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Chapter 36
The
Epidemiology
of Infectious
Disease
1
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May 7, 2015
微免科 陳怡原
[email protected]
Ext. 3352
2
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Today’s lecture will discuss:
1. What is epidemiology?
2. How to identify the etiologic agent of various
infectious diseases and how to control and
prevent epidemics
3. The epidemiology of nosocomial infections
and emerging and reemerging infectious
diseases.
4. The role of the public health system
3
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Epidemiology
• science that evaluates occurrence,
determinants, distribution, and control of
health and disease in a defined human
population
• epidemiologist
– one who practices epidemiology
• John Snow was the first epidemiologist
– studied cholera in London
4
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Centers for Disease Control and
Prevention (CDC)
• functions as national focus for
– developing and applying disease prevention
and control
– environmental health
– health promotion and health education
activities designed to improve the health of the
people
• world wide counterpart is the World Health
Organization (WHO) located in Geneva,
Switzerland
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Figure 36.1
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Epidemiology…
• determine:
–
–
–
–
causative agent
source and/or reservoir of disease agent
mechanism of transmission
host and environmental factors that facilitate
development of disease within a defined
population
– best control measures
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Epidemiology Terminology
• sporadic disease
– occurs occasionally and at irregular
intervals
• endemic disease
– maintains a relatively steady low-level
frequency at a moderately regular interval
• hyperendemic diseases
– gradually increase in occurrence frequency
above endemic level but not to epidemic
level
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More terms…
• outbreak
– sudden, unexpected occurrence of disease
– usually focal or in a limited segment of
population
• epidemic
– sudden increase in frequency above expected
number
– index case – first case in an epidemic
• pandemic
– increase in disease occurrence within large
population over wide region (usually
worldwide)
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Figure 36.2
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Remote Sensing and
Geographic Information
Systems: Charting Infectious
Diseases
• can be used to study distribution, dynamics,
and environmental correlates of microbial
diseases
11
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More on Charting Infectious
Diseases…
• remote sensing (RS)
– gathering of digital images of Earth’s
surface from satellites and transforming
data into maps
• Geographic information system (GIS)
– data management system that organizes
and displays digital map data from RS
12
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Measuring Frequency:
The Epidemiologist’s Tools
• to determine if an outbreak, epidemic or
pandemic is occurring, epidemiologists
measure disease frequency at single time
points and over time
• statistics
– mathematics dealing with collection,
organization, and interpretation of numerical
data
• three important statistical measures of
disease frequency
– morbidity rate
– prevalence rate
– mortality rate
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Morbidity rate
• an incidence rate
• number of new cases in a specific time period
per unit of population
# new cases during a specific time
# individuals in population
14
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Prevalence rate
• total number of individuals infected at any
one time
• depends both on incidence rate and duration
of illness
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Mortality rate
• number of deaths from a disease per number
of cases of the disease
# deaths due to given disease
size of total population with disease
16
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Patterns of Infectious Disease
in a Population
• infectious disease
– disease resulting from an infection by microbial agents
such as viruses, bacteria, fungi, protozoa, and helminths
• communicable disease
– can be transmitted from one host to another
• two types of epidemics
– common source epidemic
• results from single common contaminated source
such as food
– propagated epidemic
• results from the introduction of a single infected
individual into a susceptible population which is
propagated to others
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Course of infectious disease
• incubation period
– period after pathogen entry but before signs
and symptoms appear
• prodromal stage
– onset of signs and symptoms
– not clear enough for diagnosis
• period of illness
– disease is most severe and has characteristic
signs and symptoms
• convalescence
– signs and symptoms begin to disappear
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Recognition of an Infectious
Disease in a Population
• involves use of surveillance methods
• cases of a disease recognized by its
characteristic disease syndrome
– set of signs and symptoms characteristic of a
disease
– signs
• objective changes in body that can be directly
observed
– symptoms
• subjective changes experienced by patient
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Figure 36.3
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Figure 36.5
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Herd immunity
• resistance of a population to infection and to
spread of an infectious organism because of
the immunity of a large percentage of the
population
• level can be altered by introduction of new
susceptible individuals into population
• level can be altered by changes in pathogen
– antigenic shift – major change in antigenic
character of pathogen
– antigenic drift – smaller antigenic changes
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Figure 36.6
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What Pathogen Caused the
Disease?
• Koch’s postulate (or modifications of them)
are used if possible
• clinical microbiology lab
– plays important role in isolation and
identification of pathogen
• communicable disease
– can be transmitted from one host to another
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What was the Source and/or
Reservoir of the Pathogen?
• source
– location from which pathogen is
transmitted to host
• period of infectivity
– time during which source is infectious or is
disseminating the organism
• reservoir
– site or natural environmental location in
which pathogen is normally found
– sometimes functions as source of pathogen
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Human sources/reservoirs
• carrier
– infected hosts who are potential sources of
infection for others
– types of carriers
• active carrier has overt clinical case of disease
• convalescent carrier has recovered but
continues to harbor large numbers of pathogen
• healthy carrier harbors the pathogen but is not
ill
• incubatory carrier is incubating the pathogen
in large numbers but is not yet ill
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More Types of carriers…
• casual, acute, or transient carriers
– convalescent, healthy, and incubatory carriers
that harbor pathogen for brief time
• chronic carriers
– convalescent, healthy, and incubatory carriers
that harbor pathogen for long time
27
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Animal reservoirs
• numerous diseases are zoonoses
• transmission to human can be direct or
indirect
• vectors
– organisms that spread disease from one host to
another
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How Was the Pathogen
Transmitted?
• four main routes
–
–
–
–
airborne
contact
vehicle
vector-borne
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Airborne Transmission
• pathogen suspended in air and travels  1
meter
• droplet nuclei
–
–
–
–
small particles (1-4 m diameter)
can remain airborne for long time
can travel long distances
usually propelled from respiratory tract of
source organisms by sneezing, coughing, or
vocalization
• dust particles also important route of
airborne transmission
30
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Contact Transmission
• coming together or touching of
source/reservoir and host
• direct contact (person-to-person)
– physical interaction between source/reservoir
and host
– e.g., kissing, touching, and sexual contact
• indirect contact
– involves an intermediate (usually inanimate)
– e.g., eating utensils, bedding
• droplet spread
– large particles (>5 m) that travel < 1 meter
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Vehicle Transmission
• vehicles
– inanimate materials or objects involved in
pathogen transmission
• common vehicle transmission
– single vehicle spreads pathogen to multiple
hosts
– e.g., water and food
• fomites
– common vehicles such as surgical instruments,
bedding, and eating utensils
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Vector-Borne Transmission
• external (mechanical) transmission
– passive carriage of pathogen on body of vector
– no growth of pathogen during transmission
• internal transmission
– carried within vector
– harborage transmission – pathogen does not
undergo changes within vector
– biologic transmission – pathogen undergoes
changes within vector
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Why Was the Host Susceptible
to the Pathogen?
• two main factors
– defense mechanisms of host
– pathogenicity of pathogen
• nutrition, genetic predisposition and stress
also play a role in host susceptibility to
infection
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How Did the Pathogen Leave
the Host?
• active escape
– movement of pathogen to portal of exit
• passive escape
– excretion in feces, urine, droplets, saliva, or
desquamated cells
35
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Virulence and the Mode of
Transmission
• evidence suggests correlation between mode
of transmission and degree of virulence
– direct contact  less virulent
– vector-borne  highly virulent in human host;
relatively benign in vector
– greater ability to survive outside host  more
virulent
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Emerging and Reemerging
Infectious Diseases and
Pathogens
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Emerging and Reemerging
Infectious Diseases and Pathogens
Figure 36.7
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Figure 36.8
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Systematic epidemiology
• focuses on ecological and social factors that
influence development and spread of
emerging and reemerging diseases
• numerous factors have been identified
40
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Reasons for Increases in
Emerging and Reemerging
Infectious Diseases
• crowding
• habitat disruption
• shift in distribution of nosocomial pathogens
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More reasons…
• excessive or inappropriate use of
antimicrobial therapy
• medical practices that lead to
immunosuppression
• rapid global transportation systems
42
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Nosocomial Infections
• result from pathogens that develop within a
hospital or other clinical care facility and are
acquired by patients while they are in the
facility
• 5-10% of all hospital patients acquire a
nosocomial infection
• usually caused by bacteria that are members
of normal microbiota
43
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Figure 36.10
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Sources of Nosocomial…
• endogenous pathogen
– brought into hospital by patient or acquired
when patient is colonized after admission
• exogenous pathogen
– microbiota other than the patient’s
• autogenous infection
– caused by an agent derived from microbiota of
patient despite whether it became part of
patient’s microbiota following admission
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Control, Prevention, and
Surveillance
• nosocomial infections
– prolong hospital stays by 4–13 days
– result in over $4.5 billion costs
– result in 20,000–60,000 deaths annually
• proper training of personnel in basic infection
control measures
– e.g., handling of surgical wounds and hand
washing
• monitoring of patient for signs and symptoms
of nosocomial infection
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The Hospital Epidemiologist
• individual responsible for developing and
implementing policies to monitor and control
infections and communicable diseases
• reports to infection control committee or
similar group
47
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Control of Epidemics
• three types of control measures
– reduce or eliminate source or reservoir of
infection
– break connection between source and
susceptible individual
– reduce number of susceptible individuals
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Reduce or eliminate source or
reservoir
•
•
•
•
quarantine and isolation of cases and carriers
destruction of animal reservoir
treatment of sewage
therapy that reduces or eliminates infectivity
of cases
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Break connection between
source and susceptible
individuals
• chlorination of water supplies
• pasteurization of milk
• supervision and inspection of food and food
handlers
• destruction of insect vectors with pesticides
50
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Reduce number of susceptible
individuals
• raises herd immunity
• passive immunity following exposure
• active immunity for protection
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Vaccines and Immunization
• vaccine
– preparation of microbial antigens used to
induce protective immunity
– may consist of killed, living, weakened
(attenuated) microbes or inactivated bacterial
toxins (toxoids), purified cell material,
recombinant vectors or DNA
52
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More on Vaccines and
Immunization
• immunization
– result obtained when vaccine stimulates
immunity
• vaccines attempt to induce antibodies and
activated T cells to protect host from future
infection
• vaccinomics is the application of genomics
and bioinformatics to vaccine development
53
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Adjuvants
• are mixed with antigens in vaccines can to
enhance the rate and degree of immunization
• can be any nontoxic material that prolongs
antigen interaction with immune cells and
stimulates the immune response to the
antigen
• several types are available
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Immunized Hosts
• vaccination of children should begin at ~2
months
• further vaccination depends on relative risk
–
–
–
–
living in close communities
reduced immunity
international travelers
health-care workers
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Whole Cell Vaccines
• most current vaccines active against bacteria
and viruses consist of who microbes that are
either inactivated (killed) or attenuated (live
but avirulent)
57
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58
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Acellular or Subunit
Vaccines
• the use of specific, purified macromolecules
derived from pathogenic microbes helps
avoid some of the risks associated with wholecell vaccines
• Forms of subunit vaccines
– capsular polysaccharides
– recombinant surface antigens
– inactivated exotoxins (toxoids)
59
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60
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Recombinant-Vector Vaccines
• pathogen genes that encode major antigens
inserted into nonvirulent viruses or bacteria
which serve as vectors and express the
inserted gene
• released gene products (antigens) can elicit
cellular and humoral immunity
61
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DNA Vaccines
• DNA directly introduced into host cell via air
pressure or gene gun
• when injected into muscle cells, DNA taken
into nucleus and pathogen’s DNA fragment is
expressed
– host immune system responds to foreign
proteins produced
• many DNA vaccine trials are currently being
run
62
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Figure 36.11
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The Role of the Public Health
System: Epidemiological
Guardian
• network of health professionals involved in
surveillance, diagnosis, and control of
epidemics
• form county, regional, state, national, and
international public health organizations
64
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Bioterrorism Preparedness
• bioterrorism
– “intentional or threatened use of viruses,
bacteria, fungi, or toxins from living organisms
to produce death or disease in humans,
animals, and plants”
– A list of potential biological agents is shown in
Table 36.7
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Choosing Biological Agents as
Weapons
• biocrime when chosen as a means for a localized
attack vs. bioterrorism when chosen for mass
casualties
• characteristics that favor their use
– invisible, odorless and tasteless
– difficult to detect
– take hours or days before awareness that they
have been used
– fear and panic associated with the anticipation
that they were used
66
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Key Indicators of a
Bioterrorism Event
• sudden increased numbers of sick people,
especially with unusual diseases for that place
and/or time of year
• sudden increased numbers of zoonoses,
diseased animals or vehicle-borne illnesses
67
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68
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Global Health Considerations
• ~500,000 infectious disease deaths in
developed countries
• ~18 million infectious disease deaths in lessdeveloped countries
• precautions needed for
– global travel
– clean water, sanitation
– basic health care infrastructure
– vaccination
69
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Summary
• define epidemiology
• the epidemiological links in the infectious
disease cycle
• nosocomial infections and the consequences
of these infections
• the role of public health system and disease
epidemiology
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