Introduction to Epidemiology

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Introduction to Epidemiology
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Definition

• Epidemiology Epi (upon) demos (people) logy
  (study of)
• Epidemiology is a discipline that describes,
  quantifies, and postulates causal mechanisms for
  health phenomena in populations.
• Epidemiology is concerned with the distribution
  and determinants of health and diseases,
  morbidity, injuries, disability and mortality in
  populations.

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Aims Levels

• As the basic method of public health,
• epidemiology is concerned with efforts to
• Describe the health status of the population
• Explain the etiology of disease
• Predict the occurrence of disease
• Control the distribution of disease

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Foundations

• Human diseases do not occur by chance
• Factors that cause or contribute to diseases and
  injuries can be identified by means of systematic
  investigation

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Goals

• Improve the understanding of the natural history
  of disease and the factors that influence its
  distribution
• Intervention

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Historical Antecedents

• Hippocrates Environment disease causation
• Graunt Vital statistics in the mid 1600
• Snow Natural experiments to track cholera
  outbreak in London
• Koch Human disease - micro-organism / a disease

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Epidemiologic Approach

• Quantification
• Use of special vocabulary
• Interdisciplinary composition

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Epidemiologic Approach Special Vocabulary

• Epidemic Pandemic Endemic
• Epidemic Excessive occurrence of a disease.
• Epidemicity Relative to usual frequency of the
  disease in the same area, among the specified
  population, at the same season of the year.

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• Pandemic Epidemic on a worldwide scale
• Endemic Characterize a disease that is
  habitatually present in a particular geographical
  region. (Malaria and Cholera)
• Surveillance Systematic collection, analysis,
  interpretation, dissemination of data.

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Descriptive Variables for the Health of the
Community

• Demographic Social Variables
• Age and Sex
• Socioeconomic Status
• Family structure
• Racial, ethnic and religious composition
• Variables related to community infrastructure
• Availability of social and health services
• Quality of housing stock
• Social stability

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• Health Related Variables
• Homicide and suicide rates
• Infant Mortality Rate
• Mortality from Specific Condition
• Magnitude of chronic and infectious diseases
• Alcoholism and drug abuse rates
• Teenage pregnancy rates
• Birth Rate

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Population Dynamics and Epidemiology

• Three factors affect the size of populations
• Birth
• Death
• Migration
• When is the Po in equilibrium?,
• What is a fixed Po?,
• What is a dynamic Po?,
• When a Po is in steady state?

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Epidemiologic Triad Traditional Model of
Infectious Disease Causation

• The epidemiologic triangle recognized three
  factors in the pathogenesis of disease
• Host

Agent
Environment
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The Agent

• Agent must be present for an infection to occur
  Microbial agents
• Characteristics of Infectious disease agents
• Infectivity
• Pathogenicity
• Virulance
• Toxigenicity
• Resistance
• Antigenicity

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Agent Host

• After exposure from sub-clinical infection
  (inapparent) to active case of the disease.
• End Result Recovery, disability, disfigurement,
  death.
• Ability to fight infections, comprises 2 broad
  categories
• Non-specific defense mechanisms
• Disease specific defense mechanisms

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The Environment

• Domain in which the disease-causing agent may
  exist, survive or originate.
• Acts as a reservoir or niche that fosters the
  survival of infectious disease agents. The
  reservoir may be a part of the physical
  environment or may reside in animals or insects
  (vectors) or other human beings (human reservoir
  host)
• External Environment physical, biologic, social,
  economic components

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Means of Transmission Directly or Indirectly
from reservoir

• Direct Transmission Spread of infection through
  person to person contact
• Portals of exit sites where infectious agents
  may leave the body (respiratory passages, the
  alimentary canal, the openings in the
  genitourinary system, and skin lesions. Also
  through insect bites, the drawing of blood,
  surgical procedures and accidents)
• Portal of entry respiratory system (influenza,
  cold), the mouth digestive system (hepatits A),
  mucous membranes or wounds in the skin.

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• Indirect Transmission
• through an intermediary source
• Vehicle contaminated H2O, infected blood on used
  hypodermic needles.
• Fomites inanimated objects doorknob or
  clothing.
• Vectors animate, living insect or animal that is
  involved with transmission of the disease agent.

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Some Measures Used in Epidemiology

• Counts Number of individuals with a certain
  disease/exposure
• Proportions (P)
• Ones status in relation to group
• Ratio A fraction in which there is not
  necessarily any specified relationship between
  the numerator denominator.
• Rate A ratio in which time forms part of the
  denominator
• Prevalence of existing cases of a disease /
  health condition in a Po at some designated time
  (specified or not).

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• Infant Mortality Rate (IMR) risk of dying during
  the first year of life among infants born alive
  (aged 0-365 days)
• Perinatal MR measures risk of dying among late
  fetal death infant deaths within the 7 days of
  birth. (28 weeks gestation to 7 days of life)
• Noenatal MR measures risk of dying among
  infants younger than 28 days
• Postneonatal MR measures risk of dying among
  older infants (28 days to 12 months)

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• Incidence Rate (IR) rate of development of new
  cases
• Crude Mortality Rate (CMR)
• of annual death Po
• Maternal Mortality Rate MMR number of maternal
  death ascribed to childbirth
• Age-Specific Mortality Rate ASMR
• deaths in specific group per total number of
  people in that age group
• Proportional Mortality Rate PMR of death
  within a Po due to a specific disease

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Specialized Epidemiological Measures

• Years of Potential Life Lost (YPLL)
• Measures of years lost due to a cause
• It is based on projected life expectancy
• Dependency Ratio (DR)
• Proportion of dependents in a society to the
  number of adults (19-63)
• DR under 18 and over 64
• between 18 and 65