Outbreak Investigation: Discussion Group

Outbreak Investigation: Discussion Group

Ebbing Lautenbach, MD, MPH

Assistant Professor of Medicine and Epidemiology

Senior Scholar, CCEB

Associate Hospital Epidemiologist, HUP

Initial Call

• Late June, 1997: Calls from 4 MDs reporting 6 patients with bloody diarrhea and E. coli O157:H7 infection

Initial Call

• Late June, 1997: Calls from 4 MDs reporting 6 patients with bloody diarrhea and E. coli O157:H7 infection

• 1 day later: Call from Michigan Department of Community Health (MDCH)

• Increase in laboratory reports of E. coli O157:H7– June 1997 = 52

First….

Steps in Outbreak Investigation

1. Verify the Diagnosis

1. Verify the Diagnosis• Escherichia coli O157:H7 first identified as a human

pathogen in 1982 in the US • Sporadic infections and outbreaks since reported from

many parts of the world (e.g., N. America, Western Europe, Australia, Asia, and Africa)

• Cattle are the primary reservoir for E. coli O157:H7• Implicated foods are typically those derived from cattle

(e.g., beef, hamburger, raw milk); • Infection has also been transmitted through contact with

infected persons, contaminated water, and other contaminated food products.


• Infection with E. coli O157:H7 is diagnosed by detecting the bacterium in the stool.

• Only recently has E. coli O157:H7 infection become nationally notifiable in many parts of the U.S.




1. Verify the diagnosis

2. Confirm the outbreak

Trends in MDCH E. Coli O157 Cases

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# Cases

1996 1997

What could account for the increase in cases?

What could account for the increase in cases?

Real increase• Increase in population size• Changes in population

characteristics• Random variation• Outbreak

Artificial increase• Increased cx of stools• New testing protocol• Contamination of cxs• Changes in reporting

procedures

Initial Investigation

• No substantial changes in population size

• No appreciable changes in the population characteristics

• No laboratory based changes– Surveillance / testing – Reporting protocol

Initial Investigation

• Any other way to see if there is a relationship between these E. coli isolates?

Molecular Epidemiology

• DNA fingerprinting

• Pulsed Field Gel Electrophoresis (PFGE) most common in outbreak investigations

• A cluster of isolates with the same PFGE pattern suggests they arose from the same parent (same source)

• Still need an epidemiologic investigation

PFGE pattern of E. coli Isolates

Molecular Epidemiology of E. coli Isolates

• 17 of the first 19 E. coli O157:H7 isolates from June-July were indistinguishable.

• They did not match any fingerprints from a convenience sample of isolates from patients with E. coli O157:H7 infection before May.







3. Case definition

3. Develop a Case Definition

• Incubation period for E. coli O157:H7 ranges from 3-8 days with a median of 3-4 days.

• The infection often causes severe bloody diarrhea and abdominal cramps, but can also cause a non-bloody diarrhea or result in no symptoms.

• In some persons, particularly children under 5 years of age and the elderly, infection can be complicated by hemolytic uremic syndrome (occurs in about 2-7% of infections)

Case Definition?

Case Definition

• Outbreak investigation definition:

1. diarrhea (>3 loose bowel movements a day) and/or abdominal cramps

2. resident of Michigan

3. onset of symptoms between June 15 and July 15

4. stool culture yielding E. coli O157:H7 with the outbreak strain PFGE pattern.

• Advantages? Disadvantages?

Case Definition• Advantages:

– Lab confirmation increases specificity of case definition• Reduces misclassification; maximizes power to detect source.

• Disadvantages: – Lab confirmation

• Excludes patients who did’nt see MD, were not cxd, or cxd without PFGE. • Decreases the sensitivity of the case definition • Possibly leads to a misrepresentation of case characteristics.

– Limiting cases to Michigan residents • excludes visitors who became infected; inhibits recognition of extension of outbreak into other states.

– Dates reasonable? • Need more information• Could limit the number of secondary cases included in the study that could interfere with identification

of the initial source of the outbreak.


• Verify the diagnosis• Confirm the outbreak• Case definition


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive Epidemiology

Characterization of Cases

Age group(years)

GenderTOTAL

Male Female

0-9 2 (17%)* 2 (8%) 4 (11%)

10-19 2 (17%) 3 (12%) 5 (13%)

20-39 3 (25%) 9 (35%) 12 (32%)

40-59 2 (17%) 8 (31%) 10 (26%)

60+ 3 (25%) 4 (15%) 7 (18%)

TOTAL 12 (101%) 26 (101%) 38 (100%)

* percentages refer to column totals.

Of the initial 38 persons who met the case definition, 26 (68%) were female with a median age of 31 years.

Table 1. Age group and gender distribution for persons with E. coli O157:H7 infection (with PFGE pattern), Michigan, June 15 - July 15, 1997. (N=38)

APPENDIX 1Age group(years)

GenderTOTAL

Male Female

0-<1 5 (3%) 5 (3%) 10 (3%)

1-9 77 (48%) 77 (43%) 154 (45%)

10-19 36 (22%) 18 (10%) 54 (16%)

20-29 10 (6%) 20 (11%) 30 (9%)

30-39 6 (4%) 12 (7%) 18 (5%)

40-49 7 (4%) 5 (3%) 12 (4%)

50-59 7 (4%) 17 (10%) 24 (7%)

60+ 14 (9%) 24 (13%) 38 (11%)

TOTAL 162 (100%) 178 (100%) 340 (100%)

).

Age group(years)

GenderTOTAL

Male Female

0-9 2 (17%)* 2 (8%) 4 (11%)

10-19 2 (17%) 3 (12%) 5 (13%)

20-39 3 (25%) 9 (35%) 12 (32%)

40-59 2 (17%) 8 (31%) 10 (26%)

60+ 3 (25%) 4 (15%) 7 (18%)

TOTAL 12 (101%) 26 (101%) 38 (100%)

.

MI Cases

FoodNet Data

Michigan counties

The 38 cases of E. coli O157:H7 infection meeting the investigation case definition were reported from 10 counties in the lower peninsula of Michigan.

Epidemic CurveFigure 3. Date of illness onset for persons with E. coli O157:H7 infection and the outbreak PFGE pattern, MI, June 15 - July 15, 1997. (N=38)

Epidemic Curves

• How to set it up

• What it tells you– Mode of transmission

• Propagated

• Common source

– Timing of exposure– Course of exposure

Epidemic Curves

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Propagated source: single exposure, no secondary cases (e.g., measles)

Epidemic Curves

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Propagated source: secondary and tertiary cases (e.g., hepatitis A)

Epidemic Curves

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Common source: point exposure (e.g., salmonella)

Epidemic Curves

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Common source: Intermittent exposure (e.g., contaminated blood product)

Epidemic CurveFigure 3. Date of illness onset for persons with E. coli O157:H7 infection and the outbreak PFGE pattern, MI, June 15 - July 15, 1997. (N=38)


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive Epidemiology• Develop a hypothesis

Developing a Hypothesis

Ask questions!!

But of whom….

And when...

Determining the Probable Period of Exposure

• Mean/Median incubation period

• Minimum/maximum incubation period

Estimating date of exposure

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One incubation period

Rubella = 18 daysP

roba

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Estimating date of exposure

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Maximum incubation

21 days

Minimum incubation

14 days

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E. Coli Epidemic CurveFigure 3. Average incubation period = 4 days ( range 3-8 days)

Focus of Questions

Focus of Questions• demographic information

• clinical details of the illness with date of onset, duration, and severity of symptoms– visits to health care providers or hospitals, and laboratory results

• a complete food history in the last 7 days

• water exposure in the last 7 days (e.g., drinking water, exposure to recreational waters)

• exposure to other ill persons in the last 7 days

• exposure to children in day care in the last 7 days

• exposure to a farm or farm animals in the last 7 days

• travel outside the immediate area in the last 7 days

Interview ResultsVariable Cases (n=38) Female 26 (68%) Med Age 31 Rec water exposure 13 (34%) Other Ill person 6 (16%) Day care 18 (47%) Farm 2 (5%) Fair 18 (47%) Travel 9 (24%) Hamburger 25 (66%) Meat 22 (58%) Milk 32 (84%) Alfalfa sprouts 19 (50%) Lettuce 24 (63%)

Findings Thus Far

• Cases are spread over 10 counties – No uniform attendance at any common event

– Onset of symptoms among known cases extends over approximately one month.

• The median age of patients is 31 years (range 2-76); 68% of cases are among females.

• Factors present in over 50% of cases: – Female, milk, hamburger, lettuce, alfalfa sprouts

– Role of fair attendance, water exposure?

Hypothesis?

Hypothesis of Investigators

• Lettuce and/or alfalfa sprout consumption is associated with E. coli infection


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology• Develop a hypothesis


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology• Develop a hypothesis• Test the hypothesis

Pick a Control Group

Controls Selected

• 2 controls selected for every case

• Matched to the case by:

– Age group • (0-<2 years, 2-<5 years, 5-<12 years, 12-<18

years, 18-<60 years, and 60+ years)

– Sex

Methods to Identify Controls

• Random digit dialing

• Neighborhood controls

• Other patients of same physician

Selection of Controls

• The investigators identified controls for the study using random digit dialing.

• Exposure information among cases was collected for the 7 days before onset of illness.

• For controls, exposure information was collected for the 7 days before the interview and for the 7 days before the onset of illness in the matching case.

• Twenty-seven case-control sets were interviewed; the remaining case-patients could not be reached.

Interview ResultsVariable Cases (n=27) Controls (n=54) Female 18 (67%) 36 (67%) Med Age 31 31 Rec water exposure 9 (33%) 21 (39%) Other Ill person 6 (22%) 9 (17%) Day care 16 (59%) 33 (61%) Farm 2 (7%) 2 (4%) Fair 12 (44%) 24 (44%) Travel 8 (30%) 13 (24%) Hamburger 17 (63%) 36 (67%) Meat 14 (52%) 26 (48%) Milk 21 (78%) 44 (81%) Alfalfa sprouts 15 (56%) 4 (7%) Lettuce 18 (67%) 34 (62%)

E. coli and Alfalfa Sprouts?

VariableCases Controls OR (95%CI)

15 (56%) 4 (7%) 25 (4-528)

No other food item was significantly associated with

illness.


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology• Develop a hypothesis• Test the hypothesis


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology• Develop a hypothesis• Test the hypothesis• Refine hypothesis / Execute additional studies

Refine Hypothesis/Additional Studies

• What control measures might you consider at this point?

• What further studies might you do?

Additional Studies

• Culture implicated sprouts• Traceback study

– distributor, processor, and producer; examination of the chain of production of the sprouts from the farm to the table

• Applied research on E. coli– research on alfalfa sprouts and survival/growth of E.

coli O157:H7 (e.g., the ability of E. coli to survive and grow on alfalfa seeds/sprouts at each step of the production process).

Traceback Studies

• Often necessary to identify sources of contamination and quickly limit a public health threat by removing these sources.

• Ascertain the distribution and production chain for a food product to facilitate effective recall.

• Clarify the point or points at which the implicated food was likely to have become contaminated

Traceback Results

Follow up• The implicated seed lot was a blend of 5 lots from fields of four

farmers and was harvested between 1994 and 1996. The seed processor and the farmers were located in Idaho.

• Inspection of the alfalfa fields revealed three possible sources of contamination: cattle manure, irrigation water, and deer feces. – Manure is not normally applied to alfalfa fields in Idaho– Cattle feed lots were common in this area and the alfalfa fields of one

farmer were adjacent to a feed lot. – Manure may have leaked or been illegally dumped onto the alfalfa fields or

run-off water from neighboring fields, contaminated by manure, may have been used to irrigate the alfalfa fields.

– In addition, three of four farmers occasionally saw deer in their fields and one field was situated next to a wildlife refuge.

Steps in Outbreak Investigation• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology• Develop a hypothesis• Test the hypothesis• Refine hypothesis / Execute additional studies


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology• Develop a hypothesis• Test the hypothesis• Refine hypothesis / Execute additional studies• Implement control and prevention measures

What interventions are Needed?

• 2 issues:

– 1) the immediate problem with this implicated lot of seed

– 2) the larger issue of seed sprouts as vehicles for pathogenic

What interventions are Needed?• Implicated seed lot

• all remaining seeds and alfalfa sprouts from the implicated lot should be removed from the market. – Persons who have purchased sprouts from the implicated lot should be

instructed to destroy any remaining sprouts or return them to the store at which they were purchased.

• The producers of these particular seeds should be informed of the need to protect alfalfa and other seeds used in sprouting from contamination during growing, harvesting, and packing.

• Specific sources of contamination should be identified and eliminated from these growing sites.

What interventions are Needed?• Seed sprouts are high risk vehicle for foodborne diseases

• Continue applied research to find ways to successfully decontaminate the seeds/sprouts.

• Educate sprout growers on appropriate growing conditions and handling of sprouts to limit contamination.

• Educate the public about the riskiness of sprouts – Persons at high risk for complications of infection (e.g., children <5 years of age,

immunocompromised individuals, and the elderly) avoid sprouts.

• Require sprout producers to label sprouts as risky foods• Remove sprouts from the market for human consumption until their safety can

be assured.

Steps in Outbreak Investigation• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology• Develop a hypothesis• Test the hypothesis• Refine hypothesis / Execute additional studies• Implement control and prevention measures


• Verify the diagnosis• Confirm the outbreak• Case definition• Descriptive epidemiology• Develop a hypothesis• Test the hypothesis• Refine hypothesis / Execute additional studies• Implement control and prevention measures• Communicate findings

Communicate Findings• The implicated seed lot was not distributed to any other sprouting

companies. The remaining 6,000 lbs. of seed was immediately removed from the marketplace.

• The Idaho Division of Food and Drugs held meetings at which public health officials explained to seed growers the need to protect alfalfa and other seeds used in sprouting from contamination during growing, harvesting, and packing.

• Public television and radio announcements about the risk of contaminated sprouting seeds, recommending persons at high risk for complications from E. coli O157:H7 not eat sprouts.

• The Center for Food Safety and Quality Enhancement began working with the sprout industry to identify ways to make sprouts safer for human consumption.

Conclusions

• Importance of applying the multi-step approach in outbreak investigation

• Utility of new subtyping methods such as PFGE • Importance of disease reporting• Flexibility of hypothesis generation

– New vehicle for the transmission of E. coli O157:H7

• Increasing geographic dissemination of outbreaks

Documents

Outbreak Investigation: Discussion Group