Clinical features of primary HHV-6 and HHV-7 infections

Human herpesvirus-6 (HHV-6), a T-lymphotropic herpesvirus, belongs to one of two variants, A or B (HHV-6A and HHV-6B). HHV-6B is the cause of exanthem subitum (ES) which had a wide spectrum of related illnesses in the central nervous system, gastrointestinal tract, respiratory tract, and blood cells including fatal outcome, however, a clear etiologic role has not been identified for HHV-6A. HHV-6 is ubiquitous and primary infection with the virus almost always occurs before the age of 2 years. On the other hand, human herpesvirus-7 (HHV-7), isolated from CD4+ T lymphocytes from the peripheral blood of a healthy individual has been recognized as a new lymphotropic herpesvirus. The virus was distinct from the six previously identified human herpesviruses and had limited homology to human cytomegalovirus and HHV-6 by both molecular and immunological analyses. Healthy adults frequently shed the virus into saliva, and children are infected at a young age but somewhat later than HHV-6B. The primary infection with HHV-7 is linked to febrile illness with or without rash that resembles ES. A consensus is needed on whether the term should be used only for clinical features by primary infection with HHV-6 or for clinical syndromes featuring febrile exanthem by various infectious agents including HHV-6, HHV-7, enteroviruses, etc.