Naturally acquired simian varicella virus infection in African green monkeys

Simian Varicella Virus: Molecular Virology and Mechanisms of Pathogenesis

Simian varicella virus (SVV) was first isolated in 1966 from African green monkeys (Cercopithecus aethiops) imported from Nairobi, Kenya, to the Liverpool School of Tropical Medicine in the United Kingdom (UK) (Clarkson et al., Arch Gesamte Virusforsch 22:219-234, 1967). SVV infection caused severe disease that resulted in a 56% case fatality rate (CFR) in the imported animals within 48 h of the appearance of a varicella-like rash (Clarkson et al., Arch Gesamte Virusforsch 22:219-234, 1967; Hemme et al., Am J Trop Med Hyg 94:1095-1099, 2016). The deceased animals presented with fever, widespread vesicular rash, and multiple hemorrhagic foci throughout the lungs, liver, and spleen (Clarkson et al., Arch Gesamte Virusforsch 22:219-234, 1967). This outbreak was quickly followed by a second outbreak in 47 patas monkeys (Erythrocebus patas) imported from Chad and Nigeria by Glaxo Laboratories (London, England, UK), which quickly spread within the facility (McCarthy et al., Lancet 2:856-857, 1968).

Current In Vivo Models of Varicella-Zoster Virus Neurotropism

Varicella-zoster virus (VZV), an exclusively human herpesvirus, causes chickenpox and establishes a latent infection in ganglia, reactivating decades later to produce zoster and associated neurological complications. An understanding of VZV neurotropism in humans has long been hampered by the lack of an adequate animal model. For example, experimental inoculation of VZV in small animals including guinea pigs and cotton rats results in the infection of ganglia but not a rash. The severe combined immune deficient human (SCID-hu) model allows the study of VZV neurotropism for human neural sub-populations. Simian varicella virus (SVV) infection of rhesus macaques (RM) closely resembles both human primary VZV infection and reactivation, with analyses at early times after infection providing valuable information about the extent of viral replication and the host immune responses. Indeed, a critical role for CD4 T-cell immunity during acute SVV infection as well as reactivation has emerged based on studies using RM. Herein we discuss the results of efforts from different groups to establish an animal model of VZV neurotropism.

Pathogenesis of varicelloviruses in primates

Varicelloviruses in primates comprise the prototypic human varicella-zoster virus (VZV) and its non-human primate homologue, simian varicella virus (SVV). Both viruses cause varicella as a primary infection, establish latency in ganglionic neurons and reactivate later in life to cause herpes zoster in their respective hosts. VZV is endemic worldwide and, although varicella is usually a benign disease in childhood, VZV reactivation is a significant cause of neurological disease in the elderly and in immunocompromised individuals. The pathogenesis of VZV infection remains ill-defined, mostly due to the species restriction of VZV that impedes studies in experimental animal models. SVV infection of non-human primates parallels virological, clinical, pathological and immunological features of human VZV infection, thereby providing an excellent model to study the pathogenesis of varicella and herpes zoster in its natural host. In this review, we discuss recent studies that provided novel insight in both the virus and host factors involved in the three elementary stages of Varicellovirus infection in primates: primary infection, latency and reactivation.

Animal models of varicella zoster virus infection

Primary infection with varicella zoster virus (VZV) results in varicella (chickenpox) followed by the establishment of latency in sensory ganglia. Declining T cell immunity due to aging or immune suppressive treatments can lead to VZV reactivation and the development of herpes zoster (HZ, shingles). HZ is often associated with significant morbidity and occasionally mortality in elderly and immune compromised patients. There are currently two FDA-approved vaccines for the prevention of VZV: Varivax® (for varicella) and Zostavax® (for HZ). Both vaccines contain the live-attenuated Oka strain of VZV. Although highly immunogenic, a two-dose regimen is required to achieve a 99% seroconversion rate. Zostavax vaccination reduces the incidence of HZ by 51% within a 3-year period, but a significant reduction in vaccine-induced immunity is observed within the first year after vaccination. Developing more efficacious vaccines and therapeutics requires a better understanding of the host response to VZV. These studies have been hampered by the scarcity of animal models that recapitulate all aspects of VZV infections in humans. In this review, we describe different animal models of VZV infection as well as an alternative animal model that leverages the infection of Old World macaques with the highly related simian varicella virus (SVV) and discuss their contributions to our understanding of pathogenesis and immunity during VZV infection.

Animal models of skin disease for drug discovery

INTRODUCTION

Discovery of novel drugs, treatments, and testing of consumer products in the field of dermatology is a multi-billion dollar business. Due to the distressing nature of many dermatological diseases, and the enormous consumer demand for products to reverse the effects of skin photodamage, aging, and hair loss, this is a very active field.

AREAS COVERED

In this paper, we will cover the use of animal models that have been reported to recapitulate to a greater or lesser extent the features of human dermatological disease. There has been a remarkable increase in the number and variety of transgenic mouse models in recent years, and the basic strategy for constructing them is outlined.

EXPERT OPINION

Inflammatory and autoimmune skin diseases are all represented by a range of mouse models both transgenic and normal. Skin cancer is mainly studied in mice and fish. Wound healing is studied in a wider range of animal species, and skin infections such as acne and leprosy also have been studied in animal models. Moving to the more consumer-oriented area of dermatology, there are models for studying the harmful effect of sunlight on the skin, and testing of sunscreens, and several different animal models of hair loss or alopecia.

Human trigeminal ganglionic explants as a model to study alphaherpesvirus reactivation

Varicella zoster virus (VZV) latency is characterized by limited virus gene expression and the absence of virus DNA replication. Investigations of VZV latency and reactivation have been hindered by the lack of an in vitro model of virus latency. Since VZV is an exclusively human pathogen, we used naturally infected human trigeminal ganglia (TG) obtained at autopsy to study virus latency. Herein, we report optimization of medium to maintain TG integrity as determined by histology and immunohistochemistry. Using the optimized culture medium, we also found that both herpes simplex virus-1 (HSV-1) and VZV DNA replicated in TG explants after 5 days in culture. The increase in HSV-1 DNA was fourfold greater than the increase in VZV DNA. Overall, we present a model for alphaherpesvirus latency in human neurons in which the key molecular events leading to virus reactivation can be studied.

CD4 T cell immunity is critical for the control of simian varicella virus infection in a nonhuman primate model of VZV infection

Primary infection with varicella zoster virus (VZV) results in varicella (more commonly known as chickenpox) after which VZV establishes latency in sensory ganglia. VZV can reactivate to cause herpes zoster (shingles), a debilitating disease that affects one million individuals in the US alone annually. Current vaccines against varicella (Varivax) and herpes zoster (Zostavax) are not 100% efficacious. Specifically, studies have shown that 1 dose of varivax can lead to breakthrough varicella, albeit rarely, in children and a 2-dose regimen is now recommended. Similarly, although Zostavax results in a 50% reduction in HZ cases, a significant number of recipients remain at risk. To design more efficacious vaccines, we need a better understanding of the immune response to VZV. Clinical observations suggest that T cell immunity plays a more critical role in the protection against VZV primary infection and reactivation. However, no studies to date have directly tested this hypothesis due to the scarcity of animal models that recapitulate the immune response to VZV. We have recently shown that SVV infection of rhesus macaques models the hallmarks of primary VZV infection in children. In this study, we used this model to experimentally determine the role of CD4, CD8 and B cell responses in the resolution of primary SVV infection in unvaccinated animals. Data presented in this manuscript show that while CD20 depletion leads to a significant delay and decrease in the antibody response to SVV, loss of B cells does not alter the severity of varicella or the kinetics/magnitude of the T cell response. Loss of CD8 T cells resulted in slightly higher viral loads and prolonged viremia. In contrast, CD4 depletion led to higher viral loads, prolonged viremia and disseminated varicella. CD4 depleted animals also had delayed and reduced antibody and CD8 T cell responses. These results are similar to clinical observations that children with agammaglobulinemia have uncomplicated varicella whereas children with T cell deficiencies are at increased risk of progressive varicella with significant complications. Moreover, our studies indicate that CD4 T cell responses to SVV play a more critical role than antibody or CD8 T cell responses in the control of primary SVV infection and suggest that one potential mechanism for enhancing the efficacy of VZV vaccines is by eliciting robust CD4 T cell responses.

Varicella zoster virus (VZV) causes chickenpox and establishes a life-long latent infection in humans. VZV can reactivate years later to cause shingles, a debilitating and painful disease. Vaccines against both chickenpox and shingles are available but not 100% efficacious. Two doses of the chickenpox vaccine are required to provide adequate protection and the shingles vaccine reduces the incidence of this disease by 51%. To improve these vaccines, we must identify the components of the immune system that are important for the control of VZV replication. However, the contribution of T versus B cell responses is unknown. Infection of rhesus macaques with simian varicella virus is a robust model of VZV infection. Here, we used this unique animal model to show for the first time that the absence of B cells does not alter disease severity and that the loss of CD8 T cells only results in a mild increase in disease severity. In sharp contrast, the lack of CD4 T cells leads to disseminated varicella. These data highlight the importance of CD4 T cells and suggest that novel vaccines that focus on engendering a more robust CD4 T cell response against VZV might provide better protection from chickenpox and shingles.

[Varicella-zoster virus (VZV)]

Varicella-zoster virus (VZV) causes varicella in primary infection and zoster after reactivation from latency. Both herpes simplex virus (HSV) and VZV are classified into the same alpha-herpesvirus subfamily. Although most VZV genes have their HSV homologs, VZV has many unique biological characteristics. In this review, we summarized recent studies on 1) animal models for VZV infection and outcomes from studies using the models, including 2) viral dissemination processes from respiratory mucosa, T cells, to skin, 3) cellular receptors for VZV entry, 4) functions of viral genes required uniquely for in vivo growth and for establishment of latency, 5) host immune responses and viral immune evasion mechanisms, and 6) varicella vaccine and anti-VZV drugs.

Review: The neurobiology of varicella zoster virus infection

Varicella zoster virus (VZV) is a neurotropic herpesvirus that infects nearly all humans. Primary infection usually causes chickenpox (varicella), after which virus becomes latent in cranial nerve ganglia, dorsal root ganglia and autonomic ganglia along the entire neuraxis. Although VZV cannot be isolated from human ganglia, nucleic acid hybridization and, later, polymerase chain reaction proved that VZV is latent in ganglia. Declining VZV-specific host immunity decades after primary infection allows virus to reactivate spontaneously, resulting in shingles (zoster) characterized by pain and rash restricted to one to three dermatomes. Multiple other serious neurological and ocular disorders also result from VZV reactivation. This review summarizes the current state of knowledge of the clinical and pathological complications of neurological and ocular disease produced by VZV reactivation, molecular aspects of VZV latency, VZV virology and VZV-specific immunity, the role of apoptosis in VZV-induced cell death and the development of an animal model provided by simian varicella virus infection of monkeys.

Latent simian varicella virus reactivates in monkeys treated with tacrolimus with or without exposure to irradiation

Simian varicella virus (SVV) infection of primates resembles human varicella-zoster virus (VZV) infection. After primary infection, SVV becomes latent in ganglia and reactivates after immunosuppression or social and environmental stress. Herein, natural SVV infection was established in 5 cynomolgus macaques (cynos) and 10 African green (AG) monkeys. Four cynos were treated with the immunosuppressant tacrolimus (80 to 300 μg/kg/day) for 4 months and 1 was untreated (group 1). Four AG monkeys were exposed to a single dose (200 cGy) of x-irradiation (group 2), and 4 other AG monkeys were irradiated and treated with tacrolimus for 4 months (group 3); the remaining 2 AG monkeys were untreated. Zoster rash developed 1 to 2 weeks after tacrolimus treatment in 3 of 4 monkeys in group 1, 6 weeks after irradiation in 1 of 4 monkeys in group 2, and 1 to 2 weeks after irradiation in all 4 monkeys in group 3. All monkeys were euthanized 1 to 4 months after immunosuppression. SVV antigens were detected immunohistochemically in skin biopsies as well as in lungs of most monkeys. Low copy number SVV DNA was detected in ganglia from all three groups of monkeys, including controls. RNA specific for SVV ORFs 61, 63, and 9 was detected in ganglia from one immunosuppressed monkey in group 1. SVV antigens were detected in multiple ganglia from all immunosuppressed monkeys in every group, but not in controls. These results indicate that tacrolimus treatment produced reactivation in more monkeys than irradiation and tacrolimus and irradiation increased the frequency of SVV reactivation as compared to either treatment alone.

Simian herpesviruses and their risk to humans

A high level of genetic and physiological homology with humans has rendered non-human primates (NHP) an essential animal model for biomedical research. As such NHP offer a unique opportunity to study host-pathogen interactions in a species that closely mimics human biology but can yet be maintained under tight laboratory conditions. Indeed, studies using NHP have been critical to our understanding of pathogenesis as well as the development of vaccines and therapeutics. This further facilitated by the fact that NHPs are susceptible to a variety of pathogens that bear significant homology to human pathogens. Unfortunately, these same viruses pose a potential health issue to humans. In this review we discuss the simian herpesviruses and their potential to cause disease in researchers that come into close contact with them.

Simian varicella virus pathogenesis

Because varicella zoster virus (VZV) is an exclusively human pathogen, the development of an animal model is necessary to study pathogenesis, latency, and reactivation. The pathological, virological, and immunological features of simian varicella virus (SVV) infection in nonhuman primates are similar to those of VZV infection in humans. Both natural infection of cynomolgus and African green monkeys as well as intrabronchial inoculation of rhesus macaques with SVV provide the most useful models to study viral and immunological aspects of latency and the host immune response. Experimental immunosuppression of monkeys latently infected with SVV results in zoster, thus providing a new model system to study how the loss of adaptive immunity modulates virus reactivation.

Simian varicella virus infection of rhesus macaques recapitulates essential features of varicella zoster virus infection in humans

Simian varicella virus (SVV), the etiologic agent of naturally occurring varicella in primates, is genetically and antigenically closely related to human varicella zoster virus (VZV). Early attempts to develop a model of VZV pathogenesis and latency in nonhuman primates (NHP) resulted in persistent infection. More recent models successfully produced latency; however, only a minority of monkeys became viremic and seroconverted. Thus, previous NHP models were not ideally suited to analyze the immune response to SVV during acute infection and the transition to latency. Here, we show for the first time that intrabronchial inoculation of rhesus macaques with SVV closely mimics naturally occurring varicella (chickenpox) in humans. Infected monkeys developed varicella and viremia that resolved 21 days after infection. Months later, viral DNA was detected only in ganglia and not in non-ganglionic tissues. Like VZV latency in human ganglia, transcripts corresponding to SVV ORFs 21, 62, 63 and 66, but not ORF 40, were detected by RT-PCR. In addition, as described for VZV, SVV ORF 63 protein was detected in the cytoplasm of neurons in latently infected monkey ganglia by immunohistochemistry. We also present the first in depth analysis of the immune response to SVV. Infected animals produced a strong humoral and cell-mediated immune response to SVV, as assessed by immunohistology, serology and flow cytometry. Intrabronchial inoculation of rhesus macaques with SVV provides a novel model to analyze viral and immunological mechanisms of VZV latency and reactivation.

Simian varicella virus in pigtailed macaques (Macaca nemestrina): clinical, pathologic, and virologic features

Simian varicella virus (SVV; Cercopithecine herpesvirus 9) is a naturally occurring herpesvirus of nonhuman primates. Here we present the clinical, pathologic, and virologic findings from 2 cases of SVV in adult female pigtailed macaques (Macaca nemestrina). The initial case presented with hyperthermia and a diffuse inguinal rash which spread centripetally, progressing to vesiculoulcerative dermatitis of the trunk, face, and extremities. At 96 h after presentation, the animal was anorexic and lethargic and had oral and glossal ulcerations. Euthanasia was elected in light of the macaque's failure to respond to clinical treatment. Seven days after the first case was identified, a second macaque presented with a vesicular rash and was euthanized. Gross necropsy lesions for both cases included vesicular, ulcerative dermatitis with mucocutaneous extension and hepatic necrosis; the initial case also demonstrated necrohemorrhagic gastroenterocolitis and multifocal splenic necrosis. Histology confirmed herpetic viral infection with abundant intranuclear inclusion bodies. Immunofluorescence assays detected antibodies specific for SVV. PCR assays of vesicular fluid, tissue, and blood confirmed SVV and excluded varicella-zoster virus (Human herpesvirus 3). Serology for Macacine herpesvirus 1 (formerly Cercopithecine herpesvirus 1), poxvirus (monkeypox), and rubella was negative. Banked serum samples confirmed SVV exposure and seroconversion. Investigation into the epidemiology of the seroconversion demonstrated a SVV colony prevalence of 20%. The described cases occurred in animals with reconstituted immune systems (after total-body irradiation) and demonstrate the clinical effects of infection with an endemic infectious agent in animals with a questionable immune status.

Clinical and molecular aspects of varicella zoster virus infection

A declining cell-mediated immunity to varicella zoster virus (VZV) with advancing age or immunosuppression results in virus reactivation from latently infected human ganglia anywhere along the neuraxis. Virus reactivation produces zoster, often followed by chronic pain (postherpetic neuralgia or PHN) as well as vasculopathy, myelopathy, retinal necrosis and cerebellitis. VZV reactivation also produces pain without rash (zoster sine herpete). Vaccination after age 60 reduces the incidence of shingles by 51%, PHN by 66% and the burden of illness by 61%. However, even if every healthy adult over age 60 years is vaccinated, there would still be about 500,000 zoster cases annually in the United States alone, about 200,000 of whom will experience PHN. Analyses of viral nucleic acid and gene expression in latently infected human ganglia and in an animal model of varicella latency in primates are serving to determine the mechanism(s) of VZV reactivation with the aim of preventing reactivation and the clinical sequelae.

Varicella zoster virus infection: clinical features, molecular pathogenesis of disease, and latency

Varicella zoster virus (VZV) is an exclusively human neurotropic alphaherpesvirus. Primary infection causes varicella (chickenpox), after which virus becomes latent in cranial nerve ganglia, dorsal root ganglia, and autonomic ganglia along the entire neuraxis. Years later, in association with a decline in cell-mediated immunity in elderly and immunocompromised individuals, VZV reactivates and causes a wide range of neurologic disease. This article discusses the clinical manifestations, treatment, and prevention of VZV infection and reactivation; pathogenesis of VZV infection; and current research focusing on VZV latency, reactivation, and animal models.

Simian varicella in old world monkeys

Simian varicella virus (SVV) causes a natural erythematous disease in Old World monkeys and is responsible for simian varicella epizootics that occur sporadically in facilities housing nonhuman primates. This review summarizes the biology of SVV and simian varicella as a veterinary disease of nonhuman primates. SVV is closely related to varicella-zoster virus, the causative agent of human varicella and herpes zoster. Clinical signs of simian varicella include fever, vesicular skin rash, and hepatitis. Simian varicella may range from a mild infection to a severe and life-threatening disease, and epizootics may have high morbidity and mortality rates. SVV establishes a lifelong latent infection in neural ganglia of animals in which the primary disease resolves, and the virus may reactivate later in life to cause a secondary disease corresponding to herpes zoster. Prompt diagnosis is important for control and prevention of epizootics. Antiviral treatment for simian varicella may be effective if administered early in the course of infection.

Simian varicella virus reactivation in cynomolgus monkeys

SVV infection of primates closely resembles VZV infection of humans. Like VZV, SVV becomes latent in ganglionic neurons. We used this model to study the effect of immunosuppression on varicella reactivation. Cynomolgus monkeys latently infected with SVV were irradiated and treated with tacrolimus and prednisone. Of four latently infected monkeys that were immunosuppressed and subjected to the stress of transportation and isolation, one developed zoster, and three others developed features of subclinical reactivation. Another non-immunosuppressed latently infected monkey that was subjected to the same stress of travel and isolation showed features of subclinical reactivation. Virus reactivation was confirmed not only by the occurrence of zoster in one monkey, but also by the presence of late SVV RNA in ganglia, and the detection of SVV DNA in non-ganglionic tissue, and SVV antigens in skin, ganglia and lung.

The simian varicella virus genome contains an invertible 665 base pair terminal element that is absent in the varicella zoster virus genome

Simian varicella virus (SVV) causes chickenpox in monkeys, establishes latency and reactivates to produce zoster thus providing a model to study human varicella zoster virus (VZV) infection. Sequence analysis of a recombinant cosmid clone containing the left end of the SVV genome revealed a 665 base pair (bp) segment that is absent in VZV DNA. This segment inverts and contains 507 bp of unique sequences flanked on either side by 79 bp inverted repeats, making the SVV genome to be 124,785 bp in size. Part of the inverted repeat sequence (64 bp) is also present at the junction of the long and short segments of the SVV genome. The terminal DNA sequences are conserved among different SVV isolates and present in tissues from infected monkeys. The terminal region is transcriptionally active and is also present in the genomes of other animal varicelloviruses but absent in the VZV genome.

The pattern of viral persistence in monkeys intra-tracheally infected with Simian varicella virus

In situ PCR (ISPCR) and in situ hybridisation (ISH) was performed on 32 tissues from 10 monkeys, intra-tracheally (IT) infected with simian varicella virus (SVV) and 5 tissues from 3 uninfected control animals. The results showed persistence of SVV DNA up to 2 years post-infection (pi) and the localisation of SVV to be confined to neurons except at time points 9 and 10 months pi where SVV positive satellite cells were also detected. There was no evidence for transcription of SVV ORFs 63 and 21 in the ganglia of the one IT infected and 2 naturally infected monkeys investigated using RNA ISH.

Simian varicella: a model for human varicella-zoster virus infections

Simian varicella virus (SVV) causes a natural varicella-like disease in nonhuman primates. Epizootics of simian varicella occur sporadically in facilities housing Old World monkeys. SVV is antigenically and genetically related to varicella-zoster virus (VZV), the etiologic agent of varicella (chickenpox) and herpes zoster (shingles) in humans. The SVV and VZV genomes are similar in size and structure, share 70%-75% DNA homology and are co-linear with respect to gene organisation. Simian varicella is a highly contagious disease characterised by fever and vesicular skin rash and may progress to pneumonia and hepatitis. Infected monkeys may resolve the disease within 2 weeks although epizootics are sometimes associated with high morbidity and mortality. SVV, like VZV, establishes life-long latent infection, as indicated by detection of viral DNA within neural ganglia. Subsequently, SVV may reactivate to cause secondary disease and spread of the virus to susceptible monkeys. The relatedness of VZV and SVV and the similarities in the clinical symptoms and pathogenesis of human and simian varicella make SVV infection of nonhuman primates an excellent animal model to investigate VZV pathogenesis and latency, and to evaluate potential antiviral strategies.

Neuronal localization of simian varicella virus DNA in ganglia of naturally infected African green monkeys

In situ hybridization analysis of monkey ganglia 2 months after natural infection with simian varicella virus (SVV) revealed SVV DNA only in neurons. These findings parallel the detection of varicella zoster virus in neurons of latently infected human ganglia. Natural exposure to SVV provides a model system to study varicella latency.

Inactivity of the bicyclic pyrimidine nucleoside analogues against simian varicella virus (SVV) does not correlate with their substrate activity for SVV-encoded thymidine kinase

Simian varicella virus (SVV) and human varicella-zoster virus (VZV) are closely related viruses that share many structural and functional properties. 5-Substituted 2'-deoxyuridine derivatives (e.g., BVDU, BVaraU) and acyclic guanine nucleoside derivatives (i.e., ACV and GCV) show comparable antiviral efficacy against VZV and SVV in cell culture. In contrast, the novel bicyclic nucleoside analogues (BCNAs) are exquisitely inhibitory to VZV (EC50 in the lower nanomolar range) but completely inactive against SVV. The VZV-encoded thymidine kinase (TK) appeared to be essential for BCNA activation (phosphorylation) and anti-VZV activity. Also SVV TK is able to recognize the BCNAs as substrate, although with a different structure-affinity relationship. Thus, viral TK-catalyzed phosphorylation is necessary but not sufficient for the BCNAs to display antiviral activity. Our data suggest that the eventual target of the BCNAs against VZV is either absent in SVV or, alternatively, is insensitive for the (phosphorylated) BCNAs.

Clinical and molecular pathogenesis of varicella virus infection

Varicella zoster virus (VZV) is a neurotropic human herpesvirus that infects nearly all humans and causes chickenpox (varicella). After chickenpox, VZV becomes latent in cranial nerve, dorsal root, and autonomic nervous system ganglia along the entire neuraxis. Virus reactivation produces shingles (zoster), characterized by pain and rash usually restricted to 1-3 dermatomes. Zoster is often complicated by postherpetic neuralgia (PHN), pain that persists for months to years after rash resolves. Virus may also spread to the spinal cord and blood vessels of the brain, producing a unifocal or multifocal vasculopathy, particularly in immunocompromised individuals. The increased incidence of zoster in elderly and immunocompromised individuals appears to be due to a VZV-specific host immunodeficiency. PHN may reflect a chronic VZV ganglionitis, and VZV vasculopathy is due to productive virus infection in cerebral arteries. Strategies that might boost host cell-mediated immunity to VZV are discussed, as well as the physical state of viral nucleic acid during latency and the possible mechanisms by which herpesvirus latency is maintained and virus is reactivated. A current summary of varicella latency and pathogenesis produced by simian varicella virus (SVV), the counterpart of human VZV, points to the usefulness of a primate model of natural infection to study varicella latency, as well as the experimental model of intratracheal inoculation to study the effectiveness of antiviral agents in driving persistent varicella virus into a latent state.

Tropical dermatology: viral tropical diseases

Viruses are important pathogens in tropical areas; most of them, especially the tropical hemorrhagic fevers, produce mucocutaneous manifestations. More than any other kind of pathogen, viruses have the possibility for being widespread, since they have a greater probability of mutation than do bacteria, can cross species barriers easily, and infect both human beings and animals in habitats with a great biodiversity. Tropical habitats also have been subject to major ecologic changes in the last few decades, exposing humans to direct contact with these viruses and allowing hemorrhagic fevers due to new emergent viruses such as flaviviruses, filoviruses, arenaviruses, and hantaviruses to become major threats to public health. The collapse of eradication programs in many countries, as well as population increases and ecologic modifications, have led to the spread of dengue and yellow fever to large portions of the world owing to the dissemination of vectors, especially mosquitoes, with broad ecologic ranges. Viruses previously restricted to some geographic areas, such as Rift Valley fever, Crimean-Congo hemorrhagic fever, West Nile fever, and monkeypox are now affecting new countries and populations. Other viruses such as herpes B infection often affect travelers and animal handlers in most parts of the world. Dermatologic lesions occur in all these diseases and can facilitate a rapid diagnosis, leading to control of the virus and helping prevent possible outbreaks.

Varicella-zoster virus ORF47 protein kinase, which is required for replication in human T cells, and ORF66 protein kinase, which is expressed during latency, are dispensable for establishment of latency

Varicella-zoster virus (VZV) results in a lifelong latent infection in human sensory and cranial nerve ganglia after primary infection. VZV open reading frame 47 (ORF47) and ORF66 encode protein kinases that phosphorylate several viral proteins, including VZV glycoprotein gE and ORF32, ORF62, and ORF63 proteins. Here we show that the ORF47 protein kinase also phosphorylates gI. While ORF47 is essential for virus replication in human T cells and skin, we found the gene to be dispensable for establishment of latent infection in dorsal root ganglia of rodents. ORF66 protein is expressed during latency. Rodents infected with VZV unable to express ORF66 developed latent infection at a rate similar to that for the parental virus. ORF63 transcripts, a hallmark of VZV latency, were also detected in similar numbers of animals infected with the ORF47 and ORF66 mutants and with the parental virus. VZV mutants unable to express four of the six genes that do not have herpes simplex virus (HSV) homologs (ORFs 1, 13, 32, 57) were also unimpaired for establishment of latency. While a truncated HSV VP16 mutant was previously reported to be unable to establish latency in a mouse model, we found that VZV with a deletion of ORF10, the homolog of HSV VP16, was dispensable for establishment of latency. Thus, seven genes, including one expressed during latency, are dispensable for establishing latent VZV infection.

Herpes simplex virus-1 and varicella-zoster virus latency in ganglia

Two human alpha-herpesviruses, herpes simplex virus (HSV)-1 and varicella zoster virus (VZV), account for the most frequent and serious neurologic disease caused by any of the eight human herpesviruses. Both HSV-1 and VZV become latent in ganglia. In this review, the authors describe features of latency for these viruses, such as distribution, prevalence, abundance, and configuration of viral DNA in latently infected human ganglia, as well as transcription, translation, and cell type infected. Studies of viral latency in animal models are also discussed. For each virus, remaining questions and future studies to understand the mechanism of latency are discussed with respect to prevention of serious cutaneous, ocular, and neurologic disease produced by virus reactivation.

Varicella Virus–Mononuclear Cell Interaction

Varicella zoster virus (VZV) causes varicella (chickenpox), becomes latent in cranial nerve, dorsal root, and autonomic ganglia; and reactivates decades later to produce zoster (shingles). The main complication of zoster is postherpetic neuralgia (PHN), pain that persists for months and often years after zoster. VZV also causes chronic radicular pain without rash (zoster sine herpete). Viremia is associated with each stage of VZV infection. Viral DNA has been found in peripheral blood mononuclear cells (MNCs) of patients with varicella, zoster, PHN, and zoster sine herpete. In varicella, viremia contributes to the widespread distribution of skin lesions and infection of multiple organs. Although the role of viremia in other VZV-associated diseases is not as clear, the detection of VZV DNA (and sometimes VZV RNA and proteins) helps diagnose neurological diseases produced by VZV, has indicated that PHN may reflect a chronic VZV ganglionitis, and has established that VZV reactivates subclinically, especially in immunocompromised humans. In vitro studies have established that VZV can productively infect MNCs for a short time and have identified the subpopulations of MNCs that are infected. Finally, simian varicella virus (SVV) infection of nonhuman primates shares clinical, pathological, and virologic features with VZV in humans. Like VZV, SV viremia in nonhuman primates during acute infection plays an important role in the pathogenesis of SVV. Infectious virus can be isolated from MNCs, and SVV DNA can be detected in MNCs during varicella. Further, SVV DNA can be detected for months in MNCs of monkeys after experimental infection with SVV. Herein, we review the current literature related to VZV infection of MNCs during naturally occurring varicella, PHN, and zoster sine herpete in humans, including studies of experimental infection of human MNCs with VZV. We also review SVV MNC interaction during naturally occurring simian varicella and after experimental infection of primates with SVV.