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Outbreak of an infectious disease should be investigated according to a certain protocol. These steps are listed below:

1. Identification of the Outbreak:
Outbreak is defined as increase in occurrence of a disease above the background rate as determined by ongoing surveillance or retrospective data review.
Outbreak may be identified by clinical or laboratory staffs that start noticing an increase in number of cases or epidemiologist analyzing surveillance data.

2. Investigation of the outbreak proper:
For the actual investigation of outbreak, the first step is to define a case in terms of clinical and microbiological criteria.
The next step is making an epidemic curve or “Line-listing” which is to list the number of confirmed and probable cases chronologically in the form of a bar chart or table. This will identify the index case and any risk factors.
At the same time, isolates suspected to be outbreak strains should be saved and attempts should be made to isolate them from cultures of environment or food items. Once isolates from both sources have been procured, strain typing should be performed to confirm that both are the outbreak strains.

3. Interpretation of results:
Data from the outbreak investigation should be entered in database and analyzed.
This data analysis identifies risk factors. Hence, epidemiology can be described and measures to curtail the outbreak identified.

4. Institution of control measures:
Control measures are instituted as soon as risk factors are identified, e.g. in case of nosocomial infections, strict hand hygiene, vaccination or isolation measures are taken; while in foodbourne outbreaks, the offending food item is withdrawn from the market.
To assess the effectiveness of control measures, surveillance should be continued well after the outbreak has ended.

5. Reporting of results:
Ongoing and final results of the investigation should be communicated to the relevant authorities. Infection control team should be aware of ongoing developments to better control the outbreak, while director/ administrator of institute/ council must receive a finalized report as well. In some cases information of the outbreak is also revealed to the public.
Changes may be needed in infection control policies after reviewing the outbreak report and these should be implemented by the infection control committee.

Joveria Farooqi
Microbiologist, Karachi










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