Varicella is one of the most infectious diseases known (along with pertussis and measles). Transmission occurs via airborne droplets, or contact with infected respiratory tract secretions or vesicular lesions. The incubation period is usually 14 to 16 days (range 10 to 21 days), and cases are infectious from 1 to 2 days before the onset of the rash until all the lesions have crusted. A maculopapular rash, which becomes vesicular, appears ﬁrst on the face and scalp, later spreading to the trunk and abdomen and eventually to the limbs. The vesicles dry and crust after three to four days, but may be followed by further lesions.
A wide variation in the number of lesions is possible, ranging from a few to many hundred. The hallmark of the disease is the presence of lesions in varying stages of development. Lesions on mucosal surfaces (mouth, vagina) can cause considerable distress. The rash is pruritic and is usually associated with mild fever, malaise, anorexia and listlessness.
In the majority of children, varicella is a mild and self-limiting disease, but complications requiring hospitalisation and fatalities do occur. Secondary bacterial infections and VZV encephalitis are the most common morbidities. Serious complications include central nervous system involvement (encephalitis, cerebellar ataxia, stroke), pneumonia, secondary invasive bacterial infections, and even death. Primary infection in adults is rare but has a higher rate of complications, with pneumonia being the most common. VZV pneumonia often requires mechanical ventilation and carries an overall mortality rate of 10–30 percent despite appropriate antiviral therapy. Adults with VZV are 25 times more likely to develop severe disease than children.
Pregnant women and their unborn babies are particularly vulnerable to VZV (see section 21.8.6). Maternal varicella occurring in the first half of pregnancy can cause the rare but devastating congenital varicella syndrome (see Table 21.3), whereas disease very late in pregnancy (from five days before to two days after delivery) may cause severe neonatal varicella infection. Women who contract varicella while pregnant have an estimated 10–20 percent risk of developing VZV pneumonia, which is a higher rate than observed in non-pregnant women.
Others vulnerable to both VZV and HZ are those who are immune compromised, such as people taking immunosuppressive medications (eg, cancer treatment or organ transplant patients) and those with human immunodeficiency virus (HIV) infection. Varicella can be a fatal disease in the immune compromised.
VZV infection is followed by the production of VZV-specific antibody and VZV-specific T-cell mediated immunity. The latter is necessary to maintain the latency of VZV in the ganglia and therefore prevent HZ. The immune response is boosted by subclinical reactivation of latent virus or exposure to wild-type virus (contact with a case of chickenpox or shingles). The incidence of HZ increases with age as VZV-specific T cell-mediated immunity declines (see chapter 22).