Chickenpox is caused by the Varicella-zoster virus (VZV), a double-stranded DNA virus that belongs to the herpes virus group. Like other herpes viruses, VZV may persist in the body after primary infection: primary infection results in chickenpox; shingles is the result of recurrent infection.
VZV is transmitted by respiratory droplets, aerosol or direct contact with rash lesions, and patients are usually contagious from a few days before rash onset until the rash has crusted over. VZV enters the body through the respiratory tract and conjunctiva and replicates in the nasopharynx and lymph nodes. The incubation period following exposure ranges from 10-21 days.
Four to six days after infection, the virus enters the bloodstream (viremia) and spreads to other parts of the body, primarily the liver, spleen and sensory ganglia of the nervous system. VZV undergoes further replication in the internal organs, enters the bloodstream once again (secondary viremia), and causes skin infection.
Most cases of chickenpox are mild and occur in children under the age of 10. Appearance of the characteristic itchy rash is usually the first sign of infection in children; whereas, in adults, rash may be preceded by fever and malaise. Fever may also present for 2-3 days.
The rash usually appears first on the scalp and face, and gradually spreads to the trunk and extremities with the highest concentration on of lesions on the trunk. The lesions are usually very itchy, which increases the risk of secondary infection, especially in children where hand hygiene is often somewhat lacking. Additionally, lesions can occur on the back of the throat, the respiratory tract, vagina, conjunctiva and cornea.
As the rash spreads to new areas, lesions in areas where the rash first appeared begin to dry up and crust over, while fresh lesions are just beginning in other areas. Rash lesions begin as small (< 5 mm in diameter) reddish non-raised spots (macules), which then become raised (papules), fill with a clear fluid (vesicle), rupture, and finally crust over. It takes approximately 7-10 days for crusting to disappear. Healthy children usually present 200-500 lesions. In some cases, scarring may occur.
Natural infection results in life-long immunity to chickenpox; however, VZV may persist in sensory nerve ganglia, and may later reactivate to cause zoster (shingles). Approximately 10-20% of people infected with VZV will develop shingles later in life.
Complications of chickenpox include:
- pneumonia
- encephalitis
- scarring
- secondary bacterial infection of rash lesions that may also lead to more serious infections (e.g. necrotizing fasciitis, septicemia)
- congenital varicella syndrome associated with infection during the first 20 weeks of pregnancy
Serious complications are more common in adults than in children, with death occurring in previously healthy adults at a rate 30-40 times higher than in children 5-9 years of age.
Before the introduction of varicella vaccine in 1995, approximately 4 million cases of varicella occurred annually in the United States. Of these, approximately 11,000 people required hospitalization and 100 died.
Routine immunization with varicella vaccine is recommended for all children at 12-18 months of age.
More than 90% of healthy people immunized with varicella vaccine will develop long-term immunity against chickenpox.
VZV vaccine is considered extremely safe and serious complications are rare. The most common side-effects presented (27% of healthy children) include swelling and redness at the injection site. Fewer than 5% of healthy children vaccinated develop a mild varicella-like disease with rash within four weeks of immunization. Vaccine strains may induce latency with later reaction (shingles) as occurs with natural infection.
Serious vaccine complications have also been reported (e.g. encephalitis, pneumonia), but occur at much lower rates than reported following natural infection.
