Prescott’s Microbiology, 9th Edition
37
Epidemiology and Public Health Microbiology
CHAPTER OVERVIEW
This chapter discusses the epidemiological parameters used to institute effective control, prevention, and
eradication measures within an affected or potentially affected population, including a discussion of vaccines
and immunization procedures. It also discusses the epidemiology of hospital-acquired (nosocomial) infections
and bioterrorism, which have been of increasing concern in recent years.
LEARNING OUTCOMES
After reading this chapter you should be able to:
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define the agencies that are responsible for disease prevention and control
define the basic vocabulary and processes used in the science of epidemiology
evaluate the effectiveness of public health surveillance methods
deduce the impact of public health methods on the quality of U.S. life over the last 100 years
describe the use of geographical information systems to track diseases remotely
use standardized surveillance data to measure infectious disease frequency
calculate rates of incidence, prevalence, morbidity, and mortality
discriminate between a communicable and a noncommunicable disease
interpret population infection data to define epidemic and pandemic events
apply the concept of herd immunity to public health in your community
predict potential infectious disease outbreaks from molecular data revealing antigenic drift and
antigenic shift events
report on recently emerged and reemerging global infectious diseases
organize geographic data to reveal changes in microbial drug resistance and vector spread of infection
report the major causes of health-care-associated infection in the United States
distinguish community-acquired pathogens from health-care–associated pathogens, and explain why
community-acquired pathogens are of concern to hospital personnel
recommend measures for preventing and controlling health-care–-associated infections
assemble communicable disease linkages to create a chain of infection
reveal the weakest link in the chain of infection
advise vaccine use to prevent infectious disease transmission in a population
explain the role of disinfection, sanitation, and chemotherapy to control infectious disease transmission in
a population
discuss the readiness efforts necessary to be prepared for a bioterrorism attack
prioritize microorganisms based on public health threat
construct tables of prevention and control information relative to an infectious agent
alert appropriate authorities in response to suspected acts of bioterrorism
CHAPTER OUTLINE
I.
Epidemiology
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Prescott’s Microbiology, 9th Edition
A.
Epidemiology—the science that evaluates the occurrence, determinants, distribution, and control of
health and disease in a defined human population
B. Epidemiologist—one who practices epidemiology (a disease detective)
1. Centers for disease control (CDC) US agency of epidemiologists
2. World health organization – worldwide counterpart to the CDC
3. John Snow – British physician – first epidemiologist
C. 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 disease—a gradual increase in frequency above the endemic level, but not to
the epidemic level
4. Outbreak—an epidemic-like increase in frequency, but in a very limited (focal) segment of the
population
5. Epidemic—sudden increase in frequency above the endemic level
6. Index case—the first case in an epidemic
7. Pandemic—a long-term increase in frequency in a large (usually worldwide) population
II. Epidemiological Methods
A. Public health surveillance
1. Proactive evaluation of genetic background, environmental conditions, human behaviors, and
lifestyle choices, emerging infectious agents, and microbial responses to chemotherapeutic
agents
2. Data is gathered by population surveys (sentinel surveillance) and case reporting (retrospective
comparisons)
B. Remote sensing and geographic information systems: charting infectious disease data
1. Map-based tools that can be used to study the distribution, dynamics, and environmental
correlates of microbial disease
2. Remote sensing—gathering digital images of Earth’s surfaces from satellites and transforming
data to maps
3. Geographic information system (GIS)—data management system
4. This approach works best with diseases clearly associated with mapped environmental
variables (e.g., vegetation types, elevation, precipitation)
III. Measuring Infectious Disease Frequency
A. Measures of frequency are usually expressed as fractions; the numerator equals the number of
individuals experiencing the event; the denominator is the number of individuals in whom event
could have occurred
B. Statistics—the mathematics of collection, organization, and interpretation of numerical data
C. Cases of certain diseases must be reported to public health agencies, so-called "reportable diseases"
D. Morbidity rate—the number of new cases in a specific time period per unit of population
E. Prevalence rate—number of individuals infected at any one time per unit of population
F. Mortality rate—number of deaths from a disease per number of cases of the disease
IV. Patterns of an Infectious Disease in a Population
A. Infectious disease is a disease resulting from infection by microbial agents; a communicable disease
is an infectious disease that can be transmitted from person to person
B. Two types of epidemics
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
C. Recognition of an epidemic involves various surveillance methods to monitor the population for
disease occurrence and for demographic analysis
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
D.
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
E. 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
V. Emerging and Reemerging Infectious Diseases and Pathogens
A. New diseases have emerged in the past few decades (e.g., AIDS, hantavirus pulmonary syndrome,
and many “old” diseases that have increased in frequency, such as tuberculosis)
B. The Centers for Disease Control (CDC) has defined such diseases as “new, reemerging, or drugresistant 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
C. 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. Increasing population density
a. Overcrowding increases possibility of exposure
b. Sanitary measures and health-care systems can become overburdened
c. Encroachment and destruction of natural habitats exposes humans to new pathogens
d. Introduction of pathogens into new environments can alter transmission and exposure
patterns
4. Global climate change affects microbial selection and survival in the environment
5. Drug resistance has increased dramatically in nosocomial pathogens due to excessive or
inappropriate use of antimicrobial therapy
6. Rapid transportation systems aid in the spread of disease out of areas where they are endemic
VI. Health-Care–Associated Infections
A. 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; methicillin-resistant Staphylococcus
aureus (MRSA) and vancomycin-resistant enterococci (VRE) are particularly troublesome.
Often referred to as nosocomial infections
B. 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
C. 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 and
prudently using antibiotics
D. The hospital epidemiologist—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
VI. Prevention and Control of Epidemics
A. 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
B. 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
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
3. Supervision and inspection of food and food handlers
4. Destruction of insect vectors with pesticides
C. 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
D. Vaccines and immunization
1. Vaccine—a preparation containing one or more antigens of a pathogen; immunization results
from the immunity stimulated by the delivery of vaccines; vaccinomics is the application of
genomics and bioinformatics to vaccine development
2. More efficient immune responses are obtained by mixing antigens with adjuvants, nontoxic
materials that help to stimulate a strong immune response
3. Vaccines and vaccination have a long history starting with Jenner’s use of cowpox as a vaccine
against smallpox; today there are many vaccines, and vaccination is one of the most costeffective methods for preventing microbial disease
4. Immunization practices depend on the age of the individual and the risk group to which the
individual belongs
a. Children begin a vaccination series at 2 months of age; the series protects against
numerous childhood disease (e.g., measles, mumps, rubella)
b. Adults living in close quarters, having reduced immunity, traveling in other countries,
and working in certain professions (e.g., health care provider) may receive additional
immunizations
E. Types of vaccines and their characteristics
1. Whole-cell vaccines
a. Consist of whole organisms that have been inactivated (killed) or attenuated (live but
avirulent); in general, attenuated whole-organism vaccines are most effective and easy to
use, and they provide more complete immunity
b. Though considered the “gold-standard,” numerous problems are associated with wholecell vaccines
1) Fail to shield against some diseases
2) Attenuated vaccines can cause disease in immunocompromised individuals
3) Attenuated viruses can revert to virulence
2. Acellular or subunit vaccines—vaccines containing specific, purified macromolecules derived
from pathogen (capsular polysaccharides, recombinant surface antigens, and inactivated
exotoxins called toxoids); avoid many of common risks associated with whole-cell vaccines
3. Recombinant-vector vaccines—vaccines containing genetically engineered viruses or bacteria,
having genes that encode major antigens from a pathogen; elicit both humoral and cellular
immunity
4. DNA vaccines—vaccines containing recombinant DNA molecules (usually a plasmid); the
DNA is taken up by muscle cells after injection and enters host nuclei; the antigen gene is then
expressed, producing antigenic proteins that elicit both humoral and cellular immunity;
currently several human trials of DNA vaccines are underway
F. 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 Prevention and the World Health Organization)
VII. Bioterrorism Preparedness
A. Bioterrorism is the intentional or threatened use of microorganisms or toxins from living organisms
to produce death or disease in humans, animals, and plants
B. The list of biological agents that could pose the greatest public health risk is short, and includes
viruses, bacteria, parasites, and toxins
C. Indicators of a bioterrorism attack include a sudden increase in cases of an unusual disease in
humans or animals
D. Biological weapons are more destructive than chemical weapons; the United States government
recently launched an initiative to create a biological weapons defense; global organizations also are
working to detect and respond to bioterrorist attacks
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in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.
Prescott’s Microbiology, 9th Edition
CRITICAL THINKING
1.
Explain the concept of herd immunity. How can it develop naturally? How can it be stimulated
artificially? Cite examples of both methods.
2.
Explain antigenic drift and how it overcomes herd immunity and leads to new outbreaks of the same
disease.
3.
The authors present a number of surveillance methods used by epidemiologists. Discuss these methods in
the context of a particular disease of past or current importance (e.g., smallpox, AIDS) to show how the
data collected by these methods increase our knowledge of the disease process.
4.
Over the last 100 years there have been very successful campaigns against infectious diseases such as
poliomyelitis, leprosy, and tuberculosis. There have also been some works in progress such as HIV,
malaria, and drug resistant tuberculosis. How has understanding the chain of infection concept
perpetuated the success of efforts to curtail these pathogens.
CONCEPT MAPPING CHALLENGE
Map the following concepts to demonstrate your understanding of their relatedness.
Bioterrorism Population Immunity Vector Incidence Vehicle
Infectious disease Zoonosis Mortality Morbidity Epidemiology
Communicable Statistics
Vaccine WHO
CDC
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© 2014 by McGraw-Hill Education. This is proprietary material solely for authorized instructor use. Not authorized for sale or distribution
in any manner. This document may not be copied, scanned, duplicated, forwarded, distributed, or posted on a website, in whole or part.