Download EPB PHC 6000 EPIDEMIOLOGY FALL, 1997

Unit 12-13:
Infectious Disease
Epidemiology
Unit 12-13 Learning Objectives:
1. Understand primary definitions used in
infectious disease epidemiology.
2. Differentiate between direct and indirect
modes of infectious disease transmission.
3. Understand the concept of herd immunity.
4. Understand the necessary conditions, goals,
and utility for outbreak investigations.
5. Understand unique features for studying
infectious diseases from an epidemiological
perspective.
6. Understand the major components of
conducting an outbreak investigation.
Unit 12-13 Learning Objectives (cont.):
7. Interpret epidemic curves in terms of
possible outbreak patterns and sources.
8. Recognize the primary epidemiological study
designs used in outbreak investigations.
9. Understand measures of effect used in
outbreak investigations.
10.Review a real-world example of an outbreak
investigation.
DEFINITIONS
AND
BACKGROUND
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.
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.
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))
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 infection’s ability
to produce specific immunity.
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).
Timelines for Infection and Disease
Time of Infection
Dynamics of
infectiousness
susceptible
latent infectious
period
period
noninfectious
-- removed
-- dead
-- recovered
time
Dynamics of
disease
susceptible
noninfectious
incubation symptomatic -- dead
-- recovered
period
period
-- immune
-- carrier
time
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 noninfectious 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.
Factors Affecting Disease Transmission
and Symptomatic Clinical Disease
VECTOR
-- Prevalence
-- Portal(s) of entry
Host
-- Susceptibility
-- Immune response
-- Resistance
--- Portal(s) of entry
Vector
Agent
-- Virulence
Toxigenicity
-- Infectivity
Resistance
-- Pathogenicity Antigenicity
Environment
-- Balance of immune to
susceptible individuals
--- Opportunity for
exposure (e.g. crowding)
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).
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.
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
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.
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.
Unique Features of Infectious Diseases
1. A case may also be a source:
For most non-infectious (e.g. “chronic”)
diseases, a person’s risk of disease in not
influenced by the disease status of others
(e.g. a person’s 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 person’s risk is greatly affected by
the number of influenza patients around, and
if many of the people one meets have been
vaccinated.
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.
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.
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.