Human herpesvirus 6 culprit in neurologic diseases

The recently discovered human herpesvirus 6 (HHV-6) appears to be both a passenger and a pathogen. This often silent virus which belongs to the Betaherpesvirinae subfamily and infects human indiscriminately, has now been shown to be active cause of some neurologic disease in children and possibly in adults, experts said at the Interscience Conference on Antimicrobial Agents and Chemotherapy.

A survey conducted in the British Isles from October 1998 through September 2001 showed that 8% of patients (13 of 156) under the age of 2 years with encephalitis who had fever and convulsions had primary HHV-6 infection. However, much remains unknown about the actual mechanism by which the virus causes disease.

Investigators are now looking into the pathogenicity of this virus in older children and in adults. Some of the data presented at the meeting also suggest that this virus is unique among the human herpesviruses, because in some individuals HHV-6 becomes incorporated into the human chromosomes.

They think that about 1% of people are born with this virus. If a person has this virus integrated into their chromosomes, it is in their germ line. So, they have inherited it from either their mother of their father. Since the virus is in chromosomes and is therefore in every cell in the body, such people are absolutely full of vial DNA. It is even in their hair follicles, a consultant virologist at the Royal Free & University College Medical School, London said.

Just because the virus is in the chromosomes doesn’t mean that it is multiplying or that it is an active infection. This is something that is entirely new in terms of herpes viruses and does not jive with how herpes viruses have been viewed.

Right now, the whole field is open. There is so much they still don’t know. It has long been known that primary HHV-6 infection in young children causes exanthema subitum, a rash illness with high fever, but the severe neurologic disease is new and surprising. It is unknown how this virus affects normal adults. However, it always remains as a passenger in the body and in immunocompromised adults — that is, stem-cell transplant and solid organ recipients — it causes encephalitis. It is fatal in bone marrow transplant patients; 40% of those who become sick with this virus will die.

The take-home message for primary care physicians is that in some children, HHV-6 may cause a severe neurologic illness with prolonged seizures requiring supportive therapy, including ventilation and sedation. Furthermore, in the 1% of individuals where the virus is integrated into chromosomes, high levels of viral DNA will be found in blood and in cerebrospinal fluid, which may confuse the diagnosis with a variety of conditions.