Human herpesvirus 6, or HHV-6 for short, belongs to the herpesvirus family, which is divided into alpha, beta, and gamma subfamilies. HHV-6 belongs to the beta herpesvirus subfamily, which has a very narrow host range and replicates slowly in the body. The virus can cause various diseases in humans but can also persist in the body without any symptoms.
What is human herpesvirus 6
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A total of eight human pathogenic herpesviruses that can cause disease in humans have been characterized to date. There are two subtypes of HHV-6, subtype A and subtype B. The virus was discovered in 1986 and is a DNA double-stranded virus. HHV-6 infects CD4-positive T lymphocytes, specific cells of the human immune system. The virus spreads via saliva and droplet infection. HHV-6 has a worldwide distribution and is very common: More than 90% of adults carry the virus. Infection occurs from the sixth month of life, usually in infancy or early childhood. Before the sixth month of life, infants are still protected against infection by the maternal antibodies they absorbed through the placenta before birth. Between the ages of two and five, approximately 80% of children are already infected with the virus. If an initial infection with HHV-6 occurs in a pregnant woman, transmission to the embryo can occur, so that the child is already carrying the virus at birth.
Significance and function
Human herpesvirus 6 infection usually progresses without clinical symptoms after initial infection. HHV-6 remains latent in the body, which is a characteristic feature of all herpesviruses. Therefore, most adults are usually unaware that they are infected with the virus. Complete elimination is usually not possible. When the virus is first infected, it can be detected in the blood, saliva, and stool in the laboratory. Disease of the lymphatic system and central nervous system occurs. Since the virus can spread along nerve fibers and thus bypass the blood–brain barrier, it enters the spinal cord and brain. Here it infects glial cells and neurons. In the latent phase, HHV-6 is found in the salivary glands, through which it is shed and spread. In this phase, the virus does not produce particles infectious to the body. However, it can be reactivated and re-enter the infection cycle. This can happen especially in the case of a weakened immune system. For example, immunocompromised patients who have HIV infection or whose immune system is suppressed due to transplantation may experience an increase in viral replication. If reactivation of the virus occurs, this can manifest itself in the recurrence of the same or similar symptoms as in the initial infection. HHV-6 has several pathogenic mechanisms: The virus can lead to degenerative changes in cell morphology in infected cells (cytopathic effect). It can induce so-called cytokines, certain proteins responsible for cell growth and differentiation. HHV-6 can affect immune function by partially suppressing it. In addition, The virus can lead to trans-activation of other viruses in case of co-infection.
Diseases and medical conditions
Human herpesvirus 6 is best known as the causative agent of three-day fever. This usually occurs in early childhood. After several days of high fever, a characteristic rash appears as the fever subsides. Three-day fever heals on its own and is rarely associated with complications, so therapy is usually not necessary. In Europe, it is usually caused by subtype B of HHV-6. In individual cases, the disease can also occur in adults and manifests itself in flu-like symptoms. In rare cases, complications with diarrhea and vomiting may occur. In addition, eyelids and lymph nodes in the neck may swell, papules on the palate and uvula occur, and febrile convulsions may occur. Several other diseases suspected to be related to HHV-6 infection have also been observed in very rare cases. For example, HHV-6 can cause chronic fatigue syndrome, which is associated with severe fatigue and exhaustion and depression. However, this is thought to affect less than 1% of all HHV-6 infected individuals.Myocarditis, pneumonia or hepatitis are also possible. Diseases such as meningitis and encephalitis have also been observed. HHV-6 could have an influence on the development of multiple sclerosis, in addition to other factors. Likewise, researchers suspect that HHV-6 as an additional factor may promote the development of basal cell carcinoma and squamous cell carcinoma. If severe complications arise from HHV-6, antiviral therapy can be administered.
