1. EPIDEMIOLOGICAL INVESTIGATIONS
Dr. Ganesh Veerasekar., MBBS., MPH (UK)., FRSPH (UK).,
Epidemiologist,
Department of Epidemiology,
Kovai Medical Center and Hospital

2. Epidemiology
‘The study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to control of health problems’
Porta, M. (Ed.) (2008). A Dictionary of Epidemiology, 5th edn. New York: Oxford University Press

3. STUDY DESIGNS

4. Epidemiological Studies
Data on
groups
Data on
individuals
Experimental
Observational
Observational
Experimental
e.g. Community
Intervention
trial
Clinical
trial
Ecological
Cross-sectional
Measure frequencies (health & exposures) at a particular point in time
Cohort
Disease frequencies in relation to population exposures
Case-control
Compare disease frequencies in exposed and non-exposed individuals
Compare ‘exposures’ of those with and without disease

5. Ecological (correlation) Studies
Units of analyses = populations or groups
Disease occurrences are correlated with other traits in the populations or groups
Data obtained is linked from databases, cross sectional surveys etc...
Findings cannot be extrapolated to any individual- Ecological Fallacy

6. Cross-Sectional studies
Information is collected from individuals at one point in time
Used to: -
–Determine the distributions of variables within the population e.g. smoking, drinking etc.
–Estimate levels of common conditions with reasonably long durations e.g. long-term illness etc.

7. Case Control Studies Time

8. Cohort Studies

9. Study design – Usefulness
(● → more the better)
Ecological
Cross-
sectional
Case-
control
Cohort
Rare Diseases
●●●● -
●●●●● ●
Rare Exposure ●●
Multiple outcomes
Multiple exposures
●●●
Time between exposure & outcome
Measurement of incidence

10. Strengths & weaknesses
(● → less the better)
Ecological
Cross-
sectional
Case-
control
Cohort
Selection bias - ●●
●●● ●
Information (measurement) bias
Loss to follow-up
Confounding
Time required
Cost

11. MEASURING EVENTS IN POPULATIONS

12. Risk and Incidence Rate
Cumulative
Incidence (Risk)=
Incidence Rate=
Unlike risk, an incidence rate has ‘units’
Number of events per person-time (not people)
Number of new cases in time period
Number of people ‘at risk’ at the start of the period
Number of new cases
Total ‘person-time’ of observation

13. Prevalence Measures the burden of disease in a population
This is made up of all ongoing cases of disease
Point prevalence =
Number of existing cases at specified time
Total population
Period prevalence =
Number of existing cases at start of period + all new cases diagnosed within period
Average population over period

14. Rate Ratio/ Risk Ratio /Odds Ratio
The incidence of disease in exposed individuals
RR=
The incidence of disease in unexposed individuals
Relative risk > 1 Factor increases the risk of disease
Relative risk < 1 Factor decreases the risk of disease

15. An Epidemic Curve- Investigation
Point source:- eg. (Food poisoning; Water contamination)
All cases arise from single source
Period of exposure influences this
Sharp peak, gradual decline
Propagated:- eg. (Any close contact – Measles)
Transmission between cases and susceptibles
Gradually increasing peaks that widen and may coalesce
Peaks may or may not be identifiable

16. Point Source

17. Propagated Source
Grais et al Accessed at

18. Steps in Investigating An Outbreak
Prepare for field work
Establish the existence of an outbreak
Verify the diagnosis
Define and identify cases
Measure the frequency of adverse events in terms of time, place and person
Formulate hypotheses
Evaluate hypotheses
Refine hypotheses and employ additional investigations
Implement control and preventive measures
Communicate findings

19. EXERCISE
A group of soldiers belonging to the parachute regiment were on a training exercise that started 22 July. That afternoon, the soldiers met near the source of a mountain stream. After that, they split into smaller groups for the return march via different sites, some of which included other streams or lake shores. To supplement the pre-packaged rations some of the soldiers drank from the natural water sources. The following day all soldiers were back at barracks and on their usual diet.
The unit consisted of 122 soldiers, 100 of whom participated in the exercise. Twenty-five soldiers from another unit shared the same barracks and canteen.

20. Number of Cases of Gastrointestinal Illness by date of onset
Eighty-one soldiers from the parachute regiment developed febrile gastrointestinal illness at the dates and times given. None of the 25 soldiers in the other unit developed the illness during this period.
Thirty-nine of the soldiers with clinical illness had increased serology or a stool sample positive for Campylobactor jejuni.
The incubation period has a range of between 1 and 10 days
Number of Cases of Gastrointestinal Illness by date of onset
Date
Cases
July 23 7 24 43 25 17 26 5 27 1 28 3 29 2 30 31
August

21. As part of the investigation, the soldiers were grouped according to which sites they visited on the march and whether they drank the water with the following results:
Visited site
Drank water & Ill
Drank water & not Ill
No Drink & Ill
No Drink & not Ill
Mountain stream 78 9 3 10
Site A 4 12 5
Site C 2 6
Site H 26 22
Site J 15 1 55

22. Questions
A) Calculate the risk (cumulative incidence) of febrile gastrointestinal illness in the parachute regiment unit from the date of the exercise to the end of the day on 2 August.
B) Plot the epidemic curve and decide which type of outbreak this is (i.e. point source, extended source or propagated)
C) What was the likely date of exposure? Was it during the drill?
D) Which of the sites do you think was the source of the outbreak? How would you investigate further?

23. A) Calculate the risk (cumulative incidence) of febrile gastrointestinal illness in the parachute regiment unit from the date of the exercise to the end of the day on 2 August.
Risk: 81 ill of 122 soldiers in the unit = 66.4%
Including only the 100 soldiers on the exercise or all 147 that share the barracks are both acceptable options, providing you specify this in your answer – it depends on what you consider to be your unit of analysis (i.e. who is at risk).

24. B) Plot the epidemic curve and decide which type of outbreak this is (i.e. point source, extended source or propagated)
With the rapid increase in cases, this looks like a typical point source with all cases being infected simultaneously from a common source

25. C) What was the likely date of exposure? Was it during the drill?
last case date – longest incubation period=1 Aug – 10 days=22 July
first case date – shortest incubation period=23 July – 1 day=22 July
Most likely exposure was during the drill on the afternoon of 22 July

26. D) Which of the sites do you think was the source of the outbreak
D) Which of the sites do you think was the source of the outbreak? How would you investigate further?
As the relative risk associated with the mountain stream is much higher than the other sites, it is the most likely source.
Further investigations: Most important would be microbiological testing of water from the mountain stream. Epidemiologically, confidence limits on the RR and significance tests would be appropriate.
Visited site
Drank water
Attack rate
No Drink
Attack Rate
Relative Risk
Mountain Stream
90%
23%
3.89
Site A
71%
1.01
Site C
60%
1.19
Site H
93%
92%
Site J
94%
82%
1.14

27. Epidemiological Software

28. EPI INFO 7
Provided Free of cost by CDC- Center for Disease Control and Prevention, Atlanta, USA
Available for download at:-
USES:-
Quickly create event-specific forms by dragging and dropping templates.
Collect data concurrently in a multi-user environment.
Link records together to create exposure relationships.
Visualize data on case-based cluster maps

30. Thank you