Human Herpes Virus l HHV-6

Human Herpes Virus: Herpes virus is the leading cause of human viral disease, second only to influenza and cold viruses. They are capable of causing overt disease or remaining silent for many years only to be reactivated, for example as shingles. The name herpes comes from the Latin herpes which, in turn, comes from the Greek word herpein which means to creep. This reflects the creeping or spreading nature of the skin lesions caused by many herpes virus types.

There are at least 25 viruses in the family Herpesviridae (currently divided into three sub-families). Eight or more herpes virus types are known to infect man frequently.

Herpes Virus Types That Infect Humans

  • Herpes simplex virus  Type 1 (HSV-1)
  • Herpes simplex virus  Type 2 (HSV-2)
  • Epstein Barr virus (EBV)
  • Cytomegalovirus (CMV)
  • Varicella Zoster Virus (VZV)
  • Human herpes virus 6 (exanthum subitum or roseola infantum)
  • Human herpes virus 8 (Kaposi’s sarcoma-associate herpes virus).

Human herpes virus 6 (HHV-6) is the virus that most commonly causes the childhood disease roseola. Two genetically distinct variants have been discovered: human herpes virus 6A (HHV-6A) and human herpes virus 6B (HHV-6B). HHV-6B has been associated with a variety of viral illnesses, including exanthem subitum (roseola infantum), mononucleosis syndromes, focal encephalitis, and pneumonitis.

This virus shows the closest homology with cytomegalovirus and human herpesvirus 7 (HHV-7).  HHV-6 infection in infants is the most common cause of fever-induced seizures. Infection in adults is seen primarily in immunocompromised hosts who have undergone solid organ transplants or in those with HIV infection. Reactivation of latent HIV infection attributable to HHV-6 infection has been reported. No prophylaxis or treatment for infection with HHV-6 presently exists. The great majority of HHV-6 infections is silent or appears as a general mild febrile illness.

A patient has become infected by herpes virus, the infection remains for life. The initial infection may be followed by latency with subsequent reactivation. Herpes viruses infect most of the human population and persons living past middle age usually have antibodies to most of the above herpes viruses with the exception of HHV-8.

Herpes virus replication:

i) Binding to the cell surface: As with many other viruses, cell tropism is determined by the availability of the correct receptor on the surface of the cell to be infected. The virus fuses with the cell membrane at ambient pH and so there is the possibility of syncytia formation between infected cells and therefore cell to cell transmission even in the presence of neutralizing humoral antibodies. This means that cell-mediated immunity is important in suppressing herpes virus infections.

ii) Nucleocapsid enters cytoplasm: The tegument-surrounded nucleocapsid is carried to the nuclear membrane where the nucleocapsid binds. The DNA genome then enters the nucleus.

iii) Transcription: This is a very complex process, as might be expected from the large size of genome. There are three classes of proteins that need to be made for the production of a mature virus.