Epidemic Investigation and Management : Type Of Epidemic and Level Of Disease
Levels of Disease Occurrence
Diseases occur in a community at different levels at a particular point in time. Some diseases are usually present at a predictable level. This is called the expected level. But sometimes they occur in excess of what is expected. The examples of expected level are endemic and hyper endemic. When the disease occur as epidemic, outbreak, and pandemic it is considered as excess of what is
expected.
Definition of terms related to the level of occurrence of disease
1. Endemic: Presence of a disease at more or less stable level. Malaria is endemic in the lowland areas of US.
2. Hyper endemic: Persistently high level of disease occurrence.
3. Sporadic: Occasional or irregular occurrence of a disease. When diseases occur sporadically they may occur as epidemic.
4. Epidemic : The occurrence of disease or other health related condition in excess of the usual frequency in a given area or among a specific group of people over a particular period of time.
5. Outbreak : Epidemics of shorter duration covering a more limited area.
6. Pandemic: An epidemic involving several countries or continents affecting a large number of people. For example the worldwide occurrence of HIV/AIDS is a pandemic.
The definition of epidemic indicates that the term can have a broad meaning. It may include any kind of disease or injury including non – infectious diseases. There is no general rule about the number of cases that must exist for a disease to be considered an epidemic. If the number of cases exceeds the expected level on the basis of the past experience of the particular population, then it is an epidemic.
It is important to note that this level varies for different diseases and different circumstances. An epidemic may cover a small area within a city, or an entire nation or may have a worldwide distribution. It may encompass any time period ranging from few hours (chemical intoxication, bacterial food poisoning), a few weeks (influenza, hepatitis) to several years (AIDS). A disease that remains epidemic over many years eventually may be considered endemic.
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Types of epidemics
Epidemics (outbreaks) can be classified according to the method of spread or propagation, nature and length of exposure to the infectious agent, and duration.
1. Common Source Epidemics:- Disease occurs as a result of exposure of a group of susceptible persons to a common source of a pathogen, often at the same time or within a brief time period.
When the exposure is simultaneous, the resulting cases develop within one incubation period of the disease and this is called a point source epidemic. The epidemic curve in a point source epidemic will commonly show a sharp rise and fall. Food borne epidemic following an event where the food was served to many people is a good example of point source epidemic. If the exposure to a common source continues over time it will result in a continuous common source epidemic.
A waterborne outbreak that spreads through a contaminated community water supply is an example of a common source epidemic with continuous exposure. The epidemic curve may have a wide peak because of the range of exposures and the range of incubation periods.
2. Propagated/ Progressive Epidemics :- The infectious agent is transferred from one host to another. It can occur through direct person to person transmission or it can involve more complex cycles in which the agent must pass through a vector as in malaria.
Propagated spread usually results in an epidemic curve with a relatively gentle upslope and somewhat steeper tail. An outbreak of malaria is a good example of propagated epidemic.
When it is difficult to differentiate the two types of epidemics by the epidemic curve, spot map (studying the geographic distribution) can help.
3. Mixed Epidemics :- The epidemic begins with a single, common source of an infectious agent with subsequent propagated spread. Many food borne pathogens result in mixed epidemics.
Investigation of an Epidemic
The purpose is to determine the specific cause or causes of the outbreak at the earliest time and to take appropriate measure directed at controlling the epidemic and preventing future occurrence. The following questions should be answered when investigating an epidemic.
- What is the etiological agent responsible for the epidemic?
- What is/are the predominant modes of transmission?
- What specific source/s of disease can be identified?
E.g. human carriers, breeding sites for vectors, etc.
- What specific practices or environmental deficiencies have contributed to the outbreak? E.g. improper food handling, human made breeding sites for mosquitoes.
- What is the chain of events that led to the outbreak?
E.g. accumulation of susceptible hosts in an area.
Uncovering outbreaks
Outbreaks are detected in one of the following ways:
a. Through timely analysis of routine surveillance data
b. Report from clinician.
c. Report from the community, either from the affected group or concerned citizen.
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