Presented by: Per Ljungman. Hematology Centre, Karolinska University Hospital and Karolinska Institutet, Stockholm, Sweden.

Human herpesvirus 6 (HHV-6) is an important cause of encephalitis in immunocompromised patients, and in particular transplant patients. In the most severely immunocompromised patients, such as stem-cell transplant patients, the outcome of untreated patients has been reported to be poor. No controlled trial studying the effectiveness of any antiviral drug compared with placebo or no treatment has been performed. However, many reports describing single cases or small-case series have been published. The main therapeutic options are ganciclovir and foscarnet, since both drugs have shown in vitro effectiveness against HHV-6 and have also been able to reduce the HHV-6 viral load in blood. The superiority of either ganciclovir or foscarnet over the other has not been established. In the clinical setting, successful outcome with either of these agents has been reported in some, but not all reports of patients treated for HHV-6 encephalitis. Cidofovir has in vitro efficacy against HHV-6, but very limited clinical data have been published. Comorbid clinical conditions, such as renal failure or marrow suppression, may also dictate whether ganciclovir or foscarnet is used as therapy for HHV-6. Ganciclovir may be preferable in patients with renal dysfunction since foscarnet is the potentially more nephrotoxic drug. On the other hand, foscarnet may be preferable in patients with marrow suppression because it does not possess the myelosuppressive effect of ganciclovir.