Unit Epidemiology in CHN PowerPoint Presentation diphtheria

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Unit Epidemiology in CHN PowerPoint Presentation diphtheria Powered By Docstoc
					Epidemiology in CHN

    Fadia Hasna PhD
      January 2004
            Learning Objectives
1.   Define epidemiology
2.   Explore the historical roots of epidemiology
3.   Explain the host, agent and environment model
4.   Define immunity and compare passive, active cross, and
     herd immunity]
5.   Explain how epidemiologists identify populations at risk
6.   Identify the four stages of a disease
7.   List major sources of epidemiologic information
8.   Distinguish between incidence and prevalence in health
     and illness
9.   Use epidemiologic methods and distinguish between
     types of epidemiologic studies
Epidemiology = A branch of medical
• Definition: The study of the determinants
  and distribution of health, disease and
  injuries in human population
• Derived from Greek epi (upon), demos
  (people), logos (knowledge) = the
  knowledge of what happens to people
Historical roots of epidemiology and
•   Back to Hippocrates (a Greek physician) 460 -375 B,
    sometimes referred to as the first epidemiologist.
•   Not until the late 19th century that epidemiology came
    into existence.
1. Epidemic: A disease that clearly exceeds normal or
    expected frequency in a community or region
2. Pandemic: Epidemic with worldwide distribution ( for
    e.g. plague or AIDS)
•   In England alone, approximately one fourth of the
    population died from the plague
3. Endemic: Continuing presence of a disease or infectious
    agent in a given geographic area means the disease is
    endemic to that area
 Historical roots of epidemiology
• When threats of epidemics declined  epidemiologists
  started focusing on communicable diseases: diphtheria,
  infant diarrhea, typhoid, tuberculosis and syphilis
• In recent years, epidemiologists are studying the major
  causes of death and disability:
   –   Cancer
   –   Cardiovascular disorders
   –   AIDS
   –   Violence
   –   Mental illness
   –   Accidents
   –   Arthritis
   –   Congenital defects
  Nursing roots in epidemiology
• Florence Nightingale: Obtained advice on
  hospital statistics and disease classification
  from William Farr who established the field
  of medical statistics in England. She applied
  what she learnt in a detailed record of
  morbidity during the Crimean war. She used
  graphs circles and squares in her
  descriptions and in presenting her statistics.
    Host, agent and environment model
1.    Host is: A susceptible human or animal who harbors
      and nourishes a disease-causing agent
•     Hosts’ resistance depends on:
     – Age
     – Sex
     – Race
     – Genetic influence
     – People’s response to stress
     – Lifestyle: diet, exercise, sleep patterns, healthy and
         unhealthy habits
•     Concept of resistance is important in CHN practice ‫مقاومة‬
      which can be promoted through preventive measures
    Host, agent and environment model
2.    Agent is: A factor that causes or contributes to a health
      problem or condition.
•     Five types of agents:
     1. Biologic: Bacteria, viruses, fungi, protozoa, worms
         and insects. Infectious biologic agents such as
         influenza and HIV
     2. Chemical agents: liquids, solids, gases, dusts, fumes.
         Examples: poisonous sprays on garden pests,
         industrial wastes
     3. Nutrient agents: Too much or too little
     4. Physical agents: anything mechanical such as
         autmobile, rockslide, atm,ospheric ultraviolet
         radiation, geologic earthquakes, or genetically
     5. Psychological agents; events producing stress leading
         to health problems
Host, agent and environment model
•    Agents can be classified as infectious or non-infectious
•    Infectious agents cause communicable diseases (can
     spread from one person to another) such as TB or AIDS
•    Characteristics of infectious agents important to
     understand for CHN
    1. Extent of exposure to the agent ‫درجة التعرض‬
    2. The agent’s pathogenicity ( disease-causing ability)
        ‫القدرة على احداث المرض‬
    3. Infectivity (invasive ability) ‫القدرة على االنتشار‬
    4. Virulence (severity of disease) ‫درجة االيذاء‬
    5. The infectious agent structure and chemical
Host, agent and environment model
• Environment: All external factors surrounding the host that might
  influence resistance.
• Physical environment:
   – Geography
   – Climate
   – Weather
   – Safety of buildings
   – Water and food supply
   – Presence of animals
   – Plants
   – Insects
   – Microorganisms (serve as reservoirs = storage sites or vectors of
      transmission = carriers of disease)
• Psychosocial environment:
    –   Social
    –   cultural
    –   Economic
    –   Psychological conditions that affect health, work stressors, poverty
Host, agent and environment interact
with each other to cause a disease or
          health condition
•    Concept of causality means trying to find a
     cause and effect relationship in epidemiology
•    Chain of causation:
1.   Identify reservoir: (where the causal agent can
     live and multiply)
2.   Identify portal of exit from reservoir and
     mode of transmission
3.   Identify agent itself
4.   Identify portal of entry
•    Multiple causation = web of causation includes
     the concept of association (smoking and cancer)
• Immunity is: the host’s ability to resist a
  particular infectious disease-causing agent.
• This happens when body forms antibodies
  and lymphocytes that react with antigen
  molecules and make them harmless
       Four types of immunity in
          community health:
1.   Passive immunity is: short-term resistance that is
     acquired naturally ( from mother to newborn) or
     artificially (vaccine)
2.   Active immunity is: long-term resistance acquired
     naturally (acquired by host infection) long-lasting
     antibodies or artificially (vaccines) success in polio,
     diphtheria and smallpox eradication.
3.   Cross-immunity is : A person’s immunity to one agent
     provides immunity to another related agent. (cowpox
     and smallpox)
4.   Herd immunity is : Immunity level present in a
     population group (Chin 1999) immune people >
     susceptible people to a disease
     Relative Risk Ratio: used by
epidemiologists to identify populations
               at risk
• Relative risk =
Incidence in exposed group
Incidence in unexposed group
For examples smokers versus non smokers
  and heart conditions when ratio > 1 exposed
  group is at greater risk
      Natural History of Disease
1.  Susceptibility stage: Stage of pre pathogenesis : the
    disease is not present nor have individuals been exposed
2. Exposure stage: Invasion by causative agent but people
    are asymptomatic. Early pathogenesis
3. Onset stage: Signs and symptoms of the disease or
    condition develop. Clinical stage
4. Culmination stage or advanced disease stage: the
    disease or health condition is fully advanced and there is
    either return to health, a chronic form with disabling
    limitation or death.
The first two stages need primary prevention while the
    onset stage needs secondary prevention and the
    advanced disease stage needs tertiary prevention
 Types of Epidemiologic Studies
1. Cross-sectional study: exploring a health
   condition’s relationship to other variables in a
   specified population at a certain point in time
2. A retrospective study: Looking backward in
   time to find a relationship ( Is there an
3. A prospective study: Looking forward in time
   to find an relationship
4. An experimental study when possible (ethical
   clearance): investigator controls factors
   suspected of causing the condition and observes
   the results
      Sources of Information for
        Epidemiologic Studies
1. Existing Data:
  •   Vital statistics: birth, death rates
  •   Census data: age sex, type of housing, occupation,
      ethnicity, marital status, education
  •   Morbidity statistics on infectious and communicable
      diseases (reportable diseases)
  •   Disease registries (cancer)
2. Informal investigations
3. Scientific studies
     Methods in Epidemiology
1. Descriptive epidemiology: includes
   counts, definition of what one is counting
   is important also includes rates
• Rates are:statistical measures showing
   the proportion of people with a health
   problem among a population at risk
• Denominator is the population
  Prevalence and Incidence rates
1. Prevalence rate = Number of persons with a
    characteristic / Total population
Prevalence rate is: All people with a health
    condition in a given population at a given point
    in time
Period prevalence rate = prevalence rate over a
    period of time
2. Incidence rate = Number of new cases of a
    disease / total population at risk per unit of
Incidence rate is: All new cases of a disease during
    a given time
  Morbidity and Mortality rates
1. Morbidity rate: relative incidence of a
   disease in a population
2. Mortality rate: relative death rate in a
   given population at a given time
Please see figure 14.2 and keep it for future

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