Varicella-Zoster — Chickenpox and Shingles

• Viral entry and primary infection cascade: VZV enters through respiratory epithelium, with initial replication in regional lymph nodes followed by viremia (typically day 4–6). The virus disseminates to skin via infected mononuclear cells; the characteristic centripetal rash distribution (trunk > face > extremities) reflects hematogenous seeding of dermal and epidermal vasculature. VZV glycoproteins (gE, gI, gC) facilitate cell-to-cell spread and immune evasion by blocking MHC and complement molecules. The temporal progression from macule → papule → vesicle → pustule → crust reflects progressive viral cytolysis of keratinocytes with associated inflammation. Peak viremia correlates with maximum rash extent (day 7–10 of illness).

• Latency establishment and maintenance: Following primary viremia control by adaptive immunity (IgG antibodies and VZV-specific CD8+ T cells), virus establishes lifelong latency in dorsal root and cranial sensory ganglia. Latent VZV persists in approximately 1–2 neurons per ganglion in an episomal state; transcription of latency-associated transcripts (LATs) maintains viral genome stability while suppressing lytic gene expression through microRNA-mediated mechanisms. Neuronal VZV DNA remains indefinitely, protected from immune surveillance by the blood-brain barrier and neuronal immune privilege.

• Reactivation mechanism and dermatomal distribution: Reactivation occurs when local or systemic immunosuppression, nerve trauma, inflammatory cytokines (TNF-α, IL-6), or somatic mutations in latent neurons trigger switch from latency to productive infection. The virus replicates within the ganglion, causing inflammation and neuronal destruction, then travels via retrograde axonal transport along sensory nerve fibers to the corresponding dermatome. The dermatomal restriction is the pathognomonic feature—only the skin supplied by the affected ganglion exhibits the rash, explaining the unilateral, band-like distribution. Reactivation with increasing age is attributed to declining VZV-specific cell-mediated immunity; each decade after age 50, the risk increases approximately 10-fold. In immunocompromised patients (HIV/AIDS, organ transplant, chemotherapy), multiple dermatomal involvement and disseminated cutaneous or visceral disease occurs as immune surveillance fails.

• Post-herpetic neuralgia (PHN) pathophysiology: Chronic neuropathic pain results from several mechanisms: (1) viral-induced apoptosis of dorsal root ganglion neurons causing permanent sensory loss mixed with paradoxical pain; (2) persistent inflammation and glial activation (microglial and astrocytic) releasing pro-inflammatory cytokines; (3) central sensitization with amplified pain signaling; (4) spontaneous ectopic activity in damaged sensory nerves. Early and aggressive antiviral therapy may reduce PHN incidence by preventing extensive neuronal necrosis, while prompt initiation of neuropathic pain medications (gabapentin, pregabalin) addresses the underlying neurophysiology.

• Immune evasion strategies: VZV produces immunoregulatory proteins including ORF61 (blocks interferon signaling), ORF63 (interferes with IRF-3), and gE/gI (prevent complement deposition and antibody binding). These mechanisms explain delayed innate immunity in primary infection and incomplete immune control in latency, allowing viral persistence. Reactivation is therefore facilitated not only by immunosuppression but also by active viral immune evasion.

• Primary VZV infection (chickenpox): Transmitted via respiratory droplets (highly contagious; R₀ approximately 10–12 in unvaccinated populations) or direct contact with vesicular fluid. Infectivity begins 2 days before rash onset and continues until all lesions crust (approximately 7–10 days). Seasonal clustering occurs in temperate climates (winter–spring), reflecting respiratory transmission patterns. Introduction to a susceptible household member results in secondary cases 10–21 days after the index case.

• Herpes zoster (shingles) reactivation: Reactivation risk increases with advancing age (>50 years), with annual incidence of 3–4 per 1,000 in the general population, rising to 10–11 per 1,000 after age 80. Major immunosuppression is a primary reactivation driver: HIV/AIDS (especially CD4 <200 cells/μL), solid organ transplantation (15–20% cumulative incidence), hematopoietic stem cell transplantation (40% within first 5 years), and active chemotherapy. Localized immunosuppression (topical or intralesional corticosteroids, rarely phototherapy) can trigger dermatomal reactivation. Malignancy—particularly lymphoma—substantially increases risk independent of chemotherapy effects.

• Mechanical/inflammatory triggers: Trauma, radiation therapy, or surgery to a dermatome can precipitate zoster within weeks; this somatic trigger hypothesis is supported by cases of zoster following thoracic surgery or herpes zoster ophthalmicus after ophthalmologic procedures. Spinal cord trauma and demyelinating diseases (multiple sclerosis) increase reactivation risk through localized inflammation.

• Medication-related factors: Systemic corticosteroids (controversial; large doses >20 mg prednisone daily may slightly increase risk), TNF-α inhibitors (2–3 fold increased incidence), and other biologic immunomodulators. Varicella vaccine strain reactivation is rare but documented in vaccinated individuals, though the disease is typically milder.

• Demographic and genetic factors: No significant genetic predisposition identified for reactivation. Previous varicella infection is required (universal exposure without vaccination in pre-vaccine era populations); seronegative individuals cannot develop zoster. Race/ethnicity associations are primarily epidemiological (healthcare access, vaccination rates) rather than biologically intrinsic.

Chickenpox (Varicella)

• Prodromal phase: Fever, malaise, headache, and myalgias begin 1–2 days before rash (absent in young children, more prominent in adolescents/adults). The virus is replicating systemically during this presymptomatic viremic phase, making patients highly infectious before the rash appears—critical for isolation timing.

• Centripetal rash evolution (classic cardinal feature): The exanthem appears in successive crops over 3–4 days, creating the pathognomonic "crops at different stages of development." Initial presentation is erythematous macules (blanching) that rapidly progresses to papules within hours. Vesicles form on an erythematous base ("dew drops on a rose petal"—classic board description), typically 2–3 mm in diameter, extremely pruritic. Over 3–4 days, vesicles become cloudy/pustular, then crust over; crusts typically shed by day 14–21. The rash concentrates on the trunk (50% of lesions) and face; extremities and mucous membranes (buccal mucosa, palate, conjunctiva) are also affected. Lesions are superficial (not ulcerating) and heal without scarring unless secondarily infected.

• Systemic symptoms: Fever typically peaks during the rash phase (rarely exceeding 39–40°C [102–104°F]) and resolves by day 7–10. Pruritus is often intense and may interfere with sleep. Mild lymphadenopathy (cervical, axillary) is common.

• Physical examination: The rash distribution is centripetal with relative sparing of palms/soles. Oral mucosa may show shallow ulcers with surrounding erythema (enanthem). Conjunctival involvement presents as subconjunctival vesicles. Examination should assess for secondary bacterial infection (purulent drainage, surrounding cellulitis).

Herpes Zoster (Shingles)

• Prodromal pain and dysesthesia (precedes rash by 2–7 days in >50% of cases): Unilateral burning, sharp, or stabbing pain, often initially mistaken for cardiac, pulmonary, or abdominal pathology depending on dermatome—a common diagnostic trap. Dysesthesia, hyperesthesia, and paresthesias in the dermatomal distribution are highly suggestive. Fever and systemic malaise are less prominent than chickenpox; constitutional symptoms are mild unless disseminated disease develops.

• Dermatomal rash pattern (most critical distinguishing feature): Unlike chickenpox's scattered distribution, zoster presents as a unilateral, band-like eruption confined to one (occasionally 2–3) contiguous dermatome(s). Thoracic dermatomes are most common (50–60% of cases), followed by trigeminal (V1 ophthalmic, V2 maxillary, V3 mandibular—15–20%), and cervical/lumbar. The rash rapidly progresses from erythematous macules → papules → vesicles (typically larger and more grouped than chickenpox, forming clusters) → pustules → crusting over 7–10 days. Lesions remain painful throughout evolution. The rash duration is typically 2–4 weeks before complete resolution.

• Herpes zoster ophthalmicus (V1 distribution, 10–25% of zoster cases): Involves forehead, upper eyelid, and anterior scalp; may include conjunctivitis, keratitis, and iritis. Risk of postherpetic neuralgia affecting the eye, vision loss from keratitis or scleritis, and secondary bacterial infection are substantial. Hutchinson sign (vesicles on the tip of the nose indicating nasociliary branch involvement) predicts higher risk of ocular involvement.

• Herpes zoster oticus (Ramsay Hunt syndrome): VZV reactivation in the geniculate ganglion (CN VII) causing vesicles in the external auditory canal and auricle, facial paralysis, and often sensorineural hearing loss and vertigo. This is a medical emergency as early antiviral therapy improves facial nerve outcomes.

• Physical examination findings: Unilateral, dermatomal distribution is pathognomonic. Lesions are grouped and closely approximated. Regional lymphadenopathy (in draining nodes) is common. The rash does not cross the midline (except rarely with disseminated disease). Examination should note the specific dermatome(s) affected and screen for complications (eye involvement, neurologic deficits).

Atypical/Severe Presentations

• Disseminated zoster: >20 vesicles outside the primary and adjacent dermatomes; defines severe disease in immunocompromised patients (HIV CD4 <50, transplant recipients). Represents failure of local viral containment; risk of disseminated visceral disease (hepatitis, pneumonitis, encephalitis) is significant.

• Zoster sine herpete ("zoster without herpes"): Dermatomal pain and hyperesthesia without visible rash; confirmed by detection of VZV DNA/RNA in cerebrospinal fluid or by high PCR/IgM serology. Misdiagnosed as other neuropathies or acute coronary syndrome if thoracic.

• VZV meningitis/encephalitis: Aseptic meningitis (pleocytosis with lymphocytic predominance, normal glucose) or focal encephalitis, particularly in immunocompromised hosts. Can occur with or without cutaneous zoster.

• Clinical diagnosis: For typical varicella (childhood chickenpox), clinical presentation is sufficient: fever + centripetal vesicular rash in crops at different stages + clear prodrome. For typical zoster, unilateral dermatomal rash with prodromal pain is nearly pathognomonic. However, atypical presentations, immunocompromised hosts, or disseminated disease warrant laboratory confirmation.

• Polymerase chain reaction (PCR): Gold standard for diagnosis with sensitivity >95% and specificity >99%. VZV PCR can be performed on vesicular fluid, crusts, or swabs from the rash; also detects VZV in CSF (meningitis), blood (disseminated disease), or bronchoscopy specimens (pneumonitis). PCR is superior to viral culture (slow, insensitive, requires intact lesion). Real-time quantitative PCR can assess viral load, useful in transplant/HIV patients monitoring response to therapy.

• Viral culture: Traditional gold standard but slower (3–7 days) and requires fresh vesicular fluid; sensitivity decreases as lesions progress. Mainly replaced by PCR in modern practice but may still be done if PCR unavailable.

• Tzanck smear: Rapid bedside test using Wright or Giemsa stain of material from the base of a freshly unroofed vesicle; identifies multinucleated giant cells and acantholytic cells consistent with herpesvirus infection. Sensitivity approximately 50–80%; cannot differentiate VZV from HSV. Limited utility given PCR availability but can provide same-day presumptive diagnosis in resource-limited settings.

• Serologic testing: VZV-specific IgG indicates past infection (immunity); IgM suggests acute infection but is less specific. IgG avidity testing can estimate timing of infection (low avidity in early acute infection, high avidity in chronic/latent infection). Serology is not useful for acute diagnosis (IgG develops over 7–10 days) but confirms immunity status for vaccination counseling.

• Immunohistochemistry/immunofluorescence: Fluorescent antibody staining of lesion exudate or tissue can identify VZV antigen; rapid (hours) but less sensitive than PCR. Used mainly in research or when PCR unavailable.

• Direct fluorescent antibody (DFA): Can be performed on scrapings or swabs; sensitivity 80–90% but requires experienced personnel. Less specific than PCR.

• Imaging findings: Chest X-ray in suspected varicella pneumonitis shows bilateral interstitial or nodular infiltrates, typically appearing 3–7 days after rash onset. MRI of thoracic spine in suspected myelitis shows cord signal abnormality. Brain MRI in encephalitis may show temporal lobe involvement (mimicking HSV) or more diffuse changes.

• Differential diagnosis considerations:
• HSV-1/HSV-2: Unilateral herpetic lesions can mimic zoster; however, HSV typically recurs at the same site (labial, genital) rather than migrating dermatomes. PCR or HSV serology differentiate.
• Hand-foot-mouth disease: Enteroviral; lesions on palms/soles, oral ulcers, typically in children; milder systemic illness.
• Secondary bacterial infection: Purulent lesions may mimic impetigo; culture lesion fluid and surrounding skin for bacteria.
• Drug reaction: Some medications (allopurinol, anticonvulsants) cause vesicular rashes; temporal correlation with drug exposure and absence of dermatomal pattern helps differentiate.
• Erythema multiforme/Stevens-Johnson syndrome: Targetoid lesions, mucosal involvement more prominent; VZV PCR negative.

Chickenpox (Varicella)

• First-line antiviral: Acyclovir (or valacyclovir/famciclovir): Acyclovir is a nucleoside analog that, after phosphorylation by viral thymidine kinase, inhibits VZV DNA polymerase. Dosing: Oral acyclovir 20 mg/kg (max 800 mg) four times daily for 5 days, initiated ideally within 24 hours of rash onset (reduces duration by 1–2 days and severity of dissemination risk). IV acyclovir 10–15 mg/kg every 8 hours is used for severe disease, immunocompromised hosts, or disseminated varicella (meningitis, pneumonitis). Valacyclovir (1,000 mg three times daily for 5 days) has better

Chickenpox complications

• Bacterial superinfection: Scratching pruritic vesicles inoculates Streptococcus pyogenes or S. aureus; the tell is a lesion that becomes purulent, painful, and surrounded by expanding erythema. Rapidly progressive pain out of proportion, bullae, or crepitus signals necrotizing fasciitis or streptococcal toxic shock — a surgical emergency requiring debridement plus antistreptococcal therapy.
• Varicella pneumonia: Hematogenous seeding of alveolar capillaries; disproportionately affects adults, smokers, and pregnant patients. Cough, dyspnea, and hypoxemia days after the rash with diffuse nodular/interstitial infiltrates. Emergency — treat with IV acyclovir.
• Acute cerebellar ataxia: Post-infectious, immune-mediated; a child with wide-based gait, dysmetria, and nystagmus days after the rash. Self-limited, benign — contrast with encephalitis (altered mental status, seizures), which is an emergency.
• Reye syndrome: Aspirin-associated mitochondrial injury — vomiting, encephalopathy, transaminitis with normal bilirubin. Avoid salicylates in varicella (AAP Red Book).
• Congenital and neonatal varicella: Maternal infection in the first two trimesters can cause limb hypoplasia, cicatricial skin scarring in a dermatomal pattern, chorioretinitis, and cortical atrophy. Maternal rash shortly before or after delivery yields neonatal varicella, high mortality; ACOG/CDC advise VariZIG for the newborn.

Zoster complications

• Postherpetic neuralgia: Most common; dermatomal burning/allodynia persisting after crusting, risk rising steeply with age.
• Herpes zoster ophthalmicus: V1 involvement with Hutchinson sign → keratitis, uveitis, glaucoma, vision loss. Same-day ophthalmology referral (American Academy of Ophthalmology).
• Ramsay Hunt syndrome, VZV vasculopathy, myelitis, meningoencephalitis: Granulomatous arteritis after ophthalmic zoster can cause contralateral hemiplegic stroke weeks later. Acute retinal necrosis / progressive outer retinal necrosis in advanced HIV threatens rapid blindness — emergencies.
• Disseminated zoster: >20 extradermatomal vesicles in immunocompromised hosts with visceral hepatitis/pneumonitis; requires IV acyclovir and airborne plus contact isolation.

Treatment-related

• Acyclovir crystal nephropathy and neurotoxicity: Tubular crystal deposition with rapid IV infusion or volume depletion (rising creatinine); confusion and myoclonus in renal impairment. Hydrate and dose-adjust.
• Foscarnet (for thymidine kinase–deficient, acyclovir-resistant VZV): nephrotoxicity and electrolyte wasting — hypocalcemia, hypomagnesemia, seizures.
• Vaccines: Live varicella vaccine is contraindicated in pregnancy and severe immunocompromise; recombinant zoster vaccine is markedly reactogenic (CDC/ACIP).

• **"Dew drops on a rose petal" plus lesions in different stages simultaneously**: The single best varicella buzzword pair. The classic distractor is smallpox, in which lesions are all in the same stage and distributed centrifugally (face and extremities, including palms and soles) rather than centripetally.
• Best next step for atypical or immunocompromised presentations is VZV PCR of vesicle fluid or a swab of an unroofed lesion base — highest sensitivity and specificity. The Tzanck smear showing multinucleated giant cells is the trap: it cannot separate VZV from HSV, and serology cannot diagnose acute disease.
• Antivirals for zoster are most effective started within 72 hours of rash onset; beyond that window, treat anyway if new lesions are still forming or if the patient is immunocompromised, has ophthalmic involvement, or has disseminated disease.
• **Vesicles on the nose tip (Hutchinson sign) = nasociliary involvement = urgent ophthalmology.** Vesicles in the ear canal with facial palsy and vertigo = Ramsay Hunt syndrome; early antiviral plus corticosteroid improves facial nerve recovery.
• Corticosteroids do not reliably prevent postherpetic neuralgia. First-line PHN therapy is a gabapentinoid (gabapentin or pregabalin), with tricyclics (nortriptyline) or topical lidocaine as alternatives; opioids are not first line.
• Zoster in a patient under 50 with no obvious cause should prompt HIV testing — the association examiners test most often. Multidermatomal or disseminated zoster implies profound cellular immunodeficiency.
• Prevention (CDC/ACIP): Two doses of the recombinant zoster vaccine for all adults ≥50, and for immunocompromised adults ≥19 — given even after prior zoster or prior live vaccine. The live varicella vaccine is contraindicated in pregnancy and severe immunosuppression; postpartum vaccination is the answer for a seronegative pregnant patient.
• Post-exposure: Susceptible high-risk contacts (neonates of peripartum-infected mothers, pregnant patients, immunocompromised hosts) get VariZIG; healthy susceptible contacts get varicella vaccine promptly. Hospitalized varicella and disseminated zoster require airborne plus contact precautions; localized zoster in an immunocompetent host needs only lesion covering with contact precautions.
• Never give aspirin — Reye syndrome.