Write a short note on varicella (chickenpox) and herpes zoster.

Aetiology

Both are caused by the varicella-zoster virus (VZV, human herpesvirus 3), a double-stranded DNA herpesvirus:

• Primary infection → varicella (chickenpox)
• The virus then establishes lifelong latency in the dorsal root and cranial nerve ganglia
• Reactivation → herpes zoster (shingles), typically decades later, when cell-mediated immunity wanes

Varicella (chickenpox)

Transmission:

• highly contagious, by respiratory droplets and direct contact with vesicle fluid.
• Infectivity begins 1–2 days before the rash appears and continues until all lesions have crusted — the pre-eruptive infectivity is why school and household outbreaks are so difficult to contain. Incubation is 10–21 days.

Clinical features:

• Prodrome (more marked in adults) — fever, malaise, headache, anorexia for 1–2 days.
• Rash — the characteristic features:
• Centripetal distribution — densest on the trunk and face, sparser on the limbs (the opposite of smallpox, which was centrifugal)
• Rapid evolution: macule → papule → vesicle ("dewdrop on a rose petal") → pustule → crust
• PLEOMORPHISM — lesions of ALL STAGES present simultaneously in the same area — this is the single most useful diagnostic sign
• Intense pruritus
• Mucosal involvement — oral, pharyngeal, conjunctival and genital lesions
• Lesions appear in successive crops over 3–5 days.
• Adults, pregnant women, neonates and the immunocompromised have far more severe disease.

Complications:

• Secondary bacterial infection of skin lesions — the commonest complication (*Staphylococcus aureus*, *Streptococcus pyogenes*), causing cellulitis, abscess and rarely necrotising fasciitis and toxic shock.
• Varicella pneumonia — the commonest serious complication in adults and pregnancy; presents with cough, dyspnoea and hypoxia 3–5 days into the illness, with diffuse nodular infiltrates; carries significant mortality. Smoking is a major risk factor.
• Neurological — acute cerebellar ataxia (particularly in children, usually with full recovery), encephalitis, transverse myelitis, Reye's syndrome (associated with aspirin use — which is therefore contraindicated in children with varicella).
• Haemorrhagic varicella, hepatitis, thrombocytopenia, glomerulonephritis.
• Congenital varicella syndrome — infection in the first 20 weeks of pregnancy causes limb hypoplasia, cicatricial skin scarring, eye and CNS defects.
• Neonatal varicella — maternal infection from 5 days before to 2 days after delivery causes severe neonatal disease with high mortality; requires varicella-zoster immunoglobulin and aciclovir.

Management:

• symptomatic in healthy children — antipyretics (paracetamol, NEVER aspirin), antihistamines and calamine for itch, nail trimming and hygiene to prevent secondary infection.
• Oral aciclovir for adolescents and adults, smokers, pregnancy, chronic skin or lung disease, and those on steroids — most effective when started within 24 hours of the rash.
• Intravenous aciclovir for the immunocompromised, varicella pneumonia, encephalitis and neonatal disease. Antibiotics for secondary bacterial infection.
• Isolate until all lesions have crusted.

Prevention:

• live attenuated varicella vaccine for non-immune children, adolescents, healthcare workers and women planning pregnancy (not during pregnancy)
• varicella-zoster immunoglobulin for susceptible high-risk contacts — neonates, pregnant women and the immunocompromised — given as soon as possible after exposure.

Herpes Zoster (shingles)

Pathogenesis: reactivation of latent VZV in a sensory ganglion, with spread down the sensory nerve to the skin, producing a dermatomal eruption. Risk rises with age (the dominant factor), immunosuppression, HIV, malignancy, corticosteroids, chemotherapy, stress and trauma.

Clinical features:

• Prodromal pain, burning or itching in the dermatome for 2–3 days BEFORE the rash — frequently misdiagnosed at this stage as angina, renal colic, cholecystitis or a disc lesion, depending on the site.
• Unilateral vesicular rash in a DERMATOMAL distribution that does NOT cross the midline — the defining feature. Most often thoracic, then ophthalmic division of the trigeminal nerve.
• Lesions are at the same stage of development within the dermatome (in contrast to the pleomorphism of varicella).
• Pain is often severe, and may be burning, lancinating or allodynic.
• Disseminated zoster (more than 20 lesions outside the primary dermatome) indicates significant immunosuppression and requires investigation, including HIV testing.

Important syndromes:

• Herpes zoster ophthalmicus — involvement of the ophthalmic division. Hutchinson's sign (vesicles on the tip or side of the nose, indicating nasociliary nerve involvement) predicts ocular involvement. Risks keratitis, uveitis, glaucoma and blindness — requires urgent ophthalmological assessment and systemic antivirals.
• Ramsay Hunt syndrome (herpes zoster oticus) — geniculate ganglion involvement causing facial palsy with vesicles in the external auditory canal and on the pinna, with ear pain, and often vertigo, tinnitus and hearing loss. It has a worse prognosis for facial nerve recovery than Bell's palsy and requires antivirals with steroids.
• Postherpetic neuralgia — the commonest and most disabling complication: pain persisting beyond 3 months after the rash. Risk rises sharply with age, severity of the acute pain and rash, and the presence of a prodrome. Managed with gabapentin, pregabalin, amitriptyline, topical lidocaine or capsaicin, and referral to a pain clinic for refractory cases.

Management:

• oral antivirals — aciclovir, valaciclovir or famciclovir — started within 72 hours of rash onset, which reduces the duration of the rash, the severity of acute pain and (less certainly) the risk of postherpetic neuralgia.
• Intravenous aciclovir for disseminated disease, ophthalmic involvement with ocular signs, CNS involvement and the immunocompromised. Add adequate analgesia from the outset (paracetamol, NSAIDs, opioids, and early gabapentinoids for neuropathic pain), keep lesions clean and covered, and treat secondary infection.

Prevention: the recombinant zoster vaccine for adults aged 50 and over, including the immunocompromised, substantially reduces both zoster and postherpetic neuralgia.