human immunodeficiency virus type 1 cannot be transmitted by mosquitos. Hepatitis viruses and human immunodeficiency virus type 1 can be transmitted by virus-laden blood during transfusions and injections. Virus particles may also be transmitted from blood during sexual intercourse or childbirth, and consumption of raw meat may place contaminated blood in contact with the alimentary canal and respiratory tract. Health care professionals, emergency rescue workers, and dentists are exposed routinely to blood from individuals who may harbor infections. Indeed, for many of the viruses that cause fatal hemorrhagic fevers (such as members of the Bunyaviridae and Filoviridae), the primary mode of transmission to humans is via contaminated blood and body fluids. Consequently, health care workers often are among the first to become infected and show symptoms in an outbreak of such viral diseases.
Urine
Virus-containing urine is a common contaminant of food and water supplies. The presence of virus particles in the urine is called viruria. Hantaviruses and arenaviruses that infect rodents cause persistent viruria. Consequently, humans may be infected by exposure to dust that contains dried urine from infected rodents. A few human viruses, including the polyomaviruses JC and BK, and the prevalent herpesvirus, cytomegalovirus, reproduce in the kidneys and are shed in urine.
Semen
Some retroviruses, including human immunodeficiency virus type 1, herpesviruses, and hepatitis B virus, are shed in semen and are transmitted during sex. Herpesviruses that infect the genital mucosa are shed from lesions and transmitted in genital secretions, as are papillomaviruses. Though Zika virus is primarily transmitted by mosquito, a surprising observation was that it is also detected in the semen of infected males, where it can remain replication-competent for up to 6 months. Most viruses found in semen reproduce in testicular tissue, from which they are shed into the seminal fluid.
Milk
Mouse mammary tumor virus is transmitted to offspring primarily via mother’s milk into which the virus is shed, as are some tick-borne encephalitis viruses. Mumps virus and cytomegalovirus are shed into human milk but are probably not often transmitted by this route. Importantly, transmission of human immunodeficiency virus type 1 through breastfeeding is responsible for more than half of new pediatric infections with this virus. Breast milk contains components with anti-infectious, immunomodulatory, or anti-inflammatory properties that can regulate both viral reproduction and infant susceptibility. Even during antiretroviral therapy, a stable, CD4+ T cell-associated reservoir of human immunodeficiency virus type 1 is persistently present in breast milk, a likely source of infection. Only prophylactic treatment in infants is likely to protect human immunodeficiency virus type 1-exposed babies against all forms of transmission from breast milk.
Skin Lesions
Many viruses reproduce in the skin, and the lesions that form from such infections contain virions that can be transmitted to other hosts. In these cases, the virus is usually transmitted by direct body contact. For example, herpes simplex virus causes a common rash in wrestlers, known as herpes gladiatorum or “mat herpes” (Fig. 2.22). Certain poxviruses and papillomaviruses that cause warts may also be transmitted by direct, skin-to-skin contact.
Varicella-zoster virus, the agent of chicken pox, is released from the skin in a particularly effective manner. The lesions that form during an acute chicken pox infection are small, lymph-filled blisters that erupt, leaving a crusty scab. Virus concentrations in this fluid are high. Despite the avail ability of an effective vaccine, acute infections still occur in unvaccinated individuals. Alarmingly, some parents have elected to allow their children to become infected by encouraging close exposure to acutely infected peers (Box 2.13).
Tears
Researchers have found that Zika virus genetic material can be identified in tears of infected mice. This finding perhaps explains why some Zika-infected patients develop uveitis, a serious eye disease that can lead to permanent blindness, as well as less serious eye disorders such as retinal damage or optic nerve inflammation. Moreover, detection of viral nucleic acid in tears suggests that the eye might be a reservoir for Zika, and thus a portal for virus shedding. Of note, this has yet to be confirmed in humans, and presence of genetic material does not indicate that infectious virus is present. Nevertheless, the identification of a new potential “way out” for viruses indicates that viruses may be able cross numerous host barriers to access tear ducts, and may pose challenges for commonplace procedures such as corneal transplants. Adenoviruses can also be found in tears of individuals suffering from “pink eye,” the common name given for adenoviral conjunctivitis.
Figure 2.22 Mat herpes. An example of a herpesvirus infection on the arms of a young wrestler, called herpes gladiatorum. It should probably be called “herpes not-glad-at-allum.” Reprinted from Morse D, Vangipuram R, Tyring SK. 2019. Eur J Intern Med 60:e1e2, with permission.
Perspectives
Despite the complexity and diversity of viral infection cycles, at a minimum, all viruses must get in, and they must get out. This is true not only for infections of the cell (a major theme of Volume I) but also for infections of the host. In this Chapter, we discussed the many ways by which an organism may acquire pathogens. It is not hyperbole to note that pathogens, including viruses, bacteria, eukaryotic parasites, and fungi, are truly everywhere, and because they have coevolved with their hosts, all have coopted our behaviors to ensure host-to-host transmission.
Fortunately, our counter-defenses pose formidable obstacles. Viruses are trapped in mucus, repelled by dead layers of skin, brushed away by cilia, and destroyed by stomach acid. However, capturing a rook and a bishop does not end this age-old game of chess. Some viruses can bypass these defenses to reach target cells deep within organs. When viruses breach our formidable barriers, it is up to the elite forces of the host immune system, the precise strategies of the intrinsic, innate, and adaptive responses, to either end the game in checkmate or suffer the fateful capture of the king.
DISCUSSION
Chicken pox parties
Prior to the widespread use of the varicellazoster virus vaccine, some parents who wanted to control when their child would get chicken pox (mistakenly considered a childhood “rite of passage”) would host chicken pox parties, in which uninfected children would share lollipops licked by infected children. Given the presence of the virus in the oral mucosa, this practice ensured that the lollipop contained a high dose of the virus and virtually guaranteed infection. Moreover, because the incubation period for varicella is quite precise (about 14 days following exposure), parents could preplan days of from work to be with their sick child. Even today, there is a “black market” of virus-laced items (such as lollipops) available through the Web. Such practices are an almost inconceivably bad idea; infections by these viruses can be quite severe, and effective, safe vaccines do exist. Moreover, infected children pose risks to immunocompromised individuals, such as the elderly and cancer patients receiving immunosuppressive chemotherapy.
