Antiviral therapy in herpes zoster: a review

Herpes zoster is a disease caused by reactivation of the latent form of the varicella zoster virus (VZV). It is usually seen in adults, occurring mainly in the elderly. The acute phase of the disease is characterized by a rash, which is typically accompanied by pain. In some patients the pain may persist after the rash has healed, and may last for many months or years. Three antiviral agents are currently available to treat herpes zoster: aciclovir, its prodrug valaciclovir, and famciclovir. All three are effective in accelerating healing of the rash, and reducing the patient's period of infectivity. These antiviral agents also impact on the chronic pain associated with herpes zoster but appear to differ in their efficacy. Two different measures of chronic pain have been used in clinical studies: post-herpetic neuralgia (PHN) which refers to the pain occurring after the rash has healed, and zoster-associated pain (ZAP), defined as the continuum of pain occurring after the onset of herpes zoster (i.e. making no distinction between acute pain and PHN). Famciclovir has been shown to significantly reduce the risk and duration of PHN in patients over 50 years old. In another study famciclovir was shown to be significantly more effective than aciclovir in relieving ZAP when treatment was taken within 48 h of the onset of herpes zoster. Valaciclovir was also found to be better than aciclovir in reducing the duration of ZAP, but it is unclear whether this improvement over aciclovir also applied to PHN. Elderly patients therefore benefit from antiviral therapy, which should be initiated as early as possible, and can significantly reduce the risk and duration of the chronic pain associated with herpes zoster.