Chickenpox

virus belongs to the family Herpesviridae, genus varicella-zoster virus. Its appearance is similar to that of other herpes viruses belonging to the same family.
The virus consists of a double-helix DNA genome that codes for more than 70 proteins, including those that include the onset of humoral and cellular immunity by the host: it is the smallest genome among those of herpes viruses.
The particles are spherical pleomorphic to pleomorphic in shape, about 150-200 nm in diameter, and are composed of three protein layers: a nucleocapsid containing the viral double-helix DNA (dsDNA) genome; a tegument layer, consisting of numerous proteins of both viral and host origin surrounding the nucleocapsid; and an envelope comprising a lipid bilayer derived from the inserted host with outward-facing viral glycoproteins.
Upon entry of a VZV virion into the host cell, tegument proteins are released into the newly infected cell, altering the host environment, thereby inhibiting antiviral responses and affecting the fate of the viral program, i.e., lytic or latent infection.
Symptomatic reactivation of the virus, leading to the manifestation called Herpes Zoster or associated diseases, typically occurs only once or twice in the lifetime of an infected individual and probably requires two events: stimulation of a latent infected neuron and overcoming existing immune protection mechanisms. In contrast, asymptomatic reactivation is believed to occur more frequently.
However, the environmental factors and molecular mechanisms underlying virus reactivation in humans remain poorly understood.

Chickenpox is a viral infectious disease, less contagious than measles and whooping cough, but more contagious than rubella and mumps. The incubation period ranges from 14 to 16 days, with a minimum of 10 and a maximum of 21, from the time of infection to the time of the appearance of the rash. Infection originates from contact of the virus with the upper airway of a susceptible individual: the early stages of infection are characterized by multiplication of the virus in the pharyngeal nose and conjunctiva during the next 2 to 3 days. From here it reaches the regional lymph nodes early, where it continues to multiply. A primary viremia subsequently starts from these sites with spread to the cells of the reticuloendothelial system. The already immune individual who comes in contact with the virus is able to limit its multiplication and allow the local infection to be easily overcome.

VZV-related diseases still occur worldwide. In some developing countries, VZV infection is of less concern, compared with other more serious infectious diseases, but the growing number of immunocompromised individuals worldwide requires the establishment of worldwide herd immunity against VZV.
Even in developed countries where routine varicella vaccination has been performed, VZV infection has been far from eradication, the common ultimate goal of infectious disease research. To achieve these goals, several distinct research strategies have been actively pursued in the field of VZV research.

The incubation period varies from 14 to 16 days, with a minimum of 10 and a maximum of 21, from the time of infection to the time the rash appears.
Chickenpox is characterized by blistering, itchy exanthema that appears, in successive spurts, all over the body, and by general, usually mild, symptoms. Prodromal symptoms, present mainly in older children are fever, malaise, anorexia, headache, and sometimes abdominal pain.
Vesicular lesions appear first on the scalp, then on the face, trunk, and limbs: the exanthem begins with small, itchy, erythematous macules that evolve over a few hours into vesicles, filled with clear fluid, on an erythematous base. The vesicles are very superficial and fragile, so the roof easily ruptures.
During primary infection, VZV gains access and establishes a permanent latency in the ganglion of neurons along the entire human spine. Months or years later, when VZV-specific cellular immunity (but not humoral immunity) wanes, VZV can reactivate to cause shingles, characterized by painful rash.

In most cases, the diagnosis of chickenpox proves to be extremely easy. The most typical aspects of the rash are represented by:

• papulo-vesicular rash accompanied by low fever and mild general symptoms;
• rapid progression of macules into papules and of these into vesicles and then into crusts;
• appearance of the skin manifestations in successive spurts with predominant distribution to the trunk and with scalp involvement;
• concomitance in the various skin districts of elements in various stages of evolution

Chickenpox typically heals without trace.
The prognosis becomes severe under particular conditions, more because of host characteristics than because of any particular virulence of the causative agent. In general, chickenpox is a self-limiting disease, but it can sometimes be followed by serious complications, including bacterial sepsis, pneumonia, hepatitis, encephalitis, and hemorrhage, and occasionally can result in death. These serious complications and death are most evident in infants, adults, and severely immunocompromised individuals.

Home isolation for at least 5 days from the appearance of the first spurt of vesicles, with restriction of contact with other susceptible individuals, especially pregnant women and infants, is provided for varicella patients. In any case, the main tool for preventing the spread of the disease is vaccination. A single dose of varicella vaccine is 80-85% effective in preventing disease of any severity and more than 95% effective in preventing severe varicella. The vaccination program has reduced the incidence of the disease by 57-90%, hospitalizations by 75-88%, deaths by 74%, and direct inpatient and outpatient medical costs by 74%. Despite a dramatic decline in varicella disease with one-dose vaccination, continued outbreaks of varicella have been reported among the school-age population with high coverage rates. Therefore, in 2006, a two-dose varicella vaccination program was recommended by the Centers for Disease Control and Prevention (CDC). The U.S. two-dose vaccination program reduced the odds of developing chickenpox by 95 percent and appears to have significantly reduced the number, size, and duration of outbreaks.
However, the two-dose vaccination program has not yet been incorporated into routine vaccination strategies for several countries.