Human herpesvirus (HHV)-6 and HHV-7: two closely related viruses with different infection profiles in stem cell transplantation recipients.

Human herpesvirus (HHV)-6 and HHV-7 loads were evaluated retrospectively in peripheral blood mononuclear cells (PBMC) from 78 recipients of stem cell transplantation (SCT) by real-time polymerase chain reaction. The median HHV-6 load in patients was 1357 genome equivalent copies (EqCop)/10(6) PBMC but was below the quantitation threshold in 31 immunocompetent individuals, which strongly suggests that HHV-6 reactivation occurred after SCT. The HHV-6 load was higher in patients with delayed neutrophil engraftment (P=.002) or severe graft-versus-host disease (P=.009). Moreover, the occurrence of at least 1 HHV-6-related manifestation (fever, cutaneous rash, pneumonitis, or partial myelosuppression) was statistically associated with a concomitant virus load >10(3) EqCop/10(6) PBMC (P=.007). Conversely, HHV-7 reactivation was not favored, because median HHV-7 loads were similar in patients and healthy control subjects (1053 vs. 1216 EqCop/10(6) PBMC). The kinetics of Roseolovirus loads during the posttransplantation period suggested that HHV-7 may act as a cofactor of HHV-6 reactivation.