Download Lec 7 Principles of disease epidemiology

Lecture 7: Principles of disease and epidemiology
Edith Porter, M.D.
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Case study (Video)
Pathology, infection, and disease
Normal microbiota
Etiology of infectious diseases
▪ Koch’s postulates and exceptions
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Classifying infectious diseases
Patterns of disease
Spread of infections
Nosocomial infections
Emerging infectious diseases
Epidemiology
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Pathology
 Scientific study of disease
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Etiology
 Study of the cause of a disease
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Pathos = Suffering
Logos = Science
Pathogenesis
 Development of disease
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Infection
 Colonization of the body by microbes that are not routinely
present at this site
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Disease
 A change from state of health
HIV pos.  AIDS
Tuberculin pos.  Tuberculosis
 Abnormal state in which the body is not function normally
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Pathogen
 Causes disease in a healthy adult
 Expresses special virulence or pathogenicity factors
 Is not part of normal flora
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Opportunist
 Does not cause disease under normal conditions
 Causes disease at local or systemic breaches of host
defense
 Often part of normal flora
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Cooperation
 Different microbes together cause disease
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Body is sterile in utero
Colonized within hours after birth
 Lactobacilli
 Staphylococci
Outnumber body cells by at least 10– fold
Normal flora does not cause disease under normal
conditions
 Transient microbiota
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 present for a relatively short period of time (days, weeks,
or months)
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Resident microbiota
 Normal microbiota permanently colonizing the host
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Body site
Microbiota
Lead microorganism
Skin
Gram+
Staphylococcus epidermidis
Eye
Sparse (gram+)
Nasooropharynx
Gram+
Gram- anaerobes
Stomach
Sparse
Small intestine
Sparse
Large intestine
Facultative and anaerobic gram+
Facultative and anaerobic gram-
Enterococcus faecalis
E. coli, Clostridium species
Urethra
Gram+
Some anaerobic gram -
Male: Mycobacterium
smegmatis
Vagina
Gram+
Some anaerobic gram -
Lactobacillus spec.
Neisseria spec.
a-hemolytic streptococci
Haemophilus spec.
Low numbers of Candida albicans can be found everywhere
Nose
Stomach
Intestine
Trichomonas vaginalis is considered a
pathogen
 Trichomonas hominis is normal flora in large
intestine
Normal microbiota
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Competes with potential pathogens for nutrients
Directly inhibits potential pathogens
▪ Lactobacilli: lactic acid, low pH
▪ Bacteriocins
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Produces some vitamins (K, B)
Candidiasis
after antibiotic treatment
Clostridium difficile diarrhea
after antibiotic therapy
http://www.health-res.com/EX/07-28-04/37FF1.jpeg
Pseudomembranous enterocolitis
caused by C. difficile
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Administration of viable
bacteria to the benefit of
human health
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 Lactobacilli, Streptococci,
Bifidobacteria
Withstand HCl, bile salts
 Adhere to host intestinal
mucosa
 Produce useful enzymes or
physiological end products
 Restore normal microbiota
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Prophylactic application
 Lactobacilli to
prevent development
of antibiotic
associated diarrhea
Therapeutic applications
 Supplementary
therapy in chronic
UTI with E. coli
Robert Koch established the “Golden Rule” to
positively identify a microorganism as the cause
of an infectious disease
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The same pathogen must be present in every case of
disease and not in the healthy one.
The pathogen must be isolated from the diseased host
and grown in pure culture.
The pathogen from the pure culture must cause same
disease when it is inoculated into a healthy susceptible
host.
The same pathogen must be isolated from the
inoculated host in pure culture.
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Microorganism cannot be grown in the
laboratory in/on artificial culture media
 Utilization of animals or eggs for propagation
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One disease can be caused by multiple
microorganisms
 E. g. nephritis
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One microorganism can cause multiple
disease conditions
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http://2.bp.blogspot.com/_ayDfkQkrVmU/TFJ0clyjbWI/AAAAAA
AAAIs/5oG7IQ7UuHI/s1600/post+mortem+Dis+alpacas.JPG
http://www.path.cam.ac.uk/Abnormal/TB_Tuberculosi
s/TB_Tuberculosis/SN_Spine/A_TB_TB_SN_02.jpg
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Microorganism cannot be grown in the laboratory in/on
artificial culture media
 Utilization of animals or eggs for propagation
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One disease can be caused by multiple microorganisms
 E. g. nephritis
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One microorganism can cause multiple disease conditions
 M. tuberculosis can affect skin, lungs, bones etc.
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No host other than humans
 E.g. HIV
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More than one microorganisms cause one infection
 Polymicrobial infections such as abscess caused by anaerobic
bacteria
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Symptom
 A change in body function that is felt by a patient as a
result of disease
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Sign
 A change in a body that can be
Often used
interchangeable
measured or observed as a result of disease
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Syndrome
 A specific group of signs and symptoms that accompany a
disease
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Communicable disease
 A disease that is spread from one
host to another
▪ Example: Tuberculosis
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Contagious disease
 A disease that is easily spread from
one host to another
▪ Example: Chicken pox
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Noncommunicable disease
 A disease that is not transmitted from
one host to another
▪ Example: Tetanus
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Incidence
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 Number of people in a population
who develop a disease during a
particular time period
 Disease that occurs
occasionally in a population
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Prevalence
 Number of people in a population
in a population
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Epidemic disease
 Disease acquired by many
having a specific disease at a given
time
 Includes old a new cases
Endemic disease
 Disease constantly present
 Includes new cases
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Sporadic disease
hosts in a given area in a
short time
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Pandemic disease
 Worldwide epidemic
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Acute disease
 Symptoms develop rapidly
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Chronic disease
 Disease develops slowly
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Subacute disease
 Symptoms between acute
and chronic
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Latent disease
 Disease with a period of no
symptoms when the
infectious agent is inactive
 Herd immunity
 Many immune people are
present in a population
preventing the spread of a
disease
Herd Immunity
Often contagious during incubation and prodormal period!!
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Local infection
 Pathogens limited to a small area of the body
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Systemic infection
 An infection spread through the body (via blood
or lymphatic system)
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Focal infection
 Systemic infection that began as a local infection
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Bacteremia
 Bacteria in the blood
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Septicemia
 Spread of bacteria through
the blood with organ
manifestation
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Toxemia
 Toxins in the blood
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Fungemia
 Fungi in the blood
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Viremia
 Viruses in the blood
Common cause for bacteremia:
Coagulase negative staphylococci
colonizing i.v. catheter
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Factors that make the body more
susceptible to disease
 Primary and secondary infection
▪ Acute infection that causes the initial illness
and predisposes to a secondary, often
opportunistic infection
 Male versus female
 Genetic background
 Climate and weather
 Nutrition
 Lifestyle
 Occupation
 Pre-existing illness
Example:
Influenza and
Haemophilus
influenzae
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Continual sources of the disease organisms
Humans — AIDS, gonorrhea
 Carriers may have inapparent infections or latent diseases
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Animals — Rabies, Lyme disease
 Some zoonoses may be transmitted to humans
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Nonliving — Botulism, tetanus
 Soil
Lyme Disease:
Skin manifestation
Tick
Borrelia burgdorferi
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Direct
 Requires close association
between infected and
susceptible host
 Includes fecal-oral
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Indirect
 Spread by fomites
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Droplets
 Transmission via airborne
droplets
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Inanimate reservoir
 Food
 Water
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Vectors
 Arthropods
▪ Fleas : plague
▪ Ticks: Lyme disease
▪ Mosquitoes: malaria
 Mechanical
 Biological : some part of the
development of the microbe
takes place in the vector
Acquired during a hospital stay
 Source is hospital
 5-15 % of all hospital patients affected
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Diseases that are new, increasing in incidence, or
showing a potential to increase in the near future
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Appearance of new strains
by genetic recombination
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 E. coli O157:H7
 Hantavirus pulmonary
syndrome
 Avian influenzavirus H5N1
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Evolution of new serovars
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 V. cholerae O139
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Inappropriate use of
antibiotics and pesticides
tuberculosis
Spread of known diseases
by modern transportation
 Cholera
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Ecological Disaster
 Coccidioidomycosis after
 Antibiotic resistant strains
 Multidrug resistant M.
Global warming and
weather changes
Northridge earthquake
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Failures in public health
 Missed immunizations
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As told by CDC …
It seems that one of their scientists, on first arriving at CDC from a clinical
practice, found himself somewhat unsure of what epidemiology was all
about, so he sought an answer down the street at Emory University.
The first person he asked was a medical student, who told him that
epidemiology was "the worst taught course in medical school."
The second, a clinical faculty member, told him epidemiology was "the
science of making the obvious obscure."
Finally, knowing that statistics are important to epidemiology, he asked a
statistician, who told him that epidemiology is "the science of long
division" and provided him with a summary equation. Giving up on
finding a real answer, he returned to CDC.
On the way, however, he decided to try one more time. He stopped a
native Atlantan who told him that epidemiology was "the study of skin
diseases.”
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Discipline that find answers to When? Where? How transmitted?
Study—Epidemiology is the basic science of public health. It's a highly
quantitative discipline based on principles of statistics and research
methodologies.
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Distribution—Epidemiologists study the distribution of frequencies and
patterns of health events within groups in a population. To do this, they
use descriptive epidemiology, which characterizes health events in terms
of time, place, and person.
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Determinants—Epidemiologists also attempt to search for causes or
factors that are associated with increased risk or probability of disease.
This type of epidemiology, where we move from questions of "who,"
"what," "where," and "when" and start trying to answer "how" and "why,"
is referred to as analytical epidemiology.
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Health-related states—Epidemiology as it is practiced today is applied
to the whole spectrum of health-related events, which includes chronic
disease, environmental problems, behavioral problems, and injuries in
addition to infectious disease.
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Populations—One of the most important distinguishing characteristics
of epidemiology is that it deals with groups of people rather than with
individual patients.
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Control—Epidemiological data steers public health decision making
and aids in developing and evaluating interventions to control and
prevent health problems. This is the primary function of applied, or
field, epidemiology.
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John Snow
 Cholera outbreaks in London 1848-1849
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Ignaz Semmelweis
 Childbed fever (puerperal sepsis) 1846 - 1848
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Florence Nightingale
 Epidemic typhus 1858
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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
Mandatory
hand washing
introduced
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Recorded statistics on epidemic
typhus in English civilian and
military populations
Published a 1000 page report in
1858
 Statistically linked disease and
death with poor food and
unsanitary conditions
 Novel graph: coxcomb chart or
polar area diagram chart
▪ Fixed angle and variable radii
Resulted in reforms in the British
Army
 Nightingale became the first
female member of the Statistical
Society
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Experimental
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Epidemiologist is in control of the circumstances at the beginning of the study
Begins with a hypothesis
Prospective study that usually involves controls
Example: Semmelweis’ study; vaccine efficacy trials
Observational (or descriptive)
 Epidemiologist is not in control of the circumstances at the beginning of the
study
 Descriptive
▪ Collect data about affected individuals, the places and the periods in which disease
occurred (Who? Where? When?)
▪ Typically retrospective
▪ E.g. Snow’s study
 Analytical
▪ Analyzes a particular disease to determine its probable cause (How? Why?)
▪ 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