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Human herpesviruses (HHV)-6, HHV-7, and HHV-8 were identified as causes of infection in solid organ transplant (SOT) and hematopoietic stem cell transplant (HSCT) recipients nearly three decades ago. HHV-6 infection occurs early posttransplant and may cause fever of unknown origin, diarrhea, rash, pneumonitis, and encephalitis. Chromosomal integration of HHV-6 can complicate diagnostic testing for HHV-6, as these patients have markedly elevated HHV-6 DNA levels due to the presence of HHV-6 in every nucleated cell. Umbilical cord transplant recipients are at the highest risk for HHV-6 infection after HSCT, but the risk factors after SOT are ill-defined. HHV-6 is thought to mediate graft rejection and loss, delayed engraftment, and graft versus host disease and may also increase the risk of cytomegalovirus infection. However, no study has unequivocally proven the pathogenicity of HHV-6, the presence of which may simply be a marker of immune dysregulation. Even less is known about HHV-7 infection, which is largely asymptomatic and resolves without intervention. HHV-8 is well-recognized as the cause of Kaposi sarcoma (KS), multicentric Castleman disease, primary effusion lymphoma, and nonmalignant inflammatory syndromes. Extensively described after SOT, it is most common in endemic areas, where active research efforts are evaluating the benefit of screening donors and recipients for anti-HHV-8 antibodies prior to transplant. Little is known about HHV-8 infections after HSCT; KS and bone marrow failure are the only described entities. Antivirals (ganciclovir, foscarnet, and cidofovir) are of questionable efficacy against these viruses. Reduction of immunosuppression is the mainstay of treatment of HHV-8 infection.

DOI: 10.1007/978-3-030-01751-4_28-1