shifting paradigms in epidemiology: the expanded role of the public health laboratory linda gaul,...

Shifting Paradigms in Epidemiology: The Expanded Role

of the Public Health Laboratory

Linda Gaul, PhD, MPH

Epidemiologist

Infectious Disease Epidemiology and Surveillance Division

The Old Paradigm

Salmonellosis Outbreak at a Hill Country Ranch,

Texas, 2001

Outbreak Notification

• Infection Control Coordinator at a central Texas hospital called the regional office of the state health department

• ~30 people had presented to the hospital’s emergency room during the previous 24 hours with gastrointestinal symptoms

• All of the ill persons had eaten a picnic supper the previous evening at a local ranch

Components of a Foodborne Illness Outbreak Investigation

• Preliminary epidemiologic investigation: Identification of person, place and time

• Environmental investigation: Assessment of risk factors• Specimen collection and laboratory analysis:

Identification of pathogen• Implementation of control measures: Prevention of

additional cases• Case-control (epidemiologic) study: Identification of

outbreak source• Conclusion and dissemination of findings

Case Definition

• Any three of the following symptoms: Diarrhea, fever, nausea, cramps, vomiting

• Diarrhea defined as > 3 loose stools in a 24 hour period

• OR culture positive stool specimen

• AND ate the picnic supper

Attack Rate

AR (case-defined ill persons) =

109 cases / 215 persons who ate meal =

50.7%

Salmonellosis Outbreak in a Church Camp, 2001

0

10

20

30

40

50

60

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6/23 A

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6/23 P

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6/24 A

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6/24 P

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6/25 A

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6/25 P

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6/26 A

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6/26 P

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6/27 A

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6/27 P

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Onset date (by morning or afternoon)

Num

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Case Control Study and Laboratory Results for Salmonella enterica

Serotype Panama outbreak

Food item Odds Ratio/significance Culture resultsBeans 27.9* N/AJalapenos 2.3 N/ABarbecue chicken 2.2* PositiveOnions 2.1 N/A Barbecue sauce 1.9* NegativeBarbecue brisket 1.6 NegativeSmoked sausage 1.6 NegativePotato salad 1.6 Negative

Food item Odds Ratio/significance Culture resultsPasta salad 1.6 NegativeIced tea 1.6 N/APunch 1.6 N/AColeslaw 1.5 NegativeCorn muffin 1.5 NegativeCorn-on-the-cob 1.3 PositiveIce 1.2 N/ASoda ~1+ N/ABrownies 0.9 Negative

Food item Odds Ratio/significance Culture results Watermelon 0.8 N/ALemonade 0.8 N/AWater 0.8 NegativeOther food items 0.7 N/ACupcakes 0.5 N/A

*Indicates an odds ratio that is statistically significant at the 95% confidence level.+An exact odds ratio could not be calculated because one cell contained zero entries. N/A indicates food not available for testing.

Conclusions• 109 persons developed salmonellosis

following consumption of a bean dish and/or chicken served at a ranch picnic

• Either raw chicken or an ill foodhandler could have been the source of the bacteria

• The beans were cross-contaminated with the bacteria in the kitchen

Pathogen

• Salmonella enterica serotype Panama• Incubation period: 6-72 hours, usually 1-3

days• Symptoms, in descending order of

frequency: Diarrhea, cramps, fever, nausea, vomiting, headache

• Detected in 25 of 37 stool specimens submitted

• Detected in leftover chicken and corn-on-the-cob

Summary: Old Paradigm • Outbreak notification results from detection of

more than the expected number of cases for the place and time

• Pathogen often unknown until after outbreak detection

• Epidemiologic investigation aims at identifying the exposure

• Laboratory results identify the agent and may confirm the epidemiologic findings implicating a source of exposure (food item)

The New Paradigm

Raw Oysters and Typhoid FeverTexas, 2003

Outbreak Notification

• 2 cases of Salmonella Typhi identified in Houston area• Cases had same strain of S. Typhi by PFGE

• 4 additional cases of the same strain reported over the next several weeks• 1 Houston area

• 2 Austin area

• 1 San Antonio

• None of the 6 cases had recent international travel

Components of a Foodborne Illness Outbreak Investigation

• Preliminary epidemiologic investigation: Identification of person, place and time

• Environmental investigation: Assessment of risk factors• Specimen collection and laboratory analysis:

Identification of pathogen• Implementation of control measures: Prevention of

additional cases• Case-control (epidemiologic) study: Identification of

outbreak source• Conclusion and dissemination of findings

Texas cases, typhoid fever strain 309 cluster, 2003

Typhoid Fever, Texas 2003

Case Definition

• Typhoid fever, strain 309, with onset during the summer of 2003

Attack Rate (among interviewed persons)

• Cannot be calculated because number of exposed persons is unknown– AR = # ill / # exposed

Working Hypotheses of Investigation

• Cases have a common exposure

• Vehicle • Unusual food item

• Small number of cases

• Food item not cooked prior to consumption• More than one establishment involved

Epidemiologic Findings

• In-depth exposure histories obtained• Food frequency questions for home eaten food• Categories of restaurants, some specific

examples• Specific questions about raw shellfish

consumption

• All 6 cases identified raw oyster consumption during 5 weeks prior to onset– Specific dates and locations identified

Oyster Traceback and Plant Investigation

• Oysters traced for all 6 cases to the same lease in Galveston Bay

• Samples of frozen product from same harvest area and timeframe, and specimens from oyster plant workers tested negative for S. Typhi

Case Control Study Results for Salmonella Typhi Outbreak

Food item Odds Ratio/significanceRaw oysters 148.2*

Conclusions• 6 cases of typhoid fever were linked to raw

oyster consumption• Implicated oysters were harvested from the

same area of Galveston Bay by the same harvester

• The source of the oyster contamination was not identified• Could have happened during harvesting• Could have happened at the lease site

Pathogen • Salmonella enterica serovar Typhi• Reservoir: Humans• Incubation period: 3 days – 5 weeks• Symptoms: Sudden onset of acute illness,

sustained high fever, severe headache, malaise, anorexia, splenomegaly, rose spots on trunk

• Case-fatality rate 10-20% without prompt antibiotic therapy

• 2-5% of patients become permanent carriers

Summary: New Paradigm • Outbreak notification results from detection

of an increased number of cases of a particular agent or strain of an agent by a laboratory– Pathogen identified at time of outbreak

detection

• Epidemiologic investigation aims at identifying the exposure

Recent Outbreaks Illustrating the New Paradigm

• Spinach-associated E. coli O157:H7 multistate outbreak, Fall 2006

• Lettuce-associated E. coli O157:H7 multistate outbreaks (2), Fall 2006

Recent Outbreaks with Texas Cases Illustrating the New

Paradigm • Peanut butter-associated Salmonella Tennessee

– Outbreak detection based on large number of cases reported to CDC’s PulseNet with unusual serotype of Salmonella and one of 3 similar strains

– Multistate epidemiologic investigation, using common questionnaire, identified 2 brands of peanut butter as suspect vehicles• 2 brands produced in same plant

Recent Outbreaks with Texas Cases Illustrating the New

Paradigm, cont.• 629 lab-confirmed cases in 47 states

reported to date– Onsets 8/1/06 – 4/23/07– Including 24 cases in Texas

• Outbreak strains of organism detected in 21 of 222 tested specimens of peanut butter

• Contamination conditions found in plant, plant shut down for thorough cleanup