Herpes Simplex Virus — UKMLA Guide 2026

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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.

Key points

HSV-1: classically orolabial (cold sores), increasingly causes genital herpes. HSV-2: classically genital. Both establish lifelong latency in sensory ganglia with periodic reactivation
Primary genital herpes: painful vesicles/ulcers, dysuria, inguinal lymphadenopathy, systemic symptoms. Treat with aciclovir 400 mg TDS for 5 days
Herpes encephalitis (usually HSV-1): acute onset confusion, fever, temporal lobe seizures/focal neurology — MEDICAL EMERGENCY. IV aciclovir 10 mg/kg TDS immediately, do NOT wait for LP results
Eczema herpeticum: widespread HSV in atopic eczema — widespread vesicles/punched-out erosions, systemically unwell. Urgent IV aciclovir
Neonatal herpes: transmitted during delivery (especially primary genital HSV near term). High mortality. Caesarean section if active primary genital HSV at delivery

Overview

Herpes simplex viruses (HSV-1 and HSV-2) are DNA viruses of the Herpesviridae family. After primary infection, HSV establishes lifelong latency in sensory nerve ganglia (trigeminal for HSV-1, sacral for HSV-2) and periodically reactivates, causing recurrent episodes. Primary infection is often more severe than recurrences. Serious manifestations include herpes simplex encephalitis (HSE — most common cause of sporadic viral encephalitis in the UK), eczema herpeticum, and neonatal herpes.

Epidemiology

HSV-1 seroprevalence is approximately 60-80% in UK adults; HSV-2 seroprevalence approximately 10-15%. HSV-1 is increasingly recognised as a cause of genital herpes (>50% of new genital herpes diagnoses in some populations). HSE has an incidence of approximately 2-4 per million per year and carries ~70% mortality if untreated. Neonatal herpes is rare in the UK (~1-2 per 100,000 births) but has high morbidity and mortality.

Clinical Features

Symptoms

Orolabial herpes (cold sores): prodromal tingling then vesicles on the lip/perioral skin

Primary genital herpes: painful vesicles → shallow ulcers on genitalia, dysuria, inguinal lymphadenopathy, fever, myalgia

Recurrent genital herpes: milder than primary, shorter duration, localised — often preceded by prodromal tingling

HSE: acute onset fever, headache, confusion, personality change, dysphasia, seizures (temporal lobe predilection)

Eczema herpeticum: widespread monomorphic vesicles/punched-out erosions over eczematous skin, fever, malaise

Signs

Grouped vesicles on an erythematous base — classic appearance

Shallow painful ulcers (after vesicles rupture)

Tender inguinal lymphadenopathy (genital herpes)

Temporal lobe signs: dysphasia, personality change, olfactory hallucinations (HSE)

Widespread punched-out erosions with crusting on eczematous skin (eczema herpeticum)

Investigations

First-line

HSV PCR (swab from vesicle/ulcer)Most sensitive test — type-specific (distinguishes HSV-1 from HSV-2). Swab base of fresh vesicle

Clinical diagnosisClassic cold sores and recurrent genital herpes can often be diagnosed clinically

Second-line

HSV PCR of CSFFor suspected herpes encephalitis — highly sensitive and specific. Results take hours — do NOT delay treatment

MRI brainHSE: temporal lobe oedema and haemorrhagic change — characteristic finding. MRI is more sensitive than CT

EEGHSE: periodic lateralised epileptiform discharges (PLEDs) over temporal lobes

Specialist

Type-specific serologyIgG antibodies to HSV-1 or HSV-2 — indicates previous exposure. Not useful for acute diagnosis

Management

BASHH Guidelines + NICE CKS

Orolabial herpes (cold sores)

Topical aciclovir cream 5% — apply 5 times daily for 5-10 days. Most effective if started during prodrome
Oral aciclovir if severe/frequent recurrences: 400 mg TDS for 5 days (treatment) or 400 mg BD long-term (suppressive)

Genital herpes

Primary: aciclovir 400 mg TDS for 5 days (or valaciclovir 500 mg BD for 5 days). Saline bathing, analgesia, topical lidocaine gel
Recurrent: aciclovir 800 mg TDS for 2 days (episodic short-course) or suppressive therapy (aciclovir 400 mg BD) if ≥6 recurrences/year
Refer to GUM/sexual health clinic for counselling, partner notification, and full STI screen

Herpes simplex encephalitis — EMERGENCY

IV aciclovir 10 mg/kg TDS for 14-21 days — start IMMEDIATELY on clinical suspicion. Do NOT delay for LP/MRI results
Mortality drops from ~70% to ~20% with treatment — delay increases mortality and neurological sequelae
LP: CSF HSV PCR, lymphocytic pleocytosis, elevated protein, often red cells (haemorrhagic encephalitis)
MRI: temporal lobe changes

Eczema herpeticum

IV aciclovir — systemically unwell or extensive disease
Oral aciclovir if mild — but low threshold for admission and IV treatment
Do NOT use topical steroids on active eczema herpeticum

HSV in pregnancy

Primary genital herpes in third trimester: high risk of neonatal transmission → elective caesarean section recommended
Recurrent genital herpes at delivery: lower risk → vaginal delivery usually safe with suppressive aciclovir from 36 weeks

Complications

HSE: Temporal lobe necrosis → permanent cognitive and language deficits in survivors. ~20% mortality with treatment, ~70% without
Neonatal herpes: Disseminated, CNS, or localised skin/eye/mouth disease. High mortality and neurodevelopmental morbidity
Eczema herpeticum: Can cause secondary bacterial infection and sepsis — dermatological emergency
Erythema multiforme: HSV is the most common trigger for recurrent EM (target lesions)
Keratitis: HSV can cause dendritic corneal ulcers — risk of corneal scarring and visual loss. Do NOT use topical steroids

UKMLA Exam Tips

1Fever + confusion + temporal lobe seizures = HSE until proven otherwise → IV aciclovir 10 mg/kg TDS IMMEDIATELY
2HSV PCR of CSF is the diagnostic test for HSE — but NEVER delay treatment waiting for results
3Temporal lobe abnormality on MRI + fever = classic HSE imaging finding
4Eczema herpeticum: monomorphic punched-out erosions on eczematous skin — distinct from infected eczema (which has irregular crusting)
5Erythema multiforme with target lesions → most common cause is recurrent HSV (not drugs — drugs cause SJS/TEN)
6Dendritic ulcer on slit-lamp = HSV keratitis — topical aciclovir, do NOT use topical steroids (worsens viral replication)

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