Foundations of Epidemiology

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Chapter 1

• Foundations of Epidemiology

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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

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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.

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Define epidemiology

• The word epidemiology is based on the Greek words
  
• epiprefix meaning on, upon, or befall
• demosroot meaning the people and
• logossuffix meaning the study
• In other words, epidemiology is the study of what
  befalls the population

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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

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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

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Key terms in the definition

• Determinants
• Search for causes and other factors of
  health-related states or events

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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

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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

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Descriptive epidemiology

• Allows us to answer who, what, and where
  questions
• This knowledge is prerequisite to effective
  education, screening, prevention, and control
  programs

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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

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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

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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

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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

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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

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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

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Obesity Trends Among U.S. AdultsBRFSS, 1991,
1996, 2003
(BMI ?30, or about 30 lbs overweight for 54
person)
1996
2003
No Data lt10 1014
1519 2024 25
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Common source, propagated, and mixed epidemics

• Common source
• Point
• Intermittent
• Continuous
• Propagated

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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

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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

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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

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Propagated

• Examples
• tuberculosis
• whooping cough
• Influenza
• measles

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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)

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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.

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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

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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.

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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

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Different levels of diagnosis

• Suspect
• Probable
• Confirmed

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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.

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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)

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Epidemiology triangle for infectious disease
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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.

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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

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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

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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

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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

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Zoonosis

• When an animal transmits a disease to a human
• Examples Rabies, Rocky Mountain spotted fever,
  shigellosis

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Carrier

• A carrier contains, spreads, or harbors an
  infectious organism
• Example Typhoid Mary

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Six types of carriers

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

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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

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The chain of infection

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

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Advanced epidemiology triangle for chronic
diseases and behavioral disorders
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Three levels of prevention used in public health
and epidemiology

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

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Active primary prevention

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

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Passive primary prevention

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

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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

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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

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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