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 possible contributing factor to a number of neurological diseases including multiple sclerosis, febrile seizures, Alzheimer's disease, , chronic fatigue syndrome , epilepsy including mesial temporal lope epilepsy (MTLE), as well as fibromyalgia and AIDS.

HHV6a and HHV6b can infect multiple organs including the brain and central nervous system, salivary glands, tonsils, liver, kidneys, lymph nodes, endothelial cells, multiple types of white blood cells, plus several types of cell lines.

Encephalitis
HHV-6B can cause encephalitis, particularly in people who are immunocompromised, especially after a hematopoietic cell transplantation.

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 is no drug approved for treating HHV-6. Those used clinically are the drugs used for human cytomegalovirus: ganciclovir (Cytovene IV), cidofovir (Vistide IV), and foscarnet (Foscavir IV).

Notable studies

 * 1994, Prevalence of Human Herpesvirus 6 Variants A and B in Patients with Chronic Fatigue Syndrome
 * 2016, Human herpesvirus 6 and 7 are biomarkers for fatigue, which distinguish between physiological fatigue and pathological fatigue
 * 2020, Human Herpesvirus-6 Reactivation, Mitochondrial Fragmentation, and the Coordination of Antiviral and Metabolic Phenotypes in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (Full text)
 * 2020, Cytomegalovirus, Epstein-Barr Virus, and Human herpesvirus-6 Infections in Patients With Myalgic Encephalomyelitis/Chronic Fatigue Syndrome - (Abstract)
 * 2020, Human Herpesviruses 6A and 6B in Brain Diseases: Association versus Causation (Full text)

Learn more

 * HHV-6 Foundation
 * 2016, HHV-6 Mediated Mitochondrial Modulation and Its Association to ME/CFS - Project summary for Solve ME/CFS Initiative
 * 2018, A Common Virus May Play Role in Alzheimer’s Disease, Study Finds by Pam Belluck via NY Times