Chicken Pox (Varicella)

Synonyms: Varicella, varicella-zoster

Chicken pox is a highly infectious disease caused by the varicella-zoster virus. It is a DNA virus of the Herpesviridae family. Reactivation of the dormant virus after a bout of chicken pox leads to herpes zoster (shingles). Most chicken pox is mild to moderate and self-limiting but serious complications can occur in both the immunocompetent and the immunocompromised.

• Although immunisation against chicken pox has been routine in the USA since 1995, it is not practiced in the UK and most children are liable to develop the disease at some stage during childhood. Annual Incidence in the UK is 48 per 10 000 (2006).¹
• 90% of people affected before adolescence in temperate climates.²
• Varicella is highly infectious. Spread throughout households is very common with infection of up to 90% of vulnerable individuals who come into contact.
• Most infection occurs in those under 5 years of age and immunity increases with age until adulthood.³
• Epidemics occur at 2-5 year intervals.²
• Occurs worldwide and is endemic in most countries. It tends to occur in sporadic outbreaks, usually in winter or spring.
• Infectivity is from a few days before the onset of the lesions until the crusts fall off.
• It is not possible to catch shingles from chicken pox as the former represents a resurgence of a dormant virus. It may be possible to catch chicken pox from active lesions of shingles but in practice this is rare.

Risk factors

• Usually it is a self limiting disease but complications can occur in those with the following risk factors
  • Immunocompromised
  • Older age
  • Long-term steroid use⁴
  • Malignancy
• It tends to be milder in younger children than in older children and if contracted in adulthood it is significantly more unpleasant.
• It is dangerous in neonates and to the fetus if contracted in pregnancy.
• The infection tends to be severe in pregnancy - a high risk of pneumonia as well as risks to the fetus as described below
• Complications may occur in as many as 1 in 50 cases

• Infection with chicken pox and subsequent immunity can occur without clinical disease.
• The virus enters through the upper respiratory tract. Viraemia occurs 4 to 6 days later but the incubation period between exposure and the first skin lesions is around 10 to 14 days but can be as long as 21 days.²
• The first feature is often pyrexia - temperature of around 38 to 39°C is usual for up to 4 days.
• Headache, malaise and abdominal pain may be reported.
• Crops of vesicles appear over the course of 3 to 5 days - mostly on the head, neck and trunk and very sparse on the limbs (may also occur in the mouth and oropharynx).
• The lesions tend to be very itchy but perhaps less so in younger children.
• They pass through the stages of papule, vesicle, pustule and crust.
• When the crusts fall off they may leave marks that may be present for a few weeks but there is normally no long term scarring. However, in adolescents and adults there is a greater risk of scarring.
• Redness around the lesion may suggest bacterial superinfection, probably introduced by scratching.
• Females may get vulval lesions that are very unpleasant.
• Prolonged eruptions or delayed crusting may suggest impaired cell mediated immunity.

In the immunocompromised the following may also occur

• Skin lesions may continue over several weeks.
• Vesicles can be large and bleed.
• Pneumonia is more common.
• Disseminated intravascular coagulation may occur.

Note the pustules and excoriation due to scratching. Redness around the lesions, especially if hot, may suggest secondary infection.

The clusters of vesicles usually makes the diagnosis clear but differential includes

• Generalised herpes zoster or simplex
• Dermatitis herpetiformis
• Impetigo
• Guttate psoriasis
• Other viral skin infections
• Contact dermatitis
• Stevens-Johnson syndrome (usually the clinical picture is obvious)

Shingles or herpes zoster is like varicella but confined to just one dermatome. There may also be malaise. The lesions of chicken pox are at different stages and appear in clusters, tending to be central in distribution. The lesions of smallpox are all at the same stage and tend to be more peripheral. Smallpox has been eradicated and there is no known animal vector but the virus is kept in about a dozen laboratories throughout the world. In theory it could be developed for biological warfare or terrorism.

• Usually the diagnosis is obvious on clinical grounds, especially during an epidemic.
• Confirmation can be obtained by taking a scraping of a lesion and using immunohistochemical staining or polymerase chain reaction tests.
• Complications require further investigation e.g. respiratory symptoms require CXR and neurological features demand lumbar puncture.

Chickenpox in an otherwise healthy individual

• Simple advice regarding adequate fluid intake, minimize scratching if possible and that the first 1-2 days they are most infectious. They should avoid contact with pregnant women, neonates and anyone who may be immunocompromised.
• Symptomatic treatment e.g. analgesia and antipyretics such as, paracetamol or NSAIDs. There is a possible association with NSAIDs and risk of necrotising soft tissue infections.² Pruritus can be helped by sedating antihistamines and calamine lotion. However, as the latter dries it ceases to be effective. Secondary infection may require antibiotics.
• Aciclovir should be considered if the patient presents within 24 hours, or has severe chickenpox or if they are at risk of complications.
• Aciclovir is not recommended in children.
• Anyone with possible complications should be admitted to hospital e.g. encephalitis.

Chickenpox in an immunocompromised healthy individual

• Specialist advice regarding confirmation of the diagnosis and the need for starting urgent antivirals (as well as symptomatic treatment as above).
• Anyone with possible complications should be admitted to hospital e.g. encephalitis.

Chickenpox in a pregnant or breastfeeding woman

• Specialist advice regarding confirmation of the diagnosis, fetal varicella syndrome and need for starting urgent antivirals (as well as symptomatic treatment as above).
• Anyone with possible complications should be admitted to hospital e.g. encephalitis.
• Specialist advice should be sought as to whether a nursing mother should continue to breast feed.

• Secondary infection of lesions, probably from scratching occurs in 5 to 10% and is usually indolent.
• Secondary bacterial infections, especially group A streptococcal, can produce necrotizing fasciitis and toxic shock syndrome.
• Viral pneumonia can be life-threatening - most often in older children and adults, appearing 3 or 4 days after the onset of the rash. Chest pain wheezing and tachypnoea are all signs.
• Encephalitis is a serious illness that may require admission to an ICU. Symptoms include confusion, irritability, drowsiness and vomiting. Weakness or inability to walk, severe headache and neck stiffness are also possible features.
  Encephalitis occurs in 1.7 per 100,000 cases of varicella among otherwise healthy children between 1 and 14 years. The mortality rate is 5 to 20%.
• Other CNS complications e.g. benign cerebellar ataxia, myelitis, vasculitis causing strokes (may occur several months after the chicken pox)²
• Other infections e.g. osteomyelitis, sepsis, otitis media.

If caught in the first 20 weeks of pregnancy there is a 2% risk of congenital varicella syndrome.⁶ This causes a range of problems including intrauterine growth retardation, microcephaly, cortical atrophy, limb hypoplasia, microphthalmia, cataracts, chorioretinitis, and cutaneous scarring. Infection with varicella in the later stages of pregnancy can cause premature delivery or neonatal chickenpox infection.
This is particularly serious if the mother becomes infected 7 days before birth. There is no such risk with shingles.

If chicken pox appears in a pregnant woman she is offered immunoglobulin. There is no correlation between the severity of the chicken pox and the risk of fetal involvement.

• If chicken pox is caught in late pregnancy it can cause premature delivery.
• If the rash appears within a week of delivery or within 2 days after delivery, there is risk of neonatal varicella.
• There is transplacental transmission of virus but not antibody as there is no time for IgG to develop and the baby is at 30% risk of death from severe pneumonia or fulminant hepatitis.
• Treatment is with immunoglobulin and aciclovir.
• If at least a week passes between the rash and delivery then maternal IgG should give adequate protection. The initial antibody response is IgM but this does not cross the placenta.
• Intrauterine infection after 20 weeks gestation can result in neonatal herpes zoster. This usually presents in the first year of life and most commonly involves a thoracic dermatome.

There are no plans to make immunisation against chicken pox routine for British children at present. A vaccine is available and it is offered to healthcare workers who may come into contact with the disease whilst not immune. This appears to represent about 10% of the adult population.⁷ The vaccine is given as part of MMR in the United States.⁸ Two doses of vaccine are given 4 to 8 weeks apart and seroconversion is not routinely assessed. More information about the vaccine is available on the HPA site.⁹ The immunocompromised, including those on steroids and being treated for cancer are at risk especially children with leukaemia who may have a mortality of 7%. The response to vaccine is remarkably good¹⁰ and it is associated with almost 80% reduction in mortality. Furthermore vaccination of older individuals is also providing promising results.¹¹ Those who have had the disease are usually immune but second and even third attacks are reported, especially if the first was mild.

Patients should be advised to avoid contact with pregnant women, neonates and anyone who may be immunocompromised for the first 2 days of spots appearing. Children should be kept away from school for 5 days and air travel is not allowed until 6 days after disappearance of the last spot.⁵

Crude mortality rates from varicella are 2-4 per 100,000.² Varicella is more severe in the immunocompromised but mortality and severe morbidity is higher in healthy affected individuals.