Natural History (Ecology) and Modes of Transmission of Viruses
المؤلف:
Stefan Riedel, Jeffery A. Hobden, Steve Miller, Stephen A. Morse, Timothy A. Mietzner, Barbara Detrick, Thomas G. Mitchell, Judy A. Sakanari, Peter Hotez, Rojelio Mejia
المصدر:
Jawetz, Melnick, & Adelberg’s Medical Microbiology
الجزء والصفحة:
28e , p432-433
2025-10-25
51
Ecology is the study of interactions between living organ isms and their environment. Different viruses have evolved ingenious and often complicated mechanisms for survival in nature and transmission from one host to the next. The mode of transmission used by a given virus depends on the nature of the interaction between the virus and the host. Viruses may be transmitted in the following ways:
1. Direct transmission from person to person by contact. The major means of transmission include droplet or aerosol infection (eg, influenza, rhinovirus, measles, and small pox); by sexual contact (eg, papillomavirus, hepatitis B, herpes simplex type 2, and human immunodeficiency virus); by hand–mouth, hand–eye, or mouth–mouth con tact (eg, herpes simplex and Epstein-Barr virus); or by exchange of contaminated blood (eg, hepatitis B, hepatitis C, and human immunodeficiency virus).
2. Indirect transmission by the fecal–oral route (eg, enteroviruses, rotaviruses, and hepatitis A) or by fomites (eg, Norwalk virus and rhinovirus).
3. Transmission from animal to animal, with humans an accidental host. Spread may be by bite (rabies) or by droplet or aerosol infection from rodent-contaminated quarters (eg, arenaviruses and hantaviruses).
4. Transmission by means of an arthropod vector (eg, arboviruses, now classified primarily as togaviruses, flaviviruses, and bunyaviruses).
At least three different transmission patterns have been recognized among the arthropod-borne viruses:
1. Human–arthropod cycle: Examples—Urban yellow fever, dengue. This occurs in densely populated areas infested with competent vectors.
2. Lower vertebrate–arthropod cycle with tangential infection of humans: Examples—Jungle yellow fever, St. Louis encephalitis. This is a more common mechanism with humans as an accidental “dead end” host.
3. Arthropod–arthropod cycle with occasional infection of humans and lower vertebrates: Examples—Colorado tick fever, La Crosse encephalitis.
In this cycle, the virus may be transmitted from the adult arthropod to its offspring through the egg (transovarian pas sage); thus, the cycle may continue with or without intervention of a viremic vertebrate host.
In vertebrates, the invasion of most viruses evokes a violent reaction, usually of short duration. The result is decisive. Either the host succumbs or it lives until the production of antibodies that neutralize the virus. The period of the active virus is usually short, although persistent or latent infections that last for months to years may occur (hepatitis B, herpes sim plex, cytomegalovirus, retroviruses). In arthropod vectors of the virus, the relationship is usually quite different. The viruses produce little or no ill effect and remain active in the arthropod throughout the latter’s natural life. Thus, arthropods, in contrast to vertebrates, act as permanent hosts and reservoirs.
Emerging Viral Diseases
Owing to wide-reaching changes in societal structures, technology, and the environment—plus the decreased effective ness of previous approaches to disease control—the spectrum of infectious diseases is expanding today. New agents appear, and diseases once thought to be under control are increasing in incidence as pathogens evolve and spread. The term “emerging infectious diseases” denotes these phenomena.
Viral diseases emerge following one of three general pat terns: recognition of a new agent, abrupt increase in illnesses caused by an endemic agent, and invasion of a new host population.
Combinations of factors contribute to disease emergence. Some factors increase human exposure to once-obscure pathogens, others provide for dissemination of once-localized infections, and still others force changes in viral properties or host responses to infection. Factors include: (1) environmental changes (deforestation, damming or other changes in water ecosystems, flood or drought, famine); (2) human behavior (sexual behavior, drug use, outdoor recreation); (3) socioeconomic and demographic phenomena (war, poverty, population growth and migration, urban decay); (4) travel and commerce (highways, international air travel); (5) food production (globalization of food supplies, changes in methods of food processing and packaging); (6) health care (new medical devices, blood transfusions, organ and tissue transplantation, drugs causing immunosuppression, widespread use of antibiotics comorbid conditions); (7) microbial adaptation (new viral strains arising through mutation, recombination or reassortment resulting in changes in transmissibility, virulence, or development of drug resistance); and (8) public health measures (inadequate sanitation and vector control measures, curtailment of prevention programs, lack of trained personnel in sufficient numbers).
Examples of emerging viral agents in different regions of the world include Zika, chikingunya, Ebola, avian influenza, Nipah, hantavirus, human immunodeficiency virus, dengue, West Nile, Rift Valley fever, and bovine spongiform encephalopathy (the latter a prion disease).
Of potential concern also is the possible use of animal organs as xenografts in humans. Because the numbers of available human donor organs cannot meet the needs of all waiting patients, xenotransplantation of nonhuman primate and porcine organs is considered an alternative. Concerns exist about the potential accidental introduction of new viral pathogens from the donor species into humans.
Bioterrorism Agents
Bioterrorism agents are microorganisms (or toxins) that could be used to produce death and disease in humans, animals, or plants for terrorist purposes. Such microorganisms could be genetically modified to increase their virulence, make them resistant to drugs or vaccines, or enhance their ability to be disseminated in the environment.
Potential bioterrorism agents are classified into risk categories based on the ease of dissemination or transmission from person to person, mortality rates, ability to cause public panic, and requirement for public health preparedness. Viral agents in the highest risk category are smallpox and the viral hemorrhagic fevers; highest risk bacteria include the agents of anthrax, botulism, plague, and tularemia.
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