Human herpesvirus 6

Human herpesvirus 6 (HHV-6) is a set of two closely related herpesviruses, HHV6-A and HHV6-B.

Infection is extremely common and usually occurs at an early age. 64-83% of infants are infected by age 13 months.

HHV-6 has an affinity for leukocytes and nervous tissue, especially the olfactory bulb tissues, from which it is thought to disseminate to other parts of the brain.

After infection the virus remains latent but can reactivate asymptomatically even in healthy individuals.

HHV-6 has been found to activate Epstein-Barr virus from latency. Conversely, the presence of EBV renders B cells more susceptible to HHV-6 infection.

In human disease
HHV-6 has been implicated as a contributing factor to a number of neurological diseases including multiple sclerosis, chronic fatigue syndrome and epilepsy, as well as fibromyalgia and AIDS.

Multiple sclerosis
HHV-6 has been found in the oligodendrocytes of plaques in MS patients but not in healthy tissue.

Antivirals may have some therapeutic benefit. A randomized, placebo-controlled double-blind study found that acyclovir reduced the exacerbation rate in relapsing-remitting MS patients. . Valacyclovir reduced new lesions in patients with high disease activity.

Cancer
Like Epstein-Barr virus, HHV-6 is associated with lymphomas and carcinomas.

Chronic fatigue syndrome
One study found a higher prevalence of past HHV-6 infection in chronic fatigue syndrome patients but with a low viral load that did not suggest reactivation. Several studies have found that active infection is more common in CFS patients than healthy controls.

Antivirals
There have been no controlled trials of antivirals for HHV-6. Those used clinically are the drugs used for human cytomegalovirus: ganciclovir, valganciclovir, and to a lesser extent acyclovir.

Notable studies

 * 2016, Human herpesvirus 6 and 7 are biomarkers for fatigue, which distinguish between physiological fatigue and pathological fatigue
 * 1994, Prevalence of Human Herpesvirus 6 Variants A and B in Patients with Chronic Fatigue Syndrome