Causes and Risk Factors of Shingles

Shingles results from reactivation of the varicella virus, which, when it first infects the body, causes chickenpox and then goes into hiding in the nervous system. Why the virus re-emerges isn't entirely understood, but there are theories. Shingles is most common in older people, but anyone with a weakened immune system—due to health conditions or use of certain medications—can be affected. In fact, reduced immunity is considered the biggest risk factor for shingles. Researchers believe that stress could also play a role for some people.

Shingles is an especially unpleasant illness. It causes a painful and unsightly skin rash, as well as potential long-term complications, the most common one being a condition known as postherpetic neuralgia (PHN), which is characterized by a burning sensation where the shingles rash once was. That's why it's important to understand what causes chickenpox, who's most at risk of coming down with it, and how to protect yourself if you're exposed.

After a person recovers from chickenpox, the symptoms disappear but the varicella virus that caused it retreats to cells in the nervous system, where it can hang out for decades without causing problems.

When the virus re-emerges, it typically reactivates in clusters of nerve cells in the peripheral nervous system called a sensory ganglion. The ganglia most likely to host varicella are those in the cervical, thoracic, and lumbar spine.

Varicella also often affects the trigeminal ganglion that provides sensation to the face. As its name suggests, this particular clump of nerves has three branches. The one associated with eye function, the ophthalmic branch, is 20 times more likely than the other two to be affected.

The body part associated with the particular nerve cells in which the virus reawakens is where the shingles symptoms—extreme pain, unsightly rash—will be concentrated. Since the nervous system consists of tree-like branches of nerves, the blisters will follow the particular path of the nerves affected. That's why a shingles rash often resembles a swath of blisters in a very specific area, rather than spread all over the body (as in chickenpox).

What prompts the varicella virus to reactivate isn't entirely understood. The virus is a member of the same family of microbes that cause herpes infections, such as genital herpes and cold sores, which also tend to come and go, so it's not surprising that varicella would behave similarly. The big difference is, while herpes infections can recur multiple times, most people only experience shingles once.

In any event, there are two main causes of shingles:

Weakened Immune System

There is a clear association between shingles and weakened immunity to infection. Even though the varicella virus is not invading the body for the first time, the immune system still is responsible for keeping it at bay. It's when the immune system isn't strong enough to do this that the virus may take the opportunity to wreak havoc.

What this means is that the varicella virus, which after causing the chickenpox rash in the skin had traveled to ganglia in the nervous system, becomes active again and heads back to the skin. Since it travels along the nervous system, the rash stays on one side of the body and appears in the shape of a strip or band that aligns with the shape of the nerves below the skin. 

Stress

There's a long-held hypothesis that chronic stress or even a single episode of emotional distress can trigger the dormant varicella virus to become active again and bring on a shingles outbreak. Given that stress often is linked to any number of changes in health, including gastrointestinal problems, migraines, and eczema, this notion is not at all far-fetched.

In fact, there is some evidence to support it. For example, an often-cited 1998 study of otherwise healthy adults over 60 found that those who had had shingles were more than twice as likely to have had a negative life event within six months of the outbreak as peers who didn't have shingles. When asked about events within the past two to three months specifically, those in the shingles group reported the same amount of negative life events as their unaffected counterparts. This suggests that perceiving an event as stressful, rather than the event itself, may be linked to increased rate of shingles.

More recent research has largely supported this concept. Some have taken this to mean that the overall perception of stress and the ability to cope with it, may add to the underlying factors that create the perfect storm for a shingles outbreak.

A study aimed to determine whether Tai Chi, when used as a stress reduction tool, had any bearing on the incidence of shingles in older adults. While the study was small, its authors were able to report that a 15-week course of Tai Chi, practiced for 45 minutes three times a week, was associated with an increase in the cell-mediated immunity specific to varicella virus.

While the investigators were unable to correlate this to a reduction in shingles risk, the study suggested that the very practice of stress reduction can yield beneficial physiological changes to adults at risk of stress-related illnesses.

Since compromised immunity is the most common trigger for a latent varicella virus to become active, any factor associated with a weakened immune system can increase the risk of shingles. Important risk factors for shingles include:

• Being 50 or over. With age, there is a natural decline in cell-mediated immunity.
• Infection from human immunodeficiency virus (HIV). Although most people who develop shingles have the infection just once, it's not uncommon for someone with HIV to have recurrent shingles infections.
• A chronic medical condition. Cancer (especially leukemia or lymphoma) or diabetes are examples.
• Medication that suppresses the immune system. Some examples of these include chemotherapy drugs and systemic steroids, such as prednisone.
• Having an organ transplant. The medications that are necessary for preventing organ rejection suppress the immune response. 

Note that many of these risk factors are as likely to apply to young people and children as they are to older people. So, even though shingles often is regarded as an illness of advancing age, this isn't always the case.