Varicella (chickenpox, Shingl... Health Article

Definition

Varicella, or chickenpox, is an acute communicable disease characterized by a generalized vesicular rash. Because it is highly contagious, most individuals contract it in childhood. Herpes zoster, caused by reactivation of varicella-zoster virus (VZV), is a dermatomal cutaneous eruption.

Etiology

Varicella is caused by VZV, which is a member of the alpha Herpesviridae subfamily. It has the characteristic structure of a herpesvirus with an envelope, a tegument, a capsid, and a core of double-stranded DNA. The DNA is organized with terminal and internal repeats flanking unique short and long segments containing about 125,000 base pairs coding for approximately 70 genes. There are at least six glycoproteins. Its thymidine kinase has been a target for antiviral agents. There is some diversity in the restriction enzyme patterns among isolates; there is only a single serotype. Although the human is the only known natural host, a closely related virus has been identified in a simian species.

Epidemiology

Varicella is a highly contagious disease. After continuing household exposure, as would occur in a family, almost all susceptible persons are infected. The subclinical attack rate is believed to be no more than 4%. The results of nonhousehold exposure are less predictable. Chickenpox may be most contagious the day before the onset of rash. The period of contagiousness lasts for no more than 5 days after the appearance of the first lesion. Children may return to school at this time or earlier if the lesions are crusted. The incubation period is usually about 14 days. Ninety-nine per cent of the cases occur 10 to 20 days after exposure. The disease is known to be spread by direct contact. Airborne spread has also been demonstrated, most notably in hospitals.

Nosocomial spread of varicella has been well documented. It has occurred from room to room by airborne spread as well as between patients and staff. Adults with herpes zoster who are hospitalized are less likely to cause secondary cases of chickenpox than are children. The reason is that hospitalized children are more likely to be susceptible to chickenpox than hospitalized adults. Strict isolation is recommended for hospitalized patients with varicella and for children or immunocompromised adults with herpes zoster. Adults with localized herpes zoster require less stringent isolation procedures.

Most cases of chickenpox occur in childhood. Most children contract chickenpox prior to school entry, often in out-of-home care. Fewer than 2% of the cases occur after the second decade. Less than 10% of hospital workers with a negative history are seronegative. Almost all individuals with a positive history are seropositive. A single attack of chickenpox usually confers lifetime immunity.

There appears to be more efficient transmission of disease in temperate than in tropical climates. The reason for this is uncertain, but it may be due to temperature rather than urbanization. Varicella occurs most commonly during the late winter and spring months, the peak being in about March. Sporadic cases occur into the early summer and start in late fall.

Varicella is more common than other childhood diseases during the early months of life. After the first 2 weeks of life, the disease is generally mild. Maternal antibody transferred across the placenta may not be as effective in protecting infants against this disease as are antibodies against other viruses. However, nursery outbreaks have been rare. Children who have varicella during the early months of life or are exposed in utero have a greater risk of herpes zoster in childhood.