VZV infection gives rise to 2 distinct syndromes. The primary infection, chickenpox, is a contagious and usually benign febrile illness. After this infection resolves, viral particles remain in the dorsal root or other sensory ganglia, where they may lay dormant for years to decades.
In this latent period, host immunologic mechanisms suppress replication of the virus, but VZV reactivates when the host mechanisms fail to contain the virus. Such failure may result from a wide spectrum of conditions, ranging from stress to severe immunosuppression; occasionally, it follows direct trauma. VZV viremia occurs frequently with chickenpox but also may arise with herpes zoster, albeit with a lower viral load.
Once VZV is activated at the spinal root or cranial nerve neurons, an inflammatory response occurs that also encompasses the leptomeninges; both plasma cells and lymphocytes are noted. This inflammation in the dorsal root ganglion can be accompanied by hemorrhagic necrosis of nerve cells. The result is neuronal loss and fibrosis.
The frequency of dermatologic involvement is correlated with the centripetal distribution of the initial varicella lesions. This pattern suggests that the latency may arise from contiguous spread of the virus during varicella from infected skin cells to sensory nerve endings, with subsequent ascent to the ganglia. Alternatively, the ganglia may become infected hematogenously during the viremic phase of varicella, and the frequency of the dermatome involvement in herpes zoster may reflect the ganglia most often exposed to reactivating stimuli.
The appearance of the cutaneous rash due to herpes zoster coincides with a profound VZV-specific T-cell proliferation. Production of interferon alfa appears with the resolution of herpes zoster. In immunocompetent patients, specific antibodies (immunoglobulins G, M, and A [IgG, IgM, and IgA]) appear more rapidly and reach higher titers during reactivation (herpes zoster) than during the primary infection. The patient has a long-lasting, enhanced, cell-mediated immunity response to VZV.[8, 9, 10]
The anatomic location of the involved dermatome often determines the specific manifestations. When cervical and lumbar roots are involved, motor involvement, which is often overlooked, may be evident, depending on the virulence or extent of migration. In at least 1 case of motor neuron involvement, lymphocytic infiltration and myelin breakdown were observed with preservation of axons.
Herpes zoster infections are contagious to persons with no previous immunity to VZV. However, herpes zoster is estimated to be only one third as contagious as primary varicella. It is transmitted either via direct contact with the lesions or via the respiratory route.