Definition and epidemiology
Chickenpox (varicella) is an acute, highly contagious viral exanthem caused by primary infection with varicella-zoster virus (VZV), a member of the herpesvirus family. The disease is characterized by a generalized pruritic vesicular rash accompanied by constitutional symptoms.
Before widespread vaccination, chickenpox was nearly universal, with over 90% of individuals infected by adolescence. The peak incidence occurred in children aged 1-9 years, with approximately 4 million cases annually in the United States. Following introduction of the varicella vaccine in 1995, incidence has decreased by more than 90% in vaccinated populations.
Chickenpox occurs worldwide with no significant gender predilection. In temperate climates, it exhibits seasonal variation with peak transmission in late winter and early spring. The disease is more common and severe in tropical regions. Unvaccinated adults, pregnant women, immunocompromised individuals, and neonates face higher risk of complications. Secondary attack rates among susceptible household contacts approach 90%, making it one of the most contagious infectious diseases.
- Chickenpox lesions in multiple stages of evolution simultaneously distinguish it from smallpox (synchronous lesions)
- Dewdrop on a rose petal appearance describes characteristic chickenpox vesicles
- Avoid aspirin in children with varicella due to Reye syndrome risk
- Acyclovir is indicated for immunocompromised patients, adults, pregnant women, and neonates
- Varicella vaccine given within 3-5 days of exposure can prevent or attenuate disease
- VariZIG is post-exposure prophylaxis for immunocompromised, pregnant women, and neonates unable to receive vaccine
- Neonatal varicella is severe when maternal infection occurs 5 days before to 2 days after delivery
- Varicella pneumonitis is the most common cause of death in adults with chickenpox
Etiology and risk factors
Chickenpox is caused by varicella-zoster virus (VZV), a double-stranded DNA virus in the Herpesviridae family (human herpesvirus 3). VZV is transmitted primarily through respiratory droplets from coughing or sneezing, and less commonly through direct contact with vesicular fluid. The virus can also spread via airborne particles, allowing transmission without direct contact.
Risk factors for infection:
- Lack of vaccination or prior infection
- Close contact with active cases (household, school, daycare)
- Immunocompromised state
- Age under 12 years (pre-vaccine era)
- Living in temperate climates during late winter/spring
Risk factors for severe disease:
- Immunosuppression: HIV/AIDS, chemotherapy, organ transplant recipients, chronic corticosteroid use
- Age: neonates, adolescents, and adults (>15 years)
- Pregnancy: especially third trimester
- Chronic skin conditions: eczema, recent sunburn
- Chronic lung disease: asthma, cystic fibrosis
- Salicylate use: increases Reye syndrome risk
The virus remains latent in dorsal root ganglia after primary infection and can reactivate years later as herpes zoster (shingles).
Pathophysiology
VZV enters the body through the respiratory mucosa or conjunctiva. Following initial replication in regional lymph nodes, a primary viremia occurs 4-6 days after exposure, disseminating the virus to the reticuloendothelial system. A secondary viremia follows 10-12 days post-exposure, spreading virus to the skin and mucous membranes.
The characteristic vesicular rash results from viral replication in the epidermis. Infected keratinocytes undergo ballooning degeneration and acantholysis, forming intraepidermal vesicles filled with clear fluid containing high concentrations of infectious virus. Multinucleated giant cells with intranuclear inclusions (Cowdry type A) are pathognomonic on Tzanck smear or histology.
Vesicles progress through predictable stages: macules (flat red spots) → papules (raised bumps) → vesicles ("dewdrop on rose petal" appearance) → pustules → crusts. New crops of lesions appear over 3-5 days, resulting in lesions at different stages of evolution present simultaneously—a key diagnostic feature distinguishing chickenpox from smallpox (which shows synchronous lesions).
The virus establishes latency in sensory ganglia during primary infection through retrograde axonal transport. Cell-mediated immunity is crucial for controlling infection and preventing reactivation. Humoral immunity provides long-lasting protection against reinfection in immunocompetent individuals. Immunocompromised patients may experience prolonged viral replication, delayed rash resolution, and increased risk of disseminated disease including pneumonitis, hepatitis, and encephalitis.
Clinical features
The incubation period ranges from 10-21 days (average 14-16 days). A brief prodrome may precede the rash by 1-2 days, featuring low-grade fever, malaise, anorexia, and headache. The prodrome is more common and pronounced in adolescents and adults.
Characteristic rash
The rash typically begins on the face, scalp, or trunk and spreads centripetally to the extremities (centripetal distribution). Lesions are more concentrated on the trunk than on the limbs. The rash evolves through distinct stages:
- Macules: erythematous, flat, 2-4 mm
- Papules: raised, appear within hours
- Vesicles: clear fluid-filled, described as "dewdrop on a rose petal," intensely pruritic
- Pustules: cloudy fluid, develop after 24-48 hours
- Crusts: form as vesicles rupture and dry, fall off in 1-2 weeks without scarring (unless secondarily infected)
Key features: Lesions in multiple stages of evolution simultaneously; typically 200-500 lesions in unvaccinated individuals; presence on scalp is characteristic; mucous membranes (mouth, conjunctiva, genital) may be involved.
Constitutional symptoms
- Fever: usually 38-39°C (100.4-102.2°F), lasts 3-5 days
- Pruritus: often severe, leading to scratching and potential secondary bacterial infection
- Malaise and irritability
- Decreased appetite
Modified disease in vaccinated individuals
"Breakthrough varicella" occurs in 15-25% of vaccinated persons exposed to VZV. Features include fewer lesions (<50), milder constitutional symptoms, predominantly maculopapular rash with fewer vesicles, and shorter duration of illness. These individuals are less contagious but can still transmit infection.
Severe disease manifestations
Adults, immunocompromised patients, and neonates may develop severe complications including pneumonia, hepatitis, thrombocytopenia, cerebellar ataxia, or encephalitis.
Diagnosis
Chickenpox is primarily a clinical diagnosis based on the characteristic rash and exposure history. Laboratory confirmation is recommended for atypical presentations, severe cases, immunocompromised patients, or public health surveillance.
Clinical diagnosis
Diagnostic criteria:
- Acute onset of generalized maculopapulovesicular rash
- Lesions in multiple stages of evolution present simultaneously
- History of exposure 10-21 days prior
- Presence of vesicles on the scalp
- Centripetal distribution (trunk > extremities)
Laboratory confirmation
PCR (polymerase chain reaction): Most sensitive and specific method. VZV DNA detected from vesicular fluid, crusts, or respiratory secretions. Preferred test for confirmation.
Direct fluorescent antibody (DFA) testing: Detects VZV antigen in vesicular fluid or scrapings. Rapid results but less sensitive than PCR.
Tzanck smear: Demonstrates multinucleated giant cells with intranuclear inclusions. Cannot distinguish VZV from herpes simplex virus. Largely replaced by PCR but useful when molecular testing unavailable.
Viral culture: Identifies VZV from vesicular fluid. Low sensitivity, slow turnaround (5-10 days), rarely used clinically.
Serology: IgM antibodies appear within 5 days of rash onset and persist for weeks; IgG antibodies indicate past infection or immunity. Serology is useful for immunity assessment but not acute diagnosis due to delayed antibody response.
Additional testing
For hospitalized or complicated cases:
- Complete blood count: may show leukopenia or thrombocytopenia
- Liver function tests: if hepatitis suspected
- Chest X-ray: if pneumonitis suspected (diffuse nodular infiltrates)
- Lumbar puncture: if meningoencephalitis suspected
Differential considerations
Laboratory testing helps distinguish chickenpox from disseminated herpes simplex, impetigo, hand-foot-and-mouth disease, or drug eruptions when presentation is atypical.
Differential diagnosis
Several conditions can mimic chickenpox, particularly in early stages or atypical presentations:
Disseminated herpes simplex virus (HSV)
May present with vesicular rash in immunocompromised patients or neonates. Lesions are typically grouped and confined to specific dermatomes initially. Tzanck smear shows similar findings to VZV but PCR or DFA distinguishes the two viruses.
Smallpox (variola)
Historically the most important differential. Smallpox features synchronous lesion evolution (all at same stage), centrifugal distribution (extremities > trunk), and deeper, firmer pustules. Eradicated since 1980 but remains a bioterrorism concern.
Hand-foot-and-mouth disease
Caused by enteroviruses (especially coxsackievirus A16, enterovirus 71). Presents with vesicles predominantly on palms, soles, and oral mucosa. Lesions are fewer, less pruritic, and buttocks may be involved.
Impetigo
Superficial bacterial skin infection (usually Staphylococcus aureus or Streptococcus pyogenes). Presents with honey-colored crusted lesions, typically around nose and mouth. Lacks vesicular stage and systemic symptoms.
Scabies
Caused by Sarcoptes scabiei mite. Presents with intense pruritus worse at night, linear burrows, and predilection for web spaces, wrists, and genitalia. Diagnosis confirmed by microscopic identification of mites, eggs, or feces.
Insect bites (papular urticaria)
Grouped pruritic papules and vesicles, often on exposed areas. History of outdoor exposure, lack of fever and constitutional symptoms.
Drug eruption
Maculopapular or vesiculobullous drug reactions can mimic chickenpox. History of recent medication initiation, absence of typical evolution stages.
Disseminated herpes zoster
Occurs in immunocompromised patients. Initial dermatomal distribution followed by dissemination. History of previous varicella, older age.
Other considerations
Dermatitis herpetiformis: chronic, intensely pruritic vesicles associated with celiac disease. Symmetrical distribution on elbows, knees, buttocks.
Rickettsialpox: caused by Rickettsia akari, presents with eschar at bite site followed by vesicular rash. Rare, associated with mouse mite exposure.
Management
Management focuses on supportive care for uncomplicated cases and antiviral therapy for high-risk patients. Isolation prevents transmission to susceptible individuals.
Supportive care
Symptomatic treatment:
- Pruritus control: antihistamines (diphenhydramine, cetirizine), calamine lotion, colloidal oatmeal baths, keep fingernails trimmed
- Fever management: acetaminophen (paracetamol). Avoid aspirin due to Reye syndrome risk, and avoid NSAIDs due to an increased risk of severe secondary bacterial skin infections (e.g., necrotizing fasciitis)
- Hydration: encourage oral fluids
- Skin care: keep skin clean, avoid scratching to prevent secondary bacterial infection
Antiviral therapy
Acyclovir is the primary antiviral agent. Benefits are greatest when started within 24 hours of rash onset.
Indications for antiviral therapy:
- Immunocompromised patients (initiate immediately)
- Adults and adolescents (≥13 years)
- Pregnant women (second or third trimester)
- Neonates
- Chronic cutaneous or pulmonary disorders
- Patients receiving chronic salicylate therapy
- Secondary household cases (often more severe)
Acyclovir reduces fever duration, decreases number of lesions, and accelerates healing. Oral administration is standard for uncomplicated cases in eligible patients. Intravenous acyclovir is reserved for severe disease, immunocompromised hosts, or complications (pneumonitis, encephalitis, disseminated disease).
Isolation and infection control
- Isolation period: from 1-2 days before rash onset until all lesions are crusted (typically 5-7 days)
- Airborne and contact precautions: in healthcare settings
- School/daycare exclusion: until all vesicles have crusted
- Avoid contact with high-risk individuals: pregnant women, immunocompromised, unvaccinated infants
Post-exposure prophylaxis
Varicella vaccine: effective if given within 3-5 days of exposure in healthy, unvaccinated individuals without contraindications.
Varicella-zoster immune globulin (VariZIG): passive immunization for high-risk individuals unable to receive vaccine (immunocompromised, pregnant women, neonates). Most effective when given within 96 hours (up to 10 days) of exposure.
Prophylactic acyclovir: alternative for high-risk exposed individuals when VariZIG unavailable. Started 7-10 days post-exposure and continued for 7 days.
Management of complications
Secondary bacterial infection: topical or systemic antibiotics targeting Staphylococcus aureus and Streptococcus pyogenes.
Varicella pneumonitis: intravenous acyclovir, oxygen supplementation, possible ICU admission.
Encephalitis: intravenous acyclovir, supportive care.
Hemorrhagic varicella: intravenous acyclovir, VariZIG if available, supportive care in ICU setting.
Prevention
Vaccination is the primary preventive strategy. Two-dose varicella vaccine schedule (first dose at 12-15 months, second at 4-6 years) provides approximately 90% protection against infection and greater than 95% protection against severe disease. Combination MMRV vaccine is available.
Complications and prognosis
Chickenpox is typically self-limited in healthy children but can cause significant morbidity and mortality in high-risk populations.
Complications
Secondary bacterial infection (most common): Impetigo, cellulitis, necrotizing fasciitis, and toxic shock syndrome. Staphylococcus aureus and Streptococcus pyogenes are common pathogens. Risk increases with scratching and poor hygiene.
Pneumonitis: Occurs in 15-20% of adults with varicella, rarely in children. Presents with cough, dyspnea, and hypoxemia 2-6 days after rash onset. Chest X-ray shows diffuse nodular infiltrates. Mortality rate is 10-30% in untreated cases.
Neurological complications:
- Cerebellar ataxia: most common CNS complication in children, usually self-limited, occurs during or shortly after rash
- Encephalitis: rare (1-2 per 10,000 cases), presents with altered mental status, seizures; mortality 5-20%
- Transverse myelitis, Guillain-Barré syndrome, optic neuritis: rare
Reye syndrome: Acute hepatic encephalopathy associated with aspirin use during varicella or influenza. Characterized by vomiting, altered consciousness, hepatomegaly. Now rare due to avoidance of aspirin in children.
Hemorrhagic varicella: Severe complication in immunocompromised patients with bleeding into vesicles, mucous membranes, and internal organs. High mortality.
Hepatitis: Usually subclinical transaminase elevation; overt hepatitis is uncommon.
Thrombocytopenia: Mild, usually asymptomatic; rarely causes bleeding.
Congenital varicella syndrome: Occurs when maternal infection happens at 8-20 weeks gestation. Features include limb hypoplasia, cutaneous scarring, chorioretinitis, microcephaly, and developmental delay. Risk is approximately 2% during this window.
Neonatal varicella: Severe, potentially fatal infection in newborns whose mothers develop chickenpox 5 days before to 2 days after delivery. Mortality 20-30% without treatment.
Herpes zoster (shingles): VZV remains latent in dorsal root ganglia and reactivates in approximately 15-20% of individuals, usually decades later. Risk increases with age and immunosuppression.
Prognosis
Healthy children: Excellent prognosis with full recovery in 7-10 days. Mortality is rare (<2 per 100,000 cases). Immunity after natural infection is typically lifelong, though breakthrough cases and reinfection occur rarely.
Adults: Mortality rate is 15-20 times higher than in children. Pneumonitis is the leading cause of death.
Immunocompromised patients: Significant risk of disseminated disease, organ involvement, and death. Prompt antiviral therapy is essential.
Pregnant women: Increased risk of severe pneumonitis, especially in third trimester. Maternal mortality from varicella pneumonitis can reach 40% without treatment.
Vaccination impact: In vaccinated populations, hospitalization and death rates have declined by over 90%. Breakthrough varicella is typically mild with excellent outcomes.
VARICELLA: Vesicles Appearing Rapidly In Crops, Evolving Like Layers, Always pruritic
Practice questions
1. A 6-year-old boy is brought to the clinic with a 2-day history of rash and low-grade fever. He has no significant past medical history and has not received the varicella vaccine. On examination, you note approximately 100 lesions on his face, trunk, and proximal extremities. The lesions include erythematous macules, papules, vesicles with clear fluid, and some crusted lesions. Several vesicles are also noted on his scalp. He is mildly uncomfortable but playful. Vital signs are: temperature 37.8°C (100°F), heart rate 90/min, blood pressure 95/60 mmHg. What is the most appropriate next step in management?
2. A 28-year-old pregnant woman at 24 weeks gestation presents with a 1-day history of fever and pruritic rash. Her 4-year-old son was diagnosed with chickenpox 2 weeks ago. She never had chickenpox as a child and did not receive the varicella vaccine. On examination, she has approximately 50 vesicular lesions on her trunk and face, with some early crusting. Temperature is 38.5°C (101.3°F). She reports mild cough and dyspnea. Pulse oximetry shows 94% on room air. Which of the following is the most appropriate management?
3. A 35-year-old man with HIV infection (CD4 count 150 cells/μL) presents with a 5-day history of worsening rash. He reports exposure to his niece with chickenpox 3 weeks ago. On examination, he has over 500 vesicular and hemorrhagic lesions covering his entire body, including palms and soles. Some lesions appear necrotic. Temperature is 39.2°C (102.6°F), blood pressure 100/65 mmHg, heart rate 110/min. Laboratory results show hemoglobin 10 g/dL, platelets 80,000/μL, ALT 250 U/L, AST 280 U/L. What is the most appropriate immediate management?
Frequently asked questions
What causes chickenpox?
Chickenpox is caused by varicella-zoster virus (VZV), a highly contagious herpesvirus. The virus spreads through respiratory droplets from coughing or sneezing and direct contact with fluid from skin lesions. It can also transmit via airborne particles, allowing spread without direct contact. After primary infection, VZV remains dormant in nerve ganglia and can reactivate years later as shingles.
What are the first symptoms of chickenpox?
The first symptoms are typically low-grade fever, malaise, headache, and loss of appetite 1-2 days before the rash appears. The characteristic rash then begins as small red spots on the face, scalp, or trunk that rapidly progress to fluid-filled blisters (vesicles) within hours. These vesicles are intensely itchy and appear in successive crops over 3-5 days, resulting in lesions at different stages present at the same time.
How is chickenpox diagnosed?
Chickenpox is primarily diagnosed clinically based on the characteristic pruritic vesicular rash with lesions in multiple stages of evolution and a history of exposure. The presence of vesicles on the scalp and centripetal distribution (more on trunk than limbs) are key features. Laboratory confirmation using PCR testing of vesicular fluid is recommended for atypical cases, immunocompromised patients, or severe disease. Serology can assess immunity but is not useful for acute diagnosis.
Is chickenpox curable?
Chickenpox is a self-limited viral infection that resolves on its own in healthy children within 7-10 days without specific treatment. While the infection cannot be "cured" in the sense of eliminating the virus (which remains dormant in nerve cells), antiviral medication like acyclovir can reduce symptom severity and duration when started early. Treatment focuses on symptom relief (antihistamines for itching, acetaminophen for fever) and preventing complications. Vaccination provides the best prevention.
Chickenpox vs. shingles — what is the difference?
Chickenpox is the primary infection with varicella-zoster virus, causing widespread vesicular rash all over the body, mainly affecting children. Shingles (herpes zoster) is reactivation of dormant VZV from nerve ganglia, typically occurring decades later in adults. Shingles presents as a painful, blistering rash limited to one or a few adjacent dermatomes (skin areas supplied by specific nerves), usually on one side of the body. Chickenpox is highly contagious from person to person, while shingles spreads only through direct contact with active lesions and can cause chickenpox in susceptible individuals.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: B01
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.