Epidemiology: Disease Patterns and Analysis

• Epidemiology studies communicable and non-communicable diseases.
• The lecture aims to define epidemiology, list its uses, describe key features and basic measures, define its role in public health, and explain the natural history of disease and the epidemiologic triad.
• Epidemiology in Greek means: epi = upon, demos = the people, and ology = study of OR science
• Epidemiology definition: study of occurrence, distribution, and determinants of disease/health conditions in a defined population, applied to control health problems to promote/protect community health.
• Epidemiology's primary unit of concern is the population, while medicine focuses on the individual.
• The origin of epidemiology: started with the study of epidemics of communicable diseases.
• Then the scope expanded to endemic communicable and non-communicable infectious diseases.
• Modern epidemiology now includes chronic diseases, injuries, occupational/environmental health, and health-related behaviors.
• Epidemiology studies the distribution and determinants of diseases.

Division of Epidemiology

• Descriptive epidemiology focuses on occurrence and distribution (person, time, and place).
• Analytic epidemiology focuses on determinants (risk factors and causes).
• Descriptive studies describe the distribution of health-related states/events.
• Analytical studies provide the 'why' and 'how' of the states/events.
• Analytical studies test the etiological hypothesis and identify underlying causes or risk factors of diseases.

Disease Occurrence

• Endemic diseases constantly present in a geographic area.
• Epidemic diseases: Sudden unusual occurrence of disease in specific population during a specific time period.
• Outbreak: Localized epidemic.
• Pandemic: Epidemic in more than one country.
• Epidemo-endemic: Epidemicity of an endemic disease
• Sporadic: A disease that occurs infrequently and irregularly.
• Emergent diseases appear for the first time, with a less understood natural history.
• Re-emergent diseases: Diseases has disappeared and then reappear in a more severe form.

Disease Distribution

• Epidemiology is concerned with the frequency and pattern of health events in a population.
• Frequency refers to the number of health events and their relationship to the population size.
• Resulting rate aids in comparing disease occurrence across populations.
• Pattern refers to the occurrence of these events by time, place, and person.

Distribution in Time (when)

• Secular trends: Long-term trends in disease patterns influenced by changes in periodicity and natural history.
• Graphing annual cases or rates of a disease shows long-term or secular trends, which helps assess the direction of the disease and evaluate policy decisions.
• Periodic Change: Changes repeat themselves after a fixed interval of time, such as cyclical periodicity of 2 years.
• Seasonal pattern: Seasonal variation (e.g., influenza).

Distribution in place

• Involves mapping disease occurrence (using map spots, epidemic maps) and considering urban/rural differences to understand if risk is from the person, environment, or both.

Distribution with regard to person

• Focuses on factors such as age, sex, socio-economic status, psychological, physiological, genetics, lifestyle & occupation.

Determinants

• Determinants are risk factors that influence the occurrence of disease or health events.
• Illness occurs when there is the correct accumulation of risk factors in individuals.
• Risk factors, when present, increase the chance of a disease, which may be known or suspected.

Data Sources

• Census data
• Population estimates
• Vital records like birth and death registrations constitute data sources. Other registers are used such as:
  • Disease registers
  • Notification of infectious diseases
  • Hospital records
  • Special subgroup records
  • Morbidity surveys

Tools in Epidemiology

• Epidemiologic studies
• Other sources of data
• Measures for disease frequency and association
• Proportions, ratio and rate
• Analyses lead to community diagnoses of diseases. This is used to find preventive and control measures.

Proportion, Ratio & Rate

• Proportion: Kind of ratio where denominator is the total while the numerator is part of it.
• Ratio: Numbers that are expressed in relation to dividing one against another.
• Rates: Is the frequency of events during a certain time or period. eg. birth rates. special form of ratios that represent probable events

Measures of Disease Frequency

• Prevalence: number of people with a disease at a specific time
• Formula for Prevalence (P) is as follows: Number of having the disease at a specific time / Number of individuals in the population at that point of time (x1000)
• Incidence: Number of events/new cases. in define populations. within a specific time.
• Formula for Incidence (I) : Number of new cases at a specific time / Number of individuals in the population at that point of time

Questions for Quantifying Disease Frequency

• Questions Asked: "How many people in a given population have the disease at this point in time" determines Point Prevalence, "During a particular period of Time" determines Interval Prevalence.
• Answering, "How many people in a given population develop the disease during a given period of time?" determines Incidence.

Uses of Epidemiology

• To identify the cause and risk factors in the pathogenesis of a disease.
• To study the disease's natural history
• Community diagnosis
• Working of health services. To develop and evaluate health programs
• Understanding of full potential presentation of the disease

Additional uses of Epidemiology

• Identify new syndrome/ group of manifestation
• Clinical researches and Clinical picture
• Describe distribution/ the magnitude of health and disease problems in the population.
• Providing of data to assist with planning in order to prevent disease.
• Assess the health of a population.

Epidemiology in Public Health

• Epidemiology is essential for 5 functions within the public health system: Public health surveillance, Disease investigations, Case definition, Analytic studies and Program evaluation.

Natural History of Disease

• Single causation theory
• Multiple causation theory involves pre-pathogenesis and pathogenesis periods.
• Pathogenesis includes pre-clinical and clinical phases that occurs during Incubation Period
• Ending of the Infection includes: recovery, death, residual disability or Carrier state

Natural History stages of disease

• Exposure->Stage of susceptibility->Stage of subclinical disease->Stage of Clinical disease->Stage of recovery, disability or death

Triad Definition

• The epidemiologic Triad defines the "disease model". that consists of Agent/Host/Environment.
• The "Triad" is a result of dynamic system forces, such as: agents, environment and the host.
• Host: The agent is any susceptible organism invaded by an agent
• Agent: The element must be present to occur.
• Environment: All other factors that may prevent diseases, or the promoting of the disease (occurrences).

Causative Agents

• Biological agents: Viruses, Rickettsiae, Bacteria, Fungi, Protozoa
• Chemical Agents: Pesticides, Food additives, Pharmacologics
• Physical: Heat, Light, Radiation

Host Factors

• Resistance and Immunity:
  • Non specific resistance:
  • Specific Immunity:
    • 1. Natural Immunity.
    • 2. Artificial.
  • Herd Immunity
• Sociocultural factors: lifestyle factors/ Genetics Sociodemographic factors: age/education.

Herd Immunity Definition and Determinants

• Herd immunity is when diseases in a population are immune and can defend against certain diseases. A major component includes:
  • Resistance to infectious diseases
  • High proportions stemming from immunity (or previous exposure)
• Herd immunity-Determining factors:
  • Previous experience/ Immunization program.
  • Levels of persons from Immunization and health (susceptibility) Measuring Herd Immunity
• Indirect Measures: Morbidity levels
• Direct Measures: Measuring Immunoglobulin levels, etc.

Environmental Factors

• Physical factors: Geographical, Geological and Climatic/
• Biological and biotic considerations
• Social and cutural aspects.

Test your knowledge of epidemiology, including its focus, application, and key terminologies. Questions cover the definition of epidemiology, endemic diseases, analytic epidemiology, and disease distribution patterns. Also test concepts like disease outbreaks analysis and pandemic classification.