Varicella zoster virus
Varicella zoster virus
Varicella zoster virus is a member of the alphaherpesvirus group and is the cause of both chickenpox (also known as varicella) and shingles (herpes zoster).
The virus is surrounded by a covering, or envelope, that is made of lipid. As such, the envelope dissolves readily in solvents such as alcohol. Wiping surfaces with alcohol is thus an effective means of inactivating the virus and preventing spread of chickenpox. Inside the lipid envelope is a protein shell that houses the deoxyribonucleic acid .
Varicella zoster virus is related to Herpes Simplex viruses types 1 and 2. Indeed, nucleic acid analysis has revealed that the genetic material of the three viruses is highly similar, both in the genes present and in the arrangement of the genes.
Chickenpox is the result of a person's first infection with the virus. Typically, chickenpox occurs most often in children. From 75% to 90% of the cases of chickenpox occur in children under five years old. Acquisition of the virus is usually via inhalation of droplets containing the virus. From the lung the virus migrates to the blood stream. Initially a sore throat leads to a blister-like rash that appears on the skin and the mucous membranes, as the virus is carried through the blood stream to the skin. The extent of the rash varies, from minimal to all over the body. The latter is also accompanied by fever, itching, abdominal pain, and a general feeling of tiredness. Recovery is usually complete within a week or two and immunity to another bout of chickenpox is life-long.
In terms of a health threat, childhood chickenpox is advantageous. The life-long immunity conferred to the child prevents adult onset infections that are generally more severe. However, chickenpox can be dangerous in infants, whose immune systems are undeveloped. Also chickenpox carries the threat of the development of sudden and dangerous liver and brain damage. This condition, called Reye's Syndrome, seems related to the use of aspirin to combat the fever associated with chickenpox (as well as other childhood viruses). When adults acquire chickenpox, the symptoms can be much more severe than those experienced by a child. In immunocompromised people, or those suffering from leukemia, chickenpox can be fatal. The disease can be problematic in pregnant women in terms of birth defects and the development of pneumonia .
Treatment for chickenpox is available. A drug called acyclovir can slow the replication of the virus. Topical lotions can ease the itching associated with the disease. However, in mild to moderate cases, intervention is unnecessary, other than keeping the affected person comfortable. The life-long immunity conferred by a bout of chickenpox is worth the temporary inconvenience of the malady. The situation is different for adults. Fortunately for adults, a vaccine to chickenpox exists for those who have not contracted chickenpox in their childhood.
Naturally acquired immunity to chickenpox does not prevent individuals from contracting shingles years, even decades later. Shingles occurs in between 10% and 20% of those who have had chickenpox. In the United States, upwards of 800,000 people are afflicted with shingles each year. The annual number of shingles sufferers worldwide is in the millions. The disease occurs most commonly in those who are over 50 years of age.
As the symptoms of chickenpox fade, varicella zoster virus is not eliminated from the body. Rather, the virus lies dormant in nerve tissue, particularly in the face and the body. The roots of sensory nerves in the spinal cord are also a site of virus hibernation. The virus is stirred to replicate by triggers that are as yet unclear. Impairment of the immune system seems to be involved, whether from immunodeficiency diseases or from cancers, the effect of drugs, or a generalized debilitation of the body with age. Whatever forces of the immune system that normally operate to hold the hibernating virus in check are abrogated.
Reactivation of the virus causes pain and a rash in the region that is served by the affected nerves. The affected areas are referred to as dermatomes. These areas appear as a rash or blistering of the skin. This can be quite painful during the one to two weeks they persist. Other complications can develop. For example, shingles on the face can lead to an eye infection causing temporary or even permanent blindness. A condition of muscle weakness or paralysis, known as Guillan-Barre Syndrome, can last for months after a bout of shingles. Another condition known as postherpetic neuralgia can extend the pain of shingles long after the visible symptoms have abated.
See also Immunity, active, passive and delayed; Infection and resistance; Latent viruses and diseases