Viruses must be able to pass from one infected organism to another. The spread of specific viruses will be consid ered together with their other attributes in the chapters that follow, but it is useful to consider virus epidemiology in overview at this point. The tissues infected by a virus and the seriousness of the disease caused by it are attributes that determine in part the mechanism of spread of a virus. Thus, knowledge of the epidemiology of a virus is important for understanding the biology of its replication and pathology.
We can discriminate several general ways in which animal viruses are spread: oral-fecal, airborne, blood-borne (including viruses that are spread by blood-sucking arthropods), sexual, and congenital. We can also distinguish human viruses that have humans as their major or only host, and viruses that are also associated with other animals.
Viruses spread by an oral-fecal route are disseminated by ingestion of contaminated food or water. Infection begins in the gut, and it may or may not spread to other organs. Many of these viruses cause gastroenteritis. Virus is excreted in feces or urine to continue the cycle. Such viruses are usually fairly stable outside the organism because they may have to persist in an infectious form for long periods of time before being ingested by the next victim.
Airborne or respiratory viruses are spread when virus present in the respiratory tract is expelled as aerosols or in mucus. Infection begins when contaminated air is inhaled or when virus present in mucosal secretions, for example on doorknobs or on a companion's hands, is contacted and the virus is transferred to mucosal surfaces in the nose, mouth, or eyes. These viruses are often unstable outside the body; spread thus requires close person-to-person contact. Infection begins on mucosal surfaces in the upper respiratory tract or the eye. Many of these viruses are restricted to growth in the upper respiratory tract, but some are able to spread to other organs. Respiratory tract disease is caused by many of these viruses.
Blood-borne viruses establish a viremia in which infectious virus circulates in the blood. Some are transmitted by blood-sucking arthropods, which act as vectors, whereas others are transmitted by exposure to contaminated blood or other bodily fluids. Arboviruses (e.g., yellow fever virus, genus Flavivirus, family Flaviviridae) can replicate in both arthropods, such as ticks or mosquitoes, and in vertebrates. The arthropod may become infected when it takes a blood meal from a viremic vertebrate. After replication of the virus in the arthropod, it can be transmitted to a vertebrate when the arthropod takes another blood meal. Although arboviruses tend to have broad host ranges, a virus is usually maintained in only one or a few vertebrate hosts and vectored by a limited set of arthropods.
Therapeutic blood transfusion, use of hypodermic injections, and intravenous drug use are methods of spread of many blood-borne viruses. HIV and hepatitis B (family Hepadnaviridae) and C (genus Hepacivirus, family Flaviviridae) viruses, for example, are commonly spread among drug users through sharing of contaminated needles. Transfusion with contaminated blood is still possible despite diagnostic tests to identify infected blood products.
In developed countries, the blood supply is screened for HIV and hepatitis B and C viruses, as well as other viral agents for which tests exist, but in developing countries contaminated blood is often still a major problem. Blood-borne viruses that are not arboviruses are often spread sexually as well as by the methods above, but in some cases it is not clear how the viruses were spread before the introduction of blood transfusion and hypodermic needles.
Because of the need to establish a significant viremia, which requires extensive viral production in organs that can shed virus into the bloodstream, blood-borne viruses often cause serious disease. Furthermore, because spread is direct, these viruses need not be stable outside the body and usually have a short half-life outside an organism.
Many viruses are transmitted by sexual contact. Virus may be present in warts in the genital area (e.g., herpes simplex virus type 2 and human papillomaviruses) or in semen or vaginal secretions (e.g., HIV, hepatitis B virus). Infection begins in the genital mucosa but may spread to other organs. Because the opportunity for spread by sexual contact is much more restricted than for spread by other routes, viruses spread by sexual transmission almost invariably set up long-term persistent infections that cause only mild disease, at least early in infection. This allows the virus to be disseminated over long periods of time.
Many viruses can be spread vertically. Congenital infection of the fetus in utero or during passage of the infant through the birth canal occurs with viruses such as HIV, cytomegalovirus (family Herpesviridae), and rubella virus (genus Rubivirus, family Togaviridae). Vertical transmission can also occur shortly after birth, by breast feeding for example. HTLV I (family Retroviridae), which causes leukemia in humans, is such a virus.
Many of the viruses that cause human disease infect only humans in nature, or are maintained only in humans (e.g., all of the human herpesviruses, HIV, hepatitis B, poliovirus). Thus, spread is from one person to another. Many others are associated with wildlife and spread is often from animal to man (e.g., most of the arboviruses, rabies virus, the hantaviruses, and the arenaviruses). The hantaviruses (family Bunyaviridae) and arenaviruses (family Arenaviridae) are associated with small rodents, in which they cause little disease. Humans, in which these viruses cause serious illness, can become infected by inhaling aerosols containing excreta from such rodents (e.g., Lassa fever virus; Junin virus, the causative agent of Argentine hemorrhagic fever; and Sin Nombre virus, the causative agent of hantavirus pulmonary syndrome). Rabies virus (family Rhabdoviridae) is associated with unvaccinated domestic dogs and with many species of wildlife, including foxes, coyotes, skunks, racoons, and bats. It is spread by the bites of infected animals. The virus is present in salivary fluid of the infected animal, and the disease induces an infected animal to become aggressive and bite potential hosts. Interestingly, although many humans die worldwide of rabies each year contracted from the bites of rabid animals, human-to-human transmission does not occur. Some recent human infections have been contracted from wildlife only indirectly. Nipah virus (family Paramyxoviridae) is associated with flying foxes, large fruit-eating bats. The virus recently caused an epidemic of disease in pigs in Malayasia and Singapore, and pig farmers contracted the disease from the pigs. Before the epidemic died out, 258 humans developed encephalitis, of whom 40% died.
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