The treatment of an infection is often difficult and nof always successful. Because much of the damage is already done before appropriate medical intervention is begun, the microorganisms gain too much of a "head start." Another strategy for combating infectious diseases is to stop infections before they even start (i.e., disease prevention). As discussed at the beginning of this chapter, the first step in any host-microoiganism relationship is the encounter. Therefore, strategies to prevent disease involve interrupting encounters or
BOX 3-11 Strategies for Prevention of Infectious Diseases preventing transmission: Avoid direct contact with infected persons or take protective measures when direct contact is going to occur (e.g., wear gloves, wear condoms) Block spread of airborne microorganisms by wearing masks or isolating persons with infections transmitted by air Use sterile medical techniques controlling microbial reservoirs: Sanitation and disinfection Sewage treatment Food preservation Water treatment
Control of pests and insect vector populations minimizing risk before or shortly after exposure;
Immunization Cleansing and use of antiseptics Prophylactic use of antimicrobial agents minimizing the risk of infection when encounters do occur. As outlined in Box 3-11, interruption of encounters may be accomplished by preventing transmission of the infecting agents and by controlling or destroying reservoirs of human pathogens. Of interest, most of these measures do not really involve medical practices but rather social practices and policies.
If encounters do occur, medical strategies exist for minimizing the risk for disease development. One of the most effective methods is immunization. This practice takes advantage of the specificity and memory of the immune system. There are two basic approaches to immunization: active and passive. With active immunization, modified antigens from pathogenic microorganisms are introduced into the body and cause an immune response. If or when the host encounters the pathogen in nature, the memory of the immune system will ensure minimal delay in the immune response, thus affording strong protection. Alternatively, with passive immunization antibodies against a particular pathogen have been produced in one host and are transferred to a second host where they provide temporary protection. The passage of maternal antibodies to the newborn is a key example of natural passive immunization. Active immunity is generally more long lasting because the immunized host's own immune response has been activated. However, for complex reasons, active immunity only has been successfully applied for relatively few infectious diseases, some of which include diphtheria, whooping cough (pertussis), tetanus, influenza, polio, smallpox, measles, hepatitis, and certain S. pneumoniae and H. influenzae infections.
Prophylactic antimicrobial therapy, the administration of antibiotics when the risk of developing an infection is high, is another common medical intervention for preventing infection.
To prevent infectious diseases, information is required regarding the sources of pathogens, the method of transmission to and among humans, human risk factors for encountering the pathogen and developing infection, and factors that contribute to good and bad outcomes resulting from the encounter. Epidemiology is the science that characterizes these aspects of infectious diseases and monitors the effect diseases have on public health. By fully characterizing the circumstances assodated with the acquisition and dissemination of infectious diseases, there is a better chance for preventing and eliminating these diseases. Additionally, many epidemiologic strategies that have' been developed for public health also apply in hospitals for the control of hospital-based (i.e., nosocomial) infections (for more information on infection control in hospitals, see Chapter 64).
The field of epidemiology is broad and complex. Diagnostic microbiology laboratory personnel and hospital epidemiologists often work dosely to investigate problems. Therefore, familiarity with certain epidemiologic terms and concepts is important (Box 3-12).
BOX 3-12 Definitions of Selected Epidemiologic Terms
CARRIER: A person who carries the etiologic agent but shows no apparent signs or symptoms of infection or disease COMMON SOURCE: The etiologic agent responsible for an epidemic or outbreak originates from a single source or reservoir
DISEASE INCIDENCE: The number of diseased or infected persons in a population
DISEASE PREVALENCE: Percentage of diseased persons in a given population at a particular time ENDEMIC: A disease constantly present at some rate of occurrence in a particular location EPIDEMIC: A larger than normal number of diseased or infected individuals in a particular location ETIOLOGIC AGENT: A microorganism responsible for causing infection or Infectious disease MODE OF TRANSMISSION: Means by which etiologic agents are brought in contact with the human host (e.g., infected blood, contaminated water, Insect bite) MORBIDITY: The state of disease and its associated effects on the host
MORBIDITY RATE: The incidence of a particular disease state
MORTALITY: Death resulting from disease
MORTALITY RATE: The incidence in which a disease results in death
NOSOCOMIAL INFECTION: Infection in which etiologic agent was acquired in a hospital OUTBREAK: A larger than normal number of diseased or infected individuals that occurs over a relatively short period PANDEMIC: An epidemic that spans the world RESERVOIR: Origin of tine etiologic agent or location from which they disseminate (e.g., water, food, insects, animals, other humans)
STRAIN TYPING: Laboratory-based characterization of etiologic agents designed to establish their relatedness to one another during a particular outbreak or epidemic SURVEILLANCE: Any type of epidemiologic investigation that involves data collection for characterizing circumstances surrounding the incidence or prevalence of a particular disease or infection VECTOR: A living entity (animal, insect, or plant) that transmits the etiologic agent VEHICLE: A nonliving entity that is contaminated with the etiologic agent and as such is the mode of transmission for that agent
Because the central focus of epidemiology is on tracking and diaracterizing infections and infectious diseases, this field heavily depends on diagnostic microbiology. Epidemiologic investigations cannot proceed without first knowing the etiologic agents involved. Therefore, the procedures and protocols used in diagnostic microbiology to detect, isolate, and characterize human pathogens are not only essential for patient care but also play a central role in epidemiologic studies focused on disease prevention and the general improvement of public health. In fact, microbiologists who work in clinical laboratories are often the first to recognize patterns that suggest potential outbreaks or epidemics.
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