What Causes Herpes Zoster (Shingles)?

There the viruses settle down in an inactive (latent) form inside specific nerve cells (neurons) that relay information to the brain about what your body is sensing — whether your skin feels hot or cold, whether you've been touched or feel pain. These lie in clusters (ganglia) adjacent to the spinal cord and brain and are one type of sensory neurons.

It stays inactive until a period when your immunity is down. Somehow, the virus gets a signal that your immunity has become weakened and this triggers the reactivation. Doctors cannot always be sure what the trigger is in each case. They don't know why the virus reactivates in one person with these risk factors, while in another person with the same risk factors, it does not. Researchers would really like to understand why some people are NOT susceptible to shingles.

Researchers do know that the varicella-zoster virus behaves differently from other viruses, such as the flu virus. Our immune system usually kills off invading germs, but it cannot completely knock out this type of virus. The virus just becomes inactive.

When the chickenpox virus reactivates, the virus moves down the long nerve fibers (dermatome) that extend from the sensory cell bodies to the skin. The nerve path begins at specific points in the spine, continues around one side of the body, and surfaces at the nerve endings in the skin. The pattern of the rash reflects the location of that nerve path. Herpes Zoster is distinctive because it affects only one side of the body.

Once reactivated, the viruses multiply, the telltale rash erupts, and the person now has herpes zoster, or shingles. With shingles, the nervous system is more deeply involved than it was during the bout with chickenpox, and the symptoms are often more complex and severe.

Scientists call the chickenpox/shingles-causing agent varicella-zoster. Varicella is a Latin word meaning "little pox" to distinguish the virus from smallpox, the highly contagious and often fatal scourge that disfigured or killed millions of people, especially during the Middle Ages. Zoster is the Greek word for "girdle"; shingles often produces a girdle of blisters or lesions around the waist. This striking pattern also gives the disease its common name: the word shingles comes from cingulum, the Latin word for belt or girdle.

The varicella-zoster virus looks as though it were designed by a mathematician. It is a microscopic sphere encasing a 20-sided geometric figure called an icosahedron. Inside the icosahedron is the genetic material of the virus, deoxyribonucleic acid (DNA). When activated, the virus reproduces inside the nucleus of an infected cell. It acquires its spherical wrapping as it buds through the nuclear membrane.

Risk Factors

The virus seems to need a combination of risk factors in order for a reactivation to be triggered. The usual factors are an aging immune system combined with illness, stress, or even sunburn.

• The leading risk factor for shingles is a history of having had chickenpox. One out of every five people who have had chickenpox is likely to get shingles.
• Another risk factor is aging. As we age, our natural immunity gradually loses its ability to protect against infection. The shingles virus can take advantage of this and reactivate.
• Conditions that weaken immunity can also put people at risk for shingles. Shingles is especially dangerous for anyone who has had cancer, radiation treatments for cancer, HIV infection, or a transplant operation.

Risks for Women

Many mothers-to-be are concerned about any infection contracted during pregnancy, and rightly so. It is well known that certain viruses can be transmitted across the mother's bloodstream to the fetus, or can be acquired by the baby during the birth process.

Maternal chickenpox poses some risk to the unborn child, depending upon the stage of pregnancy during which the mother contracts the disease. During the first 30 weeks, maternal chickenpox may, in some cases, lead to congenital malformations. Such cases are rare and experts differ in their opinions on how great the risk is.

If the mother gets chickenpox from 21 to 5 days before giving birth, the newborn may have chickenpox at birth or develop it within a few days, as noted earlier. But the time lapse between the start of the mother's illness and the birth of the baby generally allows the mother's immune system to react and produce antibodies to fight the virus. These antibodies can be transmitted to the unborn child and thus help fight the infection. Still, a third of the babies exposed to chickenpox in the 21 to 5 days before birth develop shingles in the first 5 years of life because the virus must also be fought by immune cells.

What if the mother contracts chickenpox at precisely the time of birth? In that case the mother's immune system has not had a chance to mobilize its forces. And although some of the mother's antibodies will be transmitted to the newborn via the placenta, the newborn will have precious little ability to fight off the attack because the immune system is immature. For these babies chickenpox can be fatal. They must be given "zoster immune globulin," a preparation made from the antibody-rich blood of adults who have recently recovered from chickenpox or shingles.

Source: NIH