Human herpesviruses 6 and 7: effects on hematopoiesis and mode of transmission.

Human herpesvirus 6 (HHV-6) and human herpesvirus 7 (HHV-7) were recently discovered, and are known as etiologic agents of exanthem subitum (roseola). HHV-6 and HHV-7 are T-lymphotropic, and have been classified as betaherpesviruses. In monitoring of herpesviruses after hematopoietic stem cell transplantation, each herpesvirus had a unique temporal profile of detection. HHV-6 DNA was detected most frequently at 3 weeks, whereas cytomegalovirus and Epstein-Barr virus DNA were detected later. HHV-7 DNA was not detected throughout the observation period. In in vitro hematopoietic colony assays, HHV-6 suppressed all three lineages of hematopoiesis, i.e., erythroid, granulocyte/macrophage, and megakaryocyte, whereas HHV-7 did not have any suppressive effect. Molecular epidemiological analysis revealed that HHV-7 was transmitted horizontally from grandparents to parents to children through close contact within a household. Either parent could transmit HHV-7 to the children. Follow-up studies of the amount of viral DNA in saliva samples revealed that the amount of HHV-7 DNA was rather constant for each individual, and that "high producers" and "low producers" could be distinguished. Transferred antibodies against HHV-7 tended to be higher and remain longer after birth than those of HHV-6, and these findings are consistent with the clinical observation that HHV-6 infection occurs earlier than HHV-7 infection.