Chickenpox: Diseases and Conditions

What type of virus is varicella zoster virus (VZV)?
Varicella zoster virus is a member of the herpesviridae family. The virus has icosahedral symmetry containing centrally located double-stranded DNA with surrounding envelope. The DNA contains 125,000 base pairs and encodes approximately 75 proteins.

What are the reservoirs of VZV?
Humans are the only reservoir for VZV.

What are the common modes of transmission of VZV infection?
The virus is extremely labile and is unable to survive for a long period in scabs or fomites. Therefore, transmission via inanimate objects is unlikely. The infection is usually transmitted through inhalation of infectious respiratory secretions from an infected person (droplet infection), direct physical contact with vesicular lesion and by vertical transmission during pregnancy.

Is chickenpox a totally benign illness?
In most immunocompetent and healthy children, chickenpox is a self- limited disease. Complications do occur but they are infrequent as can be gauged by the fact that some infected children , who are otherwise healthy do need to be hospitalized. In adults, pregnant women and immunocompromised individuals, the disease is not so benign. In these persons, complications are more frequent and the risk of mortality is higher.

How long is the child with chickenpox infectious?
A child with chickenpox is infectious from two days prior to onset of rash till all the lesions are scabbed: which usually occurs 5 days after the appearance of rash.

What events occur immediately following the infection?
Following the entry of VZV into the body via the respiratory tract and/or the conjunctiva, the virus replicates in the regional lymph nodes for 4-6 days. The virus then disseminates throughout the blood stream and lymphatic system. Following this phase of primary viremia, VZV is taken up by the cells of the reticulo-endothelial system and undergoes another stage of replication in the liver and other organs. This is followed by secondary viremia; whose appearance coincides with the appearance of prodromal symptoms. These events following infection generally take 14-16 days, which is the incubation period of chickenpox (range 10-21 days). This period may be shorter in immunocompromised patients but longer (up to 28 days) in individuals who have received Varicella Zoster Immunoglobulin (VZIG).

How is herpes zoster related to VZV?
Herpes zoster is secondary to reactivation of VZV that has remained dormant in the dorsal root ganglia following the primary attack of varicella.

How Does Epidemiology of Chickenpox Vary in Different Geographical Areas?

The VZV infections are known to occur at much earlier age in temperate climate while they are supposed to occur at a later age in individuals staying in warmer tropical areas. Thus in the United States and European Union as many as 60% of the pre-school children demonstrate antibody to VZV. The proportion of seroprevalence rises to 78% by 11 years of age. In contrast, the mean age of chickenpox sufferer is higher in tropical countries, with many more cases occurring among adolescents and adults. There is paucity of Indian data. One study showed that only 29.7% of student nurses aged 17-20 years had antibodies against varicella while another study showed that less than 5% of under-five children were seropositive. Studies from southeast Asia and Latin America also indicate a much lower age specific seroprevalence rate.

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