Disease Detectives

Epidemiology

The study of distribution and determinants of health-related states in specified populations, and the application of this to control health problems.

What are the four basic reasons why disease detectives study and research outbreaks and epidemics?

Control and Prevention, Research Opportunities, Training, and Legal Concerns.

Two Basic Types of Epidemiology: Classical Epidemiology/Public Health

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. Note: There are all sorts of classification systems for epi and the above certainly are examples. One could add research epi vs applied epi to the above list. However probably the most fundamental and common system is Descriptive epi (e.g. person, place and time) vs Analytic epi (hypothesis testing - study design).

Two Basic Types of Epidemiology: 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.

Note: There are all sorts of classification systems for epi and the above certainly are examples. One could add research epi vs applied epi to the above list. However probably the most fundamental and common system is Descriptive epi (e.g. person, place and time) vs Analytic epi (hypothesis testing - study design).

Clinical Epidemiology can be further divided into what categories?

Infectious Disease Epidemiology - heavily dependent on laboratory support

Chronic Disease Epidemiology - dependent on complex sampling and statistical methods

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.

Epidemiology Terms: 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.

Epidemiology Terms: Epidemic

Large numbers of people over a wide geographical area are affected.

Epidemiology Terms: Etiology

Study of the cause of a disease.

Epidemiology Terms: Fomite

A physical object that serves to transmit an infectious agent from person to person. An example of this is lice on a comb. The comb is the fomite and the lice are the agent that can make your hair itch.

Epidemiology Terms: Latrogenic

An illness that is caused by a medication or physician.

Epidemiology Terms: Incubation Period

Time in between when a person comes into contact with a pathogen and when they first show symptoms or signs of disease.

Epidemiology Terms: Index Case

First patient in an epidemiological study (also known as patient zero or primary case).

Epidemiology Terms: Morbidity

Rate of disease in a population.

Epidemiology Terms: Mortality

Rate of death in a population.

Epidemiology Terms: 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.

Epidemiology Terms: Pandemic

An epidemic occurring over several countries or continents and affecting a large proportion of the population.

Epidemiology Terms: 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.

Epidemiology Terms: Nosocomial Disease

An infection that is acquired in a hospital.

Epidemiology Terms: Risk

The probability that an individual will be affected by, or die from, an illness or injury within a stated time or age span. Age-span is not usually a consideration in this usage. Risk of death from a particular illness is expressed as the Case Fatality Rate (Number deaths due to a disease/Number with the disease) or the Cause-specific Mortality Rate (Number deaths due to a disease/Number in population). Age span is a more common consideration in this last usage.

Note: Risk of illness is generally considered to be the same as the Incidence (see below) and the terms are used interchangeably.

Epidemiology Terms: 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.

Epidemiology Terms: Vector

An animal that transmits disease but is not the cause of the disease itself. For example, a mosquito is a vector for malaria.

Epidemiology Terms: Zoonosis

An infectious disease that is transmissible from animals to humans.

Epidemiology Terms: Symptomatic

Showing symptoms or signs of injury.

Epidemiology Terms: Asymptomatic

Showing no signs or symptoms, although can be carrier of disease.

Epidemiology Terms: Incidence (of an illness)

The number of new instances of disease in a population over a given time period. It is expressed as "X cases/Y population/ Z time".

Epidemiology Terms: Prevalence (of an illness)

The number of affected persons in the population at any given point in time. It is expressed as "X cases/Y population".

Note: There are two major ways in which prevalence is measured:

Period Prevalence and Point Prevalence:

Think of point prevalence as a snapshot of the population and its rate of a certain disease at a point in time while period prevalence tracks the prevalence over a certain duration. The only difference is that incidence (I) includes time while prevalence (P) does not. Time (D) reflects the duration of the illness or condition. If two conditions have the same incidence in a population, the one with the longer duration will have the greater prevalence. Importantly, P = I*D, so with two of the variables, it is possible to solve for the third.

What are the 13 steps to investigating an outbreak?

1. Prepare for field work - Research and Supplies, Official Arrangements, Safety Protocols, and Contacts

2. Establish the Existence of an Outbreak - Consider Severity, Potential for Spread, Public Concern, and Availability of Resources

3. Verify the Diagnosis - Verify Procedures and Eliminate Experimental Error (and Other Errors/Biases, for That Matter)

4. Construct a Working Case Definition

5. Find Cases Systematically and Record Information - Time: Tables, Epi Curves; Place: Geographical Extent of Disease & Spot Map; Identify By Demographic Information or Exposures to Risk Factor

6. Describe and Orient the Data in Terms of Person, Place, and Time - Descriptive Epidemiology

7. Develop Hypotheses (Agent/Host/Environment Triad) = Chain of Transmission

8. Evaluate Hypotheses - Analytical Studies (MUST Have a Control Group)

9. Refine Hypotheses if Necessary

10. Compare and Reconcile with Laboratory and/or Environmental Studies

11. Implement Control and Prevention Measures (ASAP!)

12. Initiate or Maintain Surveillance - Monitor Implementation: Track New Cases, Check the Outbreak's Spread Outside Targeted Area, Control and Change if Needed

13. Communicate Findings - Reports, to Important People and Public

What is the purpose of surveillance?

To gain knowledge of patterns of disease, injury, or other health problems in a community for prevention and control purposes. Surveillance is necessary to influence public health decisions and evaluate control measures.

What are the 5 steps of surveillance?

1. Identify, define, and measure the health problem of interest

2. Collect and compile data about the problem (and if possible, factors that influence it)

3. Analyze and interpret these data

4. Provide these data and their interpretation to those responsible for controlling the health problem

5. Monitor and periodically evaluate the usefulness and quality of surveillance to improve it for future use (surveillance of a problem often does not include actions to control the problem),

What are the alternate 5 steps of surveillance?

1. Data Collection - reports, electronic and vital records, registries, and surveys.

2. Data Analysis - ideally analyzed by location to find illness's location so resources are sent there.

3. Data Interpretation - identifying person, place, time to find how and why health event happened.

4. Data Dissemination (Distribution) - announcements, reports, articles and media → to important people and public.

5. Link to Action - without action, no real purpose (this version includes taking action),

Types of Surveillance: Passive

Diseases are reported by healthcare providers. This type of surveillance, though simple and inexpensive, is often limited by incomplete reporting and quality variation in reporting.

Types of Surveillance: Active

Health agencies contact health providers seeking reports. This ensures more complete reporting of conditions. Active surveillance is often used with a specific epidemiological investigation or during an outbreak.

Types of Surveillance: Syndromic

The syndrome of the disease is monitored as a proxy for the disease. The syndrome must be infrequent and severe enough to warrant investigation of each identified case, and must be unique. This form of surveillance is often used when timeliness is key, diagnosis is difficult or time-consuming, or when detecting and defining the scope of an outbreak.

Types of Surveillance: Sentinel

Professionals selected to represent a specific geographic area or group report health events to health agencies. This is used when high-quality data can't be obtained through passive surveillance. It involves monitoring trends or key health indicators and a limited network of reporting sites. Advantages include being able to implement intervention earlier and not being as reliant on doctors to diagnose disease.

What are the 9 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

What are the 4 Koch's Postulates?

1. The microbe must be present in abundance in all cases of the disease, but not in healthy organisms.

2. The microbe must be isolated from the diseased organism and grown in pure culture.

3. The cultured microorganism should cause disease when introduced into a healthy organism.

4. The microbe must be re-isolated from the inoculated, diseased experimental host and identified as identical to the original specific causative agent.

What are the 10 Evan's Postulates?

1. The prevalence of the disease should be significantly higher in those exposed to the risk factor than those not.

2. Exposure to the risk factor should be more frequent among those with the disease.

3. In prospective studies, the incidence of the disease should be higher in those exposed to the risk factor.

4. The disease should follow exposure to the risk factor with a normal or log-normal distribution of incubation periods.

5. A spectrum of host responses along a logical biological gradient from mild to severe should follow exposure to the risk factor.

6. A measurable host response should follow exposure to the risk factor in those lacking a response before the exposure or increase the response in those with a response before exposure. A host response should be infrequent in those not exposed to the risk factor.

7. In experiments, the disease should occur more frequently in those exposed to the risk factor than in the control group.

8. Reduction or elimination of the risk factor should reduce the risk of disease.

9. Modifying or preventing host response should eliminate or decrease disease.

10. All findings should make biological and epidemiological sense.

What is the triad for descriptive epidemiology?

Person, place, and time

What is the Chain of Infection Triad?

Altered form of the chain of infection

An external agent, a vector or fomite that transmits the disease, and a susceptible host for the disease.

This is used to define the major points of a disease case.

Types of Carriers/Vectors: Convalescent

Humans are also capable of spreading disease following a period of illness, typically thinking themselves cured of the disease

Types of Carriers/Vectors: Incubatory

When an individual transmits pathogens immediately following infection but prior to developing symptoms.

Types of Carriers/Vectors: Chronic

Someone who can transmit a disease for a long period of time.

Types of Carriers/Vectors: Genetic

Has inherited a disease trait but shows no symptoms.

Types of Carriers/Vectors: Transient/Temporary

Someone who can transmit an infectious disease for a short amount of time.

Basic Epidemiological Studies: Ecological

Comparisons of geographical locations

Basic Epidemiological Studies: Cross-Sectional

A survey, health questionnaire, "snapshot in time"

Basic Epidemiological Studies: Case-Cohort

Compare people with and without disease to find common exposures

Basic Epidemiological Studies: Cohort

Compare people with and without exposures to see what happens to each

Basic Epidemiological Studies: Randomized Controlled Trial

Human experiment that randomly assigns participants to an experimental or control group

Basic Epidemiological Studies: Quasi Experiments

Research similarities with traditional experimental design or RCT, but lack element of random assignment to treatment/control; participants are assigned a group based on non-random criteria

What are some advantages to trial study designs?

Most Scientifically Sound, best Measure of Exposure

What are some disadvantages to trial study designs?

Time Consuming, unethical for harmful exposures, most expensive

What are some advantages to cohort study designs?

Most Accurate Observational Study, good Measure of Exposure, correct Time Sequence, good for Rare Exposures, easy Risk Calculation

What are some disadvantages to cohort study designs?

Time Consuming, expensive, bad for Rare Diseases, possible Loss of Follow-up

What are some advantages to case-control study designs?

Can Study Rare Diseases, relatively Less Expensive and, relatively Fast, good for Rare Diseases, good for Long Latency Periods

What are some disadvantages to case-control study designs?

Possible Time-Order Confusion, error in Recalling Exposure, only 1 outcome

What are some advantages to cross-sectional study designs?

Fastest, least Expensive, good for More Than 1 Outcome

What are some disadvantages to cross-sectional study designs?

Possible Time-Order Confusion, least Confidence in Findings

What is the 2x2 table?

| | Disease | No Disease |

| Exposure | a | b |

| No Exposure | c | d |

What is the Odds Ratio in relation to the 2x2 table?

Used in case-control study

(ab)/(cd)

What is the Relative Risk in relation to the 2x2 table?

Used in cohort studies

(a/(a+b))/(c/(c+d))

What is the Attack Rate in relation to the 2x2 table?

Rate that a group experienced an illness

For the exposed: a/(a+b)

For the unexposed: c/(c+d)

What are point source epidemics?

Point source epidemics occur when people are exposed to the same exposure over a limited, well defined period of time. The shape of the curve commonly rises rapidly and contains a definite peak, followed by a gradual decline.

What are continuous common source epidemics?

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.

What are propagated (progressive source) epidemics?

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. The distance between these peaks may be a rough indication of the incubation period of the disease. As the outbreak progresses, the peaks flatten out (think of the variance around a mean over multiple generations).

What is an intermittent source epidemic?

Where people are intermittently exposed to a source. There are generally multiple peaks in this type of curve. An example of this may be spoiled food at a store giving people an infection, where there are intermittent cases based on when people are exposed to this source.

What is random error?

Random error is the result of fluctuations around a true value because of the sample population. As the term implies, it is random, so it is impossible to correct. However, random error can be reduced; some ways include increasing the sample size and making measurements more precise, either by using a more accurate measurement device or by taking more trials. While these techniques would decrease random error, they can also be expensive. Better measurement devices will cost more, and more trials and a larger sample size will mean more work. Precision is a measure of random error that is inversely related, so increasing random error decreases precision.

What is systematic error?

Systematic error is any error other than random error. It is usually consistent and repeatable and often occurs from flawed equipment or experiment design. For example, systematic error can occur if the markings on your ruler are wider. This would make the numeric measurements less than what they actually are, making all data collected inaccurate. However, trends observed may still be preserved (shifting a line vertically preserves a line, as it is a rigid motion).

What is selection bias?

Selection bias occurs when selection of participants for a study is affected by an unknown variable that is associated with the exposure and outcome being measured.

What is information bias?

Information bias occurs when bias is introduced through an error in measurement or observation. An example of information bias is recall bias. When studied, some subjects may more easily recall specific habits related to a disease or condition than subjects not affected with the disease or condition.

Incubation Period

The stage of subclinical disease, extending from the time of exposure to onset of disease symptoms, in infectious diseases.

Latency Period

The stage of subclinical disease, extending from the time of exposure to onset of disease symptoms, in chronic diseases.

What is the spectrum of disease?

The severity of illness resulting from the disease process.

In order, what are the stages of a disease?

1. Stage of Susceptibility

2. Stage of Subclinical Disease

3. Stage of Clinical Disease (diagnosis)

4. Stage of Recovery, Disability, or Death

Types of Disease Prevention: Primordial Prevention

Intervention at the very beginning to avoid the development of risk factors the population may be exposed to. Often deals with changing physical and social environments.

Note: In Disease Detectives scenarios, event supervisors will often ask you to brainstorm disease prevention methods. Even if you know very little about the disease, you can brainstorm ideas from the chain of infection for the disease. For example, if the chain of infection describes that a disease is comes in contact with humans through sand at the beach and enters the body through any openings (mouth, nose, etc.), a prevention method could be putting up signs at beaches reminding the public to wash their hands before consuming any food.

Types of Disease Prevention: Primary Prevention

Early intervention to avoid initial exposure to agent of disease preventing the process from starting.

Note: In Disease Detectives scenarios, event supervisors will often ask you to brainstorm disease prevention methods. Even if you know very little about the disease, you can brainstorm ideas from the chain of infection for the disease. For example, if the chain of infection describes that a disease is comes in contact with humans through sand at the beach and enters the body through any openings (mouth, nose, etc.), a prevention method could be putting up signs at beaches reminding the public to wash their hands before consuming any food.

Types of Disease Prevention: Secondary Prevention

During the latent stage (when the disease has just begun), process of screening and instituting treatment may prevent progression to symptomatic disease.

Note: In Disease Detectives scenarios, event supervisors will often ask you to brainstorm disease prevention methods. Even if you know very little about the disease, you can brainstorm ideas from the chain of infection for the disease. For example, if the chain of infection describes that a disease is comes in contact with humans through sand at the beach and enters the body through any openings (mouth, nose, etc.), a prevention method could be putting up signs at beaches reminding the public to wash their hands before consuming any food.

Types of Disease Prevention: Tertiary Prevention

During the symptomatic stage (when the patient shows symptoms), intervention may arrest, slow, or reverse the progression of disease.

Note: In Disease Detectives scenarios, event supervisors will often ask you to brainstorm disease prevention methods. Even if you know very little about the disease, you can brainstorm ideas from the chain of infection for the disease. For example, if the chain of infection describes that a disease is comes in contact with humans through sand at the beach and enters the body through any openings (mouth, nose, etc.), a prevention method could be putting up signs at beaches reminding the public to wash their hands before consuming any food.

Types of Disease Prevention: Quaternary Prevention

Set of health activities to mitigate or avoid consequences of unnecessary/excessive intervention of the health system. Social credit that legitimizes medical intervention may be damaged if doctors don't prevent unnecessary medical activity and its consequences (in other words, actions taken to prevent medical treatment that is actually harmful).

Note: In Disease Detectives scenarios, event supervisors will often ask you to brainstorm disease prevention methods. Even if you know very little about the disease, you can brainstorm ideas from the chain of infection for the disease. For example, if the chain of infection describes that a disease is comes in contact with humans through sand at the beach and enters the body through any openings (mouth, nose, etc.), a prevention method could be putting up signs at beaches reminding the public to wash their hands before consuming any food.

Immunity: Active Immunity

Occurs when the person is exposed to a live pathogen, develops the disease, and becomes immune as a result of the primary immune response.

Immunity: Passive Immunity

Short-term immunization by the injection of antibodies, such as gamma globulin, that are not produced by the recipient's cells. Naturally acquired passive immunity occurs during pregnancy, in which certain antibodies are passed from the maternal into the fetal bloodstream.

Immunity: Herd Immunity

Protecting a whole community from disease by immunizing a critical mass of its populace. Vaccination protects more than just the vaccinated person. By breaking the chain of an infection's transmission, vaccination can also protect people who haven't been immunized. But to work, this protection requires that a certain percentage of people in a community be vaccinated.

What is the chain of transmission?

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.

Chain of Infection: Agent

A microbial organism with the ability to cause disease.

Chain of Infection: Reservoir

A place where agents can thrive and reproduce.

Chain of Infection: 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. A more general portal of exit occurs when an infected animal is butchered or an infected person undergoes surgery.

What portals of exit do respiratory infections tend to utilize?

Respiratory infections tend to utilize the mouth and nose as portals of exit.

What portals of exit do gastrointestinal infections tend to utilize?

Gastrointestinal diseases tend to exit in feces or saliva, depending on the site of replication.

What portals of exit do sexually transmitted diseases tend to utilize?

Sexually transmitted diseases tend to have portals of exit at the urethra or genital region. Blood-borne diseases tend to exit via arthropods, needles, bleeding, or hyperdermic syringes.

What are the 3 most common portals of exit?

Skin, gastrointestinal tract, and respiratory tract.

Chain of Infection: Mode of Transmission

Method of transfer by which the organism moves or is carried from one place to another; the transfer of disease-causing microorganisms from one environment to another, particularly from an external environment to a susceptible individual.

What are the three general categories of transmission?

Contact, vehicle, and vector.

Chain of Infection: 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 pathogens have preferred portals of entry. Many pathogens are not able to cause disease if their usual portal of entry is artificially bypassed.

What is the most common portal of entry?

The mucous membrane of the respiratory tract.

Chain of Infection: Susceptible Host

A being who cannot resist a microorganism invading the body, multiplying, and resulting in infection.

Infectivity

capacity to cause infection in a susceptible host

Pathogenicity

capacity to cause disease in a host

Virulence

severity of disease that the agent causes to host