Herpes Simplex Virus (HSV): Structure, Diagram, Pathogenesis, and Lab Diagnosis

Once herpes simplex virus infects a person, it never truly leaves. After the first outbreak fades, the virus retreats into the nerves and hides there for life, silent but ready. Years later, stress, sunlight, or illness can wake it, and the cold sore or genital lesion returns in the same place as before. This ability to hide inside nerve cells and reactivate is the single most important thing to understand about HSV. It explains why the infection is lifelong, why it recurs, and why there is still no cure.

Structure of herpes simplex virus

First, a common question: herpes is a virus, not a bacterium. Herpes simplex virus (HSV) belongs to the family Herpesviridae. It is a large virus, second in size only to the poxviruses.

Its structure has four layers, from the inside out:

• Core: a linear, double-stranded DNA genome.
• Capsid: the DNA is enclosed in an icosahedral (20-sided) protein shell made of 162 capsomeres. This capsomere number is a classic identifying feature of herpesviruses.
• Tegument: a layer of proteins between the capsid and the envelope.
• Envelope: an outer lipid membrane, derived partly from the host cell's nuclear membrane and partly virally coded. It is studded with glycoprotein spikes that the virus uses to attach to and enter host cells.

A complete enveloped particle measures 120 to 200 nm in diameter. The "naked" capsid without its envelope is about 100 nm.

To enter a cell, the envelope glycoproteins bind to specific host-cell receptors, and the viral envelope fuses with the cell membrane. The capsid is then carried to the nuclear pores, where the viral DNA enters the nucleus and circularizes. All herpesvirus DNA replication happens inside the nucleus.

HSV-1 versus HSV-2

There are two types of herpes simplex virus. They are structurally similar but differ in where they usually cause disease and how they are transmitted.

The distinction is not absolute. HSV-1 increasingly causes genital herpes through oral-genital contact, and either type can infect either site. What stays reliable is the latency site: HSV-1 hides in the trigeminal ganglion, HSV-2 in the sacral ganglia.

Pathogenesis: latency and reactivation

The defining feature of herpes simplex virus is its ability to establish lifelong latency. Understanding this one process explains why herpes is a lifelong, recurring infection.

Primary infection. The virus first enters through the skin or a mucous membrane (the mouth, genitals, or eye). It replicates in the epithelial cells there, causing the blisters and ulcers of the first infection.

Travel to the nerve. From the site of infection, the virus enters the endings of local sensory nerves and travels up the nerve fibers to the sensory ganglion, the cluster of nerve cell bodies that serves that area. For oral HSV-1 this is the trigeminal ganglion; for genital HSV-2 it is the sacral ganglia.

Latency. Inside these nerve cells the virus becomes latent. It stops producing new virus particles and simply persists as quiet viral DNA in the nucleus. In this state the immune system cannot clear it, and antiviral drugs cannot reach it. This is why HSV infection is lifelong and why there is no cure.

Reactivation. Certain triggers can wake the latent virus: emotional stress, fever, sunlight (ultraviolet light), menstruation, or a weakened immune system. When reactivated, the virus travels back down the same nerve to the skin and causes a new outbreak, typically in the same place as before. This is why cold sores keep returning to the same spot on the lip.

This cycle, primary infection then latency then reactivation, is the heart of herpes pathogenesis. The virus does not need to reinfect a person to cause a new outbreak. It was there the whole time.

Clinical Feature of Herpes Simplex Virus

Signs and Symptoms

HSV-1 infections usually affect the oral cavity, lips, or face, and sometimes the genital region. HSV-2 infections mainly affect the genital region. In HSV 1, labial herpes (‘cold sore’) is the most common manifestation.

Primary infection

The primary infection is symptomless in most cases but may present as fever, enlarged submandibular lymph nodes, sore throat, gingivostomatitis with ulcers or vesicles, edema, with associated anorexia, pain, and malaise. This condition usually lasts for 10–21 days and may be accompanied by an inability to eat or drink. Dehydration may be a problem, especially in small children.

Symptomatic primary infection is most common in children of 1–5 years of age, with an incubation time of 2–12 days (mean about 4 days). There may be a prodrome of burning, itching, or tingling pain for some hours followed by groups of vesicles usually on the external borders of the lips. Lesions may also be present in the skin surrounding the lips; chin,  cheeks, or nose. Within a few days, the vesicles progress to pustules or ulcers with brownish-yellow crusts. Pain is most severe in the beginning and resolves during the next 4–5 days.

The Herpes 2 infection usually affects the genital regions. The primary genital infection may be severe, with illness usually lasting up to about 3 weeks (sometimes longer) with a shedding period of virus usually terminating shortly before or at the time of healing. The lesions are vesicles or ulcers localized to the genital tracts of both males and females. The lesions are painful and may be associated with inguinal lymphadenopathy and dysuria. Systemic complaints, including fever and malaise, usually occur. Genital herpes is one of the three classic causes of genital ulcer; for the differential with chancroid and syphilis and the wider STI context, see the list of sexually transmitted infections and their causative agents.

Complicating extragenital infections, including aseptic meningitis, have been observed in about 10–20% of cases. Paraesthesia or dysesthesia may occur after the genital affection. Especially in women, the severity of the primary infection may be associated with a high number of complications and frequent recurrences. Previous HSV1 infection reduces the severity and duration of primary HSV2 infection.

The recurrent genital affection is usually milder, with fewer vesicles or ulcers, and with a duration of 7–10 days. The recurrent lesions seldom last more than 10 days, and the shedding of the virus may terminate sooner. Sometimes, virus excretion can occur between active periods.

Neonatal herpes

Infection in the newborn may be acquired in utero, at or just after birth. The newborn has low resistance to this infection and usually develops a severe disease. The mortality rate of untreated disease is about 50%. Babies with neonatal herpes infection may develop:

• A disseminated generalized form with many affected organs, including the central nervous system (CNS).
• Encephalitis with or without herpetic lesions of the skin.
• Herpetic lesions localized to skin, mouth and eyes.

The generalized form is especially serious, and is often combined with intravascular coagulopathy, hepatic and adrenal necrosis, pneumonitis and/or encephalitis followed by permanent neurological sequelae if the patient survives. The congenital infection may induce malformations such as microcephaly or microphthalmia, or other symptoms such as jaundice, hepatosplenomegaly, bleeding diathesis, seizures, irritability, chorioretinitis and herpetic vesicles of the skin.

HSV encephalitis

In the USA HSV1 is considered the most common viral strain of fatal encephalitis. The lesion is usually a local process in the brain, consisting of hemorrhagic necrosis and edema, mimicking a brain tumor. The localization is usually one of the temporal lobes. At later stages, however, the expansion retracts leaving scar tissue and midline structures deviating to the affecting side.

In immunocompromised patients, the HSV infection may be severe, especially in cases of reduced cellular immunity. This is true both in patients with a disease affecting the immune system, e.g. AIDS, and in patients under immunosuppressive treatment. Especially bone marrow, renal and cardiac transplant recipients are at risk for severe herpes infections.

The lesions may be progressive, and involve unusual sites such as the respiratory tract, esophagus, liver, and intestinal mucosa, or occur as a disseminated infection in severely immunocompromised patients. The severity of the disease is directly related to the degree of immunosuppression, and will also last longer than usual, about 6 weeks. Malnourishment, especially in children, seems to aggravate symptoms. Even immunocompromised patients may discharge the virus asymptomatically.

The initial infection with herpesvirus may be located in the eye, with severe keratoconjunctivitis as a result. Recurrent infections of the eye may appear as ulcers of the cornea, sometimes dendritic ulcers, or as vesicles on the eyelids. Later chorioretinitis may develop. The cornea may develop opacifications after recurrence, indicating a progressive involvement. Blindness may be the consequence. Even herpetic necrosis of the retina has been observed as a very rare consequence of the infection.

Laboratory Diagnosis of Herpes Simplex Virus

Cytology and histology (Tzanck smear)

The classic bedside test is the Tzanck smear. Scrapings taken from the base of a fresh vesicle are stained (for example with Giemsa) and examined under the microscope.

• The hallmark finding is multinucleated giant cells, which indicate infection with HSV-1, HSV-2, or varicella-zoster virus (VZV).
• This test is fast and cheap, but it cannot tell these three viruses apart, and a negative result does not rule out infection. It has largely been replaced by PCR where available.

Isolation and identification of the virus

1. The virus is isolated from lesions; also from throat washings, CSF, and stool.
2. Inoculation of tissue cultures (human diploid fibroblast).
3. Identification by neutralization test or immunofluorescence staining with specific antiserum.
4. typing is done by using monoclonal antibodies or by RE analysis of viral DNA

PCR (Polymerase Chain Reaction): most sensitive/specific

• It helps in detection of amplified viral DNA by PCR in CSF or other samples
• typing is done by using type-specific primers or using common primers followed by RE analysis or hybridization probes.

Serology

1. Serology is of no use in recurrences except in cases of encephalitis. antibody appears in 4-7 days after infection, reach a peak in 2-4 weeks.
2. The complement fixation test (CFT) measures total antibodies, not differentiating type-specific antibodies.
3. EIA more efficient than CFT; type-specific antigen (Ag) used to detect type-specific antibodies. diagnostic value limited by multiple Ag shared by HSV-1 and HSV-2.
4. It also gives heterotypic anamnestic responses to VZV in HSV infection and vice-versa.

Interpretation of Diagnostic Test for Herpes Simplex Virus (HSV)

Treatment of herpes simplex virus

There is no cure for HSV, because no drug can remove the latent virus from the nerve cells. However, antiviral drugs can control outbreaks effectively by stopping the virus from replicating during active infection.

The main antiviral drugs are acyclovir and its relatives valacyclovir and famciclovir. They work by blocking the viral enzyme that copies the DNA, so they act only while the virus is actively replicating, not while it is latent. Started early, they shorten outbreaks and reduce viral shedding.

They are used in two ways. Episodic treatment means taking the drug at the start of an outbreak to shorten it. Suppressive treatment means taking it daily to prevent frequent recurrences and reduce the risk of passing the virus to others. Topical acyclovir cream can ease mild cold sores if applied early, but oral drugs work better for significant disease.

Severe HSV disease, such as encephalitis or infection in a newborn or an immunocompromised patient, is a medical emergency treated with intravenous acyclovir. In HSV encephalitis, starting acyclovir early, before test results confirm the diagnosis, saves lives.

How to Remember

162 capsomeres ("count to a herpes"): herpesviruses have 162 capsomeres in an icosahedral capsid. The oddly specific number is a favorite exam fact. If a virus question gives you "162 capsomeres, enveloped, dsDNA," it is a herpesvirus.

Where each type hides ("1 up high, 2 down low"): HSV-1 goes to the trigeminal ganglion in the face (up high). HSV-2 goes to the sacral ganglia in the pelvis (down low). Site of disease matches site of latency.

Why it comes back ("hides in the nerve, rides the nerve"): the virus travels up the nerve to hide (latency) and rides back down the same nerve to cause the next outbreak in the same spot. No reinfection needed.

Latency defeats the cure: antivirals only work on replicating virus, and latent virus is not replicating. That is the one-sentence reason there is no cure and why acyclovir controls but never clears HSV.

Treat encephalitis first, ask questions later: in suspected HSV encephalitis, start IV acyclovir immediately, before confirmation. Waiting for the test costs lives. This is a classic exam and clinical point.

Key exam facts

Where Students Get Confused

"Is herpes a virus or a bacterium?" A virus. Herpes simplex virus is a double-stranded DNA virus in the family Herpesviridae. It is not a bacterium, so antibiotics do not work against it; antivirals like acyclovir are used instead.

"How many capsomeres does herpesvirus have?" 162, arranged in an icosahedral capsid. This is a classic identifying feature of the whole herpesvirus family and a common exam fact.

"Why is there no cure for herpes?" Because the virus hides as latent DNA inside nerve cells, where it is not replicating. Antiviral drugs only stop the virus while it is actively copying itself, so they cannot reach or remove the latent form. The infection is therefore lifelong.

"What is the difference between HSV-1 and HSV-2?" Traditionally, HSV-1 causes oral herpes (cold sores) and HSV-2 causes genital herpes. But the line has blurred, and HSV-1 now causes many genital cases through oral contact. The reliable difference is the latency site: HSV-1 in the trigeminal ganglion, HSV-2 in the sacral ganglia.

"Why do cold sores keep coming back in the same place?" Because the latent virus lives in one specific nerve ganglion. When it reactivates, it travels back down the same nerve to the same patch of skin, so the outbreak recurs in the same spot.

"Does a negative Tzanck smear rule out herpes?" No. The Tzanck smear is quick but not very sensitive, and it cannot distinguish HSV-1, HSV-2, and VZV. A negative result does not exclude infection. PCR is far more sensitive and specific.

"If someone has no symptoms, can they still spread HSV?" Yes. The virus can reactivate and be shed from the skin without causing a visible sore (asymptomatic shedding), which is a major reason it spreads so widely.

References

1. Zhu, S., & Viejo-Borbolla, A. (2021). Pathogenesis and virulence of herpes simplex virus. Virulence, 12(1), 2670–2702. https://doi.org/10.1080/21505594.2021.1982373
2. Fatahzadeh, M., & Schwartz, R. A. (2007). Human herpes simplex virus infections: epidemiology, pathogenesis, symptomatology, diagnosis, and management. Journal of the American Academy of Dermatology, 57(5), 737–766. https://doi.org/10.1016/j.jaad.2007.06.027
3. Whitley, R. J. (2002). Herpes simplex virus infection. Seminars in Pediatric Infectious Diseases, 13(1), 6–11. https://doi.org/10.1053/spid.2002.29752
4. Saleh, D., Yarrarapu, S. N. S., & Sharma, S. (2023). Herpes simplex type 1. In StatPearls. StatPearls Publishing.
5. Mathew Jr, J., & Sapra, A. (2023). Herpes simplex type 2. In StatPearls. StatPearls Publishing.