[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Human herpesviruses](https://durgical.com/en/microbiology/groups/human-herpesviruses/) › Herpes simplex virus 1

[Microbiology](https://durgical.com/en/microbiology/) · [Human herpesviruses](https://durgical.com/en/microbiology/groups/human-herpesviruses/)

# Herpes simplex virus 1

- Virus
- Enveloped
- Exam priority: high

The virus behind cold sores; it sleeps in the nerves of the face and rarely causes severe encephalitis.

Updated 2026-11-01

Also called: HSV-1, cold sores, fever blisters

## Key points

- Latency in the trigeminal ganglion.
- HSV encephalitis: temporal lobe, CSF PCR and empiric acyclovir without waiting for results.
- A Tzanck smear with multinucleated giant cells is an exam classic, but PCR is today's test.
- Standard precautions for localized recurrent herpes; contact precautions for disseminated disease.

## At a glance

**Shape**
Icosahedral with tegument and envelope

**Genome**
Linear double-stranded DNA

**Envelope**
Enveloped

**Family**
Herpesviridae (alphaherpesvirus)

**Size**
About 150 to 200 nm

## Morphology

- Icosahedral capsid surrounded by tegument and a lipid envelope
- Forms multinucleated giant cells with intranuclear (Cowdry A) inclusions

## Transmission

- Saliva and contact with sores or the skin around the mouth
- Oral-genital contact: a growing cause of genital herpes
- Can spread without visible lesions (asymptomatic shedding)

**Reservoir:** Humans

### Isolation precautions

What the care team adds to standard precautions.

- Standard

## Virulence factors

- Latency in neurons of the trigeminal ganglion, with no virions made
- Reactivation by fever, sunlight, stress, menstruation, injury or surgery
- Envelope glycoproteins that allow cell-to-cell fusion
- The viral thymidine kinase activates antivirals; its mutation causes resistance

## Diseases it causes

- Cold sores, gingivostomatitis, keratitis and herpetic whitlow [Genital herpes (herpes simplex infection) →](https://durgical.com/en/conditions/genital-herpes/)
- Herpes simplex encephalitis (temporal lobe) [Viral encephalitis (herpes simplex encephalitis) →](https://durgical.com/en/conditions/viral-encephalitis/)
- Eczema herpeticum on atopic skin

## Clinical picture

- Grouped vesicles on a red base that ulcerate and crust
- Primary infection in children: gingivostomatitis with fever and painful eating
- Encephalitis: fever, confusion, seizures and temporal focal signs; a common cause of sporadic viral encephalitis

## Diagnosis

- PCR of lesion fluid (the most sensitive test)
- HSV PCR of cerebrospinal fluid for encephalitis
- MRI showing temporal lobe involvement in encephalitis
- The Tzanck smear (multinucleated giant cells) is no longer recommended: it cannot tell HSV from VZV

## Treatment

Nucleoside analogues: acyclovir, valacyclovir and famciclovir; in encephalitis, intravenous acyclovir is started as soon as it is suspected. For acyclovir-resistant strains (thymidine kinase mutation), foscarnet.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

- [Aciclovir](https://durgical.com/en/pharmacology/aciclovir/)
- [Valaciclovir](https://durgical.com/en/pharmacology/valaciclovir/)
- [Famciclovir](https://durgical.com/en/pharmacology/famciclovir/)
- [Foscarnet](https://durgical.com/en/pharmacology/foscarnet/)

## Prevention

- Avoid direct contact with active sores
- Suppressive therapy for frequent recurrences
- Standard precautions; contact precautions for disseminated disease or severe primary infection

## Memory aid

Type 1 lives upstairs (face and trigeminal ganglion), and when it climbs higher it reaches the temporal lobe.

## Test yourself

Think of the answer, then tap the question to see it.

## Which brain lobe does HSV-1 encephalitis typically affect?

The temporal lobe, often with asymmetric, hemorrhagic involvement.

## In which ganglion does HSV-1 stay latent?

The trigeminal ganglion; on reactivation it travels down the nerve and causes cold sores.

## Which test confirms HSV encephalitis?

HSV PCR of cerebrospinal fluid.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/microbiology/herpes-simplex-virus-1/)

## Related

- [Herpes simplex virus 2](https://durgical.com/en/microbiology/herpes-simplex-virus-2/)
- [Varicella-zoster virus](https://durgical.com/en/microbiology/varicella-zoster-virus/)

## Related conditions

- [Genital herpes (herpes simplex infection)](https://durgical.com/en/conditions/genital-herpes/)
- [Viral encephalitis (herpes simplex encephalitis)](https://durgical.com/en/conditions/viral-encephalitis/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Human herpesviruses](https://durgical.com/en/microbiology/groups/human-herpesviruses/)

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Herpes simplex virus](https://www.who.int/news-room/fact-sheets/detail/herpes-simplex-virus) (WHO, 2024); [Herpes - STI Treatment Guidelines](https://www.cdc.gov/std/treatment-guidelines/herpes.htm) (CDC, 2021); [Appendix A: Type and Duration of Precautions Recommended for Selected Infections and Conditions](https://www.cdc.gov/infection-control/hcp/isolation-precautions/appendix-a-type-duration.html) (CDC, 2024).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/microbiology/herpes-simplex-virus-1/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/herpes-simplex-virus-1/