Basics of Study Design in Foodborne

Illness Outbreak InvestigationsInstructor Name

Job Title

Organization

Types of outbreak activities

…and lots and lots of talking to people!

Laboratory

Environmental

Epidemiology

Foodborne outbreak definition• A foodborne

disease outbreak is defined as an incident in which two or more persons experience a similar illness resulting from the ingestion of a common food

CDC definition

How are outbreaks detected?• Notification/complaints systems

- Independent complaints

- Reports of a group of ill persons

• Pathogen-specific surveillance- Case reporting by laboratories

Recent outbreak examples• [Instructor, add local examples that match those you

created in the Presentation PowerPoint]

• […Outbreak of listeriosis linked to cantaloupes (2011)—146 people ill and 30 deaths from 28 states…]

• […Gastrointestinal outbreak at a prison—45 cases (2013) …]

• […E. coli O157 outbreak linked to cucumbers served at Denver Metro sandwich shop (2013)—9 people ill…]

• […Campylobacter raw milk outbreak in Gunnison, CO (2013)—34 cases…]

• […Multistate outbreak of Salmonella associated with ground beef (2014)—12 Colorado cases…]

Two outbreak scenarios

Traditional (focal) scenario:

•Acute local outbreak

•Detected by group themselves

•High dose, high attack rate

•Local investigations

•Often a local food handling error

•Local solution

New (widespread) scenario:

•Diffuse widespread outbreak

•Detected by lab-based subtype surveillance

•Low dose

•Low attack rate

•Increase in "sporadic" cases

•Industrial contamination event

•Industry-wide implications

General characteristics Traditional (focal) outbreaks

• May or may not know pathogen involved

• Most ill people have not visited the doctor

• Can define a group of people who were potentially exposed o e.g., wedding, classroom, event attendee

• Some idea of the types of activities these people had in common

General characteristicsWidespread (newer) outbreaks• Pathogen is known

• Cases generally dispersed over several counties or states

• Investigation may be less timely

• At outset, probably have no idea what cases have in common

• After extensive case interviewing, may have a few hypotheses about exposure to test

Recent outbreak examples• [Instructor, add local examples that match those you

created in the Presentation PowerPoint]

• […Outbreak of listeriosis linked to cantaloupes (2011)—146 people ill and 30 deaths from 28 states…]

• […Gastrointestinal outbreak at a prison—45 cases (2013) …]

• […E. coli O157 outbreak linked to cucumbers served at Denver Metro sandwich shop (2013)—9 people ill…]

• […Campylobacter raw milk outbreak in Gunnison, CO (2013)—34 cases…]

• […Multistate outbreak of Salmonella associated with ground beef (2014)—12 Colorado cases…]

Recent outbreak examples• [Instructor, add local examples that match those you

created in the Presentation PowerPoint]

• […Outbreak of listeriosis linked to cantaloupes (2011)—146 people ill and 30 deaths from 28 states…]

• […Gastrointestinal outbreak at a prison—45 cases (2013) …]

• […E. coli O157 outbreak linked to cucumbers served at Denver Metro sandwich shop (2013)—9 people ill…]

• […Campylobacter raw milk outbreak in Gunnison, CO (2013)—34 cases…]

• […Multistate outbreak of Salmonella associated with ground beef (2014)—12 Colorado cases…]

Cohort study—mechanics• Attempt to interview entire group (or random

sample), regardless of illness status‒ e.g., Wedding attendees, school class, event attendees

• Use same questionnaire used for everyone in the group (ill and well)

• Questionnaire based on known activities/foods that group members may have done or eaten- Created from menus, early interviews etc.

• Includes questions about ‘unknown’ exposures- “Did you do anything else that I forgot to ask about?”

Cohort study—analysis

• Calculate attack rates an relative risks (RR)• RR = Incidence rate (exposed) versus

Incidence rate (unexposed)

‒ e.g., 85% of those who ate ham became ill compared with 10% of those who did not eat ham; 85/10 = Relative risk of 8.5

Interviewing for cohort studies

Advantages:

•Easier to locate people for interview (have names, phone #s)

•Participants often motivated because connected to outbreak

•They might be expecting your call

•Shorter, more focused questionnaire

•Events are often relatively recent so recall may be good

•Participants may offer information about additional exposures of which you were not aware

Challenges:

•Interviewees ask lots of questions

•Worried about becoming ill

•Sometimes are angry about having been exposed

•Interviewer must spend more time reassuring, answering questions, etc.

•Must be careful about confidentiality when interviewees ask about other who are ill

Case-control study—mechanics• Define a ‘case’ based on laboratory criteria (with

restrictions by time, place and person)

• Interview all cases (may be their 3rd or 4th interview!)

• Need to identify potential controls‒ People who had opportunity to be exposed and become ill‒ Age, sex, geographic matching is common

• Controls may be identified using ‒ Random or sequential digit dialing, anchored on case’s # ‒ Neighborhood controls (same neighborhood as case)‒ Restaurant reservation lists‒ Other creative methods (depending on outbreak)

Case-control study—mechanics• Questionnaire asks same questions about

exposure for cases & controls

• Because of matching often need to ask questions about age, sex, location up front

• Often exclude persons ill during outbreak period; therefore need to ask symptom questions upfront

• Need to define exposure period you are asking about for controls

‒ Might be several weeks in the past

Case-control study—analysis

• Odds of exposure (cases) versus Odds of exposure (controls)

- Odd ratio (not direct assessment of risk)

- Calculation complicated by adjust for matching on age, sex, or other factors

Interviewing for case-control studiesAdvantages:

•Fewer total people needed for interview

•Not based on cooperation of group leader to provide ‘the list’

Challenges:•Lots of work to identify potential controls

•Must apply inclusion/exclusion criteria (age, sex, screen for illness, etc.)

•Cold calling results in hang ups and rude answers

•‘How did you get my number?’•Longer, less-focused questionnaire

•Time frame for questionnaire might be several weeks in past

Goals for all study types• Complete, detailed information from all who are

interviewed

• Describe the illness among ill people (cases)

• Identify foods or other exposures associated with illness in a timely fashion

• Implement control measures now or in the future

… All begins with a good interview!