The virus

Varicella-zoster virus is a double-stranded DNA virus and a member of the Herpesviridae family of viruses.

Epidemiology

Route of spread

Varicella-zoster virus is transmitted by the airborne route, from respiratory secretions and from vesicles on the skin. After entry through the respiratory route there is an initial period of viraemia, which seeds the virus in the reticulo-endothelial system. This is followed by a second episode of viraemia resulting in dissemination of the virus throughout the body and manifestations of the typical chickenpox vesicular rash on the skin surface.

Prevalence

Varicella-zoster virus infection occurs worldwide and the prevalence of infection varies considerably. While 95% of people in industrialized countries have had chickenpox by the age of 20 years (although about 20% will have had such a mild infection that they may be unaware of this), in the tropics, only 50% of people have had chickenpox by the age of 20 years.

Incubation period

10–23 days (mean 14 days).

Infectious period

From 2 days before the onset of symptoms until 5 days after the rash or all the skin lesions are fully crusted.

At-risk groups

• Immunocompromised persons

• Pregnant women

• Unborn babies in the first 20 weeks of pregnancy, and babies one week before or after delivery.