investigating noroviruses: a case study · 2016. 11. 21. · a case study david bergmire-sweat, mph...
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Investigating Noroviruses: A Case Study
David Bergmire-Sweat, MPH North Carolina Department of Health and Human Services
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Norovirus Facts
Small, round-structured viruses from Caliciviridae family
First recognized as pathogenic cause of gastroenteritis in 1968, Norwalk, Ohio
Transmitted via fecal-oral route Highly contagious—as few as 10 viral
particles can cause infection
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Symptoms
Incubation period ~ 24-48 hours, can be less Vomiting Watery non-bloody diarrhea Abdominal cramps Nausea Low-grade fever Symptoms typically resolve ~ 24-60 hours (1-3
days)
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Documented Modes of Transmission Foodhandler contamination of items in
restaurant Daycare centers Secondary person-to-person Aerosolized (vomiting and diarrhea) Cleaning up after ill person in a bathroom
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Immunity & Epidemiology
Seems to be strain-specific and temporary (few months)
CDC estimates 23 million cases of acute gastroenteritis annually in U.S. due to noroviruses
CDC estimates noroviruses cause ~50% of all foodborne disease outbreaks
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Detection
Best method is via reverse transcriptase polymerase chain reaction (RT-PCR)
Viral RNA Adults can shed viral particles for up to 2
weeks after symptoms resolve Infants and toddlers may shed longer
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Case Study
Small state university north of Houston, TX
Student body ~ 10,000 students
2,054 students owned meal plans
125 students present to student health center and local hospital with gastrointenteritis in 24 hr period
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Symptoms
125 ill students on March 10-11 1998 23 admitted to hospital 91% reported vomiting 85% reported diarrhea (3+ loose stools in
24 hours) 68% reported abdominal cramps
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Epidemic Curve
0
5
10
15
20
25
30
35
Date 5-Mar
6-Mar
7-Mar
8-Mar
9-Mar
10-Mar
11-Mar
12-Mar
13-Mar
14-Mar
15-Mar
Cases by Onset Date
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Laboratory Investigation
62 stool samples sent to Texas Department of Health Lab in Austin
All negative for Salmonella, Shigella, Campylobacter, Yersinia, E. coli O157:H7, Bacillus cereus, and Staphylococcus aureus
18 samples tested for Norovirus by RT-PCR; 9 were positive
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Challenges of Cafeteria Outbreaks Verifying Exposures Identifying potential
healthy controls Finding people to
interview them Hundreds of food
items Rule in/out other
exposures
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Rapid Action
Outbreak occurred toward end of week, right before Spring Break
Sanitarians from local health department investigated, realized cafeteria was about to close for week and perform massive cleaning/maintenance
Locked down all present food items, instructed staff not to throw anything out
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Epidemiologic Investigation
Unmatched Case-control study by Texas Dept. of Health
36 ill subjects 136 well controls
Matched Case-control study by CDC
29 ill subjects 29 well controls
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Findings – Matched Study Exposure Odds Ratio Confidence
Intervals P value
Deli sandwich 3/9 lunch
11.0 1.6-473 < 0.01
Deli sandwich 3/9 supper
6.0 0.7-276 0.06
Deli sandwich 3/10 lunch
8.0 1.1-355 0.02
Deli sandwich 3/10 supper
1.0 0.01-79 .75
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Findings – Unmatched Study Exposure Odds Ratio Confidence
Intervals P value
Deli sandwich 3/9 lunch
11.1 3.9-32 < 0.001
Deli sandwich 3/9 supper
7.1 1.6-33
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Interpretation
Both studies implicated deli sandwiches on 3/9 and 3/10
The more controls in a study, the better statistical power you have
No other restaurants or food items showed any association
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Cafeteria Staff Investigation
Sanitarians and epidemiologists interviewed foodhandlers
All denied illness All except one submitted stool samples for
testing One refused; cafeteria manager fired her She was responsible for slicing deli meats
and making deli sandwiches
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Discussion
CDC and TDH epidemiologists were disappointed with cafeteria manager decision to fire the one employee
Much more difficult to get cooperation Eventually did interview her She denied illness, but had infant with
diarrhea Stool sample from infant was PCR
positive for Norovirus
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Chance to Learn Something
Researchers at Baylor College of Medicine agreed to try to find virus RNA on deli meat
Never been done before
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Success!
Lab successful in designing protocol for recovering virus RNA from deli meat
Pulsed-Field Gel analysis of PCR results showed identical pattern between infant, ill students, and deli meats
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Take Home Points
Viral gastroenteritis is easy to spread Foodhandlers must be extremely careful,
even when they are not sick, with hygiene It is possible to recover Norovirus from
food Quick reaction by local health department
sanitarians preserved food samples Collaboration with research lab advanced
science of food safety research
Investigating Noroviruses: A Case StudyNorovirus FactsSymptoms Documented Modes of TransmissionImmunity & EpidemiologyDetection Case StudySymptomsEpidemic CurveLaboratory InvestigationChallenges of Cafeteria OutbreaksRapid ActionEpidemiologic InvestigationFindings – Matched StudyFindings – Unmatched StudyInterpretationCafeteria Staff InvestigationDiscussionChance to Learn SomethingSuccess!Take Home Points
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