Human herpesviruses (HHV) 6, 7, and 8 are capsid-enclosed DNA viruses that produce lytic and latent infection of lymphocytes and other cell types. Reactivation of latent infection occurs intermittently, with replication of virus in tissues and shedding in various secretions. HHV-6, HHV-7, and HHV-8 constitute a diverse group in terms of their biology, pathogenesis, and the diseases they produce; HHV-6 and HHV-7 are able to infect a broader array of cell types than HHV-8. Clinical presentation ranges from asymptomatic infection or mild illnesses, such as febrile exanthems, in the case of HHV-6 and HHV-7, extending to life-threatening disease in the immune compromised host. Selected clinical and virologic characteristics are summarized in Table 188.1 and limited antiviral treatment information is presented in Table 188.2.

HUMAN HERPESVIRUS 6

HHV-6 is a member of the Betaherpesvirinae group of the genus Roseolovirus, of which cytomegalo-virus (CMV) was the only previously recognized human pathogen. HHV-6 consists of two related variants, HHV-6A and HHV-6B, that have 90% DNA homology and cannot be distinguished by serologic tests, but have distinctive molecular, cell culture, and clinical features. Infection is ubiquitous; 70% to 100% of adults worldwide have serologic evidence of HHV-6 infection. Infection follows a 2-week incubation period and most often occurs between the ages of 6 and 15 months.