DESCRIPTION
Four years after the isolation of HHV-6, another herpesvirus was discovered. Because of the similarities in genes and gene products between HHV-6 and HHV-7 some cross-reactivity have been noted. It is not known whether it causes any disease.
HHV-7 is most closely related to HHV-6 at the genetic level, among all known herpesviruses, and its next closest sibling is human cytomegalovirus (HCMV). HHV-7 has not, as yet, been classified, although its genetic and biological properties would seem to be consistent with a similar classification proposed for HHV-6, which is in the genus Roseolovirus, within the beta herpesvirus subfamily.
Human herpesvirus 7 (HHV-7) is a recently described T-lymphotropic herpesvirus, which infects almost all children by the age of three years and persists lifelong, with the shedding of infectious virus in saliva. HHV-7 is similar to human herpesvirus 6 (HHV-6) in its genetic content and in many of its biological properties, which include the ability to cause at least some cases of exanthem subitum (roseola). Despite these similarities, important differences between HHV-7 and HHV-6 exist, including the fact that HHV-7 binds to the cellular CD4 molecule and uses this protein as a necessary component of its receptor, while HHV-6 binds to a different (and unknown) receptor. Furthermore, the pathogenesis and sequelae of HHV-7 infection remain very poorly understood.
METHOD OF TRANSMISSION
In initial studies, it was established that HHV-7 infection was widespread among populations in the USA, Europe and Japan. It was observed that it was acquired later in life than HHV-6 (older than 3 years). The source of the virus appears to be saliva, because HHV-7 can be easily found in oral secretions or seroimmune adults.
SYMPTOMS/CLINICAL CONDITIONS
No apparent disease occurs in late childhood that could be associated with an acute HHV-7 infection. It might be responsible for a syndrome mimicking roseola associated with acute HHV-6 infection, since seroconversions to HHV-7 antigens has been documented. Acute HHV-7 infection has also been associated with hepatitis in an 18-month old girl. Pending the results of further investigations, the most likely conclusion is that primary HHV-7 infection is not associated with a definable syndrome in the majority of children who acquire HHV-7 infection after 2 years of age.
