John Snow
Introduction to Epidemiology
and Population Systems
London Cholera Outbreaks
18311831-32
18481848-49
18531853-54
Thomas E. Wittum
Veterinary Preventive Medicine
The Ohio State University
Miasma theory commonly accepted at this time
London Cholera Deaths - 1854
London
Southwark
Lambeth
First observations
Developed hypotheses
Tested hypotheses
London Cholera Deaths - 1854
Population
Deaths
Rate/1000
2,362,236
10,530
4.5
266,516
173,748
4,093
461
15.4
2.6
Introduction to Epidemiology
and Population Systems
Population
Southwark
Lambeth
Deaths
Rate/1000
98,862
419
4.2
154,615
80
0.5
Course Components
Population Systems
Monday / Wednesday lecture
Epidemiologic Concepts
Thomas E. Wittum
Veterinary Preventive Medicine
The Ohio State University
Thursday / Friday problem solving sessions
1
Course Grading
Disease
Problem Solving Exercises
50 %
Question of the Week
20 %
Final Exam
20 %
Wooster Day
10 %
Health
Disease
State of optimal physical, mental, and social
wellwell-being and not merely the absence of
disease.
What Is Epidemiology?
animal ecology that relates to
states of health.
The study of interinter-relationships of all
organisms and their environment.
‹ Study of disease in populations
‹ Medical ecology
‹ Study of circumstances that affect disease
occurrence
– Who, what, where, when, how?
Host
Medical
Ecology
Health
Agent
Infectious Disease
- involves pathogenic microorganisms
Noninfectious Disease
- involves noninfectious processes
Contagious Disease
- can be transmitted between individuals
Epidemiology
‹ Epidemiology: The large part of human and
‹ Ecology:
Any deviation or interruption of the normal
structure or function of any part of an organ
or functional group of organs that is
manifested by symptoms or signs.
– Frequency, distribution, determinants
Environment
2
Basic Assumptions of Epidemiology
‹ Disease does not occur at random in a
population
‹ Disease has causal and preventive factors
Basic Principles of Epidemiology
1. Disease occurrence is related to the
environment of the species of interest
2. Count the occurrence of natural events
that can be identified through systematic
investigation
Basic Principles of Epidemiology
3. Utilize nature’
nature’s experiments whenever
possible
4. Controlled field experiments should be
performed in the natural environment of
the species of interest whenever possible
Traditional question:
What is the diagnosis and what is the treatment
Epidemiologic question:
Why did this patient get this disease at this time
Clinician should consider both questions
Risk
Triad of Disease
Agent
Host
Populations
Disease does not occur randomly in populations
Environment
Disease has causal and preventive factors that
can be identified through systematic
investigation
3
Infection vs. Disease
Infection ≠ Disease
Exposure to an infectious agent not always
the critical factor in determining the
occurrence of disease
Diagnostic Tests
Results of diagnostic and screening tests
are not perfect
Preventive Medicine
Vaccination ≠ Preventive Medicine
Vaccination is only one part of an overall
preventive medicine program
Comparison
Appropriate comparisons are required in
order to draw valid conclusions
Contributes information to the overall
diagnostic decision making process
Statistics
Help to evaluate the role of chance in
observations
Chance and Bias
Random Error (chance)
Systematic Error (bias)
Contributes information to the overall
decision making process
4
Establishing Cause
Multiple Causation
Process of building evidence
The “web”
web” of causation
Koch’
Koch’s postulates not adequate
Necessary and sufficient causes
MultiMulti-agent disease syndromes
Critical Decision Making
Collect and utilize all available
information to make appropriate
diagnostic and treatment decisions
Population Systems
The environment in which we place animals
largely determines the diseases of importance
When different types of animals are placed in
similar environments, they have similar health
problems
Population Issues
Most frequently occurring diseases
How environment contributes to the occurrence
of disease
Most important preventive medicine practices
Public health issues
Animal welfare issues
Environmental issues
5