Characterization of gross and histological lesions in Balb/c mice experimentally infected with herpesvirus saimiri 1 (HVS1)

Effective Prophylactic Therapy for Exposure to Monkey B Virus (Macacine alphaherpesvirus 1)

Zoonotic monkey B virus (Macacine alphaherpesvirus 1; BV) infections are extremely serious and usually fatal. Drugs currently used for treatment were developed for the treatment of herpes simplex virus but are less effective against BV. Effective suppression of viral replication in the skin could prevent the virus from invading the nervous system. To test this hypothesis, the efficacy of topical administration of several drugs against lethal BV infection was evaluated in female BALB/c mice that were infected by scarification. Drugs were then applied to the site of inoculation. As 3% preparations, most drugs were only minimally effective or ineffective. In contrast, ganciclovir and cidofovir were very effective. The ED₅₀ for cidofovir was 0.007%, compared with 1.1% for ganciclovir. At 0.5%, cidofovir protected against both death and neurologic signs, whereas 5% ganciclovir only protected against death but not neurologic involvement. All genotypes of BV were equally susceptible to cidofovir and ganciclovir. For maximal effectiveness, treatment with both cidofovir and ganciclovir had to be initiated within 8 h of infection. Cidofovir was completely protective when administered only on the day of infection, whereas a minimum of 5 d of treatment was required for maximal ganciclovir efficacy. These studies showed that topical cidofovir treatment started soon after BV exposure was very effective in preventing BV from invading the nervous system, whereas ganciclovir treatment was only partially effective. In addition, cidofovir was protective against a ganciclovir-resistant BV mutant, whereas ganciclovir was not. These studies showed that topical cidofovir treatment started soon after BV exposure is more effective than ganciclovir in preventing BV from invading the CNS.

Structure and sequence of the saimiriine herpesvirus 1 genome

We report here the complete genome sequence of the squirrel monkey α-herpesvirus saimiriine herpesvirus 1 (HVS1). Unlike the simplexviruses of other primate species, only the unique short region of the HVS1 genome is bounded by inverted repeats. While all Old World simian simplexviruses characterized to date lack the herpes simplex virus RL1 (γ34.5) gene, HVS1 has an RL1 gene. HVS1 lacks several genes that are present in other primate simplexviruses (US8.5, US10-12, UL43/43.5 and UL49A). Although the overall genome structure appears more like that of varicelloviruses, the encoded HVS1 proteins are most closely related to homologous proteins of the primate simplexviruses. Phylogenetic analyses confirm that HVS1 is a simplexvirus. Limited comparison of two HVS1 strains revealed a very low degree of sequence variation more typical of varicelloviruses. HVS1 is thus unique among the primate α-herpesviruses in that its genome has properties of both simplexviruses and varicelloviruses.

Type I IFN response to Papiine herpesvirus 2 (Herpesvirus papio 2; HVP2) determines neuropathogenicity in mice

Isolates of baboon alpha-herpesvirus Papiine herpesvirus 2 (HVP2) exhibit one of two distinct phenotypes in mice: extremely neurovirulent or apathogenic. Previous studies implicated the type I interferon (IFN) response as being a major factor in controlling infection by apathogenic isolates. To further investigate the possibility that the host IFN-beta response underlies the pathogenicity of the two HVP2 subtypes, the susceptibility of mice lacking the IFN-beta receptor (IFNAR(-/-)) to infection was examined. Apathogenic isolates of HVP2 (HVP2ap) replicated in IFNAR(-/-) primary mouse dermal fibroblast (PMDF) cultures as well as neurovirulent (HVP2nv) isolates. IFNAR(-/-) mice were also susceptible to lethal infection by HVP2ap isolates. Unlike Balb/c or parental 129 mice, LD(50) and ID(50) values for HVP2ap were the same in IFNAR(-/-) mice indicating that in these mice infection always progressed to death. HVP2ap replicated in the skin at the site of inoculation and invaded dorsal root ganglia as efficiently as HVP2nv in IFNAR(-/-) mice. Since the virion host shutoff (vhs) protein encoded by the UL41 gene of herpes simplex virus has been implicated in circumventing the host IFN-beta response and the phenotype of UL41 deletion mutants of HSV is very similar to that of HVP2ap isolates, the UL41 gene was deleted from HVP2nv (Delta 41) and replaced with the UL41 ORF from HVP2ap (Delta 41C). Like the parental HVP2nv virus, the Delta 41C recombinant replicated efficiently in Balb/c PMDFs and did not induce a strong IFN-beta response. The neuropathogenicity of the Delta 41C recombinant was also the same as the parental HVP2nv virus in Balb/c mice, indicating that the vhs protein does not underlie the different neuropathogenic phenotype of HVP2ap and HVP2nv. In contrast, the Delta 41 deletion virus induced a strong IFN-beta response but was still able to undergo multiple rounds of replication in PMDF cultures, albeit at a slower pace than the parental HVP2nv. This was reflected in vivo as the Delta 41 mutant had an LD(50) equivalent to that of the parental HVP2nv virus although the time to death was longer. These results indicate that while the vhs protein is involved in preventing and/or suppressing an IFN-beta response, it is not responsible for the ability of HVP2nv to overcome IFN-beta induced resistance of uninfected cells and does not underlie the divergent pathogenicity of the two HVP2 subtypes in mice.

Neuropathogenesis of herpesvirus papio 2 in mice parallels infection with Cercopithecine herpesvirus 1 (B virus) in humans

Cercopithecine herpesvirus 1 (monkey B virus; BV) produces extremely severe and usually fatal infections when transmitted from macaque monkeys to humans. Cercopithecine herpesvirus 16 (herpesvirus papio 2; HVP2) is very closely related to BV, yet cases of human HVP2 infection are unknown. However, following intramuscular inoculation of mice, HVP2 rapidly invades the peripheral nervous system and ascends the central nervous system (CNS) resulting in death, very much like human BV infections. In this study, the neurovirulence of HVP2 in mice was further evaluated as a potential model system for human BV infections. HVP2 was consistently neurovirulent when administered by epidermal scarification, intracranial inoculation and an eye splash. Quantitative real-time PCR, histopathology and immunohistochemistry were used to follow the temporal spread of virus following skin scarification and to compare the pathogenesis of neurovirulent and apathogenic isolates of HVP2. Apathogenic isolates were found to be capable of reaching the CNS but were extremely inefficient at replicating within the CNS. It is concluded that neurovirulent strains of HVP2 exhibit a pathogenesis in mice that parallels that observed in human BV infections and that this model system may prove useful in dissecting the viral determinants underlying the extreme severity of zoonotic BV infections.

Temporal progression of viral replication and gross and histological lesions in Balb/c mice inoculated epidermally with Saimiriine herpesvirus 1 (SaHV-1)

Saimiriine herpesvirus 1 (SaHV-1), an alphaherpesvirus enzootic in squirrel monkeys, is genetically related to monkey B virus and human herpes simplex virus (HSV). To study the temporal progression of viral spread and associated lesions, Balb/c mice were inoculated epidermally by scarification with a green fluorescent protein (GFP)-expressing recombinant strain of SaHV-1 and killed sequentially. Pinpoint ulcerative lesions in the inoculated epidermis progressed over a few days to unilateral or bilateral hindlimb paresis or paralysis, urinary and faecal incontinence, abdominal distension, hunched posture and eventual depression warranting euthanasia. Viral replication was present within epidermal keratinocytes, neurons of the dorsal root ganglia and thoracolumbar spinal cord, regional autonomic ganglia, lower urinary tract epithelium and colonic myenteric plexuses, as indicated by histological lesions and GFP expression. Almost all mice inoculated with 10(5) or 10(6) plaque-forming units (PFU) of SaHV-1 developed rapidly progressive disease. Two of eight mice given 10(4)PFU developed disease, but no mice receiving less than 10(4)PFU gave evidence of infection. Mice that showed no clinical signs also failed to develop an antiviral IgG response, indicating absence of active viral infection. For SaHV-1 inoculated epidermally, the ID(50), CNSD(50) and LD(50) values were identical (10(4.38)), indicating that successful infection by this route invariably resulted in lethal CNS (central nervous system) disease. Consistently severe disease in all infected animals, with regionally extensive distribution of viral replication, constituted a marked difference from the disease produced by intramuscular inoculation.

Clinicopathological characterization of monkey B virus (Cercopithecine herpesvirus 1) infection in mice

The purpose of this study was to establish a small animal model for monkey B virus (BV) infection. Mice were inoculated intramuscularly with several BV isolates. Comparisons were based upon the doses required to produce infection (ID50), non-central nervous system (CNS) clinical disease (CS50), CNS disease (CNSD50) and lethal effect (LD50). Strains differed in respect of the dose required to produce clinical disease in BALB/c mice. C57BL/6 mice were more resistant than BALB/c mice to CNS disease. Skin lesions at the inoculation site consisted of epidermal necrosis, ulceration, serocellular crusts and underlying dermatitis. CNS lesions included marked inflammation in the ipsilateral dorsal root ganglion and lumbar spinal cord (point of viral entry). The distribution of the lumbar spinal cord lesions suggested viral entry via sensory afferent neurons, ventral motor tracts, or both. The lesions in the more cranial spinal cord segments suggested ascension to the brain via bilateral spinothalamic and spinoreticular tracts. Brain lesions included encephalitis with neuronal necrosis and white matter destruction located consistently at the base of the brainstem, the reticular system, and rostrally to the thalamus and hypothalamus. Viral antigen was detected immunohistochemically in the lesions. The results indicated an ascending encephalomyelitis syndrome similar to that produced by BV in man.

Pathogenicity of different baboon herpesvirus papio 2 isolates is characterized by either extreme neurovirulence or complete apathogenicity

In comparisons of the pathogenicity of simian alphaherpesviruses in mice, two isolates of the baboon virus HVP2 were nearly as lethal as monkey B virus, a biological safety level 4 agent (J. W. Ritchey, K. A. Ealey, M. Payton, and R. Eberle, J. Comp. Pathol. 127:150-161, 2002). To confirm these results, mice were inoculated intramuscularly with 10(5) PFU of HVP2 isolates obtained from different baboon subspecies and primate centers. Some of the HVP2 isolates (6 of 13) caused paralysis and death in the mice, while 7 of 13 HVP2 isolates produced no clinical signs of disease. The apathogenic HVP2 isolates (HVP2ap) induced only low levels of serum antiviral immunoglobulin G relative to levels observed in sera from mice infected with the neurovirulent isolates of HVP2 (HVP2nv). Histological examination of tissues from mice inoculated with HVP2nv isolates showed extensive neural tissue destruction, while mice infected with HVP2ap isolates showed no lesions. Tissue samples collected at 48-h intervals postinfection suggested that HVP2ap isolates failed to replicate at the site of inoculation. There was no significant difference in the in vitro replication, plaque size, or cytopathic effect morphology of HVP2ap versus HVP2nv isolates. While HVP2 isolates replicated better in Vero monkey kidney cells than in murine L cells, plaquing efficiency of individual isolates did not correlate with the dichotomous pathogenic properties seen in mice. Phylogenetic analyses of both coding and intergenic regions (US4-6) of the HVP2 genome separated isolates into two distinct clades that correlated with the two in vivo virulence phenotypes. Taken together, these results demonstrate that two subtypes of HVP2 exist that are very closely related but differ dramatically in their ability to cause disease in a murine model.

Naturally occurring fatal herpes simplex virus 1 infection in a family of white-faced saki monkeys (Pithecia pithecia pithecia)

A family of three white-faced saki monkeys (Pithecia pithecia pithecia) died 48-96 hours after the onset of anorexia, nasal discharge, pyrexia and oral ulceration. One animal also had clonic seizures. Lesions found post-mortem consisted of oral and esophageal ulcers, hepatic and intestinal necrosis, meningoencephalitis and sporadic neuronal necrosis. Intranuclear inclusion bodies and syncytial cells were present in oral lesions and affected areas of liver. Herpes simplex virus 1 (HSV-1) was identified as the etiology of disease by virus isolation, polymerase chain reaction, or in situ hybridization in all three animals. Immunohistochemistry for detection of apoptotic DNA and activated caspase-3 showed significant levels of apoptosis in oral and liver lesions and occasional apoptotic neurons in the brain. These findings demonstrate the vulnerability of white-faced saki monkeys to HSV-1 and provide initial insight into the pathogenesis of fatal HSV-1-induced disease, indicating that apoptosis plays a significant role in cell death.

Comparative pathology of infections with baboon and African green monkey alpha-herpesviruses in mice

The comparative pathology of Herpesvirus papio 2 (HVP2) of baboons and SA8 virus of African green monkeys relative to that of herpes simplex virus (HSV1) of man was investigated in young adult mice inoculated intramuscularly and observed for 21 days. The 50% infectious dose (ID(50)) for HVP2 was approximately 10(2.0) plaque-forming units (PFU), while the ID(50) for HSV1 and SA8 was 10(2.5) and 10(3.8), respectively. There were marked differences in the ability of these three viruses to invade the central nervous system (CNS) and cause clinical neurological disease. HSV1 produced neurological signs in a few animals given 10(6)PFU, but SA8 did not. In contrast, HVP2 readily invaded the CNS and produced fatal disease with doses as low as 10(2)PFU. Two isolates of HVP2 tested had a 50% CNS disease dose (CNSD(50)) of 10(2.5) and 10(3.0)PFU and an LD(50) of 10(3.8) and 10(4.3)PFU, respectively. Histopathological examination of tissue from HVP2-infected mice revealed severe lesions of inflammation and necrosis in the central, peripheral and autonomic nervous systems, as well as of other tissues including skin, adrenal glands and the gastrointestinal tract. Viral antigens were detected immunohistochemically in lesions. This study showed that while both HVP2 and SA8 could infect mice, there were marked differences in the ability of these two closely related viruses to cause clinical disease and CNS lesions. This murine model may prove useful in the investigation of viral or host determinants responsible for the varying neurovirulence of these simian alpha-herpesviruses.