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Introduction

Background

The varicella-zoster virus (VZV) is the etiologic agent of the clinical syndrome of chickenpox (varicella). Zoster, a different clinical entity, is caused by reactivation of VZV after primary infection. VZV is a double-stranded DNA virus included in the Alphaherpesvirinae subfamily. Chickenpox is largely a childhood disease, with more than 90% of cases occurring in children younger than 10 years. The disease is benign in the healthy child, whereas increased morbidity is seen in adults and in patients who are immunocompromised.

Pathophysiology

Chickenpox is usually acquired by the inhalation of airborne respiratory droplets from an infected host. The highly contagious nature of VZV explains the epidemics of chickenpox that spread through schools as one child who is infected quickly spreads the virus to many classmates. High viral titers are found in the characteristic vesicles of chickenpox; thus, viral transmission may also occur through direct contact with these vesicles, although the risk is lower.

After initial inhalation of contaminated respiratory droplets, the virus infects the conjunctivae or the mucosae of the upper respiratory tract. Viral proliferation occurs in regional lymph nodes of the upper respiratory tract 2-4 days after initial infection and is followed by primary viremia on postinfection days 4-6. A second round of viral replication occurs in the body's internal organs, most notably the liver and the spleen, followed by a secondary viremia 14-16 days postinfection. This secondary viremia is characterized by diffuse viral invasion of capillary endothelial cells and the epidermis. VZV infection of cells of the malpighian layer produces both intercellular edema and intracellular edema, resulting in the characteristic vesicle.

Exposure to VZV in a healthy child initiates the production of host immunoglobulin G (IgG), immunoglobulin M (IgM), and immunoglobulin A (IgA) antibodies; IgG antibodies persist for life and confer immunity. Cell-mediated immune responses are also important in limiting the scope and the duration of primary varicella infection. After primary infection, VZV is hypothesized to spread from mucosal and epidermal lesions to local sensory nerves. VZV then remains latent in the dorsal ganglion cells of the sensory nerves. Reactivation of VZV results in the clinically distinct syndrome of herpes zoster (shingles).

Frequency

United States

Chickenpox is a common disease, with most cases occurring in the pediatric population. Since the introduction of widespread pediatric immunization in the United States in 1995, the incidence of varicella has declined significantly, approaching up to 90% in one study. Mortality from varicella has also similarly decreased since the initiation of the US vaccination program, with the mortality rate decreasing by approximately 66% in one study.

International

Countries with tropical and semitropical climates have a higher incidence of adult chickenpox compared with countries with a temperate climate (eg, United States, Europe).

Mortality/Morbidity

Chickenpox affecting a healthy child is usually a self-limited disease. Secondary bacterial infection of skin lesions, manifesting as impetigo, cellulitis, or erysipelas, is the most common complication in this population. Staphylococci and streptococci are the most commonly implicated bacterial pathogens. Bacterial superinfection may predispose to scarring. Localized bacterial superinfection may rarely result in septicemia, culminating in a secondary bacterial pneumonia, otitis media, or necrotizing fasciitis (see Complications).

Congenital infection with the VZV virus is also a concern. Maternal chickenpox during pregnancy may produce latency of the VZV virus in the dorsal root ganglia of the fetus. These children may remain asymptomatic, or they may develop herpes zoster at a young age without previous history of primary chickenpox infection. Primary maternal chickenpox infection during the first 20 weeks of gestation may also rarely produce the congenital varicella syndrome, which is characterized by limb hypoplasia, muscular atrophy, skin scarring, cortical atrophy, microcephaly, cataract formation, and rudimentary digits (see Special Concerns).

Race

Varicella has no racial predilection.

Sex

Varicella has no sexual predilection.

Age

Chickenpox is predominantly a pediatric disease.

Clinical

History

Chickenpox is usually diagnosed clinically on the basis of the characteristic rash and successive crops of lesions. Lesions may be found in all stages of development and healing in affected sites. A history of exposure to an infected contact within the incubation period of 10-21 days is also an important clue in the diagnosis.

• Childhood chickenpox is usually not heralded by a prodrome, but rather it begins with the onset of an exanthem.
• Chickenpox in adults and adolescents may be preceded by a prodrome of nausea, myalgia, anorexia, and headache.
• The typical patient is infectious for 1-2 days prior to the development of rash and for 4-5 days afterwards, which is usually the time at which the last crop of vesicles has crusted over.
• The triad of rash, malaise, and low-grade fever signals the onset of chickenpox.
• Small, erythematous macules appear on the scalp, the face, the trunk, and the proximal limbs, with rapid sequential progression over 12-14 hours to papules, clear vesicles, and pustules, with subsequent central umbilication and crust formation.
• New crops of lesions form, which subsequently progress to vesicles with crusting.
• Vesicles may appear on the palms and the soles and on the mucous membranes, with painful, shallow, oropharyngeal or urogenital ulcers.
• Intense pruritus commonly accompanies the vesicular stage of the rash.

Physical

• The characteristic chickenpox vesicle, surrounded by an erythematous halo, is described as a dewdrop on a rose petal (see Media File 1).
• Chickenpox is clinically characterized by the presence of active and healing lesions, in all stages of development, within affected locations.
• Lesions characteristically heal without scarring, though excoriation or secondary bacterial superinfection predispose to scar formation.
• Adults with chickenpox have a more complicated course than that occurring in children. Adults may experience a more widespread rash; prolonged fever; and an increased likelihood of complications, the most common being varicella pneumonia.
• Clinical variants of chickenpox infection also occur.
  • Hemorrhagic lesions (see Complications) are rare and are most commonly associated with patients who are immunocompromised or immunosuppressed.
  • Bullous chickenpox is a rare variant in which bullae appear instead of the characteristic vesicles. The possibility of bullous impetigo from Staphylococcus aureus must be addressed, especially in a child with persistent fever or relapse after he or she appeared to be improving. Bullous chickenpox may affect both children and adults and must be differentiated from other bullous disorders (eg, bullous pemphigoid, pemphigus).
  • The course of the disease is believed to be unchanged, although a delay in diagnosis and treatment of elderly patients and patients who are immunocompromised may lead to serious morbidity.
• Chickenpox and other viral exanthems may appear concentrated in areas where intense sun exposure occurred during the incubation period. Patients with atopic dermatitis may show an atypical distribution of varicella, in which the characteristic eruption is primarily found on lichenified areas.

Causes

Chickenpox is usually acquired by the inhalation of airborne respiratory droplets from a VZV-infected host. High viral titers are found in the characteristic vesicles of chickenpox; thus, viral transmission may also occur through direct contact with these vesicles.

Source : http://emedicine.medscape.com/article/1131785-overview