Virology - Human Herpes Virus 6
20 Apr 2011
This is an abstract. For the full paper click here
Human herpesvirus 6 (HHV-6) is a DNA containing virus belonging to the Roseolovirus genus of the ß-herpesvirus subfamily. HHV-6 exists in two different variants referred to as A and B. HHV-6 has a worldwide distribution and presents itself as a typical herpesvirus often inducing latent infections in monocytes and macrophages. Infection with HHV-6 is extremely common, with nearly 100% in seroprevalence. The virus has a tropism for lymphocytes, often targeting CD4+ cells. It is likely that the virus is transmitted via oral secretions and in one study 90% of infected adults displayed virions in their saliva. It should be noted, however, that other studies have shown lower rates (Campadelli, Mirandola, & Menotti, 1999).
A natural HHV-6 infection can be divided into 3 stages. The first stage is the acute primary infection and this is most commonly seen in infants. HHV-6 is common in infants and about half of the children afflicted with the virus suffer the disease roseola infantum, exanthem subitum, or sixth disease. This is a mild childhood disease that produces symptoms such as rash and fever for 3-5 days. Adults rarely suffer primary infections since most people are infected as infants, but when it does occur symptoms are generally much more severe. The second occurs in health children as well as adults. During this stage the virus replicates in the salivary glands and is expelled in saliva while causing no noticeable symptoms or pathology. The virus remains latent in, at least, lymphocytes and monocytes and replicates in low-levels in various tissues throughout the body. The third stage is less common, usually only seen in immunocompromised patients, and is known to cause reactivation of the virus from latency or reinfection (Campadelli, Mirandola, & Menotti, 1999).
Much attention was drawn to HHV-6 when light was shed on its ability to cause disease in immunosuppressed patients, particularly those suffering from AIDS. Primary or reoccurrence of infection in immunosuppressed people or suffers of AIDS can be life threatening. In fact, HHV-6 was first isolated in 1986 from “interleukin 2-stimulated peripheral blood mononuclear cells of patients with AIDS or lymphoproliferative disorders”(CDC 1999). In these patients, HHV-6 primary infection or reactivation can cause bone marrow suppression, encephalitis, pneumonitis, encephalopathy, hepatitis, rash, fever, and can even interfere with organ transplants resulting in rejection or death (Campadelli, Mirandola, & Menotti, 1999). Since more people are undergoing organ transplants, and thus must undergo therapeutic immunosuppression, the number of people at risk is increasing.
Furthermore, HHV-6 is one of the most neurotropic viruses in existence. Neruoinvasion has been seen in infants with primary infections, in focal encephalitis, patients suffering from AIDS, in those who have had bone marrow transplants, and even in immunologically competent children and adults. Both HHV-6A and HHV-6B have been discovered in the brain of patients who died of viral and non-viral causes. This demonstrates that both types of the virus can invade and inhabit the brain. Typically, HHV-6B can result in CNS invasion, while HHV-6A has never been documented to do so (Campadelli, Mirandola, & Menotti, 1999).
There is also a possible link between an active HHV-6 infection and multiple sclerosis, a severe CNS disease that often afflicts young adults. It is characterized by demyelination of nerves that eventually results in complete paralysis and death. Multiple sclerosis appears to be an autoimmune reaction to myelin and its connection to HHV-6 is still inconclusive.