Ovine Gammaherpesvirus 2

Overview and Taxonomy of Ovine Gammaherpesvirus 2

Ovine gammaherpesvirus 2 (OvGHV2), previously designated ovine herpesvirus 2 (OvHV-2), represents the archetypal and most clinically significant member of the Macavirus genus within the Gammaherpesvirinae subfamily of the Orthoherpesviridae (formerly Herpesviridae) [1, 2, 3]. This virus is the etiological agent of sheep-associated malignant catarrhal fever (SA-MCF), a frequently fatal, sporadic, lymphoproliferative and vasculotropic disease that affects a broad spectrum of artiodactyls, including domestic cattle, water buffalo, bison, cervids, swine, and, less commonly, horses [1, 4, 5, 6]. The global significance of OvGHV2 is underscored by its classification by the World Organisation for Animal Health (WOAH) as a pathogen of considerable economic and veterinary importance, responsible for substantial morbidity and mortality in susceptible livestock populations worldwide.

Taxonomic Position and the Malignant Catarrhal Fever Virus Complex

OvGHV2 is a member of the genus Macavirus, a taxon that was established to accommodate the gammaherpesviruses of ungulates [1, 2, 7]. The Macavirus genus is defined by a set of shared biological and genetic characteristics, including a narrow host range for the establishment of persistent, asymptomatic infections in reservoir species, and the capacity to induce a severe, immunopathological disease, malignant catarrhal fever (MCF), in a wide range of incidental, susceptible hosts [6, 8]. The viruses that are known to cause MCF are collectively referred to as the MCF virus (MCFV) complex [1, 2, 9, 10]. This complex includes, in addition to OvGHV2, alcelaphine gammaherpesvirus 1 (AlGHV1), the cause of wildebeest-associated MCF in Africa; caprine gammaherpesvirus 2 (CpHV-2), associated with MCF in deer and other cervids; and several other macaviruses identified in bison, ibex, and aoudad [10, 6, 8].

A defining molecular feature of all members of the MCFV complex is the expression of a conserved antigenic epitope recognized by the monoclonal antibody 15A (MAb-15A) [1, 2, 11]. This 15A epitope is the immunological foundation for a widely used competitive inhibition enzyme-linked immunosorbent assay (CI-ELISA) and an immunohistochemical (IHC) assay designed to detect MCFV-specific antigens in tissues [1, 12, 11]. The presence of this epitope allows for the serological and immunohistochemical screening of animals for infection by any macavirus associated with MCF, without the need to differentiate the specific viral species [11, 10]. Critically, the 15A-based IHC assay has been validated for the detection of OvGHV2 antigens in formalin-fixed, paraffin-embedded tissues from cattle with SA-MCF, demonstrating intracytoplasmic immunoreactivity within epithelial cells of the kidneys, intestines, bile ducts, lungs, and within histiocytes and lymphocytes [11]. This assay has proven essential for both diagnostic pathology and retrospective epidemiological studies, enabling the identification of OvGHV2 in cases where typical clinical signs are absent or where co-infections are present [1, 13, 9]. Indeed, the cross-reactivity of the 15A MAb with other macaviruses, such as bovine gammaherpesvirus 6 (BoGHV6), has been documented, highlighting both the utility and the interpretative nuance of this diagnostic tool [1, 7]. The absence of OvGHV2 or BoGHV6 DNA coupled with positive 15A-IHC findings in some clinical cases suggests the possibility of yet-unidentified macaviruses circulating in ruminant populations, further underscoring the complexity of the MCFV complex [1, 10].

Reservoir Host Dynamics and Transmission

The natural history of OvGHV2 is defined by an exquisitely evolved host-pathogen relationship with its reservoir, the domestic sheep (Ovis aries) [14, 2, 15]. Sheep are the asymptomatic, life-long carriers of the virus, with infection being highly prevalent in sheep populations globally. Molecular surveys have demonstrated that prevalence rates can exceed 65% in sheep flocks, as evidenced by studies in the Philippines, where a semi-nested PCR targeting the tegument protein gene detected OvGHV2 DNA in 58 of 89 (65.17%) blood samples from clinically healthy sheep [15]. Similarly, in Brazil, where SA-MCF is endemic, the virus is ubiquitous in sheep populations, and a recent study confirmed that sheep can transmit OvGHV2 to their fetuses via transplacental vertical infection, a finding that was previously undocumented for this virus [2]. This study provided the first evidence of OvGHV2-related transplacental infections in sheep, with the detection of viral DNA and MCFV antigens in the organs of twin fetuses from an asymptomatic ewe that succumbed to pregnancy toxemia [2]. These findings reveal a previously unrecognized route of viral maintenance and transmission within the reservoir host, potentially contributing to the persistence of the virus in sheep populations even in the absence of horizontal transmission.

Transmission from the reservoir to susceptible species is believed to occur primarily through the inhalation of viral particles shed in nasal secretions and aerosols from infected lambs, which are the primary shedders of the virus [14, 4]. The peak of viral shedding in sheep occurs around 6-9 months of age, which coincides with the waning of maternally derived antibodies [16]. This horizontal transmission from sheep to cattle is the principal source of SA-MCF outbreaks, and the risk is profoundly influenced by the proximity and density of sheep relative to cattle [14]. An epidemiological study in Southern Brazil demonstrated that the sheep-to-cattle ratio (SCR) is a critical determinant of outbreak risk, with a SCR greater than 0.15 within a geographical mesoregion being associated with a significantly increased chance of SA-MCF outbreaks in cattle compared to regions with a lower SCR [14]. This finding provides a quantitative, risk-based tool for predicting and managing SA-MCF in mixed-species farming systems.

Host Range and Global Occurrence

The host range of OvGHV2 is remarkably broad among artiodactyls, encompassing domestic and wild ruminants as well as suids. In cattle, SA-MCF is the most recognized and devastating manifestation, characterized by high fever, corneal opacity, lymphadenopathy, erosive stomatitis, and severe neurological signs, culminating in death in the vast majority of cases [1, 14, 5, 17]. However, recent research has increasingly documented the occurrence of subclinical and atypical infections in cattle, where OvGHV2 is detected in the absence of classical MCF clinical signs [18, 19, 13, 9]. These infections are often identified in the context of bovine respiratory disease (BRD) or enteric disease, with OvGHV2 DNA amplified from deep nasal swabs and pulmonary tissues of calves with interstitial pneumonia and diarrhea [13, 9]. In one outbreak of acute respiratory disease in dairy calves from Southern Brazil, OvGHV2 was the most frequently detected pathogen (81.2% of samples), suggesting that this macavirus may be a significant, and previously underappreciated, contributor to the BRD complex [13]. Furthermore, OvGHV2 has been associated with chronic folliculitis and ulcerative skin lesions in dairy cows, representing the first description of OvGHV2-related skin disease in Latin America [19]. These cases were notable because the affected cows had no direct contact with sheep, raising the possibility of alternative transmission mechanisms, such as fomites or other intermediate hosts [19].

Beyond cattle, OvGHV2 has been molecularly detected in asymptomatic water buffaloes (Bubalus bubalis) from Central-western Brazil, confirming that this species can also serve as a subclinical carrier [20]. This finding challenges the traditional paradigm of sheep as the sole reservoir and suggests that water buffaloes, particularly those raised in proximity to sheep, may act as incidental hosts capable of maintaining the virus within a population [20]. The virus has also been identified in free-ranging wild boars (Sus scrofa) from Southern Brazil, where subclinical OvGHV2 infections were detected in 37.5% of animals sampled [4]. The detection of OvGHV2 DNA in the oral cavity of one wild boar raises the alarming possibility that feral swine could act as bridge hosts, disseminating the virus to susceptible livestock and wildlife [4]. This finding, coupled with serological evidence of OvGHV2 exposure in dairy cattle within a 50 km radius of the home range of infected wild boars, suggests that these animals may be a previously unrecognized risk factor for SA-MCF transmission [12]. In cervids, OvGHV2 has been identified in free-ranging Chilean pudus (Pudu puda), representing the first report of this virus in Chile and underscoring the threat it poses to conservation efforts for endangered wildlife [21]. An outbreak in Mexico documented OvGHV2 in horses and several artiodactyl species, including deer, confirming that horses are indeed susceptible to SA-MCF and that the virus can cause significant disease in a diverse range of captive and free-ranging wildlife [5, 22].

Genomic Architecture and Molecular Pathogenesis

The genome of OvGHV2, like all members of the Gammaherpesvirinae, is a linear double-stranded DNA molecule of approximately 130-145 kilobase pairs, characterized by a unique long (UL) region flanked by multiple copies of direct terminal repeats [3, 23]. The genomic organization of OvGHV2 is highly homologous to that of other macaviruses, including AlGHV1 and the more commonly encountered bovine gammaherpesvirus 6 (BoGHV6) [23]. The genome is predicted to encode at least 74 proteins, including 61 open reading frames (ORFs) conserved across all gammaherpesviruses, and 12 genes unique to the Macavirus genus [23]. One of the most notable macavirus-specific genes is a homologue of ovine interleukin-10 (IL-10), a cytokine that may play a critical role in immune evasion by dampening the host inflammatory response, as well as a gene encoding an ornithine decarboxylase, which is involved in polyamine biosynthesis and may be important for viral replication [23].

Central to the pathogenesis of OvGHV2 is its capacity to establish latency, a hallmark of all herpesviruses. The latency-associated nuclear antigen (LANA), encoded by ORF73, is a multifunctional protein that is essential for viral episome maintenance during latency, tethering the viral genome to host chromosomes to ensure its segregation to daughter cells [3]. Recent structural and functional studies have provided unprecedented insights into the nuclear import of OvGHV2 LANA. Using high-resolution crystallography and quantitative binding assays, researchers have identified a novel bipartite nuclear localization signal (NLS) within the C-terminal region of LANA that mediates its interaction with importin alpha (IMPα) and importin beta 1 (IMPβ1) [3]. This IMPα/β1-dependent pathway is the primary mechanism for LANA nuclear translocation, and mutation or inhibition of this NLS significantly reduces nuclear accumulation [3]. Interestingly, the persistence of partial nuclear localization under these conditions suggests the existence of a secondary, IMPα/β1-independent nuclear import mechanism, indicating a level of functional redundancy that underscores the critical importance of LANA nuclear localization for the viral life cycle [3].

The pathogenesis of SA-MCF is a complex, immune-mediated process, rather than a direct cytopathic effect of the virus [24, 25]. In susceptible species, OvGHV2 infects T lymphocytes and monocytes/macrophages, leading to a profound, dysregulated expansion of these cell populations [24]. Infected T cells then home to and infiltrate the endothelial lining of blood vessels, triggering a severe, systemic necrotizing vasculitis [25]. This vasculitis is the hallmark lesion of MCF, and it is characterized by a mixed inflammatory infiltrate of infected and activated T cells and macrophages within the vessel wall, leading to fibrinoid necrosis, intimal proliferation, and thrombosis [26, 24, 25]. The Syrian golden hamster (Mesocricetus auratus) has been re-established as a valuable animal model for studying these pathogenic mechanisms, enabling the detailed in situ analysis of OvHV-2 infection and the demonstration that both T cells and macrophages harbor the virus and contribute to the vascular lesions [24]. Furthermore, this model has shown that OvHV-2 has a broader target cell spectrum than previously appreciated, including vascular endothelial cells and squamous epithelia, supporting the hypothesis that the disease is a form of graft-versus-host-like reaction driven by infected, activated, and dysregulated T cells [24]. The demonstration of widespread viral infection in vascular smooth muscle cells, adventitial fibroblasts, and the vasa vasorum in natural cases further refines our understanding of MCF vasculitis, suggesting that the inflammatory process is initiated via the recruitment of infected leukocytes from the vasa vasorum and the arterial lumen [25].

Molecular Pathogenesis of OvGHV2 Infection

The Fundamental Paradox of a Gammaherpesvirus Pathogen

The molecular pathogenesis of ovine gammaherpesvirus 2 (OvGHV2) represents one of the most intriguing and clinically significant host-virus interactions within the Gammaherpesvirinae subfamily. As a member of the Macavirus genus, OvGHV2 exhibits a peculiar and devastating biological duality: it establishes a lifelong, asymptomatic, and exquisitely adapted infection in its natural reservoir host (domestic sheep), yet precipitates a frequently fatal, hyper-inflammatory, lymphoproliferative disease, sheep-associated malignant catarrhal fever (SA-MCF), when transmitted to susceptible, dead-end hosts such as cattle, bison, deer, water buffalo, and, as recently demonstrated, horses [1-3, 16, 24]. This dichotomy is the central puzzle of OvGHV2 pathogenesis. The virus does not rely on horizontal transmission from clinically ill animals; instead, infectious virus is shed exclusively by the asymptomatic reservoir host (sheep), which effectively renders SA-MCF an inadvertent, immunopathological consequence of cross-species transmission [14, 6]. Understanding the molecular mechanisms that govern this transition from latency in the adapted host to uncontrolled lymphoproliferation and systemic vasculitis in the aberrant host is critical for developing therapeutic and prophylactic interventions.

The Molecular Architecture and Latency-Associated Machinery

The genomic blueprint of OvGHV2, while not yet fully annotated at the level of its closest relative (ovine herpesvirus 1, OvHV-1, whose 144,637 bp genome encodes at least 74 proteins), positions it firmly within the Macavirus lineage [23]. Comparative genomics reveals that OvGHV2 shares a conserved gammaherpesvirus core, including genes for capsid assembly, DNA replication, and tegument proteins, alongside a suite of Macavirus-specific genes that are central to its unique pathogenesis. Notably, OvGHV2 encodes a homologue of ovine interleukin-10 (vIL-10), a potent immunomodulatory cytokine that is shared only with OvHV-1 among the macaviruses [23]. The acquisition of a host-derived cytokine gene is a hallmark of gammaherpesvirus evolution, allowing the virus to directly manipulate the host immune environment to favor its own persistence.

A pivotal molecular player in the establishment and maintenance of latency is the latency-associated nuclear antigen (LANA), encoded by ORF73. Recent structural and functional studies have provided unprecedented insight into the nuclear trafficking of OvGHV2 LANA [3]. Unlike many other gammaherpesviruses, in vitro culture systems for OvGHV2 are notoriously difficult to establish, making molecular characterization challenging. However, using high-resolution crystallography and quantitative binding assays, researchers have identified a novel bipartite nuclear localization signal (NLS) within the C-terminal region of OvGHV2 LANA [3]. This NLS is essential for the interaction with host importin alpha (IMPα), mediating the nuclear import of LANA via the classical IMPα/β1-dependent pathway. Mutation of this NLS or pharmacological inhibition of the importin pathway significantly reduces nuclear accumulation of LANA, confirming that this interaction is critical for the virus to access the nuclear compartment, where it tethers the viral episome to host chromatin during latency [3]. Interestingly, partial nuclear localization persists even under these conditions, suggesting the existence of an alternative, IMPα/β1-independent nuclear import mechanism, a redundancy that underscores the evolutionary pressure to maintain this core latency function [3].

The 15A Antigenic Epitope and Landscape of Infection

A defining molecular feature of all MCF-causing macaviruses is the 15A antigenic epitope, a highly conserved structural motif that has become the cornerstone of diagnostic and immunohistochemical (IHC) detection [1, 2, 18, 11]. The monoclonal antibody 15A (MAb-15A) binds this epitope within viral proteins, enabling the identification of MCFV antigens in formalin-fixed tissues. This tool has revolutionized our understanding of OvGHV2 dissemination within the host. IHC studies using MAb-15A have consistently demonstrated that OvGHV2 antigens are not confined to lymphoid tissues. Instead, they are detected as intracytoplasmic immunoreactivity within a remarkably broad spectrum of epithelial cells, including those of the lungs (bronchial and bronchiolar epithelium), intestine (cryptal epithelium), liver (bile duct epithelium), kidneys (tubular epithelium), and even within neurons of the cerebral cortex and cardiomyocytes [11, 27]. This widespread epithelial tropism explains the diverse and multi-systemic clinical manifestations of SA-MCF, which can involve the respiratory, gastrointestinal, urinary, and nervous systems [1, 13, 11]. The detection of MCFV antigens in histiocytes and lymphocytes further implicates these leukocytes as both targets and vectors for viral dissemination [11, 24].

The Immunopathological Basis of Vasculitis: A Graft-Versus-Host-Like Scenario

The hallmark histopathological lesion of SA-MCF is systemic necrotizing lymphocytic vasculitis, which is the primary driver of morbidity and mortality [1, 28, 29]. The molecular pathogenesis of this vasculitis has been a subject of intense investigation, and a compelling model has emerged that frames SA-MCF not as a classic cytolytic viral infection, but as an immunopathological disorder driven by a dysregulated, virus-activated T-cell response. This model, often termed a "graft-versus-host-like" disease, posits that OvGHV2 infects and activates T lymphocytes, transforming them into cytotoxic effector cells that recognize and attack host vascular endothelial cells.

Recent in situ studies using RNA hybridization and immunohistology have provided granular detail on this process [24, 25]. In the Syrian golden hamster model, which recapitulates key aspects of SA-MCF, systemic OvGHV2 infection is associated with T-cell and macrophage-dominated mononuclear infiltrates in multiple organs [24]. Critically, both T-cells and monocytes/macrophages are shown to harbor the virus, and these infected leukocytes are abundant within the inflammatory infiltrates. The data further suggest that OvGHV2 has a broader target cell spectrum than previously appreciated, including vascular endothelial cells and selected squamous epithelia [24]. This finding is pivotal: it supports a mechanism where infected, activated T-cells and monocytes home to tissues and emigrate from vessels, potentially interacting directly with endothelial cells that themselves may be infected [24]. This interaction could trigger a cascade of inflammation, leading to the characteristic adventitial infiltration from the vasa vasorum, followed by recruitment of leukocytes from the arterial lumen, resulting in intimal and medial infiltration, neointimal proliferation, and eventual luminal occlusion [25].

The role of macrophages is equally critical. Evidence suggests that OvGHV2-infected T-cells, monocytes, and locally proliferating macrophages all contribute to the vasculitis [24, 25]. It is hypothesized that latently infected, activated endothelial cells may serve as the initial trigger for leukocyte recruitment. Once activated, macrophages are likely the principal source of pro-inflammatory mediators (e.g., TNF-α, IL-6) that drive the vascular damage and the profound lymphoid proliferation observed in SA-MCF [24, 25]. This model elegantly explains why the disease is often described as a "lymphoproliferative" and "vascular" disease, it is a consequence of uncontrolled immune activation rather than direct viral cytolysis.

The Spectrum of OvGHV2 Pathogenesis: From Acute Lethality to Subclinical Persistence

The molecular pathogenesis of OvGHV2 is not a monolithic process; it manifests across a wide spectrum, from acute, rapidly fatal disease to chronic, subclinical infections. The classic, textbook presentation of SA-MCF involves a peracute or acute course with high fever, corneal edema, oral and nasal erosions, and neurological signs, with a case fatality rate approaching 100% [1, 14]. However, the virus is equally capable of causing atypical, chronic, or even subclinical infections, a reality that is reshaping our understanding of its epidemiology and pathogenic potential.

Subclinical and Persistent Infections: The detection of OvGHV2 DNA and MCFV antigens in asymptomatic cattle is now well-documented, indicating that subclinical infections are far more common than previously thought [18, 4, 9]. A landmark study investigating bovine respiratory disease (BRD) in Brazil found that OvGHV2 was the most frequently identified single pathogen in an outbreak of acute respiratory disease, yet it was also detected in the deep nasal swabs of asymptomatic calves [13]. This suggests that OvGHV2 can establish a low-level, persistent infection that may be reactivated or act as a co-factor in disease. Furthermore, subclinical OvGHV2 infections have been demonstrated in free-ranging wild boars (Sus scrofa), raising the alarming possibility that these animals could act as "bridge hosts," introducing the virus from enzootic sheep populations to other susceptible wildlife and livestock [4]. The detection of OvGHV2 in the oral cavity of one wild boar further supports this potential for dissemination [4]. Similarly, OvGHV2 DNA has been amplified from the tissues of asymptomatic water buffaloes (Bubalus bubalis), expanding the known host range and suggesting that this species may also serve as an incidental, subclinical reservoir [20].

Transplacental and Fetal Infections: Perhaps one of the most recent and significant revelations in OvGHV2 pathogenesis is the demonstration of vertical transmission. Historically, OvGHV2 was not considered a reproductive pathogen. However, recent studies have provided compelling evidence of transplacental infection in both sheep and cattle [2, 10]. In a landmark study, OvGHV2 DNA was detected in tissues of two fetuses from a pregnant ewe, and MCFV antigens were identified via IHC in one of them, representing the first description of transplacental OvGHV2 infection in the natural host [2]. Further work in cattle confirmed that OvGHV2 DNA and MCFV antigens can be identified in aborted bovine fetuses, strongly suggesting that infection can occur in utero and may contribute to fetal mortality [10]. This vertical transmission pathway adds a new dimension to the epidemiology of OvGHV2 and challenges the assumption that infection occurs only post-natally via aerosol or direct contact with sheep.

Cutaneous and Chronic Manifestations: The pathogenic repertoire of OvGHV2 continues to expand. An unusual but increasingly recognized manifestation is chronic, non-fatal skin disease. A recent report documented outbreaks of chronic folliculitis in dairy cows that were seropositive for OvGHV2 but lacked any of the classic signs of SA-MCF [19]. Importantly, these cows had no direct contact with sheep, further implicating alternative transmission mechanisms or possibly reactivation of latent virus. Histologically, the skin lesions were characterized by chronic folliculitis, and MCFV antigens were detected in the lesions via IHC [19]. This cutaneous form contrasts sharply with the acute, systemic disease and suggests that OvGHV2 can cause a localized, immune-mediated inflammatory response in the skin. Similarly, chronic interstitial pneumonia has been described in a sheep, demonstrating that the virus can induce progressive pulmonary disease in its own reservoir host under conditions of immune compromise or co-infection [27].

Co-Infections and the Modulation of Pathogenesis

OvGHV2 rarely acts in isolation. The virus is frequently identified in the context of polymicrobial infections, and these co-infections can profoundly alter the clinical outcome and pathological presentation.

Interactions with Bovine Respiratory Disease (BRD) Pathogens: The role of OvGHV2 in the bovine respiratory disease complex is now a major area of investigation. Multiple studies have demonstrated that OvGHV2 is frequently co-detected with other BRD pathogens, including bovine coronavirus (BCoV), bovine viral diarrhea virus (BVDV), Mycoplasma bovirhinis, and Histophilus somni [26, 13, 9, 30]. The frequent association of OvGHV2 with interstitial pneumonia, a hallmark lesion of viral BRD, suggests that it may be a primary or significant contributing factor to respiratory disease outbreaks [18, 13]. A key study found that OvGHV2 was the most frequently identified pathogen in an outbreak of acute respiratory disease in dairy calves, and was often the sole infectious agent identified in singular infections [13]. This has led to the proposal that OvGHV2 be considered a potential agent of BRD [31]. The mechanism of synergy is likely complex. OvGHV2-induced immunosuppression or immune dysregulation, particularly the activation of T-cells and macrophages, could render the host more susceptible to secondary bacterial infections like Mannheimia haemolytica or Histophilus somni. Conversely, the inflammatory milieu created by these bacterial pathogens could reactivate latent OvGHV2, leading to a synergistic amplification of disease.

Synergy with Histophilus somni: A particularly notable co-infection pattern involves OvGHV2 and Histophilus somni. In fatal cases of septicemia with encephalitis, both pathogens have been identified in the same tissues, suggesting that OvGHV2 may predispose animals to the systemic dissemination of H. somni or that co-infection leads to a more severe disease phenotype [26]. The detection of MCFV antigens in tissues with lesions attributable to H. somni reinforces the concept that these agents can act in concert.

Implications for Diagnosis and Epidemiology: The high prevalence of subclinical OvGHV2 infections and its frequent co-detection with other pathogens have major diagnostic implications. Serological surveys using a competitive inhibition ELISA (CI-ELISA) or an indirect MCF-specific ELISA based on the AlGHV1 C500 strain have shown that OvGHV2 antibodies are prevalent in dairy cattle in Brazil, even in the absence of clinical SA-MCF [12]. The detection of OvGHV2 in goats (seroprevalence of 50% in one study) further extends its host range and potential reservoir pool [32]. The 15A MAb IHC assay, while excellent for detecting MCFV antigens, shows cross-reactivity with other macaviruses, such as bovine gammaherpesvirus 6 (BoGHV6), which must be differentiated by PCR [1, 7]. Therefore, a diagnosis of OvGHV2-associated disease requires a multi-pronged approach: compatible histopathology (lymphocytic vasculitis, interstitial pneumonia), detection of MCFV antigens via IHC, and molecular confirmation of OvGHV2 DNA via PCR or qPCR [1, 14, 18, 11].

Mechanisms of Immune Evasion and Latency Establishment

To establish lifelong latency in sheep, OvGHV2 must have evolved sophisticated mechanisms to evade the host immune response. While studies on OvGHV2-specific immune evasion are limited, parallels can be drawn from other gammaherpesviruses, such as murine gammaherpesvirus 68 (MHV68) and Kaposi's sarcoma-associated herpesvirus (KSHV). A key evasion strategy involves the inhibition of innate cytosolic DNA sensing pathways. Studies on MHV68 have shown that the virus, through its large tegument protein (ORF64), which possesses deubiquitinase (DUB) activity, actively suppresses the STING (stimulator of interferon genes)-dependent DNA sensing pathway [33]. A mutant MHV68 lacking DUB activity fails to suppress this pathway, leading to increased type I interferon production and impaired establishment of latency [33]. It is highly plausible that OvGHV2 employs a similar, if not identical, strategy via its own tegument protein to blunt the early innate antiviral response, thereby facilitating its spread to lymphoid tissues and the establishment of latency.

Furthermore, the vIL-10 homologue encoded by OvGHV2 is a potent immunomodulator [23]. By mimicking host IL-10, a cytokine with anti-inflammatory and immunosuppressive functions, OvGHV2 can directly suppress the activity of antigen-presenting cells and inhibit the production of pro-inflammatory cytokines, creating a favorable environment for viral persistence. The detection of OvGHV2 in the lung and bronchial lymph nodes of subclinically

Epidemiology and Risk Factors of Sheep-Associated Malignant Catarrhal Fever

Ovine gammaherpesvirus 2 (OvGHV2), the aetiological agent of sheep-associated malignant catarrhal fever (SA-MCF), represents a globally distributed pathogen of paramount economic and ecological significance, primarily affecting susceptible populations of Artiodactyla. The epidemiology of OvGHV2 is fundamentally characterized by a stark dichotomy: the virus establishes a lifelong, subclinical, and persistent infection in its reservoir host, domestic sheep (Ovis aries), while precipitating a frequently fatal, lymphoproliferative, and vasculotropic disease in a wide range of incidental, dead-end hosts, most notably cattle, but also including water buffalo, cervids, bison, pigs, and, uncommonly, horses [1, 14, 5, 25, 6, 29]. The disease is not transmissible from clinically affected individuals; infection is exclusively acquired through direct or indirect contact with the reservoir host or, potentially, other bridge hosts that are shedding the virus [4, 6]. This unique epidemiological pattern dictates that the risk of outbreak is inextricably linked to the spatial, temporal, and management-based intersection of susceptible species with asymptomatically infected sheep and, as emerging evidence suggests, other potential wildlife reservoirs.

Global Distribution and Endemicity

SA-MCF is a disease of worldwide distribution, mirroring the global dissemination of sheep husbandry. While the disease is reported across all major livestock-producing continents, the incidence and recognition of outbreaks are highly variable, often reflecting diagnostic capacity, surveillance intensity, and the structure of livestock production systems. Endemicity is particularly well-documented in regions with high-density, co-grazing systems of cattle and sheep. For instance, in Brazil, SA-MCF is considered endemic, with a disproportionately high number of outbreaks reported from the southern states, particularly Rio Grande do Sul (RS) and Paraná [1, 14, 19, 12]. This geographic clustering is not merely a surveillance artifact; a compelling epidemiological niche study in RS demonstrated that a sheep-to-cattle ratio (SCR) greater than 0.15 significantly increased the risk of SA-MCF outbreaks in cattle, suggesting that specific regional production systems and high relative sheep density are powerful drivers of disease occurrence [14]. Similarly, OvGHV2 DNA was detected with a remarkably high prevalence (65.17%) in sheep from the Philippines, underscoring the extensive circulation of the virus in reservoir populations even where clinical disease in cattle may be underdiagnosed [15]. In contrast, SA-MCF is reported sporadically in many European nations, although a pan-European survey detected no OvGHV2 or other macaviruses (beyond the endemic bovine gammaherpesvirus 6) in the tissues of 448 cattle from five countries, suggesting that clinical infection in cattle in that region may be rare or that transmission dynamics are less efficient [8]. However, case reports from Ireland [28], the UK [29], and Switzerland [8] confirm that the virus is present in European sheep flocks, capable of causing sporadic disease when conditions permit. The virus has also been molecularly confirmed in clinical outbreaks across Africa, including Egypt where a case fatality rate of 72.2% (13/18) was observed in affected cattle [17], and in Latin America, where recent studies have expanded its known geographic range to include Mexico [5, 22] and Chile [21]. The detection of OvGHV2 in free-ranging wild boar in Brazil [4] and its potential role as a bridge host further complicates the geographic risk profile, suggesting that the virus can disseminate beyond the immediate confines of sheep farms [4, 12].

Prevalence in Reservoir and Spillover Hosts

The prevalence of OvGHV2 in its natural sheep reservoir is consistently high across the globe. Asymptomatic shedding, primarily via nasal secretions, perpetuates the virus within flocks. In the Philippines, a PCR survey of 89 sheep from six farms revealed a carrier rate of 65.2% [15]. In Brazilian studies, sheep are almost universally implicated in outbreaks occurring on mixed-species farms, with direct contact between clinically affected cattle and apparently healthy sheep being a consistent epidemiological observation [1-3]. Notably, infection in sheep can lead to vertical transmission, as demonstrated by the molecular and immunohistochemical detection of OvGHV2 in fetuses from a pregnant ewe [2], and systemic necrotizing vasculitis has been definitively linked to OvGHV2 in sheep, challenging the long-held dogma that sheep are only asymptomatic carriers [28, 29]. This suggests that while most infections are subclinical, disease can manifest in the reservoir host under specific, poorly understood circumstances.

In spillover hosts, the prevalence of clinical disease is low, but the case fatality rate is exceptionally high, often approaching 100% in naïve populations [1, 17]. However, the true prevalence of infection is undoubtedly much higher than clinical incidence suggests, due to a significant iceberg of subclinical and atypical infections. Data from Brazil reveals that OvGHV2 DNA or MCFV antigens are frequently detected in tissues of cattle without characteristic clinical signs of MCF [18, 9]. For example, in a study of slaughtered beef cattle in Mato Grosso, 23% (10/43) of animals with interstitial pneumonia harbored OvGHV2 DNA in their lungs, despite the absence of clinical MCF at the time of slaughter [18]. Similarly, a serosurvey of 367 dairy cows from 43 closed farms (with no direct contact with sheep) in Southern Brazil found a seroprevalence of 7.9% using an MCF-specific ELISA, with 37.2% of farms having at least one seropositive cow [12]. This seropositivity indicates prior exposure and likely subclinical infection. The virus has also been detected in the nasal swabs of both clinically ill and asymptomatic dairy calves, with OvGHV2 being the most frequently identified pathogen (81.2%) in an outbreak of acute respiratory disease [13] and in a study of respiratory disease dynamics, OvGHV2 was found in 37.2% of diseased and 27.2% of asymptomatic calves [30]. These findings collectively indicate that subclinical OvGHV2 infection in cattle is far more common than previously recognized, and that the virus can contribute to a range of pathological processes (e.g., pneumonia, enteritis, folliculitis) well beyond the classic "head and eye" form of SA-MCF [18, 19, 26, 13, 9].

Risk Factors for Outbreaks and Infection

The principal risk factor for SA-MCF is co-mingling of susceptible hosts with asymptomatically infected sheep. This is a consistent theme across all epidemiological studies. Outbreaks in cattle are almost invariably traced back to direct or indirect contact with sheep, even if such contact is not immediately obvious or is historical [1, 14, 17]. The risk is not binary but is modulated by several key variables.

Animal-level factors:

• Species and Type: Bovidae, particularly Bos taurus and Bos indicus cattle, are among the most susceptible species, often succumbing to acute or peracute disease. Water buffaloes (Bubalus bubalis) are also highly susceptible, though subclinical infections may be underreported [20]. Cervids (e.g., pudu, huemul) are vulnerable [21], and cases have been documented in suids (wild boar) and equids [4, 5, 22]. Novel risk has been identified for female cattle, with a statistically significant (p = 0.048) association between female sex and detection of MCFV antigens in the lungs of beef cattle with interstitial pneumonia, suggesting a potential hormonal or management-related predisposition [18].
• Age: While SA-MCF can affect animals of any age, outbreaks often target young adults. In a study from Egypt, all 18 affected cattle were adults [17]. Calves can be affected (e.g., a 7-month-old calf in a Brazilian outbreak) [14], and importantly, transplacental infection has been documented in both sheep and cattle, confirming that in utero exposure is a risk factor for congenital infection and potentially fetal loss [2, 7, 10]. A novel study demonstrated transplacental OvGHV2 infection in a sheep fetus [2], while another identified OvGHV2 DNA and MCFV antigens in aborted bovine fetuses as early as 78 days of gestation, adding abortion to the list of potential outcomes in cattle [10].

Environmental and Management-Related Factors:

• Sheep-to-Cattle Ratio (SCR): The quantitative relationship between the density of the reservoir and the susceptible population is a critical metric. The work in Rio Grande do Sul, Brazil, elegantly demonstrated that mesoregions with an SCR > 0.15 had a significantly elevated chance of SA-MCF outbreaks in cattle compared to regions with a lower SCR [14]. This model provides a practical tool for risk assessment in co-grazing systems.
• Production System: Management intensity modulates risk. A serosurvey in Brazil revealed that dairy cows raised in intensive production systems had a more than threefold higher chance of being seropositive to OvGHV2 compared to those in semi-intensive systems [12]. This paradox might be explained by higher stocking densities, increased indirect contact (e.g., shared equipment, personnel), or stress-induced viral recrudescence in the reservoir, leading to greater environmental contamination.
• Traditional Husbandry: Traditional production systems, where cattle and sheep are housed in close proximity or share pastures, are high-risk environments [14]. Conversely, closed dairy farms with no reported sheep contact are not immune; the detection of seropositive cows on such farms in Brazil, linked spatially to the home ranges of subclinically infected wild boars, suggests that wildlife or fomites can act as vectors, bridging the gap between the reservoir and susceptible livestock [12].

Co-infections and Pathogen Synergy: The clinical expression and severity of OvGHV2 infection can be profoundly influenced by concurrent infections. SA-MCF should not be viewed in isolation. Several studies have documented dual or multiple infections where OvGHV2 is identified alongside other pathogens. Co-infection with Histophilus somni has been linked to a more pronounced septicemic presentation in cattle [26]. Simultaneous infection with bovine coronavirus (BCoV) is frequently encountered, contributing to both respiratory and enteric disease complexes [13, 9]. Bovine gammaherpesvirus 6 (BoGHV6) has also been found alongside OvGHV2 in pulmonary lesions [1, 13]. These co-infections can mask the classic histopathological hallmarks of SA-MCF or alter the clinical trajectory, making diagnosis challenging and potentially increasing lethality. Moreover, stress from other diseases or poor nutrition may trigger recrudescence of latent OvGHV2 in the sheep reservoir, increasing shedding and the risk of transmission to susceptible hosts. This interplay underscores the need to consider OvGHV2 as a significant player within broader polymicrobial disease complexes, such as bovine respiratory disease (BRD) [18, 13, 31] and enteric disease [1, 9].

The Emerging Role of Wildlife as Bridge Hosts: A paradigm shift in the epidemiology of OvGHV2 is the recognition that wildlife species can act as bridge hosts, serving as intermediaries that carry the virus from the domestic sheep reservoir to fully susceptible populations. The most compelling evidence for this comes from studies in Southern Brazil, where free-ranging wild boars (Sus scrofa) were found to harbor subclinical OvGHV2 infections. In one study, OvGHV2 was detected in 37.5% (9/24) of wild boars sampled, including in the oral cavity of one individual, suggesting potential for onward transmission [4]. A subsequent spatial analysis demonstrated that dairy farms located within a 50 km radius of the home range of these subclinically infected wild boars had a significantly increased risk of OvGHV2 seropositivity in their cattle, even when no sheep were present on the farms [12]. This suggests that boars can acquire the virus from sheep in surrounding rural areas and then disseminate it to other species. Similarly, in Chile, OvGHV2 was detected for the first time in a free-ranging pudu (Pudu puda), indicating that cervids are not only susceptible to clinical disease but can also maintain the virus asymptomatically [21]. These findings challenge the traditional view of OvGHV2 as a simple two-host system and introduce a complex, multi-host ecological dimension to its epidemiology, with significant implications for disease control in livestock and conservation medicine.

Transmission Dynamics and Incubation

Transmission of OvGHV2 is horizontal, primarily via the nasal and ocular secretions of lambs and young sheep, which are the principal shedders of the virus [14, 6]. Aerosol or direct contact over short distances are considered the primary routes for infecting cattle. The virus is labile in the environment and does not persist for extended periods, meaning that indirect transmission via fomites (contaminated feed, water, equipment) is possible but likely less efficient than direct contact. The detection of OvGHV2 DNA in the oral cavity of a wild boar raises the possibility of transmission via predation or scavenging, but this remains speculative [4].

The incubation period in susceptible species is highly variable, typically ranging from 2 to 8 weeks, but can be longer, particularly in chronic or subclinical cases. The disease can manifest as an acute, rapidly fatal illness (peracute form) with death occurring within days of the onset of clinical signs, or as a more protracted course characterized by classical symptoms (ocular, nasal, oral, and neurological signs). The identification of an outbreak with an acute respiratory syndrome in dairy cattle, where OvGHV2 was the most frequently detected pathogen, highlights that a short incubation period and predominance of pulmonary signs can mimic other respiratory diseases, leading to misdiagnosis [13].

In conclusion, the epidemiology of SA-MCF is a fascinating and complex interplay between a highly adapted, persistent virus in its reservoir and a broad range of susceptible dead-end hosts. The risk is highest where sheep and cattle are co-located, but is now understood to be influenced by management intensity, the sex of the host, concurrent infections, and the unexpected participation of wildlife as bridge hosts. The high prevalence of subclinical infections in both reservoir and spillover hosts, coupled with the potential for vertical transmission, suggests that the true burden of OvGHV2-related disease and infection is substantially underestimated worldwide. The World Organisation for Animal Health (WOAH) includes MCF as a notifiable disease due to its high case fatality rate and international trade implications. Understanding these intricate epidemiological drivers is essential for designing rational, targeted control strategies, including management of sheep-to-cattle ratios, biosecurity to prevent wildlife access, and enhanced surveillance for subclinical infections.

Clinical and Pathological Manifestations in Cattle

Infection with ovine gammaherpesvirus 2 (OvGHV2) in cattle represents one of the most complex and devastating viral disease syndromes affecting bovine populations worldwide. The clinical and pathological spectrum induced by this macavirus ranges from peracute death to chronic, debilitating disease, with the hallmark lesion being a systemic lymphoproliferative and vasculitic process that distinguishes this infection from virtually all other bovine viral diseases. The epidemiological context of OvGHV2 infections in cattle is fundamentally shaped by the relationship with the reservoir host, asymptomatic sheep, and the virus’s capacity to induce disease across multiple organ systems with remarkable variability in clinical presentation.

Epidemiological Context and Clinical Spectrum

Sheep-associated malignant catarrhal fever (SA-MCF) typically occurs with low morbidity but extremely high lethality, often approaching 100% in clinically affected cattle [1, 14]. The disease is sporadic in nature, with outbreaks usually involving small numbers of animals within a herd, yet the case fatality rate in those developing clinical signs is devastating [14, 17]. As noted by the World Organisation for Animal Health (WOAH), SA-MCF is a notifiable disease in many regions due to its economic impact on cattle industries and its potential for rapid, fatal outcomes. The incubation period in cattle following natural exposure to OvGHV2 is highly variable, ranging from 2 to 8 weeks, and is influenced by viral dose, route of exposure, and host susceptibility factors [6]. The reservoir host, domestic sheep, show no clinical signs of infection yet shed the virus primarily through nasal secretions, creating an invisible but persistent source of infection for susceptible cattle populations [14, 15, 6].

The clinical presentation of OvGHV2 infection in cattle is notoriously pleomorphic, leading to its historical classification into several overlapping clinical syndromes: the peracute form, the head-and-eye form, the intestinal form, and the neurological form [11, 17, 6]. However, contemporary research has demonstrated that these divisions are artificial, as most affected animals exhibit a combination of clinical signs from multiple categories. A recent outbreak investigation in Southern Brazil documented the simultaneous occurrence of neurological, respiratory, and enteric disease syndromes across nine farms, highlighting the syndromic complexity of this infection [1]. The morbidity in this outbreak ranged from 4.2% to 37%, with lethality consistently exceeding 72% in clinically affected animals [1, 17].

Peracute Form: The peracute presentation is characterized by sudden death with minimal preceding clinical signs, often occurring within 24-48 hours of initial symptom onset. Affected animals may be found dead or die rapidly after presenting with high fever (40-42°C), severe depression, and prostration [14, 9]. In a case described from Rio Grande do Sul, a 7-month-old dairy calf died acutely after developing fever, profuse salivation, and respiratory difficulties, with death occurring within hours of clinical recognition [14]. The peracute form frequently confounds clinical diagnosis, as gross pathological alterations may be absent or minimal at necropsy, leading to misdiagnosis as other causes of sudden death such as anthrax, clostridial disease, or lightning strike [9].

Head-and-Eye Form: The classical head-and-eye form, historically considered the most common presentation, is characterized by bilateral corneal opacity, frequently described as "blue eye," which begins as a peripheral edema that progresses centrally over several days [17, 6]. Conjunctivitis, photophobia, epiphora, and blepharospasm are consistent findings. Mucosal lesions develop in the oral cavity, nasal passages, and esophagus, manifesting as multifocal erosions, ulcerations, and diphtheritic membranes [5, 22, 17]. The oral lesions typically affect the gingiva, hard palate, tongue, and dental pad, with affected animals exhibiting hypersalivation, anorexia, and reluctance to eat [17, 6]. Nasal discharge initially serous progresses to mucopurulent, and affected animals may develop stertorous breathing due to nasal mucosal involvement [17]. The combination of ocular and oral lesions in the head-and-eye form is so distinctive that experienced veterinarians can often make a presumptive diagnosis based on clinical examination alone [9].

Intestinal Form: The intestinal or alimentary form of SA-MCF is characterized by severe diarrhea, frequently hemorrhagic, accompanied by tenesmus and dehydration [1, 13, 9]. This presentation may precede or accompany respiratory signs, as documented in a recent outbreak where episodes of diarrhea followed acute respiratory distress syndrome in dairy calves [13]. Affected animals develop profuse, watery diarrhea that may contain blood and mucus, leading to rapid dehydration, electrolyte imbalances, and metabolic acidosis [9]. The intestinal form is particularly challenging to differentiate from other causes of acute enteritis in cattle, including bovine coronavirus (BCoV), bovine viral diarrhea virus (BVDV), and Salmonella spp. infection, particularly in young animals [13, 9].

Neurological Form: The neurological form of SA-MCF is characterized by central nervous system (CNS) signs that reflect the severity of meningoencephalitis and vasculitis within the brain. Affected animals may exhibit depression, ataxia, head pressing, nystagmus, tremors, convulsions, and recumbency [11]. In one standardized study of five cattle with confirmed OvGHV2 infection, all animals developed acute neurological signs without ocular or nasal manifestations, underscoring that the neurological form can occur independently of the classical head-and-eye presentation [11]. The neurological signs typically progress rapidly over 24-72 hours, and affected animals that become recumbent rarely survive [11, 6].

The Spectrum of Vasculitis and Vascular Proliferation

The pathological hallmark of OvGHV2 infection in cattle is a unique form of systemic vasculitis that affects arteries, veins, and capillaries throughout the body, but with a particular predilection for medium-sized arteries in the brain, kidneys, and gastrointestinal tract [1, 11, 25]. The vasculitic process is not a simple inflammatory response but rather a complex, biphasic lesion that includes both an acute necrotizing component and a chronic proliferative component, the latter being a distinguishing feature of this disease.

Necrotizing Lymphocytic Vasculitis: The acute phase of vasculitis is characterized by fibrinoid necrosis of the vessel wall, with infiltration of lymphocytes, histiocytes, and occasional plasma cells into the intima, media, and adventitia [1, 26, 11]. The endothelial cells become swollen and may slough into the lumen, resulting in thrombosis and subsequent infarction of downstream tissues [26, 25]. In the central nervous system, this process leads to microhemorrhages, edema, and ischemic necrosis of neural tissue, correlating with the neurological signs observed clinically [11]. An immunohistochemical (IHC) assay utilizing the 15A monoclonal antibody (15A-MAb), which recognizes a common epitope shared by all malignant catarrhal fever viruses (MCFV), has demonstrated the presence of OvGHV2 antigens within the cytoplasm of vascular endothelial cells, histiocytes, and infiltrating lymphocytes [1, 11, 24]. Recent experimental work using in situ hybridization has confirmed that OvHV-2-infected T cells and monocytes/macrophages are the dominant infiltrating cells in the vasculitic lesions, with both cell types harboring the virus and proliferating locally [24, 25].

Proliferative Vascular Lesions (PVLs): A unique and diagnostically critical feature of chronic OvGHV2 infection in cattle is the development of proliferative vascular lesions (PVLs), which represent the end stage of the vasculitic process [1, 9, 11]. These lesions are characterized by concentric intimal proliferation of myofibroblasts and smooth muscle cells, resulting in marked thickening of the vessel wall and eventual luminal stenosis or occlusion [9, 25]. PVLs were initially thought to be associated with the chronic head-and-eye form of MCF, but recent work has demonstrated that they occur in cattle infected with OvGHV2 regardless of the clinical presentation [9, 11]. In a study of three calves with OvGHV2 infection but without typical clinical manifestations of SA-MCF, PVLs were identified only at the carotid rete mirabile of two animals, suggesting that this location may be particularly predisposed to the development of proliferative changes [9]. The identification of PVLs in cattle with subclinical or atypical OvGHV2 infection indicates that these lesions represent an ongoing, progressive process that may occur irrespective of the clinical form of the disease [9, 11].

The pathogenesis of PVLs involves a complex interplay between viral infection, immune-mediated inflammation, and vascular remodeling. Mechanistic studies using the Syrian golden hamster model have demonstrated that OvHV-2 infection leads to T-cell and macrophage-dominated infiltrates in various organs, with infected leukocytes being abundant in the infiltrates [24]. The process begins with adventitial infiltration of inflammatory cells from the vasa vasorum, followed by recruitment of leukocytes from the arterial lumen, leading to superimposed infiltration of the intima and media [25]. Chronic changes, including neointimal proliferation, develop as a consequence of ongoing inflammation and vascular injury [9, 11, 25]. The detection of OvHV-2 nucleic acids within vascular endothelial cells, medial smooth muscle cells, and adventitial fibroblasts suggests that direct viral infection of these cell types contributes to the pathogenesis of PVLs [24, 25].

Systemic Pathological Manifestations by Organ System

The systemic nature of OvGHV2 infection in cattle results in pathological changes in virtually every organ system, with the distribution and severity of lesions reflecting the pattern of viral dissemination and the host immune response.

Central Nervous System: The brain is a primary target organ in OvGHV2 infection, with lesions ranging from mild lymphocytic infiltration to severe necrotizing meningoencephalitis [11, 17]. Histopathological examination reveals non-suppurative meningoencephalitis characterized by perivascular cuffing with lymphocytes and histiocytes, gliosis, and neuronal degeneration [11]. The vessels of the brain, particularly those of the rete mirabile and the meninges, show the full spectrum of vasculitic changes, from acute fibrinoid necrosis to chronic proliferative lesions [9, 11]. The involvement of the rete mirabile is of particular importance, as PVLs in this location can lead to impaired cerebral blood flow and contribute to the neurological signs observed clinically [9]. The presence of OvHV-2 antigens within the cytoplasm of neurons, as demonstrated by IHC, provides direct evidence of viral neurotropism [11].

Ocular and Mucosal Tissues: The ocular lesions of SA-MCF are dominated by corneal edema, which results from damage to the corneal endothelium and Descemet's membrane [17, 6]. The corneal opacity is initially peripheral and progresses centrally, giving the characteristic "blue eye" appearance. Conjunctival hyperemia, chemosis, and lymphocytic infiltration of the conjunctival stroma are consistent findings. The mucosal lesions in the oral cavity, nasal passages, and esophagus are characterized by epithelial necrosis, erosion, and ulceration, with a mixed inflammatory infiltrate [5, 22, 17]. The typical erosive stomatitis observed in cattle has been reproduced in the hamster model, where infected animals develop similar mucosal lesions due to infiltration of the mucosa by infected T cells and macrophages [24].

Respiratory System: The lungs are frequently involved in OvGHV2 infection, with the most common histopathological finding being interstitial pneumonia [1, 18, 13]. In a study of 44 beef cattle from Mato Grosso, Brazil, interstitial pneumonia was diagnosed in 98% (43/44) of lungs evaluated, with MCFV antigens detected by IHC in 37% (16/43) of these cases [18]. The interstitial pneumonia is characterized by thickening of the alveolar septa due to infiltration of lymphocytes, macrophages, and occasional plasma cells, with hyperplasia of type II pneumocytes [18, 26, 13]. In some cases, the pulmonary changes are accompanied by suppurative bacterial bronchopneumonia, reflecting secondary bacterial invasion [26, 9]. A particularly important observation is that OvGHV2 can be detected in the lungs of asymptomatic cattle, suggesting that subclinical pulmonary infections are common and may serve as a source of viral persistence within the host [18, 20]. The detection of OvGHV2 in bronchoalveolar lavage fluid and in bronchial epithelial cells by IHC supports the role of the respiratory tract as a portal of entry and a site of viral replication [18, 13].

Gastrointestinal Tract: The intestinal changes in SA-MCF are dominated by atrophic enteritis, which is often most severe in the small intestine [1, 13, 9]. Histopathological examination reveals villous blunting and fusion, cryptal necrosis and dilatation, and lymphocytic infiltration of the lamina propria [13, 9]. The cryptal epithelial cells frequently contain intracytoplasmic OvHV-2 antigens, as demonstrated by IHC [13, 11]. The atrophic enteritis leads to malabsorption, contributing to the diarrhea observed clinically [13, 9]. The liver shows lymphocytic portal hepatitis, with infiltration of the portal tracts by lymphocytes and plasma cells [1, 11]. Bile duct epithelial cells may contain MCFV antigens, and the cholangiohepatitis observed in some cases may be due to direct viral injury [1, 11].

Urinary System: The kidneys are consistently affected in OvGHV2 infection, with lymphocytic interstitial nephritis being a near-universal finding [11]. The renal lesions are characterized by multifocal to coalescing infiltrates of lymphocytes and histiocytes within the interstitium, often accompanied by tubular degeneration and necrosis [11]. Proliferative vascular lesions in the kidneys may lead to glomerular ischemia and contribute to renal dysfunction. The renal tubular epithelial cells consistently show positive immunoreactivity for MCFV antigens, indicating that the kidney is a site of viral replication [11]. The detection of OvHV-2 DNA in kidney tissue by PCR confirms the presence of viral infection in this organ [26, 11].

Lymphoid Tissues: The lymphoid system undergoes profound changes in SA-MCF, including generalized lymphadenopathy and lymphoid hyperplasia [17, 6]. Histopathological examination reveals expansion of the paracortical (T-cell) zones in lymph nodes and spleen, with depletion of B-cell follicles [24, 25]. The lymphadenopathy is due to the proliferation of virus-infected T cells and macrophages, which infiltrate and expand the lymphoid tissue [24]. Splenic changes include lymphoid hyperplasia, with the presence of large, transformed lymphocytes and increased numbers of macrophages [24]. The bone marrow also shows evidence of increased T-cell and monocyte/macrophage proliferation [24].

Cutaneous Manifestations: While less common than other forms, OvGHV2 can produce chronic skin disease in cattle. A recent study from Southern Brazil documented an outbreak of chronic folliculitis in dairy cows, characterized by widespread ulcerative and erythematous skin lesions [19]. Histopathological examination revealed chronic folliculitis with lymphocytic infiltration of the hair follicles and perifollicular tissues [19]. MCFV antigens were detected in most skin lesions by IHC, and OvGHV2 DNA was amplified from a significant portion of biopsies [19]. Notably, these infected cows had no contact with sheep, suggesting that other transmission mechanisms, possibly involving subclinically infected wild boars or other ruminants, may be implicated [5-7].

Cardiovascular System: Myocardial involvement in SA-MCF is characterized by lymphocytic myocarditis, with infiltration of lymphocytes and macrophages between myocardial fibers [26, 11]. Cases with concomitant infections, particularly with Histophilus somni, may develop more severe myocardial lesions, including hemorrhagic myocarditis and myocardial necrosis [26].

Diagnostic Approaches: Immunohistochemistry and Molecular Detection

The definitive diagnosis of infections caused by Ovine Gammaherpesvirus 2 (OvGHV2) and the broader malignant catarrhal fever virus (MCFV) complex requires a sophisticated, multi-modal laboratory approach. Given the absence of a permissive cell culture system that supports efficient in vitro propagation of OvGHV2 [3, 5, 22], diagnosis has historically relied upon the integration of histopathological lesion characterization with ancillary techniques designed to detect viral antigens or nucleic acids directly within affected tissues. The diagnostic arsenal has evolved considerably, moving from classical histopathology and electron microscopy to highly specific immunohistochemical (IHC) assays and exquisitely sensitive molecular biological platforms. The selection and interpretation of these assays are heavily influenced by the pathobiology of the virus, particularly the latency state of the infection, the tissue tropism, and the availability of reagents that recognize the conserved 15A antigenic epitope shared among all MCF-causing macaviruses [1, 2, 12, 10].

Serological and Antigenic Foundation: The 15A Epitope

A cornerstone of immunodiagnosis for OvGHV2 and other MCFVs is the identification of the 15A antigenic epitope. This highly conserved epitope is expressed by all members of the Macavirus genus known to induce MCF in their respective dead-end hosts, including OvGHV2, alcelaphine gammaherpesvirus 1 (AlGHV1), and caprine gammaherpesvirus 2 (CpHV-2) [1, 2, 12, 10]. The development of the 15A monoclonal antibody (15A-MAb) has been instrumental in providing a single, versatile reagent capable of detecting a wide spectrum of MCFV infections, irrespective of the specific viral species involved. This cross-reactivity, while a powerful diagnostic tool for screening, introduces a critical analytical caveat: a positive 15A-based assay confirms the presence of an MCFV complex member but cannot, without further molecular characterization, definitively identify the specific macavirus, as demonstrated by the well-documented cross-reactivity of 15A-MAb with bovine gammaherpesvirus 6 (BoGHV6) [1]. The 15A-MAb has been successfully deployed in both competitive inhibition enzyme-linked immunosorbent assays (CI-ELISA) for serological surveillance and in IHC protocols for direct antigen detection in fixed tissues [12, 11].

Immunohistochemical (IHC) Assays for In Situ Antigen Detection

The IHC assay employing the 15A-MAb has emerged as a preeminent method for detecting MCFV tissue antigens, providing critical spatial context by localizing viral proteins within specific cell types and lesions [11, 27]. This technique has been extensively validated in cases of OvGHV2-induced sheep-associated MCF (SA-MCF) in cattle, where it consistently demonstrates positive, intracytoplasmic, intralesional immunoreactivity.

Cellular and Tissue Tropism Revealed by IHC: The 15A-MAb IHC assay has been pivotal in illuminating the in vivo target cell spectrum of OvGHV2. Positive intracytoplasmic immunoreactivity has been most consistently identified within the epithelial cells of multiple organ systems. In a landmark study standardizing this assay for SA-MCF, Headley et al. [11] demonstrated that MCFV antigens are predominantly localized within the cytoplasm of epithelial cells in the kidneys (renal tubular epithelium), intestines (cryptal epithelial cells), liver (bile duct epithelium), and lungs (bronchial and bronchiolar epithelium). This epithelial tropism is a hallmark of the disease and aligns with the clinical manifestations of enteritis, nephritis, and respiratory distress. Beyond epithelial cells, the 15A-MAb has been shown to label histiocytes, lymphocytes, and, importantly, myocardial fibers and neurons in chronic cases [27]. The detection of viral antigens within leucocytes is consistent with the hypothesis that OvGHV2 is hematogenously disseminated by infected lymphoid cells, a key feature of gammaherpesvirus biology [3, 24, 25].

The utility of IHC extends to diagnosing infections in a remarkable breadth of hosts and clinical scenarios. It has been used to confirm OvGHV2 infection in:

• Cattle with acute respiratory and enteric disease: IHC identified MCFV antigens within pulmonary and intestinal epithelial cells during an outbreak of acute respiratory disease, with subsequent molecular testing confirming OvGHV2 [13].
• Cattle with atypical or subclinical presentations: In cases of chronic folliculitis caused by OvGHV2, the 15A-MAb detected MCFV antigens in the majority of skin lesions, expanding the recognized clinical spectrum of the virus and revealing a cutaneous tropism not previously appreciated in Latin America [19].
• Asymptomatic reservoir and bridging hosts: IHC detected MCFV antigens in the lungs of free-ranging wild boars (Sus scrofa) with subclinical viral-induced pneumonia, identifying them as potential bridging hosts for virus dissemination to livestock [4].
• Concurrent infections: The assay has been instrumental in complex disease investigations. In cattle co-infected with Histophilus somni and OvGHV2, IHC successfully identified intralesional MCFV antigens, allowing pathologists to differentiate the contributions of each pathogen to the observed pulmonary and vascular lesions [26].
• Transplacental and fetal infections: A critical and expanding application of IHC is in the investigation of vertical transmission. The 15A-MAb assay has been used to detect MCFV antigens in multiple organs of aborted bovine fetuses [10] and, remarkably, in the organs of ovine fetuses, providing the first evidence of transplacental OvGHV2 infection in sheep [2]. These findings underscore the role of macaviruses in reproductive failure.

Analytical Considerations and Limitations of IHC: The primary strength of IHC lies in its ability to link the presence of viral antigen directly to histological lesions (e.g., lymphocytic vasculitis, proliferative vascular lesions, interstitial pneumonia), thereby providing cause-and-effect evidence at the tissue level [1, 26, 9, 11]. However, the assay is not species-specific due to its reliance on the pan-MCFV 15A-MAb. As noted by Headley et al. [1], animals with typical MCF-like vascular lesions may test positive by 15A-MAb IHC yet be negative for both OvGHV2 and BoGHV6 by PCR, suggesting the presence of a currently uncharacterized or novel macavirus. Conversely, IHC can yield false-negative results if the viral load is below the threshold of antigen detection or if the epitope has been masked or degraded during tissue processing. Despite these limitations, IHC remains an indispensable tool for confirmatory diagnosis, particularly in formalin-fixed tissues that are not suitable for molecular analysis.

Molecular Detection: Conventional and Real-Time PCR Assays

Molecular detection targeting OvGHV2-specific nucleic acid sequences offers the highest level of sensitivity and specificity, essential for confirming the identity of the macavirus detected by IHC. The most common molecular targets include the tegument protein gene and the ORF75 gene, both of which contain regions with sufficient sequence diversity to differentiate OvGHV2 from other macaviruses like BoGHV6 and AlGHV1 [1, 14, 20, 5, 17].

Qualitative (Conventional) PCR: Conventional, endpoint PCR assays that amplify a specific segment of the OvGHV2 genome remain a standard diagnostic tool. These assays have been applied successfully across a wide range of sample types, including fresh and frozen tissues (lung, liver, spleen, kidney, lymph node), whole blood, and even deep nasal swabs (DNS) [14, 13, 5, 27]. For example, Headley et al. [27] used a PCR targeting the OvGHV2 tegument protein gene to amplify viral DNA from the lungs, kidneys, liver, spleen, and cerebrum of a sheep with chronic interstitial pneumonia, confirming the IHC findings. In the Philippine context, Mananguit et al. [15] employed a semi-nested PCR targeting the tegument gene to identify a 65.17% prevalence of OvGHV2 in clinically healthy sheep, demonstrating the utility of the assay for epidemiological screening. A critical advantage of conventional PCR is the ability to use the amplicon for direct sequencing, providing definitive phylogenetic confirmation of the viral strain. Sequencing has proven invaluable in demonstrating the genetic identity between OvGHV2 strains circulating in cattle, sheep, and wild boars in Brazil, and their high homology (>98%) with strains from Germany, Turkey, and the Philippines [20, 15, 27].

Quantitative Real-Time PCR (qPCR): Real-time PCR (qPCR) has become the gold standard for both diagnosis and research, offering enhanced sensitivity, speed, and the capacity for viral load quantification. This is particularly important in subclinical or latent infections where viral copy numbers may be low. The use of a qPCR assay was instrumental in confirming OvGHV2 in the tissues of a calf from an outbreak in Southern Brazil and, critically, in quantifying viral DNA in the blood of asymptomatic co-habiting sheep, thereby identifying the reservoir source of the outbreak [14]. The ability to measure viral load (e.g., copies per microgram of DNA) provides data that can be correlated with disease severity and transmission dynamics. For instance, Rosato et al. [8] used a quantitative PCR to survey cattle across Europe for OvGHV2, finding it to be rare (0%) compared to the high prevalence (32%) of the commensal BoGHV-6, highlighting the power of qPCR for large-scale epidemiological surveillance.

Nested PCR (nPCR) for Enhanced Sensitivity: For applications requiring the highest sensitivity, such as detecting latent virus or viral DNA in environmental or low-quality samples, nested PCR (nPCR) is employed. This approach uses two successive rounds of PCR amplification, significantly reducing non-specific amplification and increasing the limit of detection. Hidalgo-Hermoso et al. [21] used a pan-herpesvirus nested PCR to discover OvGHV2 in the spleen of a free-ranging pudu (Pudu puda) in Chile, marking the first report of the virus in the country. Similarly, Headley et al. [7] used a nested PCR targeting the BoGHV6 polymerase gene to detect that virus in aborted bovine fetuses, demonstrating the technique’s power for uncovering macaviral involvement in challenging reproductive disease cases.

The Synergistic Integration of IHC and Molecular Methods

The most robust and authoritative diagnostic approach for OvGHV2 is not the application of a single test but the strategic integration of IHC and PCR/qPCR. This dual approach mitigates the limitations of each individual method and provides a complete diagnostic picture.

The standard workflow in many contemporary studies [1, 26, 13, 9] involves:

1. Histopathological Screening: Initial identification of characteristic lesions (lymphocytic vasculitis, proliferative vascular lesions, lymphocytic interstitial pneumonia, atrophic enteritis) in tissue sections.
2. Antigenic Confirmation by IHC: Application of the 15A-MAb IHC assay to confirm the presence of MCFV antigens within the lesional tissue, providing spatial context.
3. Aetiological Identification by PCR: Application of a species-specific PCR (e.g., for OvGHV2 tegument/ORF75 gene) on a homogenate of the same lesional tissue to confirm the specific viral species.
4. Epidemiological Linkage via Sequencing: Direct sequencing of the PCR amplicon for phylogenetic comparison with known strains.

This integrated approach has been pivotal in resolving complex disease scenarios. For example, in cattle without typical manifestations of SA-MCF, IHC revealed MCFV antigens in tissues while PCR simultaneously amplified OvGHV2 DNA, confirming an atypical, subclinical presentation [9]. It also proved critical in the investigation of aborted fetuses, where IHC identified MCFV antigens in a fetus that was negative for OvGHV2 and BoGHV6 by PCR, suggesting a potentially novel macavirus [10]. In the case of a sheep with chronic pneumonia, the combination of IHC (showing viral antigens in bronchial epithelium) and PCR (amplifying OvGHV2 DNA from lung tissue) provided the conclusive evidence needed to link OvGHV2 to the chronic pulmonary disease [27].

In Situ Hybridization (ISH) as a Confirmatory Bridge: A powerful third pillar in this integrated approach is RNA in situ hybridization (ISH), which provides the cellular resolution of IHC with the specificity of molecular hybridization. ISH uses labeled probes to bind directly to viral RNA or DNA within tissue sections. Pesavento et al. [29] developed an ISH method that identified OvGHV2 nucleic acids within lesions of sheep with systemic necrotizing vasculitis, directly correlating viral presence with the pathological process. This technique beautifully bridges the gap between antigen detection (IHC) and nucleic acid detection (PCR), demonstrating active viral transcription within specific cells (e.g., endothelial cells, smooth muscle cells) [25]. While less widely available than IHC, ISH is increasingly recognized as an authoritative method for confirming viral involvement in pathogenesis studies.

Standardization and Future Directions

For international trade and regulatory purposes, the World Organisation for Animal Health (WOAH) recognizes SA-MCF as a listed disease for which specific diagnostic methods are recommended. While a single "gold standard" test does not exist, the integration of histopathology with a validated 15A-based IHC and a species-specific real-time PCR assay is considered the most reliable approach. Standardization of protocols, including the use of positive and negative tissue controls, verification of primer specificity for OvGHV2, and adoption of standardized cut-off values for qPCR, is essential for reproducibility across laboratories.

Future diagnostic developments will likely focus on:

• Next-Generation Sequencing (NGS): NGS offers the potential for unbiased discovery of novel macaviruses from IHC-positive, PCR-negative cases [1, 10].
• Digital PCR (dPCR): dPCR provides absolute quantification of viral copy numbers without the need for standard curves, which may improve diagnostic sensitivity in cases of low viral load.
• Multiplex Assays: The development of multiplex PCR panels that can simultaneously detect OvGHV2, BoGHV6, and other macaviruses will streamline the diagnostic workflow and provide a more comprehensive picture of co-infections.
• Point-of-Care (POC) Diagnostics: For field deployment in resource-limited settings, the development of POC molecular tests (e.g., isothermal amplification) could revolutionize early outbreak detection, a priority for agencies like the Food and Agriculture Organization (FAO) focused on livestock health surveillance.

Co-infections, Cross-Reactivity, and the Role of Other Macaviruses

The clinical and epidemiological landscape of ovine gammaherpesvirus 2 (OvGHV2) is profoundly shaped by its interactions with other pathogens and the broader Macavirus genus. The diagnostic and pathogenic complexities arising from co-infections, serological cross-reactivity, and the presence of related macaviruses in susceptible and reservoir hosts represent a critical frontier in understanding malignant catarrhal fever (MCF) syndromes. The 15A monoclonal antibody (15A-MAb), which targets a conserved epitope shared by all MCF-causing members of the Macavirus genus, has been instrumental in identifying these agents but simultaneously introduces a significant interpretive challenge due to its broad cross-reactivity [1, 12, 11]. This necessitates a nuanced approach to diagnosis and a deep appreciation of the diverse viral landscape within which OvGHV2 operates.

Diagnostic Cross-Reactivity and the 15A Antigenic Epitope

The 15A-MAb-based immunohistochemical (IHC) and competitive inhibition ELISA (CI-ELISA) assays are cornerstone diagnostic tools for detecting MCF virus (MCFV) antigens, yet their pan-Macavirus specificity is both a strength and a limitation [12, 11]. As demonstrated in a comprehensive study from Southern Brazil, the 15A-MAb IHC assay detected intralesional MCFV antigens in cattle with characteristic necrotizing lymphocytic vasculitis, but subsequent molecular testing revealed that the causative agents were not always OvGHV2 [1]. In that investigation, PCR identified singular infections by OvGHV2 in three animals and bovine gammaherpesvirus 6 (BoGHV6) in three other animals, all of which exhibited positive intracytoplasmic immunoreactivity with the 15A-MAb [1]. Critically, in one animal with classic vascular lesions and positive IHC findings, neither OvGHV2 nor BoGHV6 DNA was detected, strongly suggesting the involvement of a third, previously unrecognized macavirus [1]. This finding underscores the inherent risk of misattributing MCF pathology solely to OvGHV2 based on serological or IHC results alone and highlights the expanding diversity of macaviruses capable of inducing MCF-like disease.

The cross-reactivity extends to serological surveillance. An indirect MCF-specific ELISA based on the alcelaphine gammaherpesvirus 1 (AlGHV1) C500 strain has been successfully employed to detect antibodies against OvGHV2 in dairy cattle from Southern Brazil [12]. The utility of this assay depends on the conserved nature of the 15A epitope, but it cannot differentiate among antibodies elicited by different macaviruses [12]. This serological cross-reactivity is particularly problematic in regions where multiple macaviruses circulate, such as Brazil, where BoGHV6 is highly prevalent in cattle and can produce false-positive signals in assays designed to monitor OvGHV2 exposure [1, 12, 7]. The World Organisation for Animal Health (WOAH) recognizes MCF as a disease of economic significance, and the reliance on cross-reactive serological tools means that prevalence data for OvGHV2 may be inflated or confounded by the presence of other macaviruses, complicating international disease reporting and risk assessment.

Concomitant Infections: OvGHV2 and Bovine Respiratory Disease Pathogens

OvGHV2 frequently participates in polymicrobial infections, particularly within the bovine respiratory disease (BRD) complex. Its role as a primary respiratory pathogen has been increasingly recognized, but it often acts in concert with other viral and bacterial agents. In a detailed outbreak investigation of acute respiratory disease in dairy cattle, OvGHV2 was the most frequently detected agent at 81.2% (13/16) of animals, but it was found in dual and triple infections alongside bovine gammaherpesvirus 6 (BoGHV6) and bovine coronavirus (BCoV) [13]. This study demonstrated that OvGHV2 was not merely an incidental finding but the dominant pathogen in a clinical outbreak characterized by interstitial pneumonia and enteritis, suggesting a synergistic interaction with these co-infecting agents [13].

Similarly, a study of BRD in beef cattle from Mato Grosso, Brazil, revealed that OvGHV2 was the only pathogen consistently amplified from the lungs of cattle with interstitial pneumonia, occurring in singular (n=2), dual, and triple infections [18]. While bovine viral diarrhea virus (BVDV), BCoV, and Mannheimia haemolytica were also detected, OvGHV2 was the most frequent viral finding, and its detection was statistically associated with pulmonary disease in female cattle [18]. The chronic interstitial pneumonia observed in these cases aligns with the hypothesis that OvGHV2 contributes to the etiology of IP, particularly when combined with other respiratory pathogens [18, 13].

The relationship between OvGHV2 and Histophilus somni provides another compelling example of co-infection dynamics. In two fatal cases of septicemia and encephalitis in cattle, concurrent infections with H. somni and OvGHV2 were identified [26]. The animals exhibited characteristic proliferative vascular lesions (PVLs) and necrotizing vasculitis attributable to OvGHV2, alongside the suppurative lesions typical of H. somni. This finding suggests that OvGHV2-induced vascular damage may predispose cattle to hematogenous dissemination of bacterial pathogens or that the two agents act synergistically to exacerbate systemic disease [26]. Furthermore, OvGHV2 has been detected in aborted bovine fetuses co-infected with Leptospira spp. and Neospora caninum, indicating that its role in reproductive failure may be underestimated, particularly in the presence of other fetopathy agents [10].

The Expanding Host Range: OvGHV2 in Non-Traditional Species

The ecological niche of OvGHV2 extends beyond its classical domestic ruminant hosts, with increasing evidence of infection in a diverse array of mammals, including swine, cervids, and equids. The detection of subclinical OvGHV2 infections in free-ranging wild boars (Sus scrofa) from Southern Brazil is particularly noteworthy [4]. In that study, OvGHV2 DNA was amplified from 37.5% (9/24) of asymptomatic wild boars, and viral antigens were detected in pulmonary epithelial cells, with some animals displaying viral-induced pneumonia [4]. The presence of OvGHV2 in the oral cavity of one boar raises the alarming possibility that wild boars could serve as "bridge hosts," facilitating spillover from reservoir sheep populations to other susceptible livestock or wildlife [4]. This finding has profound implications for the epidemiology of MCF, as wild boar populations are expanding globally and often share habitats with sheep and cattle.

The susceptibility of cervids to OvGHV2 is well-documented, but recent data from Chilean Patagonia reveal the virus's presence in endangered species. Molecular screening of free-ranging pudus (Pudu puda) and huemuls (Hippocamelus bisulcus) identified OvGHV2 DNA in a pudu, with a second animal yielding a Macavirus sequence 98.63% identical to OvGHV2 [21]. This represents the first report of OvGHV2 in Chile and confirms that MCF poses a novel threat to the conservation of these vulnerable species [21]. The introduction of infected sheep into pristine habitats could have catastrophic effects on naïve wild ruminant populations, a concern that aligns with WOAH guidelines on the management of wildlife-livestock interfaces.

Perhaps the most surprising expansion of OvGHV2's host range is its documented infection of horses. An outbreak of SA-MCF in Mexico involved not only artiodactyls but also horses, a species not traditionally considered susceptible [5, 22]. Affected horses exhibited mucogingival ulcers, corneal opacity, and hypersalivation, and OvGHV2 was isolated from their buffy coats, confirmed by partial sequencing of the ORF75 gene [5, 22]. This finding challenges the long-held assumption that equids are refractory to OvGHV2 and suggests that the virus's tissue tropism is broader than previously appreciated. The identification of OvGHV2 in horses may have implications for the differential diagnosis of equine herpesvirus infections and for the management of mixed-species grazing operations.

Bovine Gammaherpesvirus 6 (BoGHV6): Commensal or Pathogen?

Bovine gammaherpesvirus 6 (BoGHV6), formerly known as bovine lymphotropic virus, occupies a contentious position within the Macavirus genus. While it shares the 15A epitope and is phylogenetically related to OvGHV2, its pathogenic potential in cattle remains unclear [7, 8, 34]. Large-scale surveys across Europe have demonstrated BoGHV6 to be highly endemic, with an overall prevalence of 32% in cattle from Switzerland, the UK, Finland, Belgium, and Germany [8]. The virus was detected across all age groups, including in calves as young as one day old, and its presence was not associated with any specific disease process, leading to its classification as a commensal [8, 34]. In situ hybridization studies targeting the ORF73 gene revealed extremely limited viral transcription, with signals confined to rare lymphocytes in bronchial lymph nodes and occasional alveolar epithelial cells, and no associated pathology [34].

However, data from Brazil complicate this benign characterization. BoGHV6 DNA was detected in 76.9% (10/13) of aborted bovine fetuses from seven dairy herds, often as the only identifiable pathogen in the myocardium of a fetus with myocarditis [7]. While the authors caution that the presence of DNA alone is insufficient to establish causality, the finding that BoGHV6 was the sole agent detected in several fetal tissues, including the myocardium, spleen, and kidney, raises the possibility that this virus may contribute to reproductive failure under certain circumstances [7]. This is supported by the detection of BoGHV6 in cattle without OvGHV2 but with positive 15A-MAb IHC results, where the virus was linked to mild pathological lesions [1]. The apparent discrepancy between the European and Brazilian data may reflect differences in viral strains, host genetics, or the presence of concurrent infections. The WOAH does not currently list BoGHV6 as a notifiable pathogen, but these emerging data warrant further investigation into its potential role as an opportunistic pathogen in immunocompromised or co-infected animals.

The Genus Macavirus: A Complex of Related Viruses

OvGHV2 is but one member of a growing genus of gammaherpesviruses that infect ungulates and share the capacity to cause MCF. The MCFV complex includes AlGHV1 (wildebeest-associated MCF), caprine gammaherpesvirus 2 (CpHV-2), and bison lymphotropic herpesvirus, among others [12, 6]. The recent sequencing of ovine herpesvirus 1 (OvHV-1), a macavirus initially identified in sheep with pulmonary adenocarcinoma but later found to be a common infection, has provided new insights into genomic diversity within the genus [23]. OvHV-1 shares a similar genomic architecture with OvGHV2 and BoGHV6 and encodes a homologue of ovine interleukin-10, a gene previously thought to be unique to OvGHV2 [23]. This finding suggests that macaviruses may employ common immunomodulatory strategies to establish latency and persist in their reservoir hosts.

The presence of multiple macaviruses in the same ecological niche creates opportunities for co-infection and genetic recombination. In free-ranging wild boars, 14.8% (37.5% overall) of animals were infected with OvGHV2, and the possibility of co-infection with porcine lymphotropic herpesvirus (PLHV) was not excluded [4]. Similarly, in water buffaloes (Bubalus bubalis) from Central-Western Brazil, OvGHV2 was detected in 18.9% of asymptomatic animals, demonstrating that this species serves as a reservoir for the virus without clinical disease [20]. The coexistence of OvGHV2, BoGHV6, and potentially other macaviruses in cattle, goats, and wildlife in Brazil creates a complex epidemiological web.

The reservoir host dynamics are further exemplified by studies of black wildebeest (Connochaetes gnou) in South Africa, which are latent carriers of a gammaherpesvirus closely related to AlGHV1 [16]. Unlike OvGHV2 transmission in sheep, which occurs predominantly after weaning, black wildebeest transmit their macavirus primarily in utero or immediately after birth, as evidenced by 90% of fetuses testing positive by PCR [16]. This vertical transmission strategy may have evolved to ensure efficient maintenance of the virus in a migratory wild ungulate population and stands in stark contrast to the horizontal transmission patterns observed for OvGHV2 in sheep.

Mechanistic Implications of Co-infections and Cross-Reactivity

The biological mechanisms underlying the pathogenicity of OvGHV2 in the context of co-infections are beginning to be elucidated. The Syrian hamster model of SA-MCF has demonstrated that OvHV-2 infection is associated with a T-cell and macrophage-dominated systemic vasculitis, with the virus infecting not only leukocytes but also vascular endothelial cells and smooth muscle cells [24, 25]. This widespread infection leads to a graft-versus-host-like scenario, where activated, infected T-cells and monocytes home to tissues and trigger inflammatory cascades [24, 25]. In the presence of co-infecting agents such as H. somni or BCoV, the pre-existing vascular damage and immune activation induced by OvGHV2 may facilitate bacterial invasion and exacerbate tissue injury [26, 13].

The cross-reactivity of the 15A antibody presents a significant challenge for field diagnostics. In a study of aborted bovine fetuses from Brazil, MCFV antigens were detected by IHC in fetuses #1 and #4, but OvGHV2 DNA was amplified only from Fetus #1, while BoGHV6 was absent in both [10]. This suggests that Fetus #4 may have been infected with a different, unidentified macavirus, a hypothesis supported by the earlier findings where the 15A-MAb detected MCFV antigens in the absence of both OvGHV2 and BoGHV6 DNA [1]. The implication is that there are likely more macaviruses circulating in ruminant populations than currently recognized, and that reliance on a single diagnostic assay may lead to misidentification of the etiological agent.

From a public health and disease control perspective, this diagnostic cross-reactivity complicates efforts to accurately map the distribution of OvGHV2. The CDC and WOAH emphasize the importance of accurate laboratory diagnosis for the control of transboundary animal diseases. The high prevalence of subclinical OvGHV2 infections in cattle, evidenced by studies where 37% of animals with interstitial pneumonia harbored MCFV antigens without clinical signs of SA-MCF [18], means that serosurveys based on pan-Macavirus ELISAs may overestimate the true burden of OvGHV2-specific disease. This could lead to unnecessary trade restrictions or misguided culling programs if BoGHV6 or another macavirus is the actual cause of seropositivity.

Summary of Critical Knowledge Gaps

The interplay between OvGHV2 and other macaviruses, as well as co-infecting pathogens, remains one of the least understood aspects of MCF epidemiology. The detection of BoGHV6 in 76.9% of aborted fetuses [7] and the identification of a potential unknown macavirus in 15A-positive tissues [1] underscore the need for more comprehensive metagenomic surveys. The finding that OvGHV2 can subclinically infect wild boars [4] and horses [5] expands the reservoir spectrum and raises questions about transmission dynamics across the livestock-wildlife interface. The recent sequencing of the OvHV-1 genome [23] provides a valuable resource for developing species-specific molecular diagnostics that can differentiate among these closely related viruses. As the clinical presentations of OvGHV2 overlap with those of other macaviruses and bacterial pathogens, the development of multiplex PCR panels and the integration of IHC with next-generation sequencing will be essential for accurate diagnosis and for unraveling the complex pathogenesis of polymicrobial MCF.

References

[1] Headley SA, Fritzen JTT, Maturana ÉFL, Alvarenga NACAd, Rodrigues RV, Gomes ALPL, et al.. Clinical, epidemiological, and pathological findings of ovine gammaherpesvirus 2 infections in cattle from Southern Brazil.. Journal of Infection in Developing Countries. 2025. DOI: https://doi.org/10.3855/jidc.19951

[2] Headley SA, Queroz GR, Lima JCSdJ, Fritzen J, Zufelato PAP, Yoshitani GD, et al.. Ovine gammaherpesvirus 2 vertical infections in sheep.. Microbial Pathogenesis. 2025. DOI: https://doi.org/10.1016/j.micpath.2025.107419

[3] Nath B, Schwab R, Swarbrick C, Pavan S, Rahman N, McSharry BP, et al.. Structural insights into the nuclear import of ovine gammaherpesvirus 2 ORF73 LANA homologue. Journal of General Virology. 2026. DOI: https://doi.org/10.1099/jgv.0.002250

[4] Headley SA, Fritzen J, Silva FHP, Minarelli SLM, Biondo LM, Kmetiuk LB, et al.. Subclinical Ovine Gammaherpesvirus 2-Related Infections in Free-Ranging Wild Boars (Sus scrofa) from Southern Brazil. Pathogens. 2024. DOI: https://doi.org/10.3390/pathogens13060515

[5] Madrigal-Valencia TL, Saavedra-Montañez M, Pérez-Torres A, Hernández J, Segalés J, Hernández YD, et al.. First identification and characterization of ovine gammaherpesvirus type 2 in horses and artiodactyla from an outbreak of malignant catarrhal fever in Mexico. PLoS ONE. 2023. DOI: https://doi.org/10.1371/journal.pone.0290309

[6] Janson L. MCF. Catalysis from A to Z. 2020. DOI: https://doi.org/10.1002/9783527809080.cataz10270

[7] Headley SA, Fritzen J, Bon VR, Xavier AAC, Agnol AMD, Zucoloto NZ, et al.. Detection of bovine gammaherpesvirus 6 in tissues of aborted fetuses from dairy cows concomitantly infected by Histophilus somni.. Microbial Pathogenesis. 2022. DOI: https://doi.org/10.1016/j.micpath.2022.105621

[8] Rosato G, Subira AR, Al-Saadi M, Michalopoulou E, Verin R, Dettwiler M, et al.. Gammaherpesvirus Infections in Cattle in Europe. Viruses. 2021. DOI: https://doi.org/10.3390/v13122337

[9] Headley SA, Lemos GAAd, Agnol AMD, Xavier AAC, Depes VCA, Yasumitsu CY, et al.. Ovine gammaherpesvirus 2 infections in cattle without typical manifestations of sheep-associated malignant catarrhal fever and concomitantly infected with bovine coronavirus. Brazilian Journal of Microbiology. 2021. DOI: https://doi.org/10.1007/s42770-021-00653-6

[10] Silva FHP, Fritzen J, Figueiredo JRX, Jurkevicz RMB, Domingues AFF, Rose MP, et al.. Transplacental Infections Associated with Macavirus in Aborted Bovine Fetuses. Microorganisms. 2024. DOI: https://doi.org/10.3390/microorganisms12081608

[11] Headley SA, Oliveira TE, Li H, Lisbôa J, Queiroz GR, Fritzen J, et al.. Immunohistochemical Detection of Intralesional Antigens of Ovine Gammaherpesvirus-2 in Cattle with Sheep-associated Malignant Catarrhal Fever.. Journal of Comparative Pathology. 2020. DOI: https://doi.org/10.1016/j.jcpa.2019.11.002

[12] Headley SA, Grant D, Fritzen J, Martins FDC, Camilo S, Caldart ET, et al.. Serological Detection of Ovine Gammaherpesvirus 2 Antibodies in Dairy Farms from Southern Brazil. Microorganisms. 2024. DOI: https://doi.org/10.3390/microorganisms12122629

[13] Headley SA, Agnol AMD, Bessegato JA, Frucchi APS, Maturana ÉFL, Rodrigues RV, et al.. Association of ovine gammaherpesvirus 2 with an outbreak of acute respiratory disease in dairy cattle. Scientific Reports. 2023. DOI: https://doi.org/10.1038/s41598-023-30133-w

[14] Headley SA, Martins FDC, Rodrigues DL. Another case of sheep-associated malignant catarrhal fever in Rio Grande do Sul: the curious epidemiological niche of ovine gammaherpesvirus 2 in Southern Brazil.. Journal of Infection in Developing Countries. 2025. DOI: https://doi.org/10.3855/jidc.20646

[15] Mananguit IR, Bartolome NDF, Tubalinal GAS, Mingala C. Molecular detection and genetic characterization of ovine gammaherpesvirus-2 (OvHV-2) in sheep in the Philippines. Small Ruminant Research. 2021. DOI: https://doi.org/10.1016/J.SMALLRUMRES.2021.106383

[16] Pretorius J, Oosthuizen M, Vuuren MV. Gammaherpesvirus carrier status of black wildebeest (Connochaetes gnou) in South Africa.. Journal of the South African Veterinary Association. 2008. DOI: https://doi.org/10.4102/JSAVA.V79I3.260

[17] Rahman SAE, Ateya A, El-Beskawy M, Wernike K, Hoffmann B, Eschbaumer M. Field Observations and Genetic Characterization of Sheep-Associated Malignant Catarrhal Fever in Egypt, 2018. Veterinary Sciences. 2020. DOI: https://doi.org/10.3390/vetsci7040201

[18] Figueiredo JRX, Silva FHP, Fritzen J, Machado BM, Ferreira FP, Ferreira KRG, et al.. The Association of Macavirus and Ovine Gammaherpesvirus 2 with Pneumonia in Beef Cattle from Mato Grosso, Brazil. Pathogens. 2025. DOI: https://doi.org/10.3390/pathogens14090945

[19] Headley SA, Silva FHP, Marques MdS, Fritzen J, Pinto-Ferreira F, Yoshitani GD, et al.. Chronic Folliculitis Associated with Ovine gammaherpesvirus 2-Induced Infections in Dairy Cows from Southern Brazil. Animals. 2025. DOI: https://doi.org/10.3390/ani15192883

[20] Fritzen J, Martins FDC, Castro MMd, Tavares VRR, Alfieri A, Headley SA. Ovine gammaherpesvirus 2 in asymptomatic water buffaloes (Bubalus bubalis) from Central-western Brazil and implications for infections in buffaloes worldwide. Tropical Animal Health and Production. 2026. DOI: https://doi.org/10.1007/s11250-026-04876-3

[21] Hidalgo-Hermoso E, Cabello J, López R, Vergara-Wilson V, Vera F, Valencia C, et al.. Molecular and phylogenetic analysis of herpesviruses in endangered free-ranging cervids of Chile: ovine gammaherpesvirus-2, A novel threat to wild and domestic animal health in Chilean Patagonia. Frontiers in Veterinary Science. 2024. DOI: https://doi.org/10.3389/fvets.2023.1321172

[22] Madrigal-Valencia TL, Saavedra-Montañez M, Pérez-Torres A, Hernández J, Segalés J, Hernández YD, et al.. Isolation and characterization of Ovine gammaherpesvirus type 2 from an outbreak of Malignant Catarrhal Fever in Artiodactyla and horses in Mexico. bioRxiv. 2022. DOI: https://doi.org/10.1101/2022.12.18.520964

[23] Ribeca P, Dewar P, McNab ML, Cousens C, Russell GC, Griffiths DJ. Genome sequence and annotation of ovine herpesvirus-1. bioRxiv. 2025. DOI: https://doi.org/10.1101/2025.09.04.674317

[24] Fabian R, Bentley EG, Kirby A, Sharma P, Stewart JP, Kipar A. The golden Syrian hamster (Mesocricetus auratus) as a model to decipher relevant pathogenic aspects of sheep-associated malignant catarrhal fever. bioRxiv. 2024. DOI: https://doi.org/10.1177/03009858251315115

[25] Saura-Martinez H, Al-Saadi M, Stewart J, Kipar A. Sheep-Associated Malignant Catarrhal Fever: Role of Latent Virus and Macrophages in Vasculitis. Veterinary Pathology-Supplement. 2020. DOI: https://doi.org/10.1177/0300985820978310

[26] Headley SA, Fritzen J, Silva DC, Xavier AAC, Yasumitsu CY, Silva FHP, et al.. Histophilus somni disease conditions with simultaneous infections by ovine gammaherpesvirus 2 in cattle herds from Southern Brazil. Brazilian Journal of Microbiology. 2023. DOI: https://doi.org/10.1007/s42770-023-00915-5

[27] Headley SA, Agnol AMD, Navolar FMN, Frucchi APS, Matos AMRNd, Pereira P, et al.. Ovine gammaherpesvirus-2 infection associated with chronic interstitial pneumonia in a sheep.. Microbial Pathogenesis. 2021. DOI: https://doi.org/10.1016/j.micpath.2021.105220

[28] Sheehan M, Pesavento PA, Campion F, Lynch JP, McGettrick S, Toland B, et al.. First reported case in an Irish flock of MCF- like systemic necrotizing vasculitis in sheep associated with ovine herpesvirus 2. Irish Veterinary Journal. 2024. DOI: https://doi.org/10.1186/s13620-024-00269-w

[29] Pesavento P, Dange RB, Ferreras MC, Dasjerdi A, Pérez V, LaRoca A, et al.. Systemic Necrotizing Vasculitis in Sheep Is Associated With Ovine Herpesvirus 2. Veterinary Pathology-Supplement. 2018. DOI: https://doi.org/10.1177/0300985818795166

[30] Frucchi APS, Agnol AMD, Caldart ET, Bronkhorst DE, Alfieri A, Alfieri A, et al.. The Role of Mycoplasma bovirhinis in the Development of Singular and Concomitant Respiratory Infections in Dairy Calves from Southern Brazil. Pathogens. 2024. DOI: https://doi.org/10.3390/pathogens13020114

[31] Castro M, Oliveira TE, Headley SA. Bovine respiratory disease in Brasil: a short review. Semina-ciencias Agrarias. 2021. DOI: https://doi.org/10.5433/1679-0359.2021V42N3SUPL1P2081

[32] Garcia MS, Hernández YMD, Moctezuma ADlP, García AMT, Marín JGP, Cortez ADR, et al.. Relationship between chronic diseases, hair cortisol concentration and welfare of housed dairy goats. Austral journal of veterinary sciences. 2024. DOI: https://doi.org/10.4206/ajvs.563.02

[33] Sun C, Schattgen SA, Pisitkun P, Jørgensen J, Hilterbrand AT, Wang LJ, et al.. Evasion of innate cytosolic DNA sensing by a gammaherpesvirus facilitates establishment of latent infection. Journal of Immunology. 2015. DOI: https://doi.org/10.4049/jimmunol.1402495

[34] Fabian R, Rosato G, Stewart JP, Kipar A. Bovine Gammaherpesvirus 6 Tropism in the Natural Host. Viruses. 2024. DOI: https://doi.org/10.3390/v16111730 *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.