■ The viruses in this family all feature a practically identical morphology, but show little uniformity when it comes to their biology and the clinical pictures resulting from infections. One thing shared by all herpesviruses is the ability to reactivate after a period of latency.
■ The herpes simplex virus (HSV, two serotypes) is the pathogen that causes a vesicular exanthem (fever blisters, herpes labialis, or genitalis), encephalitis, and a generalized infection in newborns (herpes neonatorum).
■ The varicella-zoster virus (VZV) causes the primary infection chickenpox, which can then recidivate as zoster (shingles).
■ Cytomegalovirus (CMV) infections remain inapparent or harmless in the immunologically healthy, but can cause generalized, fatal infections in immunocompromised individuals.
■ The Epstein-Barr virus (EBV) is the pathogen in infectious mononucleosis and is also implicated in lymphomas (including Burkitt lymphoma) and na-sopharyngeal carcinomas.
gl ■ Human herpesvirus 6 (HHV 6) is the pathogen that causes three-day fever (exanthema subitum, roseola infantum).
Human herpesvirus 8 (HHV 8) causes the AIDS-associated Kaposi sarcoma.
Diagnosis. Isolation, amplification culture, or direct detection can be used to diagnose herpes simplex, varicella-zoster, and cytomegaloviruses; antibody assays can be used for Epstein-Barr, human herpes 6 and 8, and varicella-zoster viruses; PCR can detect herpes simplex, varicella-zoster virus, cytomegalovirus, and human herpesvirus 6.
Therapy. Effective and well-tolerated chemotherapeutics are available to treat herpes simplex, varicella-zoster virus, and cytomegalovirus (acyclovir, ganciclovir). ■
■ Biology of the Herpesviruses
Several hundred herpesvirus species have been described in humans and animals, all with the same morphology (Fig. 8.4a). They have dsDNA genomes. Replication of the DNA and the morphogenesis of the virus particle take place in the host-cell nucleus. The envelope (inner nuclear membrane) is then formed when the virus penetrates the nuclear membrane (Fig. 8.4b), whereby depending on the cell and viral type involved a more or less substantial number of viruses receive an envelope after reaching the cytoplasm, at the cell membrane or not at all. The envelope is the major determinant of viral infectivity (see Chapter 7, p. 378f.). Since the envelope contains mainly host-cell determinants, it can also be assumed that it provides a level of protection from host immune responses.
Common to all herpesviruses is a high level of generalized contamination (60-90% carriers) and the ability to persist in a latent state in the body over long periods. The different viral species persist in different cells, whereby the cell type is the decisive factor determining latency or replication of the virus. Herpes simplex virus and varicella-zoster virus do not produce any virus particles during latency, although they do produce one, or a few, mRNA types and the corresponding proteins. Cytome-galovirus and Epstein-Barr virus appear to maintain continuous production of small numbers of viruses as well, so that fresh infection of a small number of new cells is an ongoing process. These viruses would appear to produce persistent, subclinical infections concurrently with their latent status (p. 394). Reactivation of these latent viruses is apparently initiated by a number of factors (psychological stress, solar irradiation, fever, traumata, other infections, immunosuppressive therapy), but the actual mechanisms that reactivate the lytic viral life cycle are unknown.
Human herpesviruses (with the exception of the varicella-zoster virus) and many zoopathic herpes species have also been implicated in the etiology of malignancies.
Eight human herpesviruses that infect different organs are known to date, e.g., the skin (herpes simplex virus types 1 and 2, varicella-zoster virus), the lymphatic system (Epstein-Barr virus, human herpesvirus type 6, cytomegaloviruses), and the CNS (herpes simplex virus, cytomegalovirus).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.