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