Читать книгу Principles of Virology, Volume 2 - Jane Flint, S. Jane Flint - Страница 75

Some viruses, including hepatitis B virus, human immunodeficiency virus type 1, and some herpesviruses, enter the urogenital tract, most typically as a result of sexual practices. Like the alimentary tract, the urogenital tract is well protected by mucus and low pH. The vagina maintains a pH that is typically between 3.4 and 4.5; when the pH increases toward neutrality (as a result of antibiotic use or natural changes in epithelial thickness during the menstrual cycle, for example), many pathogens, including bacteria and yeast, can flourish. Moreover, the vaginal mucosa is separated from the environment by a squamous epithelium that varies in thickness during the menstrual cycle, and that presents a formidable barrier to pathogens. In cases where this lining is thin, such as the zone between the endo- and ectocervix, viruses such as papilloma virus and human immunodeficiency virus type 1 may be able to infect the epithelium and abundant CD4⁺ T cells beneath.

Sexual activity can result in tears or abrasions in the vaginal epithelium or the urethra, allowing virus particles to enter. Some viruses infect the epithelium and produce local lesions (for example, human papillomaviruses, which cause genital warts). Others penetrate deeper, gaining access to cells in the underlying tissues and infecting cells of the immune system (human immunodeficiency virus type 1) or the peripheral nervous system (herpes simplex virus type 2). Infection by the latter two viruses invariably spreads from the initial urogenital site to other tissues in the host, thereby establishing lifelong infections. Viral vaginitis (inflammation within the vaginal canal) can result from infection by herpes simplex virus type 2. This infection often causes painful lesions or sores, often visible on the vulva or the vagina, but occasionally found deeper in the vaginal canal. Because herpesviruses cannot be cleared from infected hosts, recrudescence can occur following stress, natural changes in the thickness of the canal during the menstrual cycle, or other infections. Herpes vaginitis could also affect the mouth and pharynx if oral sex is performed during a period in which virions are actively shed.

Viruses that gain entry by the urogenital tract are extremely common. Approximately one in six people between 15 and 50 years of age has genital herpes, and as this is a lifelong infection, the risk of transmission to sex partners is high. Herpesvirus infection is often asymptomatic, although the virus can still be shed and infect others. In pregnancy, infections by these viruses pose a particular risk to the developing fetus and can result in miscarriage, early delivery, or lifelong infection that begins in the neonate, dangers that can be mitigated by Caesarian delivery. Moreover, transmission of human papillomaviruses can result in genital warts and cervical cancer. Such viruses have a high transmissibility rate: there is a >20% chance that an uninfected individual will pick up the virus from an infected partner over a 6-month period. It is sobering to note that individuals may be affected by multiple sexually transmitted pathogens, and a preexisting infection with one may predispose to infection with another. For example, a genital herpes lesion provides an excellent portal for human immunodeficiency virus type 1.

Human semen is a particularly robust carrier of viruses: it is estimated that up to 27 distinct viruses can reproduce in, and be spread by, semen. These include viruses that are well known to be sexually transmitted, including human immunodeficiency virus type 1 and herpes simplex type 2, but also include emerging pathogens such as Ebola virus and Zika virus. Viruses such as influenza, dengue, and severe acute respiratory syndrome virus have been found in the testes, though it is not known if these viruses can be sexually transmitted. Even if these viruses are not sexually transmitted, their presence may nevertheless affect fertility, or increase the risk of acquiring a sexually transmitted disease. Some of these viruses, including the papillomaviruses, may even cause mutations in the DNA of sperm, which could then fertilize an egg and pass along the virus-induced mutations to future generations.