Varicella

Primary infection with varicella-zoster virus (VZV, human herpesvirus 3), highly contagious, transmitted via respiratory droplets and direct contact with vesicular fluid.

Following primary infection, virus establishes lifelong latency in dorsal root/cranial nerve ganglia, with potential later reactivation as herpes zoster (shingles).

Incubation period: 10-21 days (typically 14-16 days).

Infectious period: 1-2 days before rash onset until all lesions have crusted over (typically 5-7 days after rash onset).

• Prodrome (more common in adults, often absent/mild in children): fever, malaise, headache, anorexia, 1-2 days before rash

• Characteristic rash: evolves in crops over several days, macules → papules → vesicles ("dewdrop on a rose petal" appearance) → pustules → crusts

• Key diagnostic feature: lesions present in multiple stages simultaneously (unlike smallpox, historically, where lesions were synchronous), centripetal distribution (trunk/face predominant, then spreading to extremities), can involve scalp and mucous membranes (oral, genital)

• Intensely pruritic

• Generally more severe in adults, immunocompromised, and pregnant women than in healthy children

• Complications (more common in adults, infants, immunocompromised, pregnant women):

• Secondary bacterial skin infection (S. aureus, S. pyogenes: including risk of invasive group A strep/necrotizing fasciitis)

• Varicella pneumonia: significant risk in adults (especially smokers) and pregnancy, can be severe/fatal

• Encephalitis/cerebellar ataxia (more common in children, generally good prognosis for cerebellar ataxia; encephalitis more serious)

• Hepatitis (usually mild/subclinical)

• Reye's syndrome: risk if aspirin given during varicella infection in children, AVOID aspirin

• Congenital varicella syndrome: if maternal infection <20 weeks gestation, limb hypoplasia, skin scarring, eye abnormalities, neurological damage

• Neonatal varicella: if maternal infection occurs 5 days before to 2 days after delivery, high risk of severe/disseminated neonatal disease (insufficient time for maternal antibody transfer)

Primarily clinical based on characteristic rash and exposure history.

Confirmatory testing (if diagnostic uncertainty, immunocompromised, pregnancy, or atypical presentation):

• PCR of vesicular fluid: most sensitive/specific

• Direct immunofluorescence of vesicle scraping

• Viral culture (less commonly used now, slower)

• Serology (VZV IgM/IgG): useful for confirming immunity status (e.g., pregnant women with uncertain history/exposure) rather than acute diagnosis

Differentials: hand-foot-mouth disease (Coxsackievirus, more localized distribution), disseminated herpes zoster/simplex, impetigo, insect bites, drug eruption, DRESS syndrome (if severe/atypical), monkeypox/mpox (consider in relevant epidemiological context, lesions more uniform in stage and deeper-seated than varicella).

Uncomplicated varicella in healthy children:

• Supportive care: antipyretics (paracetamol, AVOID aspirin due to Reye's syndrome risk), adequate hydration, oral antihistamines for pruritus, calamine lotion, keep nails short/cool baths to reduce scratching (secondary infection prevention)

• Antivirals not routinely needed in healthy children <12 years with uncomplicated disease

• School/nursery exclusion until all lesions crusted (typically 5 days from rash onset)

Antiviral therapy, indicated for higher-risk groups, ideally started within 24 hours of rash onset:

• Healthy adults/adolescents ≥12 years (higher complication risk than young children): aciclovir 800mg 5x/day for 7 days, or valaciclovir 1g tds

• Immunocompromised patients: IV aciclovir 10mg/kg 8-hourly, higher risk of disseminated/severe disease

• Pregnant women with varicella (particularly if presenting within 24 hours of rash, especially in later pregnancy or with pneumonia risk): oral aciclovir if >20 weeks gestation and presenting within 24h; IV aciclovir for varicella pneumonia or severe disease at any gestation

• Neonates with perinatal exposure (maternal infection 5 days before to 2 days after delivery): varicella-zoster immunoglobulin (VZIG) and/or aciclovir, close monitoring

Post-exposure prophylaxis (non-immune, exposed individuals):

• VZIG for high-risk exposed individuals without immunity: pregnant women (particularly if exposure and no clear history of prior infection/vaccination), immunocompromised, neonates (perinatal exposure), given within 10 days of exposure (efficacy highest earlier)

• Varicella vaccine (live-attenuated) can be used for post-exposure prophylaxis in immunocompetent, non-pregnant contacts if given within 3-5 days of exposure

Prevention:

• Varicella vaccination (live-attenuated): part of routine childhood immunization schedule in many countries (typically 2-dose schedule), significantly reduces incidence and severity

• Contraindicated in pregnancy and significant immunosuppression (live vaccine)

• Isolation of infected individuals from susceptible high-risk contacts (pregnant, immunocompromised, neonates) during infectious period

Complications management: bacterial superinfection, appropriate antibiotics covering S. aureus/S. pyogenes; varicella pneumonia, IV aciclovir, respiratory support as needed, low threshold for hospital admission in adults with respiratory symptoms; encephalitis, supportive care, IV aciclovir, neurology involvement.