Chicken pox

Chickenpox is one of the classic childhood diseases, and one of the most contagious. The affected child or adult may develop hundreds of itchy, fluid-filled blisters that burst and form crusts. Chickenpox is caused by a virus.

The virus that causes chickenpox is varicella-zoster, a member of the herpesvirus family. The same virus also causes herpes zoster (shingles) in adults.

Causes, incidence, and risk factors
In a typical scenario, a young child is covered in pox and out of school for a week. The first half of the week the child feels miserable from intense itching; the second half from boredom. Since the introduction of the chickenpox vaccine, classic chickenpox is much less common.

Chickenpox is extremely contagious, and can be spread by direct contact, droplet transmission, and airborne transmission. Even those with mild illness after the vaccine may be contagious.

When someone becomes infected, the pox usually appear 10 to 21 days later. People become contagious 1 to 2 days before breaking out with pox. They remain contagious while uncrusted blisters are present.

Once you catch chickenpox, the virus usually remains in your body for your lifetime, kept in check by the immune system. About 1 in 10 adults will experience shingles when the virus re-emerges during a period of stress.

Most cases of chickenpox occur in children younger than ten. The disease is usually mild, although serious complications sometimes occur. Adults and older children usually get sicker than younger children do.

Children under one year of age whose mothers have had chickenpox are not very likely to catch it. If they do, they often have mild cases because they retain partial immunity from their mothers' blood. Children under one year of age whose mothers have not had chickenpox, or whose inborn immunity has already waned, can get severe chickenpox.

The pox are worse in children who have other skin problems, such as eczema or a recent sunburn.

Complications are more common in those who are immunocompromised from an illness or medicines like chemotherapy. Some of the worst cases of chickenpox have been seen in children who have taken steroids (for example, for asthma) during the incubation period, before they have any symptoms.

Symptoms
Most children with chickenpox act sick with vague symptoms, such as a fever, headache, tummy ache, or loss of appetite, for a day or two before breaking out in the classic pox rash. These symptoms last 2 to 4 days after breaking out.

The average child develops 250 to 500 small, itchy, fluid-filled blisters over red spots on the skin (“dew drops on a rose petal”). The blisters often appear first on the face, trunk, or scalp and spread from there. Appearance of the small blisters on the scalp, found in 80% of cases, clinches the diagnosis. After a day or two, the blisters become cloudy and then scab. Meanwhile, new crops of blisters spring up in groups. The pox often appear in the mouth, in the vagina, and on the eyelids. Children with skin problems such as eczema may get more than 1,500 pox.

Most pox will not leave scars unless they become contaminated with bacteria from scratching.

Some children who have had the vaccine will still develop a mild case of chickenpox. They usually recover much quicker and only have a few pox (< 30). These often do not follow the classic descriptions of the disease. However, these mild, post-vaccine cases are contagious.

Signs and tests
Chickenpox is usually diagnosed from the classic rash and the child's medical history. Blood tests, and tests of the pox blisters themselves, can confirm the diagnosis if there is any question.

Treatment
In most cases, it is enough to keep children comfortable while their own bodies fight the illness. Oatmeal baths in lukewarm water provide a crusty, comforting coating on the skin. An oral antihistamine can help to ease the itching, as can topical lotions. Lotions containing antihistamines are not proven more effective. Trim the fingernails short to reduce secondary infections and scarring.

Safe antiviral medicines have been developed. To be effective, they usually must be started within the first 24 hours of the rash. For most otherwise healthy children, the benefits of these medicines may not outweigh the costs. Adults and teens, at risk for more severe symptoms, may benefit if the case is seen early in its course

In addition, for those with skin conditions (such as eczema or recent sunburn), lung conditions (such as asthma), or those who have recently taken steroids, the antiviral medicines may be very important. The same is also true for adolescents and children who must take aspirin on an ongoing basis.

Some doctors also give antiviral medicines to people in the same household who subsequently come down with chickenpox. Because of their increased exposure, they would normally experience a more severe case of chickenpox.

DO NOT USE ASPIRIN for someone who may have chickenpox. Use of aspirin has been associated with Reyes Syndrome. Acetaminophen and ibuprofen may be used.

Expectations (prognosis)
The outcome is generally excellent in uncomplicated cases. Encephalitis, pneumonia, and other invasive bacterial infections are serious, but rare, complications of chickenpox.

Complications
Women who acquire chickenpox during pregnancy are at risk for congenital infection of the fetus. Newborns are at risk for severe infection, if they are exposed and their mothers are not immune. A secondary infection of the blisters may occur. Encephalitis is a serious, but rare complication. Reye's syndrome, pneumonia, myocarditis, and transient arthritis are other possible complications of chickenpox Cerebellar ataxia may appear during the recovery phase or later. This is characterized by a very unsteady walk.

Calling your health care provider
Call your health care provider if you think that your child has chickenpox or if your child is over 12 months of age and has not been vaccinated against chickenpox.

Prevention 

Because chickenpox is airborne and very contagious before the rash appears, it is difficult to avoid. It is possible to catch chickenpox from someone on a different aisle in the supermarket, who doesn’t even know they have chickenpox!

A chickenpox vaccine is part of the routine immunization schedule. It is about 100% effective against moderate or severe illness, and 85-90% effective against mild chickenpox. Parents often express concern that the immunity from the vaccine might not last. The chickenpox vaccine, though, is the only routine vaccine that does not require a booster. However, a higher dose of the vaccine given later in life may reduce the incidence of herpes zoster (shingles). Reimmunization with the high dose is currently being considered by vaccination experts.

Congenital defects in babies
These may occur if the child's mother was exposed to VZV during pregnancy. Effects to the fetus may be minimal in nature but physical deformities range in severity from under developed toes and fingers, to severe anal and bladder malformation. Possible problems include:
 * Damage to brain: encephalitis, microcephaly, hydrocephaly, aplasia of brain
 * Damage to the eye (optic stalk, optic cup, and lens vesicles), microphthalmia, cataracts, chorioretinitis, optic atrophy.
 * Other neurological disorder: damage to cervical and lumbosacral spinal cord, motor/sensory deficits, absent deep tendon reflexes, anisocoria/Horner's syndrome
 * Damage to body: hypoplasia of upper/lower extremities, anal and bladder sphincter dysfunction
 * Skin disorders: zig zag (cicatricial) skin lesions, hypopigmentation

Vaccination
Japan was among the first countries to routinely vaccinate for chickenpox. Routine vaccination against varicella zoster virus is also performed in the United States, and the incidence of chickenpox has been dramatically reduced there (from 4 million cases per year in the pre-vaccine era to approximately 400,000 cases per year as of 2005). In Europe most countries do not currently vaccinate against varicella, though the vaccine is gaining wider acceptance. Australia, Canada, and other countries have now adopted recommendations for routine immunization of children and susceptible adults against chickenpox. Other countries, such as Germany and The United Kingdom have targeted recommendations for the vaccine, e.g. for susceptible health care workers at risk of varicella exposure. Chickenpox is most often a mild disease, especially for children. Prior to the introduction of vaccine, there were around 4,000,000 cases per year in the US, mostly children, with typically 100 or fewer deaths. Though mostly children caught it, the majority of deaths (by as much as 80%) were among adults. Additionally, chickenpox involved the hospitalization of about 10,000 people each year. During 2003 and the first half of 2004, the CDC reported eight deaths from varicella, six of whom were children or adolescents. These deaths and hospitalizations have substantially declined in the US due to vaccination, though the rate of shingles infection has increased for the same reason. The vaccine has more recently been determined to be effective at preventing shingles (zoster) in persons 60 years of age and older, if administered regularly.

The long-term duration of protection from varicella vaccine is unknown, but there are now persons vaccinated more than thirty years ago with no evidence of waning immunity, while others have become vulnerable in as few as 6 years. Assessments of duration of immunity are complicated in an environment where natural disease is still common, which typically leads to an overestimation of effectiveness, and we are only now entering an era in the US where the long-term efficacy of varicella vaccine can be accurately gauged.

The vaccine is exceedingly safe: approximately 5% of children who receive the vaccine develop a fever or rash, but there have been no deaths yet (as of 1 May 2006) attributable to the vaccine despite more than 40 million doses being administered. A mean of 2,350 reports per year are attributed to varicella vaccine based on 20,004 cases reported to the Vaccine Adverse Event Reporting System (VAERS) database from May, 1995 through December, 2003. Minor events are known to be under-reported reported to VAERS.

Controversy
Mortality due to primary varicella has declined significantly in countries which make wide use of the varicella vaccine. Zoster (shingles) occurs decades after varicella and unsurprisingly zoster incidence has not declined in multiple studies. It is too early to observe the effect on postherpetic neuralgia (PHN).

It has been claimed that shingles may increase after introduction of varicella vaccine. . There is yet no evidence this has occurred, and it might occur in the absence of immunisation due to a general decrease in childhood infection for other reasons.

Vaccination is common in the United States. 41 of the 50 states require immunization for children attending government-run schools. The vaccination is not routine in the United Kingdom. Debate continues in the UK on the time when it will be desirable to adopt routine chickenpox vaccination, and in the US opinions that it should be dropped, individually, or along with all immunizations, are also voiced.

Duration of immunity
Some vaccinated children have been found to lose their protective antibody in as little as five to eight years; however, according to the World Health Organization: "After observation of study populations for periods of up to 20 years in Japan and 10 years in the United States, more than 90% of immunocompetent persons who were vaccinated as children were still protected from varicella." As time goes on, boosters may be determined to be necessary, and introduced. Persons infected after vaccine experience milder cases of chicken pox.

Catching wild chickenpox as a child has been thought to commonly result in lifelong immunity, indeed parents have deliberately ensured this in the past with "pox parties" (and similarly for some other diseases such as rubella. See below.)  Historically, exposure of adults to contagious children has boosted their immunity, reducing the risk of shingles. Second episodes of chickenpox have been rare, but occur and probably more frequently in the UK latterly and definitely more frequently in the vaccine group. In one study, 30% of children had lost the antibody after five years, and 8% had already caught "wild" chickenpox in that five year period.

The CDC and corresponding national organisations are carefully observing the failure rate which may be high compared with other modern vaccines - large outbreaks of chickenpox having occurred at schools which required their children to be vaccinated..

Immunocompromise
The mortality rate in immunocompromised patients with disseminated herpes zoster is 5-15%, with most deaths from pneumonia. Vaccines, unfortunately are less effective among these high-risk patients, as well as being more dangerous because it is an attenuated live virus (see last footnote), but clearly immunisation before immunocompromise would be desirable.

Pox parties
A "pox party" is a party held by parents for the purpose of infecting their children with childhood diseases. Similar ideas have applied to other diseases, e.g. measles, but are now discouraged by doctors and health services. The rationale behind such parties is that guests exposed to the varicella virus will contract the disease and develop strong and persistent immunity, at an age before disaster is likely particularly from chickenpox or rubella. Such parties are now less common in mainstream communities. They are essentially a revival of primitive, pre-vaccination attempts at inoculation.

The first reference to such a practice is the letter of Lady Montagu to Sarah Chiswell describing the parties people in Istanbul made for the purpose of variolation - an effective technique for gaining immunity to smallpox, which she imported to England.

Pox parties have been portrayed in TV cartoons, including South Park ("Chickenpox") and The Simpsons ("Milhouse of Sand and Fog").