Chickenpox

Overview

Chickenpox (varicella) is the primary infection with varicella-zoster virus (VZV). It is a common, usually mild illness of childhood, but it can be severe in newborns, adults, pregnant women and immunocompromised children (for example those with leukaemia or on high-dose steroids). After recovery the virus lies dormant in sensory ganglia and can return years later as herpes zoster (shingles).

Aetiology

• VZV: an enveloped, double-stranded DNA virus of the herpesvirus family; humans are the only natural host.
• Enters through the conjunctiva or upper respiratory tract, multiplies locally for a few days, spreads in a primary viraemia to nodes, liver and spleen, then a second viraemia about a week later carries it to the skin.
• After the illness the virus stays latent in the dorsal root (sensory) ganglia; reactivation produces zoster in a dermatome.

Epidemiology

• Spread by droplets, air currents and direct contact with vesicle fluid; extremely contagious, with household secondary attack rates around 90%.
• Infectious period: from 1-2 days before the rash until all lesions have crusted (usually about 5-7 days after the rash starts). Crusts (scabs) are not infectious.
• Mostly a disease of children under 10 years; one attack usually gives lifelong immunity.
• Maternal antibody gives some protection for the first few months of life.
• Zoster can occasionally be the source of chickenpox in a non-immune child.

Clinical features

• Incubation: about 14-16 days (range 10-21).
• Prodrome: mild fever, malaise and poor appetite for about a day; in children the rash is often the first sign. Adults have a heavier prodrome.

The rash

Akbar describes the first lesions along the hairline before spreading to the trunk; Nelson and Park put the first crops on the trunk. All agree the rash is heaviest on the trunk.

• Lesion count is usually a few hundred.
• Second cases in a household are often more severe than the first.
• Fever settles by the end of the first week.

Differential diagnosis

Complications

Investigations

• The diagnosis is clinical.
• White count normal or low; leucocytosis suggests bacterial superinfection.
• When needed: PCR of vesicle fluid (method of choice), immunofluorescence for viral antigen, culture, or a fourfold antibody rise.
• IgG (by ELISA or FAMA) is used to check immunity in exposed high-risk people.
• Chest X-ray if pneumonia is suspected.

Management

Healthy child

• Hydration, nail trimming and skin hygiene to reduce scratching.
• Paracetamol for fever. Never aspirin (Reye syndrome).
• Calamine lotion and an oral antihistamine (for example diphenhydramine) for itch; cool baths.
• Antibiotics for secondary skin infection.
• Routine acyclovir is not needed in healthy children.

Who gets acyclovir

Oral acyclovir (Akbar: 20 mg/kg/dose, maximum 800 mg/dose, four times a day for 5 days) is used in those at higher risk of moderate or severe disease:

• Healthy, non-pregnant people older than 12 years
• Children over 12 months with chronic skin or lung disease
• Children on corticosteroids (short courses or inhaled)
• Children on long-term salicylates

Start it as early as possible, ideally within 24 hours of the rash; there is little benefit after 72 hours.

Intravenous acyclovir (Akbar: 500 mg/m2 every 8 hours, for 7-10 days or until no new lesions for 48 hours) is used for:

• Immunocompromised children (leukaemia, transplant, high-dose steroids)
• Severe disease: pneumonia, encephalitis, progressive varicella
• Neonatal varicella

Varicella in the newborn

Neonatal (perinatal) varicella

• Highest risk when the mother's rash appears from 5 days pre-delivery to 2 days post-delivery: the baby gets a large dose of virus across the placenta before maternal antibody has formed.
• The baby's rash appears in the first 2 weeks; mortality can be high.
• Give VZIG to the newborn as soon as possible (Akbar: half a vial under 2 kg, one vial over 2 kg); IVIG if VZIG is unavailable.
• If lesions appear, treat with IV acyclovir (Akbar: 10 mg/kg every 8 hours).
• Maternal zoster around delivery does not carry this risk, because the mother already has antibody.

Congenital varicella syndrome

• Follows maternal chickenpox in the first 20 weeks of pregnancy (Akbar: about 0.4% before 13 weeks and 2% between 13 and 20 weeks).
• Features: zigzag (cicatricial) skin scars in a dermatome, limb hypoplasia, eye defects (chorioretinitis, cataract, microphthalmia), brain damage (microcephaly, cortical atrophy, seizures), low birth weight, bladder and swallowing problems.
• Zoster in the first year of life without postnatal exposure suggests prenatal infection.

Prevention

• Exclusion: keep the child away from school until all vesicles have crusted; isolate in hospital.
• Varicella vaccine (live attenuated): Akbar recommends 2 doses for healthy children from 12 months, 3 months apart up to 12 years, 4 weeks apart from 13 years; Nelson gives doses at 12-15 months and 4-6 years. About 85% effective against any disease and over 95% against severe disease. It is not in the EPI schedule listed by Akbar; check the current schedule at EPI Pakistan (checked 8 Nov 2026).
• Not given in pregnancy or cellular immunodeficiency; if not given with MMR on the same day, separate them by at least 28 days.
• Post-exposure vaccine within 3-5 days can prevent or soften illness in healthy children.
• VZIG (Nelson: within 96 hours of exposure) for susceptible high-risk contacts: immunocompromised children, susceptible pregnant women, and newborns of mothers with peripartum varicella. VZIG may lengthen the incubation period up to 28 days.

Herpes zoster in children

• Reactivation of latent VZV; uncommon in children (about 5% of cases occur under 15 years); commoner in immunocompromised children.
• Painful vesicles in one or two adjacent dermatomes, usually thoracic or lumbar, on one side.
• Usually mild in children; postherpetic neuralgia is unusual. Acyclovir speeds healing.

Prognosis

• Usually self-limiting. Akbar gives mortality of about 1 in 50,000 in children and about 1 in 13,000 in infants.
• Much more dangerous in adults, newborns and the immunocompromised.

Quick revision

• VZV: DNA herpesvirus; latent in dorsal root ganglia; zoster on reactivation.
• Incubation about 2 weeks; infectious 1-2 days before rash until all crust.
• Crops, all stages together, trunk-heavy, itchy; fever with each crop.
• Commonest complication: skin sepsis (Staph, strep). Neuro: cerebellar ataxia.
• No aspirin: Reye syndrome.
• Acyclovir: over 12 years, chronic skin or lung disease, steroids, salicylates (oral); immunocompromised, severe or neonatal (IV).
• Peripartum maternal varicella (5 days before to 2 days after birth): VZIG to baby.
• Congenital varicella: dermatomal scars, limb hypoplasia, eye and brain defects.

Written by The House Job from standard references (Basis of Paediatrics by Pervez Akbar; Nelson Essentials of Paediatrics; Park's Preventive and Social Medicine).

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