Chickenpox - Varicella Zoster Virus

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What is Chickenpox - Varicella Zoster Virus?

Chickenpox or varicella is an itchy, red, lumpy rash that is caused by the varicella virus. The chickenpox virus is highly contagious for those who are not immune to it. You can only become immune to chickenpox through already having the disease and developing antibodies to it or through vaccination, which has only been introduced in the last decade.

Chickenpox is thought of as a common and fairly harmless childhood illness. The direct symptoms of chickenpox cause some discomfort without any complications. However, rarely the disease will predispose people to quite serious conditions that can occur at the disease onset or years later. Once a person has developed chickenpox the virus that causes the illness never goes away but lies dormant in the dorsal column of the spinal cord. If this virus is reactivated later in life (most commonly decades later) a condition called Herpes Zoster (shingles) can occur. It is for these reasons that a vaccine for the disease is worthwhile, however whether the vaccine reduces the risk of HZ has not yet been established.

Who gets Chickenpox - Varicella Zoster Virus?

Chickenpox is very much viewed as a childhood disease, understandably so with 95% of varicella cases occurring in people under the age of 20 and the majority of these in children under 10 years of age.

In terms of overall population incidence there have been quite dramatic changes in the past decade due to the introduction of the varicella vaccine. Before the vaccine was introduced, 83% of all 10 - 14 year olds in Australia were estimated to have contracted chickenpox at some stage of their lives, further increasing to 95.5% of all 40 year olds. The highest incidence in Australia was in the 0 -4 age bracket, with an annual rate of 78/100,000 for indigenous Australians and 43/100,000 for non-indigenous Australians. After this age the rate decreased dramatically. There were 4 deaths from varicella in Australia between 2003 -2005. Recent data has shown there has been significant decreases in varicella-related hospitalisations since this time and this can be attributed to the vaccine. However, accurate incidence data in Australia is yet to be determined.

After the varicella vaccine was introduced in the US, chickenpox was estimated to be reduced by 84%. Furthermore, the incidence of varicella complications, including death, decreased by 66%.

Approximately 5 years after the introduction of the vaccine there was an increased number of breakthrough varicella cases. Breakthrough varicella in immunised patients was an indication that the immunity provided by the vaccine was diminishing over time. Accordingly, a second dose of the vaccine was then implemented into the vaccination schedule.

Approximately 1 in 2,000 pregnant women contract chickenpox, a statistic that is not expected to fall until the last of the non-vaccinated generation surpass the childbearing age.

Predisposing Factors

Not having been vaccinated against varicella is a predisposing factor for the disease. For those that are vaccinated there is a risk of developing a less severe outbreak of the disease which is termed breakthrough varicella. Breakthrough varicella is no longer a risk 5 years after vaccination.

Before the vaccination became available, up to 95.5% of the population were estimated to have contracted chickenpox at some stage of their lives. This suggests that the only predisposing factor to the disease is catching it from someone else that has it; this can either occur via airborne transmission or from direct contact with lesions.

Immunocompromised patients with underlying disease factors or those requiring immunosuppressive therapies are more likely to experience complications as a result of varicella. The virus is more likely to spread to the lungs, liver, kidneys, heart or brain in those that have T-cell defects.

Progression

Chickenpox usually begins with headache, malaise and fever and within 48 hours the rash will have developed on the face, torso and limbs. The chickenpox rash contains many lesions which develop at different stages. New fluid filled blisters develop for the first seven days of rash progression, after a week the blisters will begin to dry up. The whole process can last up to 20 days.

The lesions are small (less than 1 cm in diameter) and begin development as clear vesicles (fluid filled blisters) on an erythematous or "red" base, the vesicles then progress to pustules (pus filled blisters) and then crust over. The rash is highly contagious until all the lesions have crusted over.

The rash often spreads from a centre point and contains lesions at various stages of the rash progression.

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