Game 19 · Public health and research practice
Where did this outbreak come from?
Steps of outbreak investigation and reading the epidemic curve.
Read with: Data presentation, Study designs19 DNB questions
Play
Investigate the outbreak
Each round is one part of an outbreak investigation. Slide a marker back from the epidemic curve to the probable time of exposure, find the food that carried the agent from an attack rate table, put the steps of the investigation in order, or name the type of epidemic curve. You have three lives.
Loading the first round…
All agents (Agent X, Agent Q and so on), meals and case counts are made up for teaching. They are not real organisms.
The steps
The 13 steps of an outbreak investigation
CDC's Principles of Epidemiology in Public Health Practice (3rd edition, Lesson 6) lists these steps. They are in a conceptual order. In practice several are done at the same time, and control measures should start as early as possible.
- Prepare for field work.
- Establish the existence of an outbreak: compare the observed number of cases with the expected number, usually from the previous few weeks or months.
- Verify the diagnosis.
- Construct a working case definition: clinical criteria, restricted by time, place and person. A confirmed case usually has laboratory verification. A probable case has typical clinical features without it.
- Find cases systematically and record information (a line list).
- Perform descriptive epidemiology: time (epidemic curve), place (spot map) and person.
- Develop hypotheses.
- Evaluate hypotheses epidemiologically: cohort study with attack rates for a defined group such as a meal, or a case-control study.
- As necessary, reconsider, refine and re-evaluate hypotheses.
- Compare and reconcile with laboratory and/or environmental studies.
- Implement control and prevention measures.
- Initiate or maintain surveillance.
- Communicate findings.
CDC defines an epidemic as an increase, often sudden, in the number of cases above what is normally expected in that population in that area. An outbreak has the same definition but is often used for a more limited area. A pandemic is an epidemic spread over several countries or continents.
Reading the curve
The shape of the epidemic curve tells you how people were exposed
| Feature | Common source | Propagated |
|---|---|---|
| Exposure | Everyone exposed to one source | Each case infects the next |
| Shape | Point source: steep upslope, more gradual downslope. Continuous: a plateau. Intermittent: irregular bursts | Progressively taller peaks about one incubation period apart |
| Duration | Point source: all cases within one incubation period | Cases over more than one incubation period |
| Example | A contaminated dish at a wedding lunch | Measles in an unvaccinated community |
For a point-source outbreak, CDC gives two ways to estimate the probable period of exposure. Identify the peak or the median case and count back one average incubation period. Then start at the earliest case and count back the minimum incubation period. Ideally the two times are similar. Widen the estimate on either side, because incubation periods vary.
How to draw the curve itself (touching bars, a sensible time unit) is in Data presentation.
Finding the vehicle
Food-specific attack rates
Attack rate = ill ÷ people exposed during the outbreak
Risk ratio = attack rate in those who ate the food ÷ attack rate in those who did not
- Attack rate
- the incidence proportion over the short epidemic period. Despite its name it is a proportion, not a rate per unit time
- Risk ratio
- also called relative risk. Above 1 means eaters were more likely to fall ill
The suspect food has three features: a high attack rate among those who ate it, a low attack rate among those who did not (so the risk ratio is high), and most of the cases ate it. A dish usually eaten together with the vehicle (raita with biryani) can show a raised risk ratio too. The vehicle has the highest ratio and explains most cases.
Risk ratios and their confidence intervals are covered in Study designs.
Writing the answer
How to write this in a 10-mark DNB answer
- Define epidemic and outbreak (more cases than expected), and say how you establish that one exists.
- List the steps in order, as above, and note that control measures start early, not at the end.
- Write a case definition for the disease in the question, with time, place and person, and confirmed and probable cases.
- Draw an epidemic curve: point source for a food-poisoning question, propagated for measles. Mark how you would estimate the time of exposure.
- For a food-borne outbreak, draw a food-specific attack rate table with the risk ratio, and name the three features of the vehicle.
- End with control measures for the specific disease, surveillance, and the contents of the written report.
The one thing to remember
The epidemic curve tells you how people were exposed. The attack rate table tells you what exposed them.
Past papers
DNB questions this game helps you answer
Questions from NBE theory papers for DNB, DrNB and Diploma courses, most often asked first. The tag after each question is the specialty and the year it was set.
- ×1M pox virus outbreak investigation.
- ×1Steps in investigation of an epidemic.
- ×1Steps involved in investigation of a outbreak.
- ×1Steps in investigation of an Epidemic.
- ×1Steps of outbreak investigations and role of molecular typing.
- ×1Discuss briefly about the role of Microbiologist in outbreak investigations.
- ×1What is basic reproduction number? Describe its application in the investigation of an outbreak giving an example.
- ×1Differentiate between ‘epidemic’ & ‘pandemic’. As a hospital administrator, what steps will you take in investigation of an epidemic outbreak?
- ×1Define an epidemic. How will you investigate an outbreak of an epidemic in your area?
- ×1How will you investigate an outbreak in the NICU?
- ×1Investigation of an epidemic of measles in a city.
- ×1How will you investigate an outbreak of Pseudomonas infection in your endoscopy unit?
- ×1Investigate an outbreak of cases of diarrhoea and vomiting after consumption of a common meal by a large number of people in a social function in a community.
- ×1State the steps to investigate an outbreak of diarrhoea in a community. What control measures should be recommended?
- ×1Steps in an investigation of outbreak of food poisoning in a school.
- ×1What are the steps of an outbreak investigation in case of healthcare associated infections?
- ×1You have investigated an outbreak of Japanese Encephalitis in your area. What would be the components of your report, please explain briefly.
- ×1Steps in the investigation of an epidemic.
- ×1Describe the steps involved in the investigation of an outbreak of leptospirosis.
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