CHAPTER 1: FOUNDATIONS OF EPIDEMIOLOGY

Objectives

• Define epidemiology

• Be familiar with the different elements of the definition

• Define risk factors and indicate why it does not mean cause

• Define epidemic, endemic, and pandemic

• Describe common-source, propagated, and mixed epidemics

• Describe why a standard case definition and adequate levels of reporting are important in epidemiology investigations

• Describe disease transmission concepts

• Define the three levels of prevention used in public health and epidemiology

• Be familiar with the basic vocabulary used in epidemiology

• Explain the role of epidemiology in public health practice and individual decision-making

What is Health?

• A holistic conception of health was adopted by the World Health Organization (WHO) in their 1948 definition of health, which is “a state of complete physical, mental and social wellbeing, not merely the absence of infirmity or disease.”

What is Public Health?

• Public health is the science and art of promoting health and extending life on the population level

• Public health is concerned with threats to health in the population (a group of people sharing one or more characteristics)

• The mission of public health is to ensure conditions that promote the six dimensions of health in the population

What is the Meaning of Population?

• Population refers to a collection of individuals that share one or more observable personal or observational characteristics from which datat may be collected and evaluated

  • Social

  • Economic

  • Family (marriage and divorce)

  • Work and labor force

  • Geographic factors

How does Public Health Relate to Epidemiology?

• Epidemiology focuses on individuals who share one or more observable characteristics (e.g., a social group, an income level, a type of worker, or where they live) from which data are collected, analyzed, and interpreted

• Epidemiologic data support objectives of preventing disease, disability, and death and promoting well-being (a state of health, happiness, or prosperity)

• Hence, epidemiology may be thought of as the foundation of public health

What is 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

Distribution

• Refers to frequency and pattern

  • Frequency - the number of health-related states or events and their relationship with the size of the population

  • Pattern - a description of the health-related state or event described by:

    • Person - who?

    • Places - where?

    • Time - when?

    • Clinical criteria - what?

Determinants

• Factors that produce (cause) a health outcomes

• The emphasis on causality (the relating of causes to the effects they produce) in epidemiology is critical to effective prevention and corrective measures (interventions) for specific situations

• Identifying causal associations requires making a “judgment” based on the totality of evidence

Health-Related States or Events

• Disease states

  • Cholera, influenza, pneumonia, mental illness

• Conditions associated with health

  • Physical activity, nutrition, environmental poisoning, seat belt use, and provision and use of health services

• Events

  • Injury, drug abuse, and suicide

Prevention and Control

• Prevention involves measures to avoid the occurrence of diease. Public health aims to prevent disease through population-based risk reduction interventions

• Control aims to prevent further spread of disease in areas where an outbreak currently exists

Risk Factor

• A behavior, environmental exposure, or inherent human characteristic that increases the chance of a person developing an adverse health outcome

  • Modifiable risk factors

  • Non-modifiable risk factors

• Is a risk factor sufficient to cause disease?

Epidemic, Endemic, and Pandemic

• Epidemic - health related state or event in a defined population above the expected over a given period

• Endemic - persistent, usual, expected health-related state or event in a defined population over a given period

• Pandemic - epidemic affecting many people, multiple countries, continents, or regions

Outbreak

• An epidemic that is geographically and time constrained

Common Source, Propagated, and Mixed Epidemics

• How epidemics spread through a population

• Common source

  • Point

  • Intermittent/Continuous

• Propagated

  • Spread form person to person

• Mixed Epidemics

  • A mixture of common source and mixed

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

• Examples

  • Anthrax, traced to milk or meat from infected animals

  • Botulism, traced to soil-contamined food

  • 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

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

• Examples

  • Tuberculosis

  • Whooping cough

  • Influenza

  • Measles

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 3,000 women attending a music festival. Over the next few weeks, subsequent generations of Shigella cases spread by person-to-person transmission from festival attendees

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

Concepts and Principles of Case as Used in Epidemiology

• A case is a person who has been diagnosed, 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 after a disease has been introduced into a population and who become infected from contact with the primary case

Different Levels of Diagnosis

• Suspect

  • An individual who has all of the signs and symptoms of a disease or condition, yet not diagnosed

• Confirmed

  • All criteria met

Some Disease Transmission Connects

• Disease transmission usually occurs by

  • Direct, person-to-person contact

    • e.g., STDs

  • Fomite

  • Vector → invertebrate

  • Reservoir

Zoonosis

• When an animal transmits a disease to a human

• Examples: rabies, Rocky Mountain spotted fever, shigellosis

Mode of Disease Transmission

• Direct transmission

  • Direct physical contact like touching with contaminated hands, skin-to-skin contact, kissing, or sexual intercourse

• Indirect transmission

  • Airborne

  • Vector-borne

  • Vehicle-borne

Chain of Infection

• Once a pathogen leaves its reservoir, it follows its mode of transmission to a host, either by direct transmission (person-to-person contact) or by indirect transmission (airborne droplets or dust particles, vector, fomites, and food)

• The final link in the chain of infection is, thus, the susceptible individual or host, usually a human or an animal

Three Levels of Prevention Used in Public Health and Epidemiology

• Primary Prevention: Health promotion, advisory and counseling services, educational programs to drive lifestyle changes for the prevention of chronic disease

• Secondary Prevention: Health assessment and screening to facilitate early identification of chronic diseases

• Tertiary Prevention: Management of chronic diseases, and rehabilitation support services to slow down the progression of diseases

Three Levels of Prevention Used in Public Health and Epidemiology

• Primary prevention (occurs prior to exposure)

  • Immunization

  • Sanitation

  • Education

  • Media Campaigns

  • Warning labels

Active vs. Passive Primary Prevention

• Requires behavior change on part of subject

  • Wearing protective devises

  • Health promotion

  • Lifestyle changes

  • Community health education

  • Ensuring healthy conditions at home, school, and workplace

• 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 - can already present. The goal is to detect the cancer before clinical symptoms arise to improve prognosis and prevent conditions from progression and from spreading

Tertiary Prevention

• To reduce the limitation of disability from disease

• The disease has already occurred

  • Physicla therapy for stoke victims

  • Halfway houses for recovering alcoholics

  • Shelter homes for the developmentally disabled

  • Fitness programs for heart attack patients

Role of Epidemiology in Public Health

• Assessment

  • Identify who is at greatest risk for experiencing the public health problem

  • Identify where the public health problem is greatest

  • Identify when the public health problem is greatest

  • Monitor potential exposures over time

  • Monitor intervention-related health outcomes over time

• Cause

  • Identify the primary agents associated with diseases, disorders, and conditions

  • Identify the mode of transmission

  • Combine laboratory evidence with epidemiologic findings

• Clinical picture/natural history

  • Identify who is susceptible to the disease

  • Identify the types of exposures capable of causing the disease

  • Described the pathologic changes that occur, the stage of subclinical disease, and the expected length of this subclinical phase of the disease

  • Identify the types of symptoms that characterize the disease

  • Identify probable outcomes (recovery, disability, or death) associated with different levels of the disease

• Evaluation

  • Identify the efficacy of the public health program

  • Measure the effectiveness of the public health program

Public Health Surveillance

• The ongoing systematic collection, analysis, interpretation, and dissemination of health data

Evaluating Public Health Programs

• The ability to produce a desired result

• Efficacy: In a controlled environment

• Effectiveness: In the “real world”

Epidemiologic Research Involves

• Investigating the clinical characteristics of the disease

• Identifying the natural history of the disease

• Developing a case definition and identify and confirm cases in a disease outbreak

• Identifying risk factors for disease

• Evaluating the eddicacy and effectiveness of an intervention

Applied-Oriented Epidemiology Jobs Involve

• Utilizing results from epidemiologic studies to define and trace disease for informing decisions

• Identifying who is at risk for the public health problem

• Identifying where the public health problem is greatest

• Identifying when the public health problem is greatest

• Monitoring rates of disease, injury, and death

Applied-Oriented Epidemiology Jobs Involve

• Monitoring potential biological, chemical, physical, or behavioral exposures for diseases and other health-related events over time

• Providing information that is 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

Questions That Need Epidemiology

• Diagnosis

  • Is there such a problem as myalgic encephalitis?

  • Is prostate-specific antigen a good test for prostate cancer?

• Causes

  • Why did this patient suffer a stroke?

  • Is obesity the cause of metabolic syndrome?

• Treatment

  • Is this the best treatment for Parkinson’s disease?

  • Is my surgery as good as that of everyone else?

• Prognosis

  • What are the chances of a recurrent heart attack?

  • How long will this knees joint prosthesis last?

Summary

• Epidemiology is the branch of medicine that invovles the study of the distribution and determinants of health-related states or events adn the prevention and control of health problems in human populations

• Epidemiology has its roots in the study of infectious diseases, but in the past century has extended its reach to include the study of noninfectious diseases and events, behaviors, and conditions associated with health

• Epidemiology focuses on identifying individuals who share one or mroe measurable personal or observational characteristics from which data are collected and analyzed

• It involves the process of describing and understanding public health problems and of applying study findings to better prevent and control these problems

• Epidemiologic information is useful in guiding policies and setting priorities designed to solve public health problems and for allocating scarce health resources for protecting and promoting the public’s health