Human herpesvirus six

Human Herpesvirus Six (HHV-6) is one of the eight known viruses that are members of the human herpesvirus family.

Groupings
HHV-6 is a member of the betaherpesviridae (subfamily of the herpesvirinae) which also includes HHV-7 and Cytomegalovirus (HHV-5 or HCMV). There are two subtypes of HHV-6 termed HHV-6A and HHV-6B.

Occurrence
HHV-6B is responsible for up to 93% of primary infections in Europe and North America. Such infections usually cause fever, with exanthem subitum (Roseola/rash) only being observed in 10% of cases. HHV-6 primary infections account for up to 20% of infant hospitalizations in the United States and are associated with several more severe complications, such as encephalitis, lymphadenopathy, myocarditis and myelosuppression.

After primary infection, latency is established in myeloid and bone marrow progenitors and exists for the life time of the host. The virus periodically re-activates from this latent state, with HHV-6 DNA being detectable in 20-25% of healthy adults in the United States. In the immunocompetent setting, these re-activations are often asymptomatic, but in immunosuppressed, individuals there can be serious complications.

HHV-6 re-activation causes severe disease in transplant recipients and can lead to graft rejection, often in consort with other betaherpesviridae. Likewise in HIV/AIDS, HHV-6 re-activations cause disseminated infections leading to end organ disease and death. Although up to 100% of the population are exposed (seropositive) to HHV-6, most by 3 years of age, there are rare cases of primary infections in adults. In the United States, these have been linked more with HHV-6A, which is thought to be more pathogenic and more neurotropic and has been linked to several central nervous system-related disorders.

HHV-6 has also been found in multiple sclerosis patients and has been implicated as a co-factor in several other diseases, including chronic fatigue syndrome, fibromyalgia, AIDS , and temporal lobe epilepsy but no definitive link has been established.