Transcript Epidemiology The Basics

Patterns of Microbe-Human Interactions in
Causing Infection and Disease
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Contact, Infection, Disease- A
Continuum
◦ Body surfaces are constantly
exposed to microbes
◦ Inevitably leads to infection:
pathogenic microorganisms
penetrate the host defenses,
enter the tissues, and multiply
◦ Pathologic state that results
when the infection damages or
disrupts tissues and organsdisease
◦ Infectious disease: the
disruption of a tissue or organ
caused by microbes or their
products
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Infection is the invasion of the
host by a pathogen
Disease results only if the
invading pathogen alters the
normal functions of the body
Disease is also referred to as
morbidity
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Diseases can be classified in
number of ways
◦ The taxonomic groups of the
causative agent
◦ The body system they affect
◦ Their longevity and severity
◦ How they are spread to their
host
◦ The effects they have on
populations (rather than on
individuals)
The study of epidemics.
The study of distribution and
prevalence of infectious
disease in a given population.
The study of comparisons of
baseline infections to cases.
Interested in protecting the
public from outbreaks of
infection.
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Epidemiologists concerned with
virulence, portals of entry and
exit, and the course of the
disease
Also interested in surveillance:
collecting, analyzing, and
reporting data on the rates of
occurrence, mortality, morbidity,
and transmission of infections
Reportable diseases: by law,
must be reported to authorities
Centers for Disease Control and
Prevention (CDC) in Atlanta,
Georgia
– Weekly notice: the Morbidity and
Mortality Report
– Shares statistics with the World
Health Organization (WHO)
All outbreaks follow a pattern.
If we can understand the
pattern, then we can intervene
to prevent further cases.
Short term increase in a
given infectious disease
in a give population.
Presence of infectious
disease in a given
population or in a
given geographical
location all the time.
Ebola in Africa
Lyme disease in Texas
Rabies in Austin (bats)
Worldwide or global
increase in an infectious
disease over a long
period of time.
HIV/AIDS
Tuberculosis
Figure 13.16
Morbidity – number of cases
of specific infectious disease
in a give population at any
given time.
Mortality – number of deaths
from a specific infectious
disease in a given population
at any given time.
Prevalence – number of cases
at any give time (people are
either getting better or
becoming infected.)
Incidence – number of new
cases of a specific infectious
disease. This is important
because it can tell us how fast
the disease is spreading.
◦ Track occurrence of
diseases using two
measures
 Incidence – number of new
cases of a disease in a
given area during a given
period of time
 Prevalence – number of
total cases of a disease in
a given area during a
given period of time
◦ Occurrence also evaluated
in terms of frequency and
geographic distribution
[INSERT FIGURE 14.17]
◦ Exceptions to Koch’s postulates
 Using Koch’s postulates is not
feasible in all cases
 Some pathogens can’t be
cultured in the laboratory
 Some diseases are caused by a
combination of pathogens and
other cofactors
 Ethical considerations prevent
applying Koch’s postulates to
pathogens that require a
human host
 Difficulties in satisfying Koch’s
postulates
 Diseases that can be caused by
more than one pathogen
 Pathogens that are ignored as
potential causes of disease
Epidemiologists work to
figure out what is going on so
they can intervene. Every
infectious disease has a
pattern (not random.)
Epidemiologists want to
know:
Place
People
Time
◦ Following infection, a
sequence of events called
the disease process
occurs
◦ Many infectious diseases
have five stages following
infection
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Incubation period
Prodromal period
Illness
Decline
Convalescence
-----------Incubation period ----------
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exposure
preclinical
symptoms
clinical
symptoms
Tx
(treatment)
outcomes
1. convalese
2. death
3. impairment
Exposure –
Preclinical symptoms – before you go to the doctor. You may not realize that you have
an infectious agent, but your body has begun to have immune response. A preclinical
symptom can be any cytopathic effect.
Clinical symptoms – symptoms that can be appreciated. In TB clinical symptoms could
include: cough, blood in phlegm, fever, night sweats.
Incubation period – spans from exposure to the time when clinical symptoms are detected.
Intervention – this is period when people seek medical attention. Options during
intervention include: quarantine and medicate individuals that were exposed or
vaccination.
Outcome – possible outcomes:
Get better (convalesce)
Death
Impairments (mental and physical)
◦ Descriptive epidemiology
 Careful tabulation of data
concerning a disease
 Record information about
the location and time of
the cases of disease
 Collect patient information
 Try to identify the index
case (or first case) of the
disease
◦ Analytical epidemiology
 Seeks to determine the
probable cause, mode of
transmission, and
methods of prevention
 Useful in situations in
which Koch’s postulates
can’t be applied
 Often retrospective –
investigation occurs after
an outbreak has occurred
◦ Experimental
epidemiology
 Involves testing a
hypothesis concerning the
cause of a disease
 Application of Koch’s
postulates is experimental
epidemiology
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Transmission is either from a
reservoir or a portal of exit to
another host’s portal of entry
Three groups of transmission
◦ Contact transmission
 Direct, indirect, or droplet
◦ Vehicle transmission
 Airborne, waterborne, or
foodborne
◦ Vector transmission
 Biological or mechanical
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Pathogens leave host
through portals of exit
Many portals of exit are the
same as portals of entry
Pathogens often leave hosts
in materials the body
secretes or excretes
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Communicable disease: when an
infected host can transmit the
infectious agent to another host and
establish infection in that host
– Transmission can be direct or indirect
– Contagious agent: highly
communicable
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Noncommunicable disease: does not
arise through transmission of the
infectious agent from host to host
– Acquired through some other, special
circumstance
– Compromised person invaded by his or
her own microbiota
– Individual has accidental contact with a
microbe in a nonliving reservoir
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Contact transmission
Indirect transmission
– Vehicle: any inanimate material
commonly used by humans that
can transmit infectious agents
(food, water, biological
products, fomites)
– Contaminated objects
(doorknobs, telephones, etc.)
• Food poisoning
• Oral-fecal route
– Air as a vehicle
• Indoor air
• Droplet nuclei
• Aerosols
[INSERT FIGURE 14.13]
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Reservoir: the primary habitat in
the natural world from which a
pathogen originates
Source: the individual or object
from which an infection is
actually acquired
Living Reservoirs
– Carrier: an individual who
inconspicuously shelters a pathogen
and spreads ith to others without
any notice
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Asymptomatic carriers
Incubation carriers
Convalescent carriers
Chronic carrier
Passive carrier
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Vector: a live animal that transmits an infectious agent from one
host to another
◦ Majority are arthropods
◦ Larger animals can also be vectors
Biological vector: actively participates in a pathogen’s life cycle
Mechanical vectors: transport the infectious agent without being
infected
[INSERT TABLE 14.10]
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Zoonosis: an infection indigenous
to animals but naturally
transmissible to humans
◦ Human does not contribute to
the persistence of the microbe
◦ Can have multihost involvement
◦ At least 150 worldwide
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Human hosts in regular
contact with
environmental sources
Soil
Water