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Microbiology, Fifth Edition
Microbiology, 5/e
Lansing M Prescott, Augustana College
Donald A Klein, Colorado State University
John P Harley, Eastern Kentucky University

The Epidemiology of Infectious Disease

Study Outline

  1. Introduction
    1. Epidemiology-the science that evaluates the occurrence, determinants, distribution, and control of health and disease in a defined human population
    2. Health-organism and all its parts functioning normally; a state of physical and mental well-being, not merely the absence of disease
    3. Disease-an impairment of the normal state of an organism or any of its components, which hinders the performance of vital functions
    4. Epidemiologist-one who practices epidemiology (a disease detective)
  2. Epidemiological Terminology
    1. Epidemiologists use a variety of terms to describe different types of disease occurrence
      1. Sporadic disease-occurs occasionally at irregular intervals in a human population
      2. Endemic disease-maintains a relatively steady low-level frequency at a moderately regular interval
      3. Hyperendemic-a gradual increase in frequency above the endemic level, but not to the epidemic level
      4. Epidemic-sudden increase in frequency above the endemic level
      5. Index case-the first case in an epidemic
      6. Outbreak-an epidemic-like increase in frequency, but in a very limited (focal) segment of the population
      7. Pandemic-a long-term increase in frequency in a large (usually worldwide) population
    2. Epizootology-study of the factors that influence disease in animal populations
      1. Enzootic-moderate prevalence in animals
      2. Epizootic-sudden outbreak of disease in animals
      3. Zoonoses-diseases of animals that can be transmitted to humans
  3. Measuring Frequency: The Tools of Epidemiologists
    1. Measures of frequency are usually espressed as fractions; the numerator equals the number of individuals experiencing the event; the denominator is the number of individuals in whom the event could have occurred
    2. Statistics-the mathematics of collection, organization, and interpretation of numerical data
    3. Morbidity rate-the number of new cases in a specific time period per unit of population
    4. Prevalence rate-number of individuals infected at any one time per unit of population
    5. Mortality rate-number of deaths from a disease per number of cases of the disease
  4. Infectious Disease Epidemiology
    1. Infectious disease is a disease resulting from infection by microbial agents
    2. Epidemiologists study the natural history of an infectious disease
      1. What organism caused the disease?
      2. What is the source and/or reservoir of the disease?
      3. How is the disease transmitted?
      4. What host and environmental factors facilitated development of the disease within a defined population?
      5. How can the disease best be controlled or eliminated?
  5. Recognition of an Infectious Disease in a Population
    1. Recognition involves various surveillance methods to monitor the population for disease occurrence and for demographic analysis
    2. Surveillance involves identification of the signs and/or symptoms of a disease
      1. Signs-objective changes in the body (e.g. fever, rash, etc.) that can be directly observed
      2. Symptoms-subjective changes (e.g. pain, appetite loss, etc.) experienced by the patient
      3. Disease syndrome-a set of signs and symptoms that is characteristic of a disease
    3. Characteristic patterns of an infectious disease
      1. Incubation period-the period after pathogen entry but before signs and symptoms appear
      2. Prodromal stage-onset of signs and symptoms but not yet clear enough for diagnosis
      3. Period of illness-disease is most severe and has characteristic signs and symptoms
      4. Period of decline (convalescence)-signs and symptoms begin to disappear
    4. Remote sensing and geographic information systems: charting infectious disease-map-based tools that can be used to study the distribution, dynamics, and environmental correlates of microbial disease
      1. Remote sensing-gathering digital images of Earth's surfaces from satellites and transforming data to maps
      2. Geographic information system-data management system
      3. This approach works best with diseases clearly associated with mapped environmental variables (e.g., vegetation types, elevation, precipitation)
    5. Correlation with a single causative organism-uses methods described in chapter 34 for the organism's isolation and identification
  6. Recognition of an Epidemic
    1. Common-source epidemic-characterized by a sharp rise to a peak and then a rapid but not as pronounced decline in the number of cases; usually results from exposure of all infected individuals to a single common, contaminated source, such as food or water
    2. Propagated epidemic-characterized by a gradual increase and then a gradual decline in the number of cases; usually results from the introduction of one infected individual into a population, who then infects others; these in turn infect more, until an unusually large number of individuals within the population are infected
    3. Herd immunity-the resistance of a population to infection and to the spread of an infectious organism due to the immunity of a large percentage of the population; this limits the effective contact between infective and susceptible individuals
    4. Antigenic shift-genetically determined changes in the antigenic character of a pathogen so that it is no longer recognized by the host?s immune system (e.g., new flu strains); smaller changes are called antigenic drift; can lead to increases in disease frequency because the population of susceptible hosts increases
  7. The Infectious Disease Cycle: Story of a Disease
    1. What pathogen caused the disease? Epidemiologists must determine the etiology (cause) of a disease
      1. Koch?s postulates (or modifications of them) are used if possible; the clinical microbiology laboratory plays an important role in the isolation and identification of the pathogen
      2. Communicable disease-one that can be transmitted from one host to another
    2. What was the source and/or reservoir of the pathogen?
      1. Source-location from which organisms are immediately transmitted to the host
      2. Period of infectivity-the time during which the source is infectious or is disseminating the organism
      3. Reservoir-site or natural environmental location where organism is normally found
      4. Carrier-an infected individual who is a potential source of a pathogen
        1. Active carrier-a carrier with an overt clinical case of the disease
        2. Convalescent carrier-an individual who has recovered from the disease but continues to harbor large numbers of the pathogen
        3. Healthy carrier-an individual who harbors the pathogen but is not ill
        4. Incubatory carrier-an individual who harbors the pathogen but is not yet ill
        5. Casual (acute, transient) carriers-any of the above carriers who harbor the pathogen for a brief period (hours, days, or weeks)
        6. Chronic carriers-any of the above carriers who harbor the pathogen for long periods (months, years, or life)
      5. Zoonoses-infectious diseases that occur in animals and can be transported to humans; the animals serve as a reservoir for the disease; transmission from animal to host can be direct or indirect
    3. How was the pathogen transmitted?
      1. Airborne transmission-suspended in air; travels a meter or more
        1. Droplet nuclei-may come from sneezing, coughing, or vocalization
        2. Dust particles-may be important in airborne transmission because microorganisms adhere readily to dust
      2. Contact transmission-touching between source and host
        1. Direct (person to person)-physical interaction between infected person and host
        2. Indirect-involves an intermediate
        3. Droplets-large particles that travel less than one meter through the air
      3. Vehicle transmission-inanimate materials or objects are involved in transmission
        1. Common vehicle transmission-a single vehicle serves to spread the pathogen to multiple hosts, but does not support its reproduction
        2. Fomites-common vehicles such as surgical instruments, bedding, eating utensils
        3. Food and water are also common vehicles
      4. Vector-borne transmission-living transmitters, such as arthropods or vertebrates
        1. External (mechanical) transmission-passive carriage of the pathogen on the body of the vector with no growth of the pathogen during transmission
        2. Internal transmission-carried within the vector
          1. ) Harborage-pathogen does not undergo morphological or physiological changes within the vector
          2. ) Biologic-pathogen undergoes morphological or physiological changes within the vector
    4. Why was the host susceptible to the pathogen?-depends on defense mechanisms of the host and the pathogenicity of the pathogen
    5. How did the pathogen leave the host?
      1. Active escape-movement of pathogen to portal of exit (e.g., many helminthes)
      2. Passive escape-excretion in feces, urine, droplets, saliva, or desquamated cells
  8. Virulence and the Mode of Transmission
    1. There is evidence of a correlation between mode of transmission and degree of virulence
      1. A pathogen that is spread by direct contact (e.g., rhinoviruses) cannot afford to make the host so ill it cannot be spread effectively
      2. If mode of transmission does not depend on health and mobility of host, then greater degree of virulence is possible
      3. A pathogen that is vector-borne needs to be able to replicate extensively within host; therefore, host is likely to remain healthy long enough for transmission to occur
    2. Evidence also suggests that the pathogen?s ability to survive outside the host is correlated with virulence
      1. Pathogens that do not survive well outside the host and that do not use a vector are likely to be less virulent
      2. Pathogens that can survive for long periods of time outside the host tend to be more virulent
    3. Human cultural patterns and behavior are also correlated with virulence
  9. Emerging and Reemerging Infectious Diseases and Pathogens
    1. New diseases have emerged in the past few decades (e.g., AIDS, hantavirus pulmonary syndrome, and many ?old? diseases that have increased in frequency (e.g., tuberculosis)
    2. The Centers for Disease Control has defined such diseases as ?new, reemerging, or drug-resistant infections whose incidence has increased in the last two decades or whose incidence threatens to increase in the near future?; these diseases are the focus of systematic epidemiology, which is concerned with the ecological and social factors that influence the development and emergence of disease
    3. Reasons for increases in emerging and reemerging infectious diseases
      1. Economic and military forces can cause population shifts and disruptions of normal public health measures
      2. Changes in sexual behavior, use of IV drugs, and changes in food preferences affect disease transmission
      3. Globalization of food processing and supply centers has created potential for widespread outbreaks
      4. Increasing population density
        1. Overcrowding increases possibility of exposure
        2. Sanitary measures and health care systems can become overburdened
        3. Encroachment and destruction of natural habitats exposes humans to new pathogens
        4. Introduction of pathogens into new environments can alter transmission and exposure patterns
      5. Drug resistance has increased dramatically in nosocomial pathogens due to excessive or inappropriate use of antimicrobial therapy
      6. Increasing numbers of immunosuppressed individuals (e.g., AIDS patients, transplant recipients) provide a habitat in which opportunistic pathogens can evolve to them become frank pathogens
      7. Rapid transportation systems aid in the spread of disease out of areas where they are endemic
  10. Control of Epidemics
    1. Reduce or eliminate the source or reservoir of infection through:
      1. Quarantine and isolation of cases and carriers
      2. Destruction of an animal reservoir, if one exists
      3. Treatment of sewage to reduce contamination of water
      4. Therapy that reduces or eliminates infectivity of individuals
    2. Break the connection between the source and susceptible individuals through sanitization, disinfection, vector control, and other measures; examples include:
      1. Chlorination of water supplies
      2. Pasteurization of milk
      3. Supervision and inspection of food and food handlers
      4. Destruction of insect vectors with pesticides
    3. Reduce the number of susceptible individuals-increase herd immunity
      1. Passive immunization to give temporary immunity following exposure
      2. Active immunization to protect individuals and host population
    4. Role of the public health system: epidemiological guardian-a network of health professionals involved in surveillance, diagnosis, and control of epidemics (e.g., The Centers for Disease Control and the World Health Organization)
  11. Emerging Threat of Bioterrorism
    1. Bioterrorism is the intentional or threatened use of microorgansisms or toxins from living organisms to produce death or disease in humans, animals, and plants
    2. Biological weapons are more destructive than chemical weapons; the United States government recently launched an initiative to create a biological weapons defense
    3. The list of biological agents that could pose the greatest public health risk is short, and includes viruses, bacteria, parasites, and toxins
    4. Vaccines are not a solution to bioterrorism because of the difficulty involved in vaccinating large populations and because of the broad spectrum of potential agents
    5. In the US, the Federal Drug Administration is charged with fostering development of vaccines, drugs and diagnostic products, and other measures needed to respond to bioterrorism threats
  12. Global Travel and Health Considerations
    1. In developed countries, effective public health systems are in place; because of this, travel in developed countries does not pose a health risk
    2. In underdeveloped countries, such public health systems do not exist, and people traveling in those countries are at greater risk
    3. Travelers can take several kinds of precautions in underdeveloped countries; of these, vaccinations are very important
    4. Space travel-zero-gravity environments impact human physiology, including immune responses; furthermore, these environments alter the pathogen (e.g., increasing resistance to antibiotics); safety of space travelers is insured by selection of healthy crew members and preflight quarantine
  13. Nosocomial Infections
    1. Produced by infectious agents that develop within a hospital or other clinical care facility and that are acquired by patients while they are in the facility; infections that are incubating within the patient at the time of admission are not considered nosocomial
    2. Source
      1. Endogenous-patient?s own microbiota
      2. Exogenous-microbiota other than the patient?s
      3. Autogenous-cannot be determined to be endogenous or exogenous
    3. Control, prevention, and surveillance should include proper handling of the patient and the materials provided to the patient, as well as monitoring of the patient for signs of infection
    4. The hospital epidemiologist (other terms are also used) is an individual (usually a registered nurse) responsible for developing and implementing policies to monitor and control infections and communicable disease; usually reports to an infection control committee or other similar group