The herpesvirus family contains six important human pathogens: herpes simplex virus types 1 and 2, varicella-zoster virus, cytomegalovirus, Epstein-Barr virus, and human herpesvirus 8 (the cause of Kaposi's sarcoma).
All herpesviruses are structurally similar. Each has an icosahedral core surrounded by a lipoprotein envelope (see Color Plate 24). The genome is linear double-stranded DNA. The virion does not contain a polymerase. They are large (120–200 nm in diameter), second in size only to poxviruses.
Color Plate 24
Herpes simplex virus—Electron micrograph. Three HSV virions are visible. Short arrow points to the envelope of an HSV virion. Long arrow points to the nucleocapsid of the virion. Provider: CDC/Dr. John Hierholzer.
They replicate in the nucleus, form intranuclear inclusions, and are the only viruses that obtain their envelope by budding from the nuclear membrane. The virions of herpesviruses possess a tegument located between the nucleocapsid and the envelope. This stucture contains regulatory proteins, such as transcription and translation factors, which play a role in viral replication.
Herpesviruses are noted for their ability to cause latent infections. In these infections, the acute disease is followed by an asymptomatic period during which the virus remains in a quiescent (latent) state. When the patient is exposed to an inciting agent or immunosuppression occurs, reactivation of virus replication and disease can occur. With some herpesviruses, e.g., herpes simplex virus, the symptoms of the subsequent episodes are similar to those of the initial one; however, with others, e.g., varicella-zoster virus, they are different (Table 37–1).
Table 37–1. Important Features of Common Herpesvirus Infections.