EPIDEMIOLOGICAL INVESTIGATIONS Dr. Ganesh Veerasekar., MBBS., MPH (UK)., FRSPH (UK)., Epidemiologist, Department of Epidemiology, Kovai Medical Center and Hospital Email: drganeshv@knchhospitals.com

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

STUDY DESIGNS

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

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

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.

Case Control Studies Time http://phprimer.afmc.ca/sites/default/files/primer_versions/57579/primer_images/image3.jpg?1319221344

Cohort Studies http://phprimer.afmc.ca/sites/default/files/primer_versions/57579/primer_images/image2.jpg?1319221344

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

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

MEASURING EVENTS IN POPULATIONS

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

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

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

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

Point Source http://www.ph.ucla.edu/epi/snow.html

Propagated Source Grais et al. 2006 Accessed at www.msf.org.uk/UploadedFiles/Session_4_no_2__Grais_200804164026.ppt

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

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.

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

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

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?

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).

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

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

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

Epidemiological Software

EPI INFO 7 Provided Free of cost by CDC- Center for Disease Control and Prevention, Atlanta, USA Available for download at:- http://wwwn.cdc.gov/epiinfo/html/downloads.htm 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

Thank you