Newly discovered human herpesviruses: Pathogenesis and treatments for central nervous system infection due to HHV-6

Human herpesvirus-6 (HHV-6) is the causative agent of the common childhood infectious disease, exanthem subitum. Soon after the virus was isolated from humans it was found to be closely related to human cytomegalovirus, and thus was classified within the β subgroup of human herpesviruses. HHV-6 possesses neuro-tropism in-vitro, and it has been suggested that primary infection can cause complications of the central nervous system (CNS), including febrile seizures and encephalitis/encephalopathy. In addition, this virus has recently been associated with limbic encephalitis, which occurred in mainly immunocompromised adult patients. It is proposed that direct invasion of the virus into the CNS may play an important role in causing these neurological complications. Although pathogenesis of HHV-6 encephalitis/encephalopathy remains unclear, it is thought that two different mechanisms such as direct invasion (primary encephalitis) and immune mediated impairment (secondary encephalitis) might play important role in causing CNS manifestations. It is possible that anti-viral drugs (ganciclovir and foscarnet) might be effective to patients with primary encephalitis due to HHV-6. Therefore, rapid diagnosis of active HHV-6 infection is crucial for patient management.