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DIFFERENT TIME TRENDS:

 The pattern of disease may be described by the time of its occurrence i.e. by week, month, year

      A. SHORT TERM FLUCTUATIONS: Short term fluctuation in the occurrence of a disease is an epidemic.

        1. Common source epidemics:

           a. Single exposure or “point source” epidemics:

              The epidemic curve rises & falls rapidly with no secondary waves.

              Epidemics tends to be explosive clustering of cases within a narrow interval of time.

              All the cases develop within one incubation period of disease. Example- epidemic of food poisoning.

           b. Continuous or multiple exposure epidemics:

              Exposure from the same source- may be prolonged, continuous, repeated or intermittent.

              May be initiated from a common source & continue as a propagative epidemic. Example- a common source in gonorrhoea outbreak in fact her clients over a period of time.

        2. Propagated Epidemics:

           a. Person to person:

              Transmission of infectious agent.

              Speed of spread depends upon herd immunity. Opportunities for contact & secondary attack rate. Example- epidemics of Hepatitis A & Polio.

           b. Arthropod vector: Malaria by female anopheles mosquito.

           c. Animal reservoir: dog is reservoir of rabies.

        3. Slow (modern) epidemics:

      B. PERIODIC FLUCTUATIONS:

        1. Seasonal trends:

              Seasonal variation related to environmental condition- temperature, humidity, rainfall, overcrowding, life cycle of vector. Example- measles & varicella in early spring.

        2. Cyclic trend:

              Occurs in cycle spread over short periods of times- days, weeks, months or years. Example- measles in pre-vaccination era appeared in cycles with major peaks every 2-3 years.

      C. LONG TERM OR SECULAR TRENDS:

            Changes in the occurrence of disease over a long period of time, generally several years or decades. Example- coronary heart disease, diabetes.

INTERPRETATION OF TIME TRENDS:

  1. Which disease are increasing, which decreasing & which are the emerging health problems & of effectiveness of measures to control old ones.
  2. Formulation aetiological hypotheses & seeks explanations whether these changes were due to changes in the aetiological agent or variations in diagnosis, reporting, case fatality or changes in age distribution or some other determinants, specific & non-specific (changes in quality of life, socio-economic status & personal habits).

        Example- “Time clustering” of cases of adeno carcinoma of vagina in young women led to incrimination of its cause.

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