Decriptive epidemiology | Anatomy2Medicine
Descriptive epidemiology provides a way of organizing and analyzing these data in order to understand variations in disease frequency geographically and over time, and how disease (or health) varies among people based on a host of personal characteristics

Decriptive epidemiology

    • Changes in occurrence of a disease over long periods of time are known as –  

 

  • Secular trends (MCQ)
  • Time distribution of disease:

 

      • Short term fluctuations:
        • Epidemic:
          • Occurrence of no. of cases of a disease ‘clearly in excess of normal expectancy (NE)’
          • Normal expectancy is derived by looking at average of no. of cases of the disease in previous 3-5 years in that geographical area.
          • If NE = zero, ‘even one case is considered epidemic’

 

  • Statistically speaking, epidemic is when no. of cases ‘exceed twice the standard deviation’
  • No. of cases > Mean + 2SD ( > + 2)

 

          • Occurrence of a new disease in a population (as NE = Zero)
          • Reoccurrence of an eliminated/ eradicated disease in a population (as NE = Zero)

 

  • Long term fluctuations (SECULAR TRENDS):

 

      • Implies changes in occurrence of a disease (progressive increase or decrease) over a long period of time, generally several years or decades
      • Is the consistent tendency to change in a particular direction or a definite movement in one direction.

        • Communicable diseases (Poliomyelitis, Diphtheria, Pertussis) are reducing in India in past few decades
        • Non-communicable diseases (Diabetes, Hypertension, Obesity) are increasing in India in past few decades

 

  • Types of epidemics:

 

      • Common-source epidemics:
        • Single exposure or ‘Point source’ epidemics ii. Continuous or multiple exposure epidemics

 

  • Propagated epidemics:

 

        • Person-to-person
        • Arthropod vector
        • Animal reservoir

 

  • Slow (modern) epidemics

 

        • periodic fluctuations:
        • seasonal trends’.
      • Is seasonal variation/fluctuation in occurrence of a disease
        • Measles (early spring)
        • Upper respiratory infections (winters)
        • Gastrointestinal infections (summers)

 

  • Cyclical trends:

 

      • Is occurrence of a disease in cycles spread over short periods of time, which may be days, weeks, months or years
      • Measles (every 2-3 years)
      • Rubella (every 6-9 years)
      • Influenza pandemics (every 10-15 years)
  • Point source epidemic  (MCQ)
      • Epidemic curve rises and falls sharply
      • Clustering of cases within a short period of time

 

  • All cases usually develop within one incubation period
  • Single exposure or ‘Point source’ epidemics: (MCQ)

 

      • Sharp rise and sharp fall’ in no. of cases

 

  • Clustering of cases’ in a narrow interval of time

 

    • All ‘cases develop within one incubation period’ of the disease
    • Examples: Food poisoning, Measles, Chicken pox, Cholera, Bhopal gas tragedy
  • ‘Common source’, continuous or repeated exposure epidemics: (MCQ)
    • ‘Sharp rise’ in no. of cases
    • Fall in no. of cases is interrupted by ‘Secondary waves/peaks’
    • Examples:
    • Contaminated well in a village
    • nationally distributed brand vaccine or food
    • prostitute in a gonorrhoea outbreak

      • Legionnaire’s disease outbreak in Philadelphia (1976)

 

  • Propagated epidemics: (MCQ)

 

      • ‘Gradual rise and gradual fall’ over a long time (Tail off)

 

  • Results from ‘person-to-person transmission’

 

      • Transmission continues till no. of susceptibles is depleted or susceptibles are no longer exposed to infected individuals

 

  • Speed of spread depends upon herd immunity, secondary attack rate, opportunities for contact
  • Examples: HIV, tuberculosis

 

  • Epidemic curve: Is drawn between no. of cases in epidemic and time elapsed (time distribution of epidemic cases)
      • Endemic:
        • Constant presence of a disease or infectious agent in a defined geographical area
        • Is the ‘usual or expected frequency’ of a disease in a population
        • Hyper-endemic:

  • Constant presence of a disease or infectious agent at high incidence/ prevalence AND affects all age groups equally
      • Holo-endemic;
        • A high level of infection beginning early in life AND affecting most of the children population

  • Common Single liner MCQs for MD Entrance
      • Bhopal gas tragedy is an example of  Point source epidemic
      • Seasonal trend is  Seasonal variation of disease occurrence may be related environmental conditions
      • Descriptive epidemiology includes Disease ,Time , Place
      • Pointed Source epidemic :
        • A graph shows an uniform curve with no secondary curves
        • Secondary Waves are not seen
        • All the cases occur simultaneously
        • Plateau is not seen
      • Food poisoning is art example of:    Common source, single exposure epidemic
      • Secular trend of disease refers to occurrence of: Random occurrence of disease
      • Disease occurs in cycles over short period of time Cyclic trend