Chickenpox is an acute disease caused by the varicella-zoster virus and characterised by a distinctive rash. It is not known exactly how the term chickenpox originated and in many parts of the world the disease is referred to simply as varicella. 

Chickenpox is a very infectious disease, up to 90% of people who do not have immunity to chickenpox will develop the disease if they come into contact with the varicella-zoster virus.¹ Chickenpox is transmitted by personal contact with an infectious person, or via droplets spread from coughs and sneezes. Exposure significant enough to cause infection includes mother/newborn contact, continuous contact in the home, contact in the same room for 15 minutes or more, and face-to-face contact such as having a conversation.¹ It is estimated that one case of chickenpox can potentially infect 10-12 susceptible people.²

Chickenpox is infectious from one to two days before the rash appears, until the blisters are dry or crusted over. The time from becoming infected until symptoms appear is usually between one to three weeks, but can be longer in immunocompromised people.¹

After chickenpox has subsided, the virus remains in the sensory nerves. Years or decades later, it can reactivate and cause shingles (herpes zoster). It is therefore possible for a person to develop chickenpox following contact with a person who has shingles, but it is not possible to develop shingles following contact with a person who has chickenpox.¹

Chickenpox is predominantly a childhood illness. Its incidence is highest before 10 years of age. In Ireland, only hospitalised cases of chickenpox are notifiable to the medical officer of health. 

In 2015, 69 hospitalised cases of chickenpox were reported in Ireland.³ There is often a seasonal incidence of chickenpox, Ireland experiences a peak in the number of cases between the months of January to April.² It is difficult to estimate the true incidence of chickenpox in the community, but it is undoubtedly a very common illness. 

Some parents hope their children will catch chickenpox at a convenient time, and therefore intentionally expose the child to an infectious person. Chickenpox parties and chickenpox lollipops (lollipops available on the internet which purport to contain the chickenpox virus) have gained some notoriety however these are not something that should be promoted.

Complications

In healthy people, chickenpox usually resolves without treatment and does not cause complications, although chickenpox can be more serious in adults than in children. Adults with chickenpox are more likely to be admitted to hospital, often due to complications such as pneumonia, hepatitis, or encephalitis. Older age is a risk factor for severe varicella disease. 

Bacterial skin infections can also occur following chickenpox, most commonly in young children, and immunocompromised people are at greater risk of developing complications following chickenpox. They may have an atypical rash with more lesions which may bleed. People who smoke are at risk of lung complications following chickenpox.¹

Chickenpox can also be dangerous in pregnancy, both for the mother who is at higher risk of developing complications such as varicella pneumonia, and for the baby who is at risk of serious complications from foetal varicella syndrome. 

Foetal varicella syndrome can cause skin scarring, vision problems, neurological problems including learning difficulties and bladder dysfunction, and underdevelopment of limbs, although foetal varicella syndrome is rare.¹ Chickenpox is most dangerous in the first 28 weeks of pregnancy.⁴ If maternal infection occurs at 20-27 weeks of pregnancy the baby can also go on to develop shingles of infancy or early childhood.

Newborn babies can contract varicella if the mother becomes infected one to four weeks prior to delivery. Up to half of these babies will be infected and around one-quarter will develop clinical varicella of the newborn, even though they have acquired maternal antibodies. If the baby is born up to seven days before or six days after onset of the mother’s rash, they are more likely to develop severe infection, which may be fatal.^1,4

The majority of women will have immunity to chickenpox prior to childbearing age. Women from tropical and subtropical areas are at increased risk of developing chickenpox because they are less likely to have developed immunity.⁴ Non-immune pregnant women who have been significantly exposed to chickenpox may be offered vaccination.²

Diagnosing chickenpox

In most cases chickenpox can be diagnosed from the characteristic rash. If there is doubt, a history of recent exposure to chickenpox (or shingles), or cases occurring in close contacts may help confirm the diagnosis. You should ask about typical features of chickenpox, including symptoms which occurred before the rash such as nausea, aches and pains, loss of appetite/feeding problems, raised temperature, and headache. 

On examination, chickenpox looks like  small red patches (macules) or spots usually starting on the head and progressing to the, face, trunk, and limbs. The rash usually progresses over 12 to 14 hours to form crops of clear vesicles (blisters) which are often intensely itchy. Vesicles can also occur on the palms and soles, and mucous membranes such as in the mouth, vagina, and eyes can also be affected causing painful, shallow oral or genital ulcers. The number of lesions can vary from a few to hundreds.² Crusting usually occurs within five days of the onset of the rash, and crusts fall off after one to two weeks. Adults may experience a more widespread rash and more prolonged fever than children.

Laboratory tests can be used for confirmation of diagnosis, but are not usually required for people who have uncomplicated chickenpox and are not in hospital. 

Management

Healthy children and adults with chickenpox should be encouraged to follow simple measures to alleviate symptoms. Encourage the person to drink adequate fluids to avoid dehydration and to dress appropriately to avoid overheating or shivering. Smooth, cotton fabrics should be worn to minimise itching, and nails kept short to prevent damage from scratching. The person should take paracetamol if pain or fever is causing distress. A calamine lotion can be applied to the rash to alleviate itch, and an antihistamine can be given over one year of age.

Immunocompromised adults or adolescents may be prescribed oral aciclovir (800mg, five times a day for seven days) if they present within 24 hours of the rash appearing, with the aim of reducing the potential for complications.⁵ Aciclovir is not effective if given more than 24 hours after the onset of the rash.

Inform the person that the most infectious period is one to two days prior to the rash appearing, but that they will still be infectious until all of the lesions are dry and crusted over, which is usually around five days after the rash first appeared. Advise the person to avoid contact with pregnant women, babies aged less than four weeks, or anyone who may be immunocompromised (including people receiving cancer treatment or high-dose oral steroids).^1,4

Children should be kept away from school or nursery until the lesions have crusted over.⁶ Parents should be made aware to look out for sudden high temperature, redness and tenderness around lesions in young children, often after some initial improvement, as this can be a sign of bacterial superinfection which needs immediate medical attention. 

People suffering from serious complications of chickenpox will usually be admitted to hospital. Specialist advice should be sought for management of pregnant women or newborns who have chickenpox. Antihistamines are not recommended for use in pregnancy or breastfeeding.

Prevention

There is a vaccine available for chickenpox but this is not given as part of the routine childhood immunisation schedule in Ireland. The vaccine is given to people in at-risk groups including healthcare workers, laboratory staff, immunocompromised people and their close contacts, HIV-infected children, children in some residential units and women of childbearing age if they do not already have immunity. For more information, see www.hse.ie 

References

1. Public Health England. Varicella: the green book, chapter 34. Published August 2015. Available from: www.gov.uk/government/publications/varicella-the-green-book-chapter-34 [Accessed January 04, 2017] 
2. Health Service Executive. Immunisation guidelines. Chapter 23 – varicella zoster. Published August 2015. Available from: http://www.hse.ie/eng/health/immunisation/hcpinfo/guidelines/chapter23.pdf [Accessed January 04, 2017]
3. Health Protection Surveillance Centre. Chickenpox-hospitalised cases. Published 2015. Available from: http://www.hpsc.ie/A-Z/VaccinePreventable/VaricellaChickenpox/SurveillanceReports/AnnualReports/ [Accessed January 04, 2017]
4. Royal College of Obstetricians & Gynaecologists (2015) Chickenpox in pregnancy. Published January 2015. Available from: https://www.rcog.org.uk/en/guidelines-research-services/guidelines/gtg13/[Accessed January 04, 2017]
5. Public Health England. Management of infection guidance for primary care for consultation and local adaptation. Published May 2016. Available from: https://www.gov.uk/government/publications/managing-common-infections-guidance-for-primary-care [Accessed January 04, 2017]
6. Health Service Executive. Management of infectious disease in childcare facilities and other childcare settings. Published May 2012. Available from: http://www.hpsc.ie/A-Z/VaccinePreventable/VaricellaChickenpox/GuidancePublications/ [Accessed January 04, 2017]