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MICR 454L
Emerging and Re-Emerging
Infectious Diseases
Lecture 2: Epidemiology
(Tortora et al., Chapter 14 )
Dr. Nancy McQueen & Dr. Edith Porter
Overview
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Concepts of epidemiology
The cycle of microbial disease
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Reservoirs of infection
Transmission
Portals of entry
Portals of exit
Nosocomial infections
Epidemiology of fear
Concepts of Epidemiology
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Epidemiology is an investigative methodology
designed to determine the source and cause of
diseases and disorders that produce illness,
disability and death in human populations.
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Why at this time and at this place?
Factors considered include age, sex, race, personal
habits, geographic location, seasonal changes,
modes of transmission.
Epidemiologists are also concerned with methods
for controlling the disease.
History of Epidemiology
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Hippocrates in 460 – 377 B.C.
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Edward Jenner in late 18th century
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Childbed fever (puerperal sepsis)
John Snow
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Cow pox protects from small pox
First vaccination
Ignaz Semmelweis
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Malaria associated with swampy environments
Cholera outbreaks in London
Florence Nightingale
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Epidemic typhus
in mid 19th century
Ignaz Semmelweis and
Puerperal Sepsis
Mandatory
hand washing
introduced
John Snow and Cholera
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Cholera epidemics in London 1846
– 1849
Snow analyzed the death records
and interviewed survivors
 Created map
 Most individuals who died of
cholera used water from Broad
street pump
 Survivors did not drink water but
beer instead or used another
pump
Identified the Broad street water
pump as likely source
After closing this pump number of
cholera cases dropped significantly
Florence Nightingale and
Epidemic Typhus
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Recorded statistics on epidemic
typhus in English civilian and
military populations
Published a 1000 page report in
1858
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Statistically linked disease
and death with poor food
and unsanitary conditions
Novel graph: coxcomb chart
or polar area diagram chart
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Fixed angle and variable radii
Resulted in reforms in the British
Army
Nightingale became the first
female member of the Statistical
Society
Investigation Types in
Epidemiology
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Descriptive
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Analytical
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Collect data about affected individuals, the places and the periods in which
disease occurred
Typically retrospective
E.g. Snow’s study
Analyzes a particular disease to determine its probable cause
Case control method – look for factors that might have preceded the disease
Cohort method – study of two populations, one having had contact with the
disease agent and the other that has not
E.g., Nightingale’s study
Experimental
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Begins with a hypothesis
Prospective study that usually involves controls
Semmelweis’ study
Case Reporting
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Health care workers are required to report specified diseases to
local, state, and national offices
Centers for Disease Control
and Prevention (CDC)
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Branch of the U.S. Public Health Service
Collects and analyzes epidemiological information in the U.S.
Publishes Morbidity and Mortality Weekly Report (MMWR)
www.cdc.gov
Morbidity: incidence of a specific notifiable disease
Mortality: deaths from notifiable diseases
Morbidity rate: number of people affected/total population in a given
time period
Mortality rate: number of deaths from a disease/total population in a
given time
Notifiable Diseases
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Reported to the CDC
To detect reemergence
of disease and be able
to respond early
List includes
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AIDS
Anthrax
Botulism
Cholera
Cryptosporidiosis
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Diptheria
Ehrlichiosis
Encephalitis (arboviral)
Enterohemorrhagic E. coli
Hepatitis, viral
Malaria
Meningococcal disease
Pertussis
Streptococcal diseases,
invasive Group A
Tuberculosis
Frequency and Distribution of
Disease
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Sporadic disease
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Disease that occurs
occasionally in a
population at irregular
intervals
Epidemic disease
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Endemic disease
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Disease continually
present in a population at
a steady level
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Disease acquired by
many hosts in a given
area in a short time
Disease with sudden
increase of morbidity
(illness rate) and
mortality (death rate)
above the normal level
Pandemic disease
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Worldwide epidemic
Classification of diseases by frequency
and distribution
Source and Spread of Major
Epidemics
Propagated or
Herd Immunity
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Large fraction of a population is immune to a
given disease, and it is difficult for the
disease to spread (proportion immune
individuals needed depends upon type of
infection)
Cycle of Microbial Disease
Reservoir
Portal of exit
Transmission
Disease
Portal of entry
Reservoirs of Infection
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Where do you get the infection from?
Site in nature where microbes survive (and possibly multiply)
Continual sources of infection
Humans
 Small pox, gonorrhea
 Active carriers: acutely ill
 Healthy carriers: no symptoms
Living reservoirs
 inapparent infections or latent diseases
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Example: Typhoid Mary
Animals
 Zoonoses
 Rabies, Lyme disease
Fomite
 Soil, water
 Botulism, tetanus
Non-living reservoirs
Transmission
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How do you get the infection?
Direct
 Requires close association
between infected and
susceptible host
 Person – to – person
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Horizontal
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Vertical
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Mother – to – child
Animal bites
Indirect
 Vehicle (fomites, water)
 Airborne (aerosols)
 Vector
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Fecal-oral
Droplet
Epidemiological Vectors
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Arthropods
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Invertebrates with jointed appendages
Ticks, flies, mosquitoes, fleas
Mechanical vector
Biological vector
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Plague
Malaria
Trypanosomiasis
Microbe proliferates in arthropod
Necessary component in the life cycle of the microbe
Sometimes transovarial transmission
Lyme Disease:
Skin manifestation
Tick
Borrelia burgdorferi
Portals of Entry and Exit
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Respiratory tract
Intestinal tract
Urinary tract
Genital tract
Skin
Blood
Mucous membranes
Nosocomial Infections
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Acquired by compromised hosts during a hospital
stay
5-15% of all hospital patients
50% of all major hospital complications
At 2 – 4 million admissions/year estimated 20,000 –
40,000 deaths per year in the US
Body Site Distribution of
Nosocomial Infections
Factors Contributing to
Nosocomial Infections
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Patient population is immune comprised
Misuse of antibiotics with development of
antibiotic resistant strains
Medical devices as new portals of entry
Hospital staff as reservoir
Control Measures against
Nosocomial Infections
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Appropriate use of antibiotics
Safe and effective decontamination and
sterilization
Staff hygiene
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Hand washing
Screening of all hospital patients?
Epidemiology of Fear
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Fear with epidemic proportions
Example: West Nile Virus (CDC national data)
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2006: 5000 infections with 180 deaths
2007: 3600 infections with 121 deaths
Not a public threat!
Take Home Message
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Epidemiological classification of disease is based on
frequency and distribution as sporadic, endemic,
epidemic, and pandemic.
The cycle of microbial disease includes reservoir,
transmission, portal of entry and portal of exit.
Transmission may be direct from person to person
or indirect through an intermediate agent such as a
fomite or a vector.
Nosocomial infections are hospital acquired
infections.
Additional Resources
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The Microbial Challenge, by Krasner, ASM Press,
Washington DC, 2002.
Brock Biology of Microorganisms, by Madigan and
Martinko, Pearson Prentice Hall, Upper Saddle River,
NJ, 11th ed, 2006.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/J/JohnSnow.gif
http://www.gazellebookservices.co.uk/Military/originals/Military/19th%20Century/Flore
nce%20Nightingale%20and%20the%20Crimean%20War.jpg
<http://www.britannica.com/eb/art-70822>.