Varicella - Zoster Virus ( Herpes Zoster ) Infections 23

Varicella-zoster virus is one of six herpesviruses isolated from humans. The core of a typical herpesvirus contains a linear, double-stranded DNA, whereas the viral capsid is icosadeltahedral and contains 162 capsomeres with a hole running down the long axis (1). The varicella-zoster virus (VZV) is associated with two distinct clinical syndromes: varicella (chickenpox) and herpes zoster (shingles). Whereas varicella is a ubiquitous and highly contagious primary infection affecting the general population (especially in childhood), herpes zoster is less common endemic clinical condition that usually occurs in older and/or immunocompromised individuals. AIDS patients with CD4+ counts of 500 cells/mm3 or less, or organ-transplant recipients (especially bone-marrow allograft recipients) are at significant risk of VZV infections. Furthermore, patients who have received prior repeated acyclovir treatment have the highest risk of harboring acyclovir-resistant strains (2,3). Herpes zoster usually is manifested with a painful vesicular eruption customary limited to a single dermatome, although cases of generalized eruptions have also been observed. It does not associate to exogenous exposure but appears to be secondary to reactivation of VZV that remained latent after an earlier attack of varicella (4). In general, the pathogenesis and mechanism of reactivation of herpes zoster are not well-understood. Predisposing factors associated with the appearance of herpes zoster are generally linked to compromised immune defenses (5) and include Hodgkin’s disease and other lymphomas, immunosuppressive therapy, trauma to the spinal cord and adjacent structures, and heavy-metal poisoning (5–8). In some instances, the host immune response is still viable enough to halt cutaneous lesions, but not the necrosis and inflammatory response in the ganglion. Such cases, known as zoster sine herpete, are characterized with radicular pain without associated skin lesions (7,9,10). The disease tends to be more severe in patients with malignancies, those with immune deficiencies, or receiving immunosuppressive therapy. Cutaneous dissemination, which occurs in up to 50% of immunocompromised patients, usually does not affect the morbidity and mortality in this population. However, patients with visceral disease (particularly pneumonitis) have increased mortality rate (4). The most common complication of herpes zoster is the postherpetic neuralgia that occurs in nearly 50% of patients 60 yr and older; it has been rarely observed in patients under 40 yr. Other complications, especially in immunocompromised hosts include chronic zoster (11), and persistent CNS infection (12,13). De La Blanchardiere et al. (14) conducted a multicenter retrospective study to evaluate the clinical features and prognostic significance of VZV-associated neurological complications. Results of the study showed that encephalitis, myelitis, radiculitis, and meningitis were the most predominant neurological manifestations.