EPB PHC 6000 EPIDEMIOLOGY FALL, 1997

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Unit 12-13 Infectious Disease Epidemiology
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• Unit 12-13 Learning Objectives
• Understand primary definitions used in infectious
  disease epidemiology.
• Differentiate between direct and indirect modes
  of infectious disease transmission.
• Understand the concept of herd immunity.
• Understand the necessary conditions, goals, and
  utility for outbreak investigations.
• Understand unique features for studying
  infectious diseases from an epidemiological
  perspective.
• Understand the major components of conducting an
  outbreak investigation.

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• Unit 12-13 Learning Objectives (cont.)
• Interpret epidemic curves in terms of possible
  outbreak patterns and sources.
• Recognize the primary epidemiological study
  designs used in outbreak investigations.
• Understand measures of effect used in outbreak
  investigations.
• Review a real-world example of an outbreak
  investigation.

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DEFINITIONS AND BACKGROUND
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Definitions
Endemic Habitual presence of a disease in a
given geographic area. Epidemic Occurrence of a
group of illnesses of similar nature within a
given community or region in excess of normal
expectancy, and derived from a common or from a
propagated source. Pandemic A worldwide
epidemic. Herd immunity Resistance of a group
of to an attack by a disease to which a large
proportion of members of the group are immune.
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Definitions (cont.)
Virulence Severity of the disease produced by
the organism. Carrier Individual who harbors
the organism but is not infected, as measured by
serologic studies or evidence of clinical
illness. Classic Example Typhoid Mary was a
carrier of Salmonella typhi who worked as a cook
in NYC in different households over many years
considered to have caused at least 10 typhoid
fever outbreaks that included 51 cases and 3
deaths.
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Definitions (cont.)
Infectivity Capacity of agent to enter and
multiply in a susceptible host (hence produce
infection/disease) (polio and measles have high
infectivity) Pathogenicity Capacity of agent to
cause clinical disease in the infected host
(measles has high pathogenicity) Toxigenicity
Capacity of agent to produce a toxin or poison
(e.g. toxin produced by the microorganism rather
than microorganism itself (such as botulism and
shellfish poisoning))
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Definitions (cont.)
Resistance Ability of agent to survive adverse
environmental conditions (hepatitis agents
generally very resistant whereas influenza
viruses are typically fragile). Note
resistance is also applied to the
host. Antigenicity Ability of agent to induce
antibody production in the host (e.g.
re-infection with measles virus is very rare).
The related term immunogenicity refers to
infections ability to produce specific immunity.
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Modes of Disease Transmission
Direct Person-to-person contact
(propagated) Indirect a) Common vehicle such
as contaminated air or water supply (can occur
from single, multiple, or continuous
exposure) b) Vector such as a mosquito (e.g.
West Nile virus) The likelihood of disease
transmission depends on several related factors
(see upcoming slides).
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Timelines for Infection and Disease
Time of Infection
Dynamics of infectiousness
noninfectious -- removed -- dead -- recovered
latent period
infectious period
susceptible
time
noninfectious -- dead -- recovered -- immune --
carrier
Dynamics of disease
incubation period
symptomatic period
susceptible
time
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Timelines for Infection and Disease
Definitions from Previous Slide Latent period
time interval from infection to development of
infectious (note this definition differs from
that used for non-infectious diseases). Infectious
period time during which the host can infect
another host. Incubation period time from
infection to development of symptomatic
disease. Symptomatic period period in which
symptoms of the disease are present.
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Factors Affecting Disease Transmissionand
Symptomatic Clinical Disease
-- Susceptibility -- Immune response --
Resistance --- Portal(s) of entry
VECTOR -- Prevalence -- Portal(s) of entry
Host
Vector
Agent
Environment
-- Balance of immune to susceptible
individuals --- Opportunity for exposure
(e.g. crowding)
-- Virulence Toxigenicity --
Infectivity Resistance -- Pathogenicity
Antigenicity
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Characteristics of Herd Immunity
-- If a large percent of the population is
immune, the entire population is likely to be
protected. -- Once a high proportion of all
people in the community are immune, the
likelihood is small that an infected person will
encounter a susceptible person. -- Due to herd
immunity, highly protective immunization can
occur without requiring 100 immunization rates
(estimated 94 immunity for measles to interrupt
the chain of transmission).
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Characteristics of Herd Immunity
-- For herd immunity, disease agent must be
restricted to a single host species, and
transmission must be relatively direct from one
member of host species to another (e.g. no
reservoir outside the human host in which the
organism can exist). -- Herd immunity operates
optimally when there is random mixing of the
population.
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Conditions for an Outbreak

• Agent and susceptible hosts are present in
  sufficient number
• The agent can effectively be conveyed from a
  source to susceptible hosts
• Why do outbreaks occur?
• Increase in amount or virulence of agent
• Recent introduction of the agent into a new
  setting
• Enhanced mode of transmission, exposing more
  susceptible individuals
• Factors that increase host exposure or involve
  introduction through new portals of entry

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Outbreak Investigation
GOALS 1. Rapidly identify the source and
reservoir of the outbreak. 2. Implement
interventions to control and eliminate the
outbreak. 3. Develop policies to prevent future
outbreaks. Note It is important to maintain an
open mind when investigating the source of an
outbreak natural biologic toxins, heavy metals,
and chemical poisoning may mimic signs and
symptoms of infectious agents.
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Outbreak Investigation
Utility of Performing Outbreak Investigations 1.
Has goal of determining cause of outbreak,
eliminating or interrupting transmission, and
providing post-exposure prophylaxis. 2. Uncovers
new infectious agents and diseases. 3. Identifies
spread of a known virus to a new geographic area
(e.g. West Nile encephalitis in New York
City). 4. Improves epidemiological understanding
by uncovering new means of disease
transmission. 5. Leads to public health
regulations/recommendations to prevent future
disease outbreaks.
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Unique Features of Infectious Diseases
1. A case may also be a source For most
non-infectious (e.g. chronic) diseases, a
persons risk of disease in not influenced by the
disease status of others (e.g. a persons risk of
CHD is not influenced by the fact that his/her
neighbor experienced a myocardial infarction).
However, for infectious diseases such as
influenza, a persons risk is greatly affected by
the number of influenza patients around, and if
many of the people one meets have been vaccinated.
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Unique Features of Infectious Diseases
2. Some people may be immune For most
non-infectious risk factors (e.g. toxins or
radiation), there will be levels when all exposed
will become ill. However, for some infectious
diseases, such as measles, once a person has had
the disease, he/she will never get it again, even
if exposed in the middle of an epidemic. Thus,
in terms of measuring incidence, not everyone is
at risk of developing the disease.
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Unique Features of Infectious Diseases
3. There is sometimes a need for urgency Many
chronic diseases are concerned with the impact
of environmental (e.g. lead exposure) and
behavioral (e.g. smoking) risk factors that
require big time-consuming public health
programs. However, with outbreaks of some
infectious diseases, such as Ebola or
Legionnaires disease, the time frame for
investigation and preventive action may be a
matter of hours or days this may give little
time for elaborate analyses.
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Unique Features of Infectious Diseases
4. Preventive measures (often) have a clear
scientific basis. For many chronic diseases
such as CHD, the relative impact of factors such
as diet and cholesterol on disease etiology (and
hence prevention), remains equivocal and a matter
of debate. However, for many infectious
diseases, the causative agent and characteristics
of transmission are well established this leads
to clear (but not always easy) targets for
disease prevention.