Varicella zoster virus

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Varicella zoster virus
Varicella (Chickenpox) Virus PHIL 1878 lores.jpg
Electron Micrograph of VZV.
Virus classification
Group: Group I (dsDNA)
Order: Herpesvirales
Family: Herpesviridae
Subfamily: Alphaherpesvirinae
Genus: Varicellovirus
Species: Human herpesvirus 3 (HHV-3)
Synonyms
  • Varicella zoster virus (VZV)

Varicella zoster virus or varicella-zoster virus (VZV) is one of eight herpesviruses known to infect humans and vertebrates. VZV only affects humans, and commonly causes chickenpox in children, teens and young adults and herpes zoster (shingles) in adults and rarely in children. VZV is known by many names, including chickenpox virus, varicella virus, zoster virus, and human herpesvirus type 3 (HHV-3).

VZV infects the nerves, and causes a wide variety of symptoms. After the primary infection (chickenpox), the virus goes dormant in the nerves, including the cranial nerve ganglia, dorsal root ganglia, and autonomic ganglia. Many years after the patient has recovered from chickenpox, VZV can reactivate to cause a number of neurologic conditions.[1]

Human disease[edit]

Main article: Chickenpox

Primary Varicella Zoster Virus infection results in chickenpox (varicella), which may result in complications including encephalitis or pneumonia (either direct viral pneumonia or secondary bacterial pneumonia). Even when clinical symptoms of chickenpox have resolved, VZV remains dormant in the nervous system of the infected person (virus latency), in the trigeminal and dorsal root ganglia.[2]

In about 10–20% of cases, VZV reactivates later in life, producing a disease known as shingles or herpes zoster. VZV can also infect the central nervous system, with a 2013 article reporting an incidence rate of 1.02 cases per 100 000 inhabitants in Switzerland, and an annual incidence rate of 1.8 cases per 100,000 inhabitants in Sweden.[3] For comparison, Amyotrophic lateral sclerosis (ALS) has an annual incidence rate of 0.2 to 2.4 per 100,000 population in industrialized countries. [4]

Other serious complications of varicella zoster infection include postherpetic neuralgia, Mollaret's meningitis, zoster multiplex, and inflammation of arteries in the brain leading to stroke,[5] myelitis, herpes ophthalmicus, or zoster sine herpete. In Ramsay Hunt syndrome, VZV affects the geniculate ganglion giving lesions that follow specific branches of the facial nerve. Symptoms may include painful blisters on the tongue and ear along with one sided facial weakness and hearing loss.

Recent advances in research and diagnosis[edit]

Until the mid 1990s, infectious complications of the CNS caused by VZV reactivation were regarded as rare. The presence of rash, as well as specific neurological symptoms, were required to diagnose a CNS infection caused by VZV. Since 2000, PCR testing has become more widely used, and the number of diagnosed cases of CNS infection has increased. [6]

Classic textbook descriptions state that VZV reactivation in the CNS is restricted to immunocompromised individuals and the elderly, however, recent studies have found that most patients are immunocompetent, and less than 60 years old. Old references cite vesicular rash as a characteristic finding, however, recent studies have found that rash is only present in 45% of cases. [7] In addition, systemic inflammation is not as reliable an indicator as previously thought: the mean level of C-reactive protein and mean white blood cell count are within the normal range in patients with VZV meningitis. [8] MRI and CT scans are usually normal in cases of VZV reactivation in the CNS. CSF pleocytosis, previously thought to be a strong indicator of VZV encephalitis, was absent in half of a group of patients diagnosed with VZV encephalitis by PCR. [9]

The frequency of CNS infections presented at the emergency room of a community hospital is not negligible, so a means of diagnosing cases is needed. PCR is not a foolproof method of diagnosis, but because so many other indicators have turned out to not be reliable in diagnosing VZV infections in the CNS, screening for VZV by PCR is recommended. Negative PCR does not rule out VZV involvement, but a positive PCR can be used for diagnosis, and appropriate treatment started (for example, antivirals can be prescribed rather than antibiotics). [10]

The introduction of DNA analysis techniques has shown some complications of varicella-zoster to be more common than previously thought. For example, sporadic meningoencephalitis (ME) caused by varicella-zoster was regarded as rare disease, mostly related to childhood chickenpox. However, meningoencephalitis caused by varicella-zoster is increasingly recognized as a predominant cause of ME among immunocompetent adults in non-epidemic circumstances.[11]

Diagnosis of complications of varicella-zoster, particularly in cases where the disease reactivates after years or decades of latency, are difficult. A rash (shingles) can be present or absent. Symptoms vary, and there is significant overlap in symptoms with herpes-simplex symptoms.[11]

Although DNA analysis techniques such as Polymerase Chain Reaction can be used to look for DNA of herpesviruses in spinal fluid or blood, the results may be negative, even in cases where other definitive symptoms exist.[12] Notwithstanding these limitations, the use of PCR has resulted in an advance in the state of the art in our understanding of herpesviruses, including VZV, during the 1990s and 2000s. For example, in the past, clinicians believed that encephalitis was caused by herpes simplex, and that patients always died or developed serious long term function problems. People were diagnosed at autopsy or by brain biopsy. Brain biopsy is not undertaken lightly: it is reserved only for serious cases that cannot be diagnosed by less invasive methods. For this reason, knowledge of these herpes virus conditions was limited to severe cases. DNA techniques have made it possible to diagnose “mild” cases, caused by VZV or HSV, in which the symptoms include fever, headache, and altered mental status. Mortality rates in treated patients are decreasing.[11]

Morphology[edit]

VZV is closely related to the herpes simplex viruses (HSV), sharing much genome homology. The known envelope glycoproteins (gB, gC, gE, gH, gI, gK, gL) correspond with those in HSV; however, there is no equivalent of HSV gD. VZV also fails to produce the LAT (latency-associated transcripts) that play an important role in establishing HSV latency (herpes simplex virus). VZV virons are spherical and 180–200 nm in diameter. Their lipid envelope encloses the 100 nm nucleocapsid of 162 hexameric and pentameric capsomeres arranged in an icosahedral form. Its DNA is a single, linear, double-stranded molecule, 125,000 nt long. The capsid is surrounded by a number of loosely associated proteins known collectively as the tegument; many of these proteins play critical roles in initiating the process of virus reproduction in the infected cell. The tegument is in turn covered by a lipid envelope studded with glycoproteins that are displayed on the exterior of the virion, each approximately 8 nm long.

Genome[edit]

The genome was first sequenced in 1986.[13] It is a linear duplex DNA molecule, a laboratory strain has 124,884 base pairs. The genome has 2 predominant isomers, depending on the orientation of the S segment, P (prototype) and IS (inverted S) which are present with equal frequency for a total frequency of 90-95%. The L segment can also be inverted resulting in a total of four linear isomers (IL and ILS). This is distinct from HSV's equiprobable distribution, and the discriminatory mechanism is not known. A small percentage of isolated molecules are circular genomes, about which little is known. (It is known that HSV circularizes on infection.) There are at least 70 open reading frames in the genome.

There are at least five clades of this virus.[14] Clades 1 and 3 include European/North American strains; clade 2 are Asian strains, especially from Japan; and clade 5 appears to be based in India. Clade 4 includes some strains from Europe but its geographic origins need further clarification.

Evolution[edit]

Commonality with HSV1 and HSV2 indicates a common ancestor, five genes do not have corresponding HSV genes. Relation with other human herpes viruses is less strong, but many homologues and conserved gene blocks are still found.

There are five principle clades (1-5) and four genotypes that do not fit into these clades.[15] The current distribution of these clades is Asia (clades 1,2,and 5) and Europe (clades 1, 3 and 4). Allocation of VZV strains to clades required sequence of whole virus genome. Practically all molecular epidemiological data on global VZV strains distribution obtained with targeted sequencing of selected regions.

Phylogenetic analysis of VZV genomic sequences resolves wild-type strains into 9 genotypes (E1, E2, J, M1, M2, M3, M4, VIII and IX).[16][17] Complete sequences for M3 and M4 strains are unavailable, but targeted analyses of representative strains suggest they are stable, circulating VZV genotypes. Sequence analysis of VZV isolates identified both shared and specific markers for every genotype and validated a unified VZV genotyping strategy. Despite high genotype diversity no evidence for intra-genotypic recombination was observed. Five of seven VZV genotypes were reliably discriminated using only four single nucleotide polymorphisms (SNP) present in ORF22, and the E1 and E2 genotypes were resolved using SNP located in ORF21, ORF22 or ORF50. Sequence analysis of 342 clinical varicella and zoster specimens from 18 European countries identified the following distribution of VZV genotypes: E1, 221 (65%); E2, 87 (25%); M1, 20 (6%); M2, 3 (1%); M4, 11 (3%). No M3 or J strains were observed.[16] Of 165 clinical varicella and zoster isolates from Australia and New Zealand typed using this approach, 67 of 127 eastern Australian isolates were E1, 30 were E2, 16 were J, 10 were M1, and 4 were M2; 25 of 38 New Zealand isolates were E1, 8 were E2, and 5 were M1.[18]

The mutation rate for synonymous and nonsynonymous mutation rates among the herpesviruses have been estimated at 1 × 10−7 and 2.7 × 10−8 mutations/site/year, respectively, based on the highly conserved gB gene.[19]

Treatment[edit]

Within the human body it can be treated by a number of drugs and therapeutic agents including aciclovir for the chicken pox, famciclovir, valaciclovir for the shingles, zoster-immune globulin (ZIG), and vidarabine. VZV immune globulin is also a treatment.[20]

Vaccination[edit]

Main articles: varicella vaccine and Zostavax

A live attenuated VZV Oka/Merck strain vaccine is available and is marketed in the United States under the trade name Varivax. It was developed by Merck, Sharp & Dohme in the 1980s from the Oka strain virus isolated and attenuated by Michiaki Takahashi and colleagues in the 1970s. It was submitted to the US Food and Drug Administration for approval in 1990 and was approved in 1995. Since then, it has been added to the recommended vaccination schedules for children in Australia, the United States, and many other countries. Varicella vaccination has raised concerns in some that the immunity induced by the vaccine may not be lifelong, possibly leaving adults vulnerable to more severe disease as the immunity from their childhood immunization wanes. Vaccine coverage in the United States in the population recommended for vaccination is approaching 90%, with concomitant reductions in the incidence of varicella cases and hospitalizations and deaths due to VZV. So far, clinical data has proved that the vaccine is effective for over 10 years in preventing varicella infection in healthy individuals and when breakthrough infections do occur, illness is typically mild.[21] In 2007, the ACIP recommended a second dose of vaccine before school entry to ensure the maintenance of high levels of varicella immunity.[22]

In 2006, the United States Food and Drug Administration approved Zostavax for the prevention of shingles. Zostavax is a more concentrated formulation of the Varivax vaccine, designed to elicit an immune response in older adults whose immunity to VZV wanes with advancing age. A systematic review by the Cochrane Library shows that Zostavax reduces the incidence of shingles by almost 50%.[23]

See also[edit]

References[edit]

  1. ^ "The protean neurologic manifestations of varicella-zoster virus infection". Cleveland Clinic Journal of Medicine. July 2007. 
  2. ^ Steiner I; Kennedy PG; Pachner AR (2007). "The neurotropic herpes viruses: herpes simplex and varicella-zoster". Lancet Neurol 6 (11): 1015–28. doi:10.1016/S1474-4422(07)70267-3. PMID 17945155. 
  3. ^ "Infection of the central nervous system caused by varicella zoster virus reactivation: a retrospective case series study". International Journal of Infectious Diseases. July 2013. 
  4. ^ "Mapping amyotrophic lateral sclerosis lake risk factors across northern New England". International Journal of Health Geographics. 2014. 
  5. ^ "The varicella zoster virus vasculopathies: clinical, CSF, imaging, and virologic features.". Neurology. March 2008. 
  6. ^ "Infection of the central nervous system caused by varicella zoster virus reactivation: a retrospective case series study". International Journal of Infectious Diseases. July 2013. 
  7. ^ "Infection of the central nervous system caused by varicella zoster virus reactivation: a retrospective case series study". International Journal of Infectious Diseases. July 2013. 
  8. ^ "Clinical Features of Viral Meningitis in Adults: Significant Differences in Cerebrospinal Fluid Findings among Herpes Simplex Virus, Varicella Zoster Virus, and Enterovirus Infections". Clinical Infectious Diseases, the Infectious Diseases Society of America. 2008. 
  9. ^ "Infection of the central nervous system caused by varicella zoster virus reactivation: a retrospective case series study". International Journal of Infectious Diseases. July 2013. 
  10. ^ "Infection of the central nervous system caused by varicella zoster virus reactivation: a retrospective case series study". International Journal of Infectious Diseases. July 2013. 
  11. ^ a b c "Varicella zoster vs. herpes simplex meningoencephalitis in the PCR era. A single center study". Journal of the Neurological Sciences. August 2011. 
  12. ^ "Recurrent Herpes Simplex Virus Type 2 Meningitis: A Case Report of Mollaret’s Meningitis". Jpn. J. Infect. Dis. July 2002. 
  13. ^ Davison AJ, Scott JE (1986) The complete DNA sequence of varicella-zoster virus. J Gen Virol 67:1759-1816
  14. ^ Chow, V. T.; Tipples, G. A.; Grose, C. (2012). "Bioinformatics of varicella-zoster virus: Single nucleotide polymorphisms define clades and attenuated vaccine genotypes". Infection, Genetics and Evolution 18: 351. doi:10.1016/j.meegid.2012.11.008. 
  15. ^ Grose, C. (2012). "Pangaea and the Out-of-Africa Model of Varicella-Zoster Virus Evolution and Phylogeography". Journal of Virology 86 (18): 9558–9565. doi:10.1128/JVI.00357-12. PMC 3446551. PMID 22761371. 
  16. ^ a b Loparev, V. N.; Rubtcova, E. N.; Bostik, V.; Tzaneva, V.; Sauerbrei, A.; Robo, A.; Sattler-Dornbacher, E.; Hanovcova, I.; Stepanova, V.; Splino, M.; Eremin, V.; Koskiniemi, M.; Vankova, O. E.; Schmid, D. S. (2009). "Distribution of varicella-zoster virus (VZV) wild-type genotypes in northern and southern Europe: Evidence for high conservation of circulating genotypes". Virology 383 (2): 216–225. doi:10.1016/j.virol.2008.10.026. PMID 19019403. 
  17. ^ Zell, R.; Taudien, S.; Pfaff, F.; Wutzler, P.; Platzer, M.; Sauerbrei, A. (2011). "Sequencing of 21 Varicella-Zoster Virus Genomes Reveals Two Novel Genotypes and Evidence of Recombination". Journal of Virology 86 (3): 1608–1622. doi:10.1128/JVI.06233-11. PMC 3264370. PMID 22130537. 
  18. ^ Loparev, V. N.; Rubtcova, E. N.; Bostik, V.; Govil, D.; Birch, C. J.; Druce, J. D.; Schmid, D. S.; Croxson, M. C. (2007). "Identification of Five Major and Two Minor Genotypes of Varicella-Zoster Virus Strains: A Practical Two-Amplicon Approach Used to Genotype Clinical Isolates in Australia and New Zealand". Journal of Virology 81 (23): 12758–12765. doi:10.1128/JVI.01145-07. PMC 2169114. PMID 17898056. 
  19. ^ McGeoch DJ, Cook S (1994) Molecular phylogeny of the alphaherpesvirinae subfamily and a proposed evolutionary timescale. J Mol Biol 238:9-22
  20. ^ Centers for Disease Control and Prevention (CDC) (March 2012). "FDA approval of an extended period for administering VariZIG for postexposure prophylaxis of varicella". MMWR Morb. Mortal. Wkly. Rep. 61 (12): 212. PMID 22456121. 
  21. ^ "Prevention of varicella: Recommendations of the Advisory Committee on Immunization Practices (ACIP). Centers for Disease Control and Prevention". MMWR Recomm Rep 45 (RR–11): 1–36. July 1996. PMID 8668119. 
  22. ^ Marin M; Güris D; Chaves SS; Schmid S; Seward JF (June 2007). "Prevention of varicella: recommendations of the Advisory Committee on Immunization Practices (ACIP)". MMWR Recomm Rep 56 (RR–4): 1–40. PMID 17585291. 
  23. ^ Gagliardi AM, Gomes Silva BN, Torloni MR, Soares BG (2012). Gagliardi, Anna MZ, ed. "Vaccines for preventing herpes zoster in older adults". Cochrane Database Syst Rev 10: CD008858. doi:10.1002/14651858.CD008858.pub2. PMID 23076951. 

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