Varicella-Zoster Virus
On this page
Direct answer
Varicella-zoster virus is an alpha herpesvirus that causes chickenpox on primary infection and herpes zoster when it reactivates from latency in sensory ganglia. Primary infection produces crops of vesicles at different stages across the body, which is the clinical clue that distinguishes it from the uniform rash of smallpox. Acyclovir, valacyclovir and famciclovir treat the disease, and a live attenuated vaccine prevents chickenpox.
What you must remember
- Classification: Human alphaherpesvirus 3, an enveloped double-stranded DNA virus spread by respiratory droplets and by direct contact with vesicle fluid.
- Chickenpox: An incubation period of 14 to 16 days is followed by a prodrome, then crops of dew-drop vesicles on an erythematous base, mostly on the trunk and face.
- Infectivity: Patients are infectious from about one to two days before the rash until every lesion has crusted.
- Herpes zoster: Reactivation within one dermatome, most common in older adults and immunocompromised people, with post-herpetic neuralgia as the main chronic complication.
- Hutchinson sign: Vesicles on the tip of the nose, supplied by the nasociliary branch of the trigeminal nerve, warn of ocular involvement.
- Ramsay Hunt syndrome: Zoster of the geniculate ganglion causes ear vesicles, facial palsy and vertigo, and can damage hearing.
- Congenital varicella: Maternal infection before about 20 weeks of gestation can cause limb hypoplasia and scarring, while infection from five days before to two days after delivery carries a high risk of severe neonatal disease.
- Diagnosis: Polymerase chain reaction on vesicle fluid is the most reliable test, whereas the Tzanck smear shows multinucleated giant cells but cannot tell varicella-zoster from herpes simplex.
Varicella and zoster compared
Both diseases come from the same latent virus, and the state of immunity decides which one appears. Exam questions usually give the age, the rash pattern and the immune status, so start there.
| Feature | Varicella (primary) | Herpes zoster (reactivation) |
|---|---|---|
| Trigger | First exposure, usually in childhood | Waning cell-mediated immunity, often in older adults |
| Viral behaviour | Viraemia seeds the skin | Latent in dorsal root or cranial nerve ganglia |
| Rash pattern | Generalised crops of vesicles at mixed stages | Grouped vesicles along one dermatome |
| Midline | Crosses the midline | Stops at the midline |
| Main complication | Pneumonia in adults, secondary bacterial infection | Post-herpetic neuralgia |
| Antiviral | Acyclovir for severe or high-risk patients | Valacyclovir, famciclovir or acyclovir within 72 hours of rash |
The table reads as one virus in two settings: circulating in the blood during primary infection, or sitting in a ganglion during latency. Waning cell-mediated immunity explains the timing of zoster, because T cells normally hold the latent virus in check.
Where the exam tries to catch you
Varicella-zoster questions usually turn on age, immune status and the shape of the rash.
- Trap: Using a Tzanck smear to confirm varicella and rule out herpes simplex. Correct: Multinucleated giant cells appear in both infections, so only PCR or direct fluorescent antibody testing of vesicle fluid separates them.
- Trap: Giving aspirin for fever in a child with chickenpox. Correct: Aspirin raises the risk of Reye syndrome, so paracetamol is the antipyretic of choice.
- Trap: Treating a vesicle on the nasal tip as an ordinary skin infection. Correct: This is the Hutchinson sign, so urgent ophthalmology review is needed for keratitis and uveitis.
- Trap: Advising conception soon after a live varicella vaccine. Correct: Live vaccines are avoided in pregnancy, and women are advised to avoid pregnancy for about one month after vaccination.
Frequently asked questions
Which test confirms varicella-zoster virus infection in a vesicle?
Polymerase chain reaction on vesicle fluid or scrapings is the most sensitive test for varicella-zoster virus. Direct fluorescent antibody testing is an older alternative, while the Tzanck smear cannot confirm the virus.
Why does herpes zoster follow a dermatomal distribution?
Varicella-zoster virus stays latent in the dorsal root or cranial nerve ganglia and reactivates along the sensory nerve, so the rash stays within one dermatome. Thoracic dermatomes are the most common site, and the rash rarely crosses the midline.
How early must an antiviral be started in herpes zoster?
Herpes zoster is treated with valacyclovir, famciclovir or acyclovir and works best when started within 72 hours of rash onset. Early therapy shortens the illness and reduces acute pain, though its effect on later post-herpetic neuralgia is less certain.
Why is varicella dangerous in pregnancy?
Varicella in pregnancy can cause pneumonia in the mother, and infection before about 20 weeks can cause congenital varicella syndrome in the fetus. Infection from five days before to two days after delivery can cause severe varicella in the newborn, so varicella-zoster immune globulin is given to exposed women who are not immune.
Why is aspirin avoided in children with chickenpox?
Aspirin given during varicella raises the risk of Reye syndrome, a rare but serious encephalopathy with fatty degeneration of the liver. Paracetamol is the preferred antipyretic in children with chickenpox.