Human herpesviruses 6, 7, and 8 (HHV-6, HHV-7, and HHV-8) are DNA viruses enclosed in a capsid that produce lytic and latent infection of lymphocytes and other cell types. Latent infection has been implicated in the etiology of several malignancies, although causation of malignancy has been established only for HHV-8 at this time. Reactivation of latent infection occurs intermittently, with replication of virus in various tissues and secretions. HHV-6, -7, and -8 constitute a diverse group in terms of their biology and pathogenesis and the diseases they produce; HHV-6 and -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 exanthem subitum in the case of HHV-6, extending to life-threatening disease in the immune compromised host, such as HHV-8–associated Kaposi's sarcoma or HHV-6 encephalitis. Selected clinical and virologic characteristics are summarized in Table 186.1, and limited antiviral treatment information is presented in Table 186.2.

HUMAN HERPESVIRUS 6

HHV-6 is a member of the Betaherpesvirinae group, of which cytomegalovirus was the previously recognized human pathogen, and is placed in the genus Roseolovirus. 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. Seventy to 100% of adults have serologic evidence of HHV-6 infection with both variants worldwide. Infection follows a 2-week incubation period and most often occurs between the ages of 6 and 15 months.