Shingles, also known as herpes zoster, is a viral infection characterized by a painful skin rash that typically appears as blisters in a localized area. This condition arises from the reactivation of a specific virus that remains dormant within the body.
The Herpesvirus Family
Shingles is caused by the varicella-zoster virus (VZV), which is the same virus responsible for chickenpox. VZV belongs to the Herpesviridae family. This family also includes herpes simplex virus type 1 (HSV-1), which commonly causes oral herpes (cold sores), and herpes simplex virus type 2 (HSV-2), primarily associated with genital herpes. All viruses in this family establish lifelong, latent infections.
After an initial infection, such as chickenpox, VZV does not leave the body; instead, it retreats and lies dormant in nerve cells. This latent state can persist for decades without causing symptoms. Under certain conditions, such as aging or a weakened immune system, the dormant VZV can reactivate. When VZV reactivates, it travels along the nerve pathways to the skin, resulting in the characteristic rash and pain of shingles. Similarly, HSV-1 and HSV-2 also establish latency in nerve cells after initial infection and can periodically reactivate, leading to recurrent outbreaks of sores.
Diagnostic Testing for Herpesviruses
Diagnosing herpesvirus infections, including shingles (VZV) and herpes simplex (HSV), typically involves several laboratory methods designed to identify the specific virus or the body’s immune response to it. One of the most common and reliable tests for direct viral detection is the polymerase chain reaction (PCR) assay. PCR tests are highly sensitive and specific, as they detect the unique genetic material (DNA) of either VZV or HSV in samples taken directly from a lesion, such as fluid from a blister. This means a PCR test can accurately distinguish between VZV and HSV DNA, providing clear identification of the causative virus.
Viral culture is another method where a sample from a lesion is placed in a special medium to allow any virus present to grow. While it can identify VZV and HSV, viral culture is generally less sensitive than PCR and takes longer to yield results, sometimes several days. For instance, VZV is slower to replicate in culture compared to HSV, and HSV can sometimes overgrow VZV in a mixed sample, making VZV detection difficult. Despite this, viral culture has historically been considered a standard for HSV detection.
Antibody tests, also known as serology, detect antibodies produced by the body’s immune system in response to a viral infection. These tests typically look for immunoglobulin G (IgG) and immunoglobulin M (IgM) antibodies in the blood. IgG antibodies indicate a past infection or vaccination, while IgM antibodies suggest a recent or active infection.
Interpreting Test Results
When a person has shingles, the infection is caused by the varicella-zoster virus (VZV), not the herpes simplex virus (HSV). Therefore, if a diagnostic test is specifically designed to detect HSV (type 1 or type 2), a person with shingles would typically not test positive for HSV. Highly specific tests, such as PCR assays, are designed to identify the distinct genetic material of VZV or HSV, ensuring an accurate differentiation between the two viruses. This specificity means that a positive VZV PCR result confirms shingles, while a negative HSV PCR result simultaneously rules out an active HSV infection as the cause of the rash.
However, confusion can arise with certain antibody tests due to the biological relationship between VZV and HSV. Since both viruses belong to the Herpesviridae family, there can be some antigenic cross-reactivity, particularly with less specific serological assays. This means that antibodies developed in response to a VZV infection might, in some instances, show a weak reaction to HSV antigens in a general antibody test. Such a result does not indicate an active HSV infection but rather reflects the body’s immune history with viruses from the same family.
Clinical diagnosis, which involves evaluating a patient’s symptoms and the appearance of the rash, remains an important component alongside laboratory testing. For instance, shingles typically presents as a painful rash in a specific band-like pattern on one side of the body, which differs from the usual presentation of HSV infections. When laboratory results are considered in conjunction with these clinical observations, healthcare providers can accurately differentiate between an active shingles infection (caused by VZV) and an HSV infection, even if a person has had exposure to both viruses in their lifetime.