Chapter 1

Foundations of Epidemiology

Objectives

Define epidemiology Explain the importance of descriptive epidemiology Explain the importance of analytic epidemiology Explain the role of epidemiology in public health practice and

individual decision making Define epidemic, endemic, and pandemic Describe common source, propagated, and mixed epidemics Define the concepts and principles of case as used in

epidemiology Describe the epidemiology triangle for infectious disease Describe the advanced epidemiology triangle for chronic

diseases and behavioral disorders Define the three levels of prevention used in public health and

epidemiology Understand basic vocabulary used in epidemiology

Define epidemiology Epidemiology is the

study of the distribution and determinants of health-related states or events in human populations, and the application of this study to prevent and control health problems.

Define epidemiology

The word epidemiology is based on the Greek words epi—prefix meaning on, upon, or befall; demos—root meaning the people; and logos—suffix meaning the study

In other words, epidemiology is the study of what befalls the population

Key terms in the definition

Study - Epidemiology involves sound methods of scientific investigation. Methods rely on careful observation and

the use of valid comparison groups to determine whether the observed health events differ from what might be expected

Key terms in the definition

Distribution Study of frequency and pattern of health

events in the population Frequency – number, and number in

relation to the population Pattern – the health-related state or event

by person, place, and time characteristics

Key terms in the definition

Determinants Search for causes and other factors of

health-related states or events

Key terms in the definition

Health-related states or events Epidemiology involves study of

disease states (e.g., cholera, influenza, pneumonia)

Epidemiology also involves events injury, drug abuse, and suicide

Epidemiology also involves behaviors and conditions associated with health

physical activity, nutrition, seat belt use, and provision and use of health services

The importance of descriptive epidemiology

Descriptive epidemiology involves characterization of the distribution of health-related states or events by Person - who Place - where Time – when Clinical criteria - what

Descriptive epidemiology

Allows us to answer who, what, and where questions

This knowledge is prerequisite to effective education, screening, prevention, and control programs

Analytic epidemiology

Analytic epidemiology involves identifying and quantifying associations, testing hypotheses, and identifying causes of health-related states or events

Explains why and how health-related states or events occur

Activities performed in epidemiology

Identifying risk factors for disease, injury, and death Describing the natural history of disease Identifying individuals and populations at greatest risk for

disease Identifying where the public health problem is greatest Monitoring diseases and other health-related events over time Evaluating the efficacy and effectiveness of prevention and

treatment programs Providing information useful in health planning and decision

making for establishing health programs with appropriate priorities

Assisting in carrying out public health programs Being a resource person Communicating public health information

Role of epidemiology in public health practice and individual decision making

Epidemiological findings contribute to preventing and controlling disease, injury,

disability, and death How?

By providing information leading to informed public health policy and planning, as well as individual health decision making

Epidemiologic information for influencing public

health policy and planning and individual decisions

Public health surveillance Causes of disease Completing the clinical picture Program evaluation

Efficacy Effectiveness

Accurate assessment requires a standard case definition

A standard set of criteria, or case definition, assures that cases are consistently diagnosed, regardless of where or when they were identified and who diagnosed the case

Epidemic, endemic, and pandemic

Epidemic – Health-related state or event in a defined population above the expected over a given period of time

Endemic – Persistent, usual, expected health-related state or event in a defined population over a given period of time

Pandemic – Epidemic affecting a large number of people, many countries, continents, or regions

1996

1991

2003

Obesity Trends* Among U.S. AdultsBRFSS, 1991, 1996, 2003

(*BMI 30, or about 30 lbs overweight for 5’4” person)

No Data <10% 10%–14% 15%–19% 20%–24% ≥25%

Common source, propagated, and mixed epidemics

Common source Point Intermittent Continuous

Propagated

Common source

Tend to result in more cases occurring more rapidly and sooner than host-to-host epidemics

Identifying and removing exposure to the common source typically causes the epidemic to rapidly decrease

Common source

Examples anthrax, traced to milk or meat from

infected animals botulism, traced to soil-contaminated food and cholera traced to fecal contamination

of food and water

Propagated

Arise from infections being transmitted from one infected person to another

Transmission can be through direct or indirect routes

Host-to-host epidemics rise and fall more slowly than common source epidemics

Propagated

Examples tuberculosis whooping cough Influenza measles

Disease transmission

Disease transmission usually occurs by direct, person-to-person contact (e.g.,

STDs) fomite-borne (e.g., Hepatitis A spread by a

contaminated eating utensil) vehicle-borne (e.g., HIV/AIDS spread

through needle sharing drug users) vector-borne transmission (e.g., Malaria

spread through mosquitoes)

Mixed epidemics

Occurs when a common source epidemic is followed by person-to-person contact and the disease is spread as a propagated outbreak

Example – Shigellosis occurred among a group of 3000 women attending a music festival. Over the next few weeks, subsequent generations of shigella cases spread by person-to-person transmission from festival attendees.

Concepts and principles of case as used in epidemiology

A case is a person who has been diagnosed as having a disease, disorder, injury, or condition

Primary case, index case

The first disease case in the population is the primary case.

The first disease case brought to the attention of the epidemiologist is the index case.

The index case is not always the primary case.

Secondary case

Those persons who become infected and ill once a disease has been introduced into a population and who became infected from contact with the primary case

Different levels of diagnosis

Suspect Probable Confirmed

Different levels of diagnosis

As more information (such as laboratory results) becomes available to the physician, he or she generally upgrades the diagnosis. When all criteria are met and they meet the case definition, the case is classified as a confirmed case.

Suspect case

An individual or group of individuals who have all of the signs and symptoms of a disease or condition yet has not been diagnosed as having the disease, nor have the cause of the symptoms connected to a suspected pathogen (i.e., any virus, microorganism, or other substance that causes disease)

Epidemiology triangle for infectious disease

Triangle is based on the communicable disease model Shows the interaction and interdependence of

agent, host, environment, and time as used in the investigation of diseases and epidemics. Agent is the cause of the disease Host is an organism, usually a human or an

animal, that harbors a disease Environment is those surroundings and conditions

external to the human or animal that cause or allow disease transmission

Time accounts for incubation periods, life expectancy of the host or the pathogen, and duration of the course of the illness or condition.

Stopping an epidemic

An epidemic can be stopped when one of the elements of the triangle is interfered with, altered, changed, or removed from existence, so that the disease no longer continues along its mode of transmission and routes of infection

Some disease transmission concepts

Fomites Objects such as clothing, towels,

and utensils that may harbor a disease agent and are capable of transmitting it; usually used in the plural

Example – transmission of cutaneous anthrax from drums to an individual

Vector An invertebrate animal (e.g., tick,

mite, mosquito, bloodsucking fly) capable of transmitting an infectious agent among vertebrates

Can spread an infectious agent from an infected animal or human to other susceptible animals or humans through its waste products, bite, body fluids, or indirectly through food contamination

Reservoir

The habitat (living or nonliving) on which an infectious agent lives, grows, and multiplies and is dependent on for its survival in nature

Humans often serve as both reservoir and host

Zoonosis

When an animal transmits a disease to a human

Examples – Rabies, Rocky Mountain spotted fever, shigellosis

Carrier

A carrier contains, spreads, or harbors an infectious organism

Example – Typhoid Mary

Six types of carriers

Active carrier Convalescent carrier Healthy carrier Incubatory carrier Intermittent carrier Passive carrier (same as healthy

carrier)

Modes of disease transmission

Direct transmission – direct physical contact such as touching with contaminated hands, skin-to-skin contact, kissing, or sexual intercourse

Indirect transmission – occurs when pathogens or agents are transferred or carried by some intermediate item, organism, means, or process to a susceptible host, resulting in disease

The chain of infection

There is a close association between the triangle of epidemiology and the chain of infection

Advanced epidemiology triangle for chronic diseases and behavioral disorders

Three levels of prevention used in public health and epidemiology

Primary prevention (occurs prior to exposure) Immunization Sanitation Education Media campaigns Warning labels

Active primary prevention

Requires behavior change on part of subject Wearing protective devises Obtaining vaccinations

Passive primary prevention

Does not require behavior change Vitamin fortified foods Fluoridation of public water supplies

Secondary prevention

Occurs to reduce the progress of disease The disease already exists in the person

Cancer screening – cancer already present. The goal is to detect the cancer before clinical symptoms arise in order to improve prognosis

Tertiary prevention

To reduce the limitation of disability from disease

The disease has already occurred Physical therapy for stoke victims Halfway houses for recovering alcoholics Shelter homes for the developmentally

disabled Fitness programs for heart attack patients

Conclusion

Epidemiology involves application of scientific methods for describing the frequency and pattern of health-related states or events

Epidemiology identifies causes of health-related states or events and modes of transmission

Epidemiology guides public health planning and decision making

Epidemiology assists individuals in making informed health behavior changes