Notes on Descriptive Epidemiology (Part 2)

Introduction

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Overview of Descriptive Epidemiology

• Topics Covered: Time, place, and percent distribution of diseases.

Time Distribution of Diseases

Definition

• Time distribution describes when diseases occur.
• Trends can be:
  • Short-term fluctuations (e.g., epidemics)
  • Periodic fluctuations
  • Long-term fluctuations (secular trends)

Short-term Fluctuations

Epidemics

• Definition: Occurrence of cases in excess of normal expectancy in a community or region.
• Types of Epidemics:
  1. Common Source Epidemics:
     • Single Exposure (Point Source Epidemics):
       • Example: Food poisoning from a single contaminated food item in a hostel.
       • Epidemic Curve: Rise in cases after exposure, sudden peak, then decline to zero.
  2. Multiple Exposure Epidemics (Continuous Epidemics):
     • Example: Contaminated water from a well leading to continuous illness (e.g., cholera).
     • Epidemic Curve: Sharp rise, gradual decline.
  3. Propagated Epidemics:
     • Infectious diseases transmitted person-to-person (e.g., COVID-19).
     • Epidemic Curve: Gradual rise and slow decline.

Periodic Fluctuations

• Seasonal Trends: Certain diseases peak during specific seasons (e.g., flu in winter, measles in spring).
• Cyclic Trends: Diseases that appear in cycles, often every few years (e.g., measles).

Long-term Fluctuations (Secular Trends)

• Changes in disease occurrence over many years (decades).
• Can show increasing or decreasing trends.
• Examples:
  • Downward trend: Plague and cholera due to better control.
  • Upward trend: Increased diagnoses of non-communicable diseases (e.g., diabetes, heart disease).

Place Distribution of Diseases

Importance

• Geographical patterns provide clues to disease causes.

Types of Place Distribution

• International Variation: Differences in disease prevalence between countries (e.g., malaria in humid climates).
• National Variation: Differences within a country (e.g., endemic goiter).
• Rural vs. Urban Variations:
  • Rural: Zoonotic diseases, tetanus.
  • Urban: Road traffic accidents, diabetes.
• Local Distribution: Spot maps can illustrate case clustering in specific areas.

Migration Studies

• Investigate whether environmental changes account for disease variations.
• Compare disease incidence rates among migrants to original populations.

Person Distribution of Diseases

Description

• Characterizes diseases based on individual traits including:
  • Age: Different age groups are susceptible to different diseases.
  • Gender: E.g., diabetes more common in women, lung cancer more common in men.
  • Ethnicity: Certain genetic predispositions (e.g., Tay-Sachs in Jews).
  • Marital Status: Varies mortality rates.
  • Occupation: Risks associated with certain jobs (e.g., coal mine workers).
  • Social Class: Affects life expectancy and disease prevalence.
  • Behavior: Lifestyle choices impact disease risk (e.g., smoking, sedentary lifestyle).
  • Migration: Introduces new environmental threats and can spread diseases.

Conclusion

• Summary of time, place, and person distributions in epidemiology.
• Announcement for next part covering types of descriptive epidemiological studies.
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