Viruses survive in nature only if fhey can be transmitted from one host to another, whether of the same or another species. Transmission cycles require virus entry into the body, replication, and shedding with subsequent spread to another host. Molecular and cellular aspects of entry and shedding were described in Chapters 3 and 6; here we mention only those aspects that are relevant to epidemiology.
Virus transmission (Fig. 14-2) may be horizontal or vertical. Vertical transmission describes transmission from mother to offspring. However, most transmission is horizontal, that is, between individuals within the population at risk, and can occur via direct contact, indirect contact, or a common vehicle, or may be airborne, vector-borne or iatrogenic. Some viruses are transmitted in nature via several modes, others exclusively via one mode.
Direct contact transmission involves actual physical contact between an infected and a susceptible person, by hand or body contact, including kissing and various types of sexual contact, such as occurs in transmission of some herpesviruses, papillomaviruses, and poxviruses.
Indirect contact transmission occurs via fomites, such as shared eating utensils, children's toys, handkerchiefs, towels, bed linen, improperly sterilized surgical equipment, or shared syringes and needles; transmission of many enteric, respiratory, and hepatitis vimses is by indirect contact.
Common vehicle transmission includes fecal contamination of food and water supplies (fecal-oral transmission, e.g., of hepatitis A and E viruses, rotaviruses, and caliciviruses).
Airborne transmission, resulting in infection of the respiratory tract, occurs via droplets and droplet nuclei (aerosols) emitted from infected persons during talking, coughing, or sneezing (e.g., influenza). Large droplets settle quickly, but microdroplets evaporate forming droplet nuclei (<5 fjiirt in diameter) which remain suspended in the air for extended periods. Droplets may travel only a meter or so; droplet nuclei may travel longer distances, but enveloped respiratory viruses do not survive for more than a few hours, being sensitive to inactivation at ambient temperatures. Environmental sources of airborne transmission include virus-contaminated dust, thought to be the source of arenavirus infections.
Arthropod-borne transmission involves the bite of arthropod vectors (e.g., mosquitoes transmit dengue, yellow fever, and some encephalitis viruses, ticks transmit other encephalitis viruses, and sandflies transmit sandfly fever; see Table 14-6).
Other terms are used to describe transmission by mechanisms that embrace more than one of the above routes. Iatrogenic transmission occurs as a direct result of some activity of the attending doctor, nurse, or other person in the course of caring for patients, usually via nonsterile equipment, multiple-use syringes, blood transfusions, the use of human blood products, or inadequate handwashing. Nosocomial transmission occurs while a person is in a
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