[Human herpesvirus-6-associated diseases in hematopoietic stem cell transplantation: an update].

Human herpesvirus (HHV)-6 belongs to the Betaherpesvirinae subfamily of human herpesviruses. Primary HHV-6 infection commonly causes exanthem subitum. Like other herpesviruses, HHV-6 is capable of persisting in the host after the primary infection. Under conditions of immunosuppression, latent HHV-6 can be reactivated. Between 30% and 70% of patients who undergo allogeneic hematopoietic cell transplantation (allo-HCT) experience HHV-6 reactivation at 2-4 weeks after transplantation. Accumulating evidence indicates that HHV-6 is an actual cause of encephalitis after allo-HCT. Risk factors for HHV-6 encephalitis include cord blood transplantation and an inflammatory milieu, which occurs in the early period after allo-HCT. Although HHV-6 encephalitis is associated with a poor prognosis, no validated treatments or preventative measures have as yet been established. HHV-6 reactivation may also cause myelitis, bone marrow suppression, lung disease, hepatitis, delirium, and graft-versus-host disease. However, such associations have not been consistently demonstrated and causality remains uncertain. This review updates the latest information regarding the clinical syndrome accompanying HHV-6 reactivation, with a particular focus on HHV-6 encephalitis, in the form of a series of questions and answers.