Disease Detectives
Disease Detectives focuses on epidemiology, that is, the study of diseases and how they spread. The
2012-2013 focus topic is environmental quality.
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Event Format
You are allowed to bring 2 non-graphing calculators and 1 page of Notes.
Competitors in Division B should be able to do the following for competition:
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List and recognize examples of different modes of transmission
Calculate health-related rates (attack, incidence, prevalence,
case fatality)
Calculate a simple relative risk and describe what it means
Interpret epi curves, temporal patters and other simple graphic
presentations of health data
List, discuss, and recognize examples of disease-causing agents
(physical and biological)
Demonstrate an understanding and ability to use terms such as
endemic, epidemic, pandemic, and concepts such as population
versus sample and association versus cause
Describe various types of prevention and control strategies, such
as immunization, behavior change, etc., and situations where
they might be used
For the national level, B division competitors should also be able to do the following:
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Understand how units affect the relative magnitude of a set of
rates with different units
Calculate appropriate measures of risk when given the study
design
Complete tables when given all data needed to complete
calculations
Propose a reasonable intervention to a public health problem
Recognize gaps in information
In addition to these concepts, competitors in Division C should be able to:
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Identify types of Epidemiology
o Classical Epidemiology- Looking at sociological,
enviornmental, and behavioral aspects.
o Analytical Epidemiology- Using quantitative data and
equations to look at epidemiological scenarios.
 Recognize differences between study designs
 Calculate measures of risk, such as relative risk or odds ratio,
when given a description of the study design
 Calculate measures based on data that is not given but that can
be readily extracted
 Recognize how gaps in information influence the ability to
extend conclusions to the general population
At the national level, C division competitors should also be able to do the following:
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Recognize unmentioned factors that may influence results
Convert between rates with different basic units (e.g. incidence
per 10,000 persons/year to incidence per 100 persons/week)
Propose a means to evaluate the effectiveness of an intervention
or control program
The Basics
Epidemiology
Epidemiology is the study of distribution and determinants of health-related states in specified
populations, and the application of this to control health problems. There are four basic reasons for
why disease detectives study and research outbreaks and epidemics. These reasons are: Control and
Prevention, Research Opportunities, Training, and Legal Concerns.
Two Basic Types of Epidemiology
1. Classical Epidemiology - population oriented, studies community
origins of health problems related to nutrition, environment,
human behavior, and the psychological, social, and spiritual
state of a population. The event is more aimed towards this type
of epidemiology.
2. Clinical Epidemiology - studies patients in health care settings in
order to improve the diagnosis and treatment of various diseases
and the prognosis for patients already affected by a disease.
These can be further divided into:
o Infectious Disease Epidemiology - heavily dependent on
laboratory support
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Chronic Disease Epidemiology - dependent on complex
sampling and statistical methods
Basic Epidemiology Terms
Cluster - An aggregation of cases over a particular period closely grouped in time and space, regardless
of whether the number is more than the expected number
Outbreak - More cases of a particular disease than expected in a given area or among a specialized
group of people over a particular period of time.
Epidemic - Large numbers of people over a wide geographical area are affected
Pandemic - An epidemic occurring over several countries or continents and affecting a large proportion
of the population.
Surveillance - The systematic and ongoing collection, analysis, interpretation, and dissemination of
health data. The purpose of public health surveillance is to gain knowledge of the patterns of disease,
injury, and other health problems in a community so that we can work towards their prevention and
control.
Plague - A serious, potentially life-threatening infectious disease that is usually transmitted to humans
by the bites of rodent fleas. It was one of the scourges of our early history. There are three major
forms of the disease: bubonic, septicemic, and pneumonic.
Vector - An animal that transmits disease. For example a mosquito is a vector for malaria.
Fomite - A physical object that serves to transmit an infectious agent from person to person.
Risk - The probability that an individual will be affected by, or die from, an illness or injury within a
stated time or age span.
Zoonosis - An infectious disease that is transmissible from animals to humans.
Incubation Period - Time in between when a person somes into contact with a pathogen and when
they first show symptoms or signs of disease.
Endemic Disease - Present at a continuous level throughout a population/geographic area; constant
presence of an agent/health condition within a given geographic area/population; refers to the usual
prevalence of an agent/condition.
2013 Topic- Environmental Quality
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Environmental Causes of Health Problems
o Smoking, air pollution
o Flooding and drought
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Extreme heat, UV exposure
Pesticides, chemical spills, lead contamination
Water pollutants, heavy metals
Ventilation pollutants
Noise induced hearing loss
Waste and toxic substance
Food contamination
Other pathogenic agents as prions
Foods most associated with food borne illness
Raw meat and poultry
Raw eggs
Unpasteurized milk
Raw shellfish
Raw fruits and vegetables
Unpasteurized fruit juice
Food processing concerns
Foods that mingle the products of several individual animals
A pathogen in one animal can contaminate may contaminate
a whole batch of food mingling the products of several
animals as bulk raw milk, pooled raw eggs or ground beef
A single hamburger may contain meat from hundreds of
animals
A glass of raw milk may contain milk from hundreds of cows.
A broiler chicken carcass can be exposed to the drippings and
juices of many thousands of other birds that went through the
same cold water tank after slaughter.
Washing fruits and vegetables can decrease but not eliminate
contamination
Processing food under less than sanitary conditions can cause
outbreaks
Raw sprouts that are eaten without cooking may contain
growing microbes
Storage and transport methods for food
Reducing the risk of food borne illness
Cook meat, poultry, and eggs thoroughly
Separate – don’t cross-contaminate one food with another
Chill – refrigerate leftovers promptly
Clean – wash produce
Report – suspected food borne illnesses to the local health
department
How to prove x caused y, or Causation
Hill's Criteria for Causation
Nine criteria must be met to establish a cause-and-effect relationship. This is commonly known as Hill's
Criteria for Causation:
1. Strength of Association - relationship is clear and risk estimate is
high
2. Consistency - observation of association must be repeatable in
different populations at different times
3. Specificity - a single cause produces a specific effect
4. Alternative Explanations - consideration of multiple hypotheses
before making conclusions about whether an association is causal
or not
5. Temporality - cause/exposure must precede the effect/outcome
6. Dose-Response Relationship - an increasing amount of exposure
increases the risk
7. Biological Plausibility - the association agrees with currently
accepted understanding of biological and pathological processes
8. Experimental Evidence - the condition can be altered, either
prevented or accelerated, by an appropriate experimental
process
9. Coherence - the association should be compatible with existing
theory and knowledge, including knowledge of past cases and
epidemiological studies
Hill's Criteria for Causation Explanations and History
Epidemiological Triad
The Epidemiological Triad is the traditional model of infectious disease causation. It is commonly
known as the agent/host/environment triad. It includes three components:
1. an external agent
2. a susceptible host
3. an environment that brings the host and agent together, so that
disease occurs
Chain of Transmission Triad
This is another common triad, which is an altered form of the Chain of Infection described below. It is
a companion to the Epidemiological Triad. It also has three components:
1. an external agent
2. a vector or fomite that transmits the disease
3. a susceptible host for the disease
This is used to define the major points of a disease case.
Epidemiological Study Designs
Basic Studies
1. Ecological- comparisons of geographical locations
2. Cross Sectional - a survey, "snapshot in time"
3. Case-control - compare people with and without disease to find
common exposures
4. Cohort - compare people with and without exposures to see what
happens to each
5. Randomized controlled trial - human experiment
Advantages and Disadvantages to Study Designs
Study Designs
Advantages
Disadvantages
Time Consuming
Most Scientifically Sound
Trial
Best Measure of Exposure
Most Accurate Observational
Study
Cohort Study
Good Measure of Exposure
Case-Control
Study
Cross-Sectional
Study
Unethical for Harmful Exposures
Most Expensive
Time Consuming
Expensive
Can Study Rare Diseases
Possible Time-Order Confusion
Relatively Less Expensive and
Relatively Fast
Possible Error in Recalling Past
Exposures
Fastest
Possible Time-Order Confusion
Least Expensive
Least Confidence in Findings
2*2 Table
Table which has two columns and rows for people with or without exposure and with or without
disease; shows amount of people with each characteristic.
Disease
No Disease
Exposure
a
b
No Exposure
c
d
Using the 2*2 Table, we can calculate odds ratio and relative risk. These calculations allow comparisons
between the control group and the group afflicted with the condition. One is the neutral value and
means that there is no difference between the groups compared; when the value is greater than one it
means that there has been some difference between the two groups, whether it was caused by bias,
chance, or an actual relationship between the exposure and outcome is yet to be seen. The P-value
tells us whether the results of the study can be used. The P-value is the measure of how confident you
are that your findings are correct. You can only trust your findings to be correct if the P-value is less
than .05.
Odds Ratio - used in case-control study, ad/bc
Relative Risk - used in cohort study, (a/(a+b))/(c/(c+d))
Attack Rate - the rate that a group experienced an outcome or illness equal to the number sick divided
by the total in that group. (There should be a high attack rate in those exposed and a low attack rate
in those unexposed.) For the exposed: a/(a+b) For the unexposed: c/(c+d)
Using Epi-Curves
An epi-curve is a histogram that shows the course of an outbreak by plotting the number of cases of a
condition according to the time of onset.
Epi-Curves fall into three classifications:
Point source epidemics occur when people are exposed to the same exposure over a limited, well
define period of time. The shape of the curve commonly rises rapidly and contains a definite peak,
followed by a gradual decline.
Continuous common source epidemics occur when the exposure to the source is prolonged over an
extended period of time and may occur over more than one incubation period. The down slope of the
curve may be very sharp if the common source is removed or gradual if the outbreak is allowed to
exhaust itself.
Propagated (progressive source) epidemics occur when a case of disease serves later as a source of
infection for subsequent cases and those subsequent cases, in turn, serve as sources for later cases.
The shape of this curve usually contains a series of successively larger peaks, reflective of the
increasing number of cases caused by person-to-person contact, until the pool of those susceptible is
exhausted or control measures are implemented.
Disease and Disease Transmission
Chain of Infection
Agent leaves reservoir through portal of exit, and is conveyed by some mode of transmission, and
enters the appropriate portal of entry to infect a susceptible host.
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Agent - A microbial organism with the ability to cause disease.
Reservoir - A place where agents can thrive and reproduce.
Portal of Exit - A place of exit providing a way for an agent to
leave the reservoir; the route a pathogen takes out of an
infected host. Portals of exit tend to be fairly well defined. What
serve as portals of exit are often not terribly surprising, at least,
once something is known of how and where a pathogen replicates
and enters new hosts. Respiratory infections tend to utilize the
mouth and nose as portals of exit. Gastrointestinal diseases tend
to exit in feces or saliva, depending on the site of replication.
Sexually transmitted diseases tend to have portals of exit at the
urethra or genital region. Blood-bourne diseases tend to exit via
arthropods, needles, bleeding, or hyperdermic syringes. A more
general portal of exit occurs when an infected animal is
butchered or an infected person undergoes surgery. The three
most common portals of exit are the skin, gastrointestinal tract,
and respiratory tract.
Mode of Transmission - Method of transfer by which the organism
moves or is carried from one place to another; the transfer of
disease-causing microrganisms from one environment to another,
particularly from an external environment to a susceptible
individual. There are three general categories of transmission:
contact, vehicle, and vector.
Portal of Entry - An opening allowing the microorganism to enter
the host; the route a pathogen takes to enter a host. Just as with
the portals of exit, many pathogents have preferred portals of
entry. Many pathogens are not able to cause disease if their
usual portal of entry is artificially bypassed. The most common
portal of entry is the mucous membrane of the respiratory tract.
Susceptible Host - A person who cannot resist a microorganism
invading the body, multiplying, and resulting in infection.
Chain of Infection: Diagram and Explanation
Characteristics of Agents
1. Infectivity - capacity to cause infection in a susceptible host
2. Pathogenicity - capacity to cause disease in a host
3. Virulence - severity of disease that the agent causes to host
Modes of Disease Transmission
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Contact Transmission - sub-categories include direct (person-toperson), indirect (fomite), or droplet.
Direct Contact - occurs through touching, kissing, and/or
sexual intercourse. To prevent direct contact transmission,
wear gloves and masks, use condoms, etc.
Indirect Contact - occurs from a reservoir via inanimate
objects called fomites. Fomites are basically almost anything
an infected individual or reservoir can touch, upon which can
be left a resdiue of contagious pathogent. Exceptions include
the various inanimates ferred to as vehicles: food, air, and
liquids. Typically, it is more difficult to avoid indirect contact
transmission than it is to avoid direct contact transmission. A
certain degree of organismal durability may be necessary to
survive passage on a fomite. The best way to prevent indirect
contact transmission is by avoiding contact with fomites,
avoiding contact of hands with mucous membranes, especially
when handling or potentially handling fomites, the use of
barriers when handling fomites, and disinfecting fomites
before handling.
Droplet Transmission - consequence of being coughed,
sneezed, or spit on. To be considered droplet transmission,
mucous droplets must still be traveling with the velocity
imparted on it leaving the mouth. As a rule of thumb, this is
up to one meter after exiting the mouth. Any further and this
is considered airborne transmission. Given interaction within
one meter of people is certainly more difficult to avoid
droplet transmission than it is to avoid either direct or
indirect transmission. Not surprisingly, it is especialy
respiratory diseases that are transmitted by droplets.
Vehicle Transmission - transmission via a medium such as food,
air, and liquid, which are al routinely taken into the body, and
thus serve as vehicles into the body.
Airborne Transmission - occurs via droplets (typically mucous
droplets) where droplets are liquids that remain airborne
whether as aerosols (very small droplets) or associated with
dust particles. An example is within airliners where
economizing measures reduces the turnover of cabin air and
consequently increases air recycling. Organisms which can
find their way into the air and remain viable thus have
repeated opportunities to infect passengers. It requires
greater organismal durability that droplet transmission simply
because of the length of time the microorganism is exposed
to the air, before infecting a new host, is longer. Increased
durability is to the effects of desiccation, exposure to
sunlight, etc. This is why breathing does not typically result
in the acquisition of disease.
o Food-bourne Transmission - any number of pathogens are
found in food and not killed during processing may be
transmitted via food product. Salmonella especially tends to
be part of the normal flora of chickens and consequently
associated with chicken products.
o Water-bourne Transmission - fecal contaminated water.
Generally, this is via sewage contaminated water supplies. It
is especially gastrointestinal pathogens that are present in
feces and therefore which rely on this type of transmission.
 Vector Transmission - no entry.
o Portals of Entry to the Nervous System - the brain is typically
fairly resistant to bacterial infection. There are four common
portals of entry to the nervous system. For an organism to
take advantage of these routes, they must display
increasingly specialized adaptations as read from first to last:
parenteral, via the blood, via the lymphatic systems, and up
the peripheral nerve axons. Ordering of blood and lymphatic
system was arbitrary and not intended ot imply that one
serves as a significantly more difficult portal to take
advantage of than the other.
Disease Prevention
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Primary prevention - early intervention to avoid initial exposure
to agent of disease preventing the process from starting
Secondary prevention - during the latent stage (when the disease
has just begun), process of screening and instituting treatment
may prevent progression to symptomatic disease
Tertiary prevention - during the symptomatic stage (when the
patient shows symptoms), intervention may arrest, slow, or
reverse the progression of disease
Cook meat, poultry, and eggs thoroughly.
Don't cross-contaminate one food with another.
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Chill and refrigerate leftovers promptly.
Clean and wash all produce.
Report suspected food-borne illnesses to the local health
department.
Ten Steps to Investigating an Outbreak
Remember that this is a conceptual order, so steps have to be done simultaneously!
1. Prepare for Field Work
2. Establish the Existence of an Outbreak - Consider Severity,
Potential for Spread, Public Concern, and Availability of
Resources
3. Verify the Diagnosis
4. Define and Identify Cases - Case Definition and Line Listing
5. Describe and Orient the Data in Terms of Person, Place, and
Time - Descriptive Epidemiology
6. Develop Hypotheses (Agent/Host/Environment Triad) = Chain of
Transmission
7. Evaluate Hypotheses - Analytical Studies (MUST Have a Control
Group)
8. Refine Hypotheses and Carry Out Additional Studies
9. Implement Control and Prevention Measures (ASAP!)
10. Communicate Findings
Ten Steps to Outbreak Investigation - Explanation of Steps