Human herpesviruses are host-specific viruses of the Herpesviridae family, all of which are human pathogens. In addition to labial herpes, this group of infections includes genital herpes, both of whose pathogens remain in their host for life. An alternation between active and inactive states is characteristic of human herpesviruses of each species.
What are human herpesviruses?
The virus family Herpesviridae consists of enveloped viruses with a genome of double-stranded, linear DNA. The individual representatives are among the largest viruses. Currently, about 170 viral species are known from the herpesvirus family. They are related to various vertebrates, including mammals, birds, reptiles and fish. Most species of the herpesvirus family are host-specific and thus cannot change from species to species. Many of the representatives can cause different diseases. Virions of the virus family measure up to 200 nm in diameter and bear an irregularly indented viral envelope that is highly sensitive. Between the envelope and the capsid is a relatively large matrix space containing structural proteins. The tegument proteins are partly embedded in the membrane or capsid-bound. A host-specific species of the Herpesviridae is considered to be the species of human herpesviruses, which can exclusively infect humans. These neurotropic viruses are human pathogenic and include, in addition to labial herpes (herpes simplex type1), genital herpes (herpes simplex type2), varicella-zoster virus, and the causative agent of Pfeiffer’s glandular fever and cytomegalovirus. A total of eight human herpes viruses exist.
Occurrence, distribution, and characteristics
A distinctive feature of herpesviridae is persistence. After initial infection, they persist in the host for life without causing disease. Transmission with human herpesviruses initially occurs via epithelial cells. Thus, alpha-herpesviruses first infect skin or mucosal cells and proliferate vigorously at this site. Due to the strong virus multiplication, the infected cells of the organism die. The immune system recognizes the infection, but before it can successfully intervene, the viruses continue to spread. They spread from the epithelial cells of the skin and mucosa to certain neurons. Within the nuclei of individual neurons, they deposit their own viral DNA in the form of episomal DNA alongside the neuronal DNA. In this way, the viral DNA reaches the nucleus and closes there to form a ring. In the closed ring form, the viral DNA remains in the infected neurons for years. From this point on, the virus behaves silently and is therefore not detected by the human immune system. This type of infection is referred to as a latent infection. This infection only changes into an active form under certain influences. Activating influences include, above all, immunosuppression, as can occur in the context of stress. As soon as the immune system of the affected person is weakened, it can no longer successfully fight the viruses. Human herpes viruses wait, so to speak, within neurons for times and conditions that are not dangerous for them. In the case of immunosuppression due to stress, illness, too much UV light or hormone fluctuations, the inactive virus goes back into an active state. After reactivation, it destroys the nerve cells, frees itself, and from there passes again to the epithelial cells. In this way, an acute herpes disease breaks out. As soon as the host’s immune system is stronger again, the viruses retreat into the nerve cells once more and behave passively. This cycle continues throughout life. Human herpes viruses are human pathogens in any case. However, due to the characteristic alternation between active and inactive states, they do not cause symptoms in certain phases, although they are still present in the body. The most common mode of infection with human herpesviruses is close contact with an infected person. Particularly via the vesicles of an HSV 1 infection, other people can become infected, with labial herpes, for example, when kissing. Transmission of HSV 2, genital herpes, occurs in most cases through sexual contact.
Diseases and symptoms
About 85 percent of the population worldwide is infected with HSV-1. Another 25 percent carry an HSV-2 infection. About one-third of those affected suffer from recurrent symptoms.Basically, the symptoms of a herpes infection depend on the virus species. Herpes Simplex Virus 1 is the best known and most widespread member of the Herpesviridae family. In its active phases, this virus causes vesicles of the skin and mucous membrane. Cold sores lead to blistering in the area of the lips. In genital herpes or HSV 2, ulcerations occur on the penis or vagina. In isolated cases, rectal manifestation accompanies genital manifestation of HSV 2. Occasionally, the viruses cause inflammation within the brain. This type of brain inflammation usually involves the temporal lobe or frontal brain. Such inflammation manifests as functional impairment and deficits in the affected areas of the brain. Flu-like symptoms are followed by a febrile infection. Psychomotor slowing and the symptoms of a brain-organically triggered psychosyndrome are the result. In addition, speech disorders may occur in this form of progression. In individual cases, focal seizures occur. Secondary generalization is conceivable. Brain inflammation always occurs when the viruses are absorbed through the nose. In this case, they reach the brain along the olfactory mucosa. However, among 200 000 people, on average, only one is affected by HSV infections with encephalitis.
