Herpes simplex virus

Herpes simplex virus
TEM micrograph of a herpes simplex virus.
Virus classification
Group: Group I (dsDNA) Family: Herpesviridae Subfamily: Alphaherpesvirinae Genus: Simplexvirus
Species
Herpes simplex virus 1 (HSV-1) Herpes simplex virus 2 (HSV-2)

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]

Overview[edit | edit source]

Herpes simplex virus 1 and 2 (HSV-1 and HSV-2) are two strains of the herpes virus family, Herpesviridae, which cause extremely painful infections in humans.^[1] They are also called Human Herpes Virus 1 and 2 (HHV-1 and HHV-2).

An infection is marked by watery blisters in the skin or mucous membranes (such as the mouth or lips) or on the genitals.^[1] Lesions heal with a scab characteristic of herpetic disease. However, the infection is persistent and symptoms may recur periodically as outbreaks of sores near the site of original infection. After the initial, or primary, infection, HSV becomes latent present in the cell bodies of nerves in the area. When it becomes reactivated, it is produced in the nerve cell and transported to the skin via the nerve's axon.^[2]

Herpes is contagious if the carrier is producing and shedding virus. This is especially likely during an outbreak but possible at other times. There is no cure yet, but there are treatments which reduce the likelihood of viral shedding. An HSV infection on the lips is commonly known as a "cold sore" or "fever blister". The blisters resemble those of chickenpox, an infection caused by another member of the alpha-Herpesviridae subfamily, Varicella Zoster Virus (VZV), also known as Human Herpes Virus 3 (HHV-3).

Transmission[edit | edit source]

HSV is transmitted by contact with lips or genitals when the sores are present, or possibly when sores are not visible (known as viral shedding). HSV can be present in semen, vaginal fluids, shedded skin in the pelvic region from eczema, and saliva. In addition, herpes may be transmitted during childbirth, which can be fatal to the infant. The immature immune system of the child is unable to defend against the virus and even if treated, infection can result in brain damage. Transmission occurs when passing through the birth canal. But, risk of infection is minimal if there are no symptoms or exposed blisters during delivery. The first outbreak after exposure to HSV is commonly more severe than future outbreaks, as the body has not had a chance to produce antibodies; this first outbreak carries a low (~1%) risk of developing aseptic meningitis.^[1]

Microbiology[edit | edit source]

Viral structure[edit | edit source]

Animal herpes viruses all share some common properties. The structure of herpes viruses consists of a relatively large double-stranded, linear DNA genome encased within an icosahedral protein cage called the capsid, which is wrapped in a lipid bilayer called the envelope. The envelope is joined to the capsid by means of a tegument. This complete particle is known as the virion.^[3] HSV-1 and HSV-2 each contain at least 74 genes (or open-reading frames, ORFs) within their genomes,^[4] although speculation over gene crowding allows as many as 84 unique protein coding genes by 94 putative ORFs.^[5] These genes encode a variety of proteins involved in forming the capsid, tegument and envelope of the virus, as well as controlling the replication and infectivity of the virus. These genes and their functions are summarized in the table below.

The genomes of HSV-1 and HSV-2 are complex, and contain two unique regions called the long unique region (U_L) and the short unique region (U_S). Of the 74 known ORFs, U_L contains 56 viral genes, whereas U_S contains only 12.^[4] Transcription of HSV genes is catalyzed by RNA polymerase II of the infected host.^[4] Immediate early genes, which encode proteins that regulate the expression of early and late viral genes, are the first to be expressed following infection. Early gene expression follows, to allow the synthesis of enzymes involved in DNA replication and the production of certain envelope glycoproteins. Expression of late genes occurs last; this group of genes predominantly encode proteins that form the virion particle.^[4]

Five proteins from (U_L) form the viral capsid; UL6, UL18, UL35, UL38 and the major capsid protein UL19.^[3]

Cellular entry[edit | edit source]

Entry of HSV into the host cell involves interactions of several glycoproteins on the surface of the enveloped virus, with receptors on the surface of the host cell. The envelope covering the virus particle, when bound to specific receptors on the cell surface, will fuse with the host cell membrane and create an opening, or pore, through which the virus enters the host cell.

The sequential stages of HSV entry are analogous to those of other viruses. At first, complementary receptors on the virus and the cell surface bring the viral and cell membranes into proximity. In an intermediate state, the two membranes begin to merge, forming a hemifusion state. Finally, a stable entry pore is formed through which the viral envelope contents are introduced to the host cell.^[10] In the case of a herpes virus, initial interactions occur when a viral envelope glycoprotein called glycoprotein C (gC) binds to a cell surface particle called heparan sulfate. A second glycoprotein, glycoprotein D (gD), binds specifically to a receptor called the herpesvirus entry mediator receptor (HVEM) and provides a strong, fixed attachment to the host cell. These interactions bring the membrane surfaces into mutual proximity and allow for other glycoproteins embedded in the viral envelope to interact with other cell surface molecules.

Once bound to the HVEM, gD changes its conformation and interacts with viral glycoproteins H (gH) and L (gL), which form a complex. The interaction of these membrane proteins results in the hemifusion state. Afterward, gB interaction with the gH/gL complex creates an entry pore for the viral capsid.^[10] Glycoprotein B interacts with glycosaminoglycans on the surface of the host cell.

Genetic inoculation[edit | edit source]

After the viral capsid enters the cellular cytoplasm, it is transported to the cell nucleus. Once attached to the nucleus at a nuclear entry pore, the capsid ejects its DNA contents via the capsid portal. The capsid portal is formed by twelve copies of portal protein arranged as a ring; the proteins contain a leucine zipper sequence of amino acids which allow them to adhere to each other.^[11] Each icosahedral capsid contains a single portal, located in one vertex.^[12][13] The DNA exits the capsid in a single linear segment.^[14]

Replication[edit | edit source]

Consequent to a cell being infected, groups of Herpes virus proteins, termed immediate-early, early, and late proteins, are produced following specific time periods. Research using a new flow cytometry methodology in KSHV indicates the possibility of an additional lytic stage, delayed-late.^[15] These stages of lytic infection, particularly late lytic, are distinct from the latency stage. For example, in the case of HSV-1, no protein products are detected during latency.

The early proteins transcribed are used in the regulation of genetic replication of the virus. On entering the cell, an α-TIF protein joins the viral particle and aids in immediate-early Transcription. The virion host shutoff protein (VHF-UL41) is very important to viral replication. This enzyme shuts off protein synthesis in the host, degrades host mRNA, helps in viral replication, and regulates gene expression of viral proteins. The viral genome immediately travels to the nucleus but the VHF protein remains in the cytoplasm.^[16][17]

The late proteins transcribed are used in the capsid formation and forming the receptors on the surface of the virus. The packaging of the viral particles, which include the genome, core and the capsid, occur in the nucleus. In the nucleus, cleavage of genome concatemers occurs and these are placed into pre-formed capsids. HSV-1 undergoes a process of primary and secondary envelopment. It acquires a primary envelope by budding into the inner nuclear membrane. This then fuses with the outer nuclear membrane releasing a naked capsid into the cytoplasm. The virus acquires its final envelope by budding into cytoplasmic vesicles.^[18]

Latent infection[edit | edit source]

HSV may persist in a quiescent but persistent form known as latent infection, notably in neural ganglia.^[1] During latent infection of a cell, HSV express Latency Associated Transcript (LAT) RNA. LAT is known to regulate the host cell genome and interferes with natural cell death mechanisms. By maintaining the host cells, LAT expression preserves a reservoir for the virus, which allows later recurrences to produce further infections.

A protein found in neurons may bind to Herpes DNA and regulate latency. Recent studies have found that the Herpes DNA contains a sequence that is involved in silencing the expression of a gene associated with lytic infection, ICP4. The sequence contains elements which bind to human nerve cell protein factors: the human neuronal protein Neuronal Restrictive Silencing Factor (NRSF), and human Repressor Element Silencing Transcription Factor (REST). When the proteins are able to bind to the viral DNA elements, histone deacytalization occurs atop the ICP4 gene sequence.^[19][20]

Reactivation[edit | edit source]

The virus can be reactivated due to the effects of other illnesses such as cold and influenza, excema, menstruation, emotional and physical stress, exposure to bright sunlight, gastric upset, fatigue or injury, consequently resulting in the appearance of surface sores.

Treatment[edit | edit source]

Antimicrobial Regimen[edit | edit source]

• Genital Herpes^[21]

• 1.First Clinical Episode of Genital Herpes

• Preferred Regimen (1): Acyclovir 400 mg PO tid for 7–10 days
• Preferred Regimen (2): Acyclovir 200 mg PO five times a day for 7–10 days
• Preferred Regimen (3): Valacyclovir 1 g PO bid for 7–10 days
• Preferred Regimen (4): Famciclovir 250 mg PO tid for 7–10 days
• Note:Treatment can be extended if healing is incomplete after 10 days of therapy.

• 2.Established HSV-2 Infection

• 2.1 Suppressive Therapy for Recurrent Genital Herpes

• Preferred Regimen (1): Acyclovir 400 mg PO bid
• Preferred Regimen (2): Valacyclovir 500 mg PO qd
• Preferred Regimen (3): Valacyclovir 1 g PO qd
• Preferred Regimen (4): Famciclovir 250 mg PO bid
• Note(1):Daily therapy with Acyclovir for as long as 6 years and with Valacyclovir OR Famciclovir for 1 year
• Note(2):Valacyclovir 500 mg qd might be less effective than other Valacyclovir OR Acyclovir dosing regimens in persons who have very frequent recurrences (i.e., ≥10 episodes per year).

• 2.2 Episodic Therapy for Recurrent Genital Herpes

• Preferred Regimen (1): Acyclovir 400 mg PO tid for 5 days
• Preferred Regimen (2): Acyclovir 800 mg PO bid for 5 days
• Preferred Regimen (3): Acyclovir 800 mg PO tid for 2 days
• Preferred Regimen (4): Valacyclovir 500 mg PO bid for 3 days
• Preferred Regimen (5): Valacyclovir 1 g PO qd for 5 days
• Preferred Regimen (6): Famciclovir 125 mg PO bid for 5 days
• Preferred Regimen (7): Famciclovir 1 g PO bid for 1 day
• Preferred Regimen (8): Famciclovir 500 mg once, followed by 250 mg PO bid for 2 days

• 3. Severe Disease (disseminated infection, pneumonitis, or hepatitis) or CNS complications (e.g., meningoencephalitis).

• Preferred Regimen: Acyclovir 5–10 mg/kg IV q8h for 2–7 days or until clinical improvement is observed, followed by oral antiviral therapy to complete at least 10 days of total therapy. HSV encephalitis requires 21 days of intravenous therapy. Impaired renal function warrants an adjustment in acyclovir dosage.

• 4. Special Considerations

• 4.1 HIV Infection

• 4.1.1 Daily Suppressive Therapy in Persons with HIV

• Preferred Regimen (1): Acyclovir 400–800 mg PO bid /tid
• Preferred Regimen (2): Valacyclovir 500 mg PO bid
• Preferred Regimen (3): Famciclovir 500 mg PO bid

• 4.1.2 Episodic Infection in Persons with HIV

• Preferred Regimen (1): Acyclovir 400 mg PO tid for 5–10 days
• Preferred Regimen (2): Valacyclovir 1 g PO bid for 5–10 days
• Preferred Regimen (3): Famciclovir 500 mg PO bid for 5–10 days
• Note: For severe HSV disease, initiating therapy with Acyclovir 5–10 mg/kg IV q8 h might be necessary.

• 4.2 Genital Herpes in Pregnancy

• Suppressive therapy of pregnant women with recurrent genital herpes
• Preferred Regimen (1): Acyclovir 400–800 mg PO bid /tid
• Preferred Regimen (2): Valacyclovir 500 mg PO bid
• Note:Treatment recommended starting at 36 weeks of gestation.

• 4.3 Neonatal Herpes

• Known or suspected neonatal herpes

• Preferred Regimen: Acyclovir 20 mg/kg IV q 8 h
• Note (1): Treatment for 14 days if disease is limited to the skin and mucous membranes, or
• Note (2): Treatment for 21 days for disseminated disease and that involving the central nervous system.

• 4.4 Acyclovir-resistant genital herpes

• Preferred Regimens: Foscarnet 40–80 mg/kg IV q8 h until clinical resolution is attained
• Alternative Regimen (1): Cidofovir 5 mg/kg IV once weekly might also be effective.
• Alternative Regimen (2): Imiquimod topical preparations should be applied to the lesions qd for 5 consecutive days.

• 4.5 Management of Sex Partners

• Preferred Regimen (1): Acyclovir 400 mg PO tid for 7–10 days
• Preferred Regimen (2): Acyclovir 200 mg PO five times a day for 7–10 days
• Preferred Regimen (3): Valacyclovir 1 g PO bid for 7–10 days
• Preferred Regimen (4): Famciclovir 250 mg PO tid for 7–10 days
• Note:The sex partners of persons who have genital herpes can benefit from evaluation and counseling. Symptomatic sex partners should be evaluated and treated

• 4.6 Allergy, Intolerance, and Adverse Reactions

• Allergic and other adverse reactions to oral Acyclovir, Valacyclovir, and Famciclovir are rare. Desensitization to acyclovir has been described.

Vaccine research[edit | edit source]

Herpevac, a vaccine for HSV-2 is currently (as of February 2007) undergoing clinical testing in women in the United States and Canada.^[22][23] Previous studies have determined that this vaccine is approximately 70% effective in women, but does not prevent the disease in men.^[24]

References[edit | edit source]

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