradigm shift in KHVD management is the recognition that genetic improvement of the host population offers a durable, non-pharmaceutical control strategy. Heritability estimates for resistance to KHVD are substantial, 0.43 on the observed scale and 0.72 on the underlying liability scale, indicating significant additive genetic variance that can be exploited through selective breeding [47]. Crucially, genomic selection (GS) using RAD-seq outperforms traditional pedigree-based best linear unbiased prediction (pBLUP) by 8-18%, with prediction accuracies remaining robust even with reduced SNP densities, provided there are close genetic relationships between training and validation populations [32].

The biological basis of this resistance is beginning to be elucidated. Genome-wide association studies have identified a significant quantitative trait locus (QTL) on linkage group 44, harboring the TRIM25 gene, a known interferon-inducible protein that targets viral capsids for degradation, suggesting that enhanced innate immune sensing is a key mechanism [36]. Furthermore, transcriptomic profiling of resistant Amur wild carp (AS) versus susceptible koi reveals that the early, robust activation of the type I interferon signaling cascade and the complement system is a distinguishing feature of resistant genotypes [28]. In contrast, susceptible strains exhibit a delayed and muted innate response, allowing the virus to establish a foothold [28].

Breeding programs have already demonstrated practical success. A synthetic Indonesian common carp population, created by blending five local strains (Rajadanu, Majalaya, Sutisna, Wildan, and Sinyonya), achieved a 62% survival rate after KHV cohabitation challenge, significantly surpassing the 20-26.7% survival of its constituent strains [60]. Similarly, crossbreeding koi with red common carp yields progeny (RK and KR) with cumulative mortalities of only 6-8% compared to 28% in pure koi, an effect linked to elevated expression of IL-12 p35, IFN-αβ, and TLR9 [38, 57]. These findings underscore that genetic selection is not antithetical to production goals; favorable genetic correlations between KHVD resistance and fillet yield (0.44) suggest that selection for improved carcass quality may incidentally enhance disease resistance [47].

Vaccination Strategies: From Live Attenuated to Oral Probiotic Platforms

The development of safe, efficacious, and scalable vaccines against KHV has been the focus of intense research. The only widely available commercial vaccine (KoVax) is a live attenuated product, but its use is restricted in many jurisdictions due to concerns over reversion to virulence and the inability to differentiate vaccinated from infected animals (DIVA). Recent advances have sought to overcome these limitations.

Live Attenuated Vaccines with DIVA Capacity: Serial passage of virulent KHV in cell culture has yielded attenuated strains with a partial deletion in open reading frame 150 (ORF150), a region associated with virulence [26]. This vaccine, when administered via immersion or oral routes, induces protective antibody responses and mild to no clinical signs. Critically, the deletion allows for differentiation via PCR, vaccinated animals lack the ORF150 fragment that is present in wild-type viruses [26]. Similarly, a double deletion mutant lacking both thymidine kinase (TK, ORF55) and deoxyuridine-triphosphatase (DUT, ORF123) has been constructed using a cell culture-adapted Taiwanese strain (KHV-T). This mutant (KHV-TΔDUT/TK) achieves high titers (>10⁷ PFU/mL) suitable for economic production, is almost avirulent, and yet induces solid protection against wild-type challenge, with a triplex real-time PCR enabling DIVA discrimination [11].

DNA Vaccines and Delivery Systems: DNA vaccines encoding structural proteins such as ORF81, ORF149, and ORF25 have been extensively evaluated. While early studies showed promise, the delivery route and formulation are paramount. An ORF25-based DNA vaccine administered via three intramuscular injections provided protection against bath challenge but failed against a more natural cohabitation challenge, highlighting the necessity of using realistic challenge models [69].

Breakthroughs in delivery technology have dramatically improved the practicality of DNA vaccination. The coupling of a pcDNA3.1-ORF149 plasmid to single-walled carbon nanotubes (SWCNTs) has enhanced protective efficacy by 33.9% over naked DNA when injected [63]. Even more significantly, this SWCNT-based platform has been adapted for immersion vaccination. A 10 mg/L immersion dose of SWCNTs-pcDNA3.1-ORF149 achieved 56% relative protection in juvenile koi, a remarkable feat considering the physical barriers (skin, gill epithelium) that typically preclude nucleic acid uptake from water [64].

Oral Probiotic Vaccines: The ultimate goal for mass vaccination in aquaculture is an oral formulation that eliminates handling stress. A landmark study developed an oral probiotic vaccine using Lactobacillus rhamnosus expressing KHV ORF81, encapsulated in chitosan-alginate microcapsules. These capsules protected the live bacteria through the acidic stomach environment, enabling colonization of the intestine and subsequent antigen delivery. This strategy induced significant antigen-specific IgM with neutralizing activity, achieving an 85% protection rate against KHV challenge [55]. This approach is scalable, cost-effective, and stress-free, making it a prime candidate for widespread field application.

Inactivated and Subunit Vaccines: More traditional approaches retain utility. Formalin-inactivated whole KHV virus combined with complete Freund's adjuvant (CFA) significantly enhances hematocrit, lysozyme activity, and antibody titers, leading to improved survival [54]. Subunit vaccines based on recombinantly expressed ORF25 protein are also under development, with bioinformatic predictions indicating strong B-cell epitope potential [62]. Additionally, the use of embryonated chicken eggs (ECE) as a low-cost propagation system for vaccine production has been explored, with allantoic fluid yielding high KHV genomic copy numbers three days post-inoculation [67]. This method could dramatically reduce production costs, particularly in developing nations.

Pharmacological Interventions and Alternative Therapeutics

Antiviral chemotherapy for fish remains a nascent field. A multidose protocol using acyclovir (10 mg/kg) administered intracoelomically at 1, 3, and 6 days post-first mortality only delayed the onset of death and reduced viral load transiently; overall mortality was unaffected, indicating the limited utility of this nucleoside analogue at the tested regimen [59].

In contrast, natural compounds show considerable promise. Emodin, an anthraquinone from Rheum species, has demonstrated potent anti-KHV activity in vivo. At 40 mg/kg, emodin suppressed viral replication, increased survival, and reduced oxidative damage via activation of the Nrf2/Keap1-ARE pathway and concomitant suppression of NF-κB-driven inflammatory cytokines (IL-6, IL-8, TNF-α) [39]. This dual antioxidant-anti-inflammatory mechanism offers a holistic therapeutic strategy.

Probiotics and their postbiotic metabolites represent another frontier. Postmetabolites from Lactiplantibacillus plantarum exhibit remarkable antiviral activity with selectivity indices (SI) between 26.5 and 221.4, acting both by direct virucidal effects and by blocking viral attachment to host CCB cells, reducing titers by Δlg = 4.25–5.25 [65]. Furthermore, these metabolites protect cells prior to viral challenge, suggesting potential as a prophylactic feed additive. This is supported by the observation that lactic acid bacteria (LAB) postmetabolites from Lactobacillus gasseri also show significant inhibition of KHV replication [65]. The gut microbiota-modulating effects of these probiotics, as demonstrated by the mitigation of nanoplastic-induced dysbiosis, may further bolster host resistance [24].

Outbreak Response and Long-Term Eradication

When biosecurity fails and an outbreak occurs, containment is paramount. The Croatian experience with KHVD in 2016 demonstrates that rapid detection, strict culling, and coordinated eradication programs can successfully eliminate the virus from a defined area [45]. The latency stage of KHV, however, complicates eradication; survivors may harbor the virus at undetectable levels and shed it upon stress or temperature shifts [25, 35]. Serological monitoring (ELISA) combined with non-lethal gill qPCR is therefore essential for identifying carrier fish in post-outbreak populations [48].

The use of KHV as a biological control agent for invasive common carp, as proposed in Australia, represents an extreme and controversial application of this virus. Epidemiological modeling of this biocontrol strategy must account for factors such as water temperature, host density, and the presence of reservoirs to predict the likelihood of sustained epidemic mortality rather than enzootic persistence [51]. The ethical and ecological implications of releasing a notifiable pathogen into open waters remain a subject of intense debate.

Ultimately, the most robust defense against KHVD is a comprehensive health management program that integrates selective breeding for resistance, risk-based surveillance using POC diagnostics, strategic vaccination (particularly oral probiotic platforms), and rigorous biosecurity that accounts for vector species and environmental persistence. As the global climate and aquaculture intensification pressures increase, the principles of prevention and early detection, rather than reactive treatment, will remain the most effective strategy against this devastating pathogen.

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