Shingles, also known as herpes zoster, is caused by the Varicella-Zoster Virus (VZV). This virus is responsible for two distinct diseases: the initial infection, chickenpox, and the later reactivation, shingles. A person must have been infected with VZV at some prior point for shingles to occur.
How the Varicella-Zoster Virus Causes Both Conditions
The life cycle of the Varicella-Zoster Virus is a two-stage process that links both chickenpox and shingles. The first stage is the primary infection, which manifests as chickenpox, characterized by a widespread, itchy, vesicular rash. During this initial illness, the virus enters the body, likely through the respiratory tract, and spreads through the bloodstream, eventually reaching the skin.
Although the immune system clears the visible rash, the virus remains in the host. Instead, VZV travels from the skin lesions and enters the sensory nerve endings. It then moves along the nerve fibers to the dorsal root ganglia, which are clusters of nerve cells located near the spinal cord and brain.
Once inside these nerve cells, the virus transitions into a dormant state called latency, where it can remain inactive for decades. During this latent period, the body’s immune system, specifically VZV-specific cell-mediated immunity, works to keep the virus suppressed and prevent it from replicating. Shingles is the reactivation of this latent VZV, where the virus travels back down the nerve pathway to the skin, causing a painful, localized rash.
The Possibility of Undetected Chickenpox
Since shingles is defined as the reactivation of a latent virus, a prior infection with VZV is required. However, many people who develop shingles do not recall ever having chickenpox.
This situation is explained by subclinical infection, where an individual is infected with VZV but experiences a case of chickenpox so mild that it produces no rash or symptoms. The virus still establishes latency in the nerve ganglia, meaning the person unknowingly carries the dormant VZV. As a result, they remain vulnerable to a future shingles outbreak, despite their lack of a chickenpox history. Blood tests can detect VZV-specific antibodies, which indicate a previous infection or vaccination, even if the person has no memory of the disease.
Who Is at Risk for Shingles Reactivation
VZV latency is controlled by the strength of the body’s immune response. When this immune response weakens, the virus reactivates, leading to shingles. The most significant risk factor for VZV reactivation is advanced age, as the immune system naturally declines over time, a process known as immunosenescence. The incidence of shingles increases significantly in individuals over 50 years old.
Immune-suppressing conditions and treatments are also major risk factors for reactivation at any age. These include chronic diseases like HIV/AIDS and various cancers, such as leukemia and lymphoma. Additionally, patients receiving immunosuppressive medications, such as those used for organ transplantation or autoimmune disorders, face a higher risk of the dormant virus reawakening. Psychological stress and physical trauma also correlate with an increased risk of shingles outbreaks.
Prevention and Treatment Methods
The most effective strategy for preventing shingles is vaccination, which is recommended for healthy adults 50 years and older. The current recombinant zoster vaccine, Shingrix, is highly effective, offering over 90% protection against shingles and its complications. This vaccine is administered in two doses, typically two to six months apart, and is recommended even for those who have already had shingles or do not recall having chickenpox.
If an outbreak of shingles does occur, prompt treatment with antiviral medication is essential to reduce the duration and severity of the illness. Medications such as acyclovir, valacyclovir, and famciclovir are the preferred options. These antivirals work best when started within 72 hours of the first appearance of the rash. Early intervention can reduce acute pain, accelerate the healing of lesions, and lower the risk of complications, such as the persistent nerve pain called postherpetic neuralgia.