Epidemiology

PATTERNS OF DISEASE OCCURRENCE IN A POPULATION

Written by MicroDok

Disease occurrence in a population occurs in the following patterns:

  1. Sporadic pattern
  2. Endemic pattern
  3. Epidemic pattern
  4. Pandemic pattern.

SPORADIC PATTERN

A sporadic disease is a disease which occurs irregularly and affects only relatively few persons in a population. It is characterized by the following features:

  1. It occurs rarely
  2. It occurs at a low frequency
  3. It takes a long inter occurrence period
  4. It is not very predictable
  5. It’s duration of occurrence is short

Examples of sporadic diseases includes: Rabies, Tetanus, Ebola and Lassa fever.

 

Figure 3. Sporadic disease curve.

 ENDEMIC PATTERN

An endemic disease is a disease that is constantly present in a population but involves relatively few persons. This disease exists at all times in a population at a frequency that may be high, low, or moderate but with minor fluctuations in frequency with time. When the occurrence of the disease is low, it is termed hypoendemic. When the occurrence is high, it is termed hyperendemic and when it is moderate, it is termed mesoendemic. Examples of endemic diseases includes: typhoid fever, malaria, and tuberculosis.

Figure 4. Endemic diseases curve.

EPIDEMIC PATTERN

A disease is said to occur epidemically when the occurrence is higher than the endemic or expected occurrence level within a short period of time. It is a sudden increase in the occurrence of a disease above the expected level. This means that a disease can graduate from been endemic to being epidemic. If a disease occurs for the 1st time in a population, that outbreak could amount to an epidemic because it occurs beyond its expected level/frequency.

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Examples of epidemic diseases includes: Avian flu/influenza, severe acute respiratory syndrome (SARS), yellow fever, and cholera

There are three types of epidemic:

  1. Common source epidemic
  2. Point source epidemic
  3. Propagated epidemic.

COMMON SOURCE EPIDEMIC

Common source epidemic is an epidemic in which the affected hosts acquired the disease from the same source. For example, it can result from a single common contaminated source such as food (food poisoning) or water (Legionnaire’s disease).

Figure 5. Common source epidemic curve.

POINT SOURCE EPIDEMIC

Point source epidemic is an extension of common source epidemic from which the affected acquired the disease from the same source at the same point in time. Example is a disease acquired from a party.

Figure 6. Point Source Epidemic Curve.

Figure 6. Point Source Epidemic Curve.

PROPAGATED EPIDEMIC

In propagated epidemic, the frequency of the epidemic fluctuates with time and at sometimes with location. Propagated epidemic is very common with vector-borne diseases. Example of propagated epidemics includes: increase in mumps or chicken pox cases that coincides with new populations of sensitive children who arrive in class rooms each fall.

 

Figure 6. Propagated epidemic curve.

PANDEMIC DISEASE OCCURRENCE

A pandemic disease is an epidemic that has affected more than one continent of the world. It is a series of epidemics that affects several continents or countries of the world. Examples include avian influenza/flu, SARS and HIV (AIDS) infection. While pandemic involves more than one continent of the world, epidemic involves only one continent. The curve for a pandemic disease occurrence is the same thing as that for a propagated epidemic curve. 

Figure 7. Pandemic Epidemic Curve.

REFERENCES

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