CHICKENPOX (Merck Manual)

An acute viral infection, usually beginning with mild constitutional symptoms that are followed shortly by an eruption appearing in crops and characterized by macules, papules, vesicles, and crusting.

Etiology and Epidemiology

Chickenpox and herpes zoster are caused by the varicella-zoster virus, chickenpox being the acute invasive phase of the virus and herpes zoster (shingles) representing reactivation of the latent phase.

Chickenpox, which is extremely contagious, is believed to be spread by infected droplets and is most communicable during the short prodrome and early stages of the eruption. The usual incubation period is 14 to 16 days, and communicability is considered possible from 10 to 21 days after exposure. When the final lesions have crusted, the patient can no longer transmit the disease. Isolation for 6 days after the first vesicles appear is usually sufficient to control cross-infection. Indirect transmission (by immune third persons) does not occur.

Epidemics occur in winter and early spring in 3- to 4-yr cycles (the period required to develop a new group of susceptibles). Susceptibility is high from birth until the disease is contracted, but some infants may have partial immunity, probably acquired transplacentally, until age 6 mo.

Symptoms and Signs

Mild headache, moderate fever, and malaise may occur 11 to 15 days after exposure, about 24 to 36 h before the first series of lesions appears. The prodrome, usually unrecognized in young children, is more likely to occur in children > 10 yr and is usually more severe in adults.

The initial rash, a macular eruption, may be accompanied by an evanescent flush. This rash evolves within a few hours to characteristic itchy, teardrop vesicles containing clear fluid and standing out from their red areolae; at this time, diagnosis can usually be made. The typical chickenpox lesions progress from macule to papule to vesicle and begin crusting within 6 to 8 h. Lesions erupt in successive crops, some macules just appearing as earlier crops begin to crust. The eruption may be generalized in severe cases; otherwise, the face and extremities are partially spared. When only a few lesions are present, the upper trunk is the most frequent site. Ulcerated lesions may also be present on the mucous membranes, including the oropharynx and upper respiratory tract, palpebral conjunctiva, and rectal and vaginal mucosa. In the mouth, the vesicles rupture immediately, are indistinguishable from those of herpetic stomatitis, and often cause pain on swallowing. Laryngeal or tracheal vesicles may cause severe dyspnea. Lesions are frequently present on the scalp, resulting in tender, enlarged suboccipital and posterior cervical lymph nodes. The acute phase of illness usually lasts 4 to 7 days. New lesions usually cease to appear by the 5th day, the majority are crusted by the 6th day, and most crusts disappear < 20 days after onset.


Secondary streptococcal infection of the vesicles may lead to erysipelas, sepsis, acute hemorrhagic nephritis, or, rarely, necrotizing fasciitis. Staphylococci may also infect the vesicles and cause pyoderma or bullous impetigo. Pneumonia as a complication of severe chickenpox is encountered in adults, newborns, and immunocompromised patients but is unusual in young children. Myocarditis, transient arthritis or hepatitis, and hemorrhagic complications have also been reported. Hemorrhagic varicella should raise suspicion of varicella-associated thrombocytopenic purpura, secondary bacterial sepsis, underlying malignancy, or immunodeficiency.

Post-chickenpox encephalopathy occurs in < 1/1000 cases. Like the encephalitis following measles, it tends to occur toward the end of the disease or 1 to 2 wk after its termination. One of the most common neurologic complications is acute postinfectious cerebellar ataxia. Transverse myelitis, cranial nerve palsies, and multiple sclerosis-like clinical manifestations have also occurred. Encephalitis may be fatal, but the prognosis for complete recovery from CNS complications is generally good and is far better than in measles encephalitis. Reye's syndrome, an unusual but severe complication, may begin 3 to 8 days after onset of the rash (see Reye's Syndrome under Miscellaneous Infections, below).

Diagnosis and Prognosis

Immunofluorescent detection of viral antigen in lesions, culture, or serologic findings confirms the diagnosis. Impetigo, infected eczema, insect bites and stings, drug rashes, contact dermatitis, erythropoietic porphyria (hydroa estivale), and occasionally coxsackievirus and disseminated herpes simplex virus infections must be considered in the differential diagnosis.

Chickenpox in childhood is usually benign; however, severe and fatal cases may rarely occur. The risk of severe or fatal disease increases in adults and in patients with depressed T-cell immunity (eg, lymphoreticular malignancy) or in those receiving corticosteroids or chemotherapy.


Chickenpox can be prevented or the disease attenuated by IM administration of zoster immune globulin, derived from the sera of patients recovering from herpes zoster, or varicella-zoster immune globulin (VZIG), prepared from pooled plasma containing high titers of specific antibody. The recommended dose is 125 U/10 kg (one 125-U vial is about 1.25 mL) with a maximum dose not to exceed 625 U. To be effective, such preparations should be given within 96 h of exposure; their use is primarily for exposed susceptible persons with leukemia, immunodeficiency syndromes, or other severe debilitating illness and susceptible pregnant women. Also, newborns whose mothers developed chickenpox within 5 days before delivery or 2 days after delivery are candidates for prophylaxis. Large doses of pooled human globulin can modify the disease if given IM shortly after exposure, but the quantity required is so great (0.6 to 1.2 mL/kg) that this is not generally recommended. VZIG and IM or IV immune globulin preparations have no therapeutic value after the illness has begun. In a few children, acyclovir has been used prophylactically after chickenpox exposure for 1 wk starting 7 days after exposure. Further studies are needed to determine its efficacy.

A live attenuated varicella vaccine is recommended by the American Academy of Pediatrics for universal vaccination of all healthy children who lack a reliable history of chickenpox. One dose is recommended at age 12 to 18 mo. For children > 18 mo without varicella immunization, one dose may be given any time between age 19 mo and 13 yr. Healthy adolescents older than age 13 yr and young adults who have not been immunized and have no history of varicella infection should receive two doses of vaccine 4 to 8 wk apart.

The Advisory Committee on Immunization Practices also recommends that susceptible adults be vaccinated (see also Ch. 152). Serologic testing before vaccination in adults to determine immune status is optional and in most cases not cost-effective. The vaccine is contraindicated in patients with moderate to severe concurrent illness, immunocompromised patients, pregnant women, patients on high doses of systemic corticosteroids, and those using salicylates. Household contacts of immunocompromised patients can rarely transmit the vaccine varicella strain; if a rash develops following vaccination of a household contact of an immunocompromised patient, all further contact should cease. Vaccine-associated chickenpox may follow vaccination in the immunocompetent individual; however, it is usually mild (< 10 papules or vesicles) and of short duration with few, if any, accompanying systemic symptoms.


Mild cases require only symptomatic treatment. Wet compresses may be applied to control itching, which may be extreme, and to prevent scratching, which may lead to widespread infection and scarring.

In severe cases, systemic antihistamines may be used. Because of the frequency of staphylococcal or streptococcal superinfection of the vesicles, patients should be bathed often with soap and water and kept in clean underclothing; hands should be kept clean and nails clipped. Antiseptics should not be applied to individual lesions unless they become secondarily infected; staphylococcal or beta-hemolytic streptococcal superinfection is treated with appropriate systemic antibiotics.

Oral acyclovir has been found to slightly decrease length and severity of chickenpox when given to immunocompetent hosts within 24 h of the onset of rash. However, given the commonly benign nature of the disease, it is not routinely recommended in healthy children with uncomplicated chickenpox. Oral acyclovir should be considered in healthy persons at risk for moderate to severe disease, including children >= 12 yr, in those with skin disorders (particularly eczema) or chronic lung disease, and in those receiving salicylate therapy or short intermittent or aerosolized corticosteroid therapy. The dose is 80 mg/kg divided qid with a maximum dose of 3200 mg. In patients known to be immunocompromised, acyclovir IV 1500 mg/m2/day divided q 8 h should be given. Many pharmacists recommend an IV dosage of 30 mg/kg/day divided q 8 h for children who are immunocompromised and < 1 yr. Zoster immune globulin can be used to attenuate the disease (see Prophylaxis, above).