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EPI 5240: Introduction to Epidemiology Gastroenteritis in Texas: an outbreak case-study October 26, 2009. Dr. N. Birkett, Department of Epidemiology & Community Medicine, University of Ottawa. Part 1.

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Dr n birkett department of epidemiology community medicine university of ottawa
EPI 5240:Introduction to EpidemiologyGastroenteritis in Texas: an outbreak case-studyOctober 26, 2009

Dr. N. Birkett,

Department of Epidemiology & Community Medicine,

University of Ottawa

Part 1
Part 1

On the morning of March 11, the Texas Department of Health (TDH) got a phone call from a university student. He reported that he and his roommate (who lived in a frat house) were suffering from nausea, vomiting and diarrhea. Both became ill during the night. Neither had seen an MD.

They believed they were ill due to food eaten at a local pizzeria the previous night. They wanted advice about whether to take a biology mid-term that afternoon.

Part 1 q s
Part 1 (Q’s)

  • What questions would you ask the student?

  • What advice would you provide?

  • Do you think that this complaint should be investigated further?

Part 2 1
Part 2 (1)

Staff were skeptical, thinking students wanted an excuse to miss the exam  but decided to make a few calls.

  • Pizzeria was closed until 1100,

  • No answer at the student health centre

Part 2 2
Part 2 (2)

  • ER at hospital A revealed 23 university students had been seen for gastro in past 24 hours.

    • Only 3 similar patients seen from March 5-9, none from university

  • 10:30, MD from Student Health Center called. 20 cases of students with vomiting and diarrhea seen in previous day.

    • Normally, see only 1-2 similar patients per week.

Is this an outbreak at the university?

Part 3
Part 3

TDH visit hospital A that afternoon to review medical records. Symptoms:

Vomiting: 91%

Diarrhea: 85%

Abdo cramps: 68%

Headaches: 66%

Muscle aches: 49%

Bloody diarrhea: 5%

Temperature: 98.8 to 102.4 (median 100)

CBC: elevated white cell count

Stool specimens: pending

Part 3 q s
Part 3 (Q’s)

Broad categories of diseases to consider?


  • Requires organism growth

    • Incubation period days, not hours

  • Symptoms: diarrhea, nausea, vomiting, cramps. Can get fever and raised white count


  • Toxin already in food  disease in minutes to hours

  • Symptoms: vomiting (**). Can include neurological symptoms

    Pizza as source toxin.

    Symptoms infection

Part 4
Part 4

  • Local hospitals asked to report any cases of vomiting/diarrhea seen in past week. Asked to collect stool samples on new cases.

  • Preliminary cultures on 17 ill students were negative for 9 common bacterial causes of gastro, including:

    • Salmonella, shigella, E. Coli and Listeria

What do the negative stool culture results mean?

Part 6
Part 6

Within 24 hrs, 75 persons with vomiting or diarrhea had been reported to the TDH. All were students from the university campus. There were no cases among faculty, staff or the local community. Median age was 19, 69% were freshmen and 62% were female.

Part 6 q s
Part 6 (Q’s)

  • Is there anything else you would like to see from this preliminary data?

    • An epidemic curve!

Part 6 q s1
Part 6 (Q’s)

  • Is there anything else you would like to see from this preliminary data?

    • An epidemic curve!

  • How do you interpret the curve?

  • What about the case on March 6?

  • Next steps?

    • Contact university

      • What would need to be discussed?

    • Get more info

      • Case series interviews to generate hypotheses

        • Does the pizzeria hold up as a source?

Part 7 1
Part 7 (1)

  • University is in small town (pop=27,354). Enrollment about 12,000. 2,386 live on campus in 36 residences.

  • University has municipal water and sewage.

    • No breaks or work on water/sewage lines in past year

    • No recent road work or digging near campus

Part 7 2
Part 7 (2)

  • Two cafeterias on campus (run by same company) plus 6 fast-food places

    • 2000 students on food plan

    • Most on-campus students eat in main cafeteria

      • Hot entrees

      • Grill, deli bar and salad bar

    • Second cafeteria used by off-campus students and staff.

      • Hot entrees, grill and salad bar

      • No deli bar.

  • Spring break starts in two days (March 13). Cafeteria will be closed for 10 days.

Part 7 3
Part 7 (3)

  • Interview of 7 early cases revealed:

  • 4 M; 3F. 6 freshmen;

    • 2 psych major,1 English/Animal husbandry major; 3 undecided

  • 5 different residence halls. All ate mainly in main cafeteria. All had eaten from deli bar; 2 from salad bar and 3 from grill. No common food items were noted

  • Only psych students shared any classes. 5 were in sorority/fraternity; 3 attended an ‘all school’ mixer on March 6; 2 went to a Sci Fi film festival on March 7.

Part 7 q s
Part 7 (Q’s)

  • State your leading hypothesis on pathogen, mode of transmission, source and period of interest.

    • Viral infection

    • Food or beverage spread

    • Served at the main university cafeteria between March 5 and 10

    • Perhaps related to the Deli bar

  • What actions would you take next?

    • Epi study with controls

    • Lab examination of foods (including ice)

    • Inspect operational methods; interview staff

Part 8 environmental 1
Part 8 (environmental - 1)

  • 31 staff work in cafeteria, 24 are foodhandlers.

    • 30 interviews. One person refused to participate (worked at the deli bar)

  • Asked about responsibilities, which meals they served and where, use of gloves, hand washing practices, work schedule in key week and illnesses. They provided stool samples.

  • No illnesses in staff.

  • Deli Bar

    • Separate refrigerator and prep area.

    • Sandwiches made to order by foodhandler

    • New meat, cheese and condiments were added to partially depleted stock daily (leftover food items were not discarded first)

    • Food items were not refrigerated or kept on ice while deli was open

    • Containers were not routinely cleaned

  • Deli Bar was closed by the Health Department on March 12.

    • Do you agree with this action?

Part 9 epi
Part 9 (epi)

  • Need more information, including a control group

    • Options?

  • Matched case-control study (done on March 12)

    • Case definition

      • Vomiting or diarrhea (3+ loose stools in a 24 hr period) with onset after March 5 in a university student seen at ER or the health centre. Had to be in dormitory during survey

    • Controls

      • Roommates of cases who did not become ill

    • Telephone interviews (why?)

    • 29 pairs of subjects

Dr n birkett department of epidemiology community medicine university of ottawa

Epidemic curve, based on 75 cases





Part 10
Part 10

  • By March 13, the outbreak had grown to include 125 patients. CDC was invited to assist. They performed RT-PRC testing of stool samples for viruses.

  • Unmatched case-control study done to explore source of infection.

    • Limited to students eating at main cafeteria

    • Case definition modified

      • Had to be member of university meal plan (need not be in residence)

    • Controls were in meal plan but had not got ill.

    • 40 randomly selected cases

    • 160 controls

    • Telephone interviews

Final results 1
Final results (1)

  • Water/ice negative for coliforms

  • All stool cultures negative for bacteria

  • 50% of fresh stool cultures on ill students were positive for Norwalk-like virus.

  • Ham samples were positive for Norwalk-like virus (same DNA sequence as stool samples)

  • Staff member who refused interview agree to provide information

    • She sliced ham for three meals on March 9/10

    • Prepared sandwiches on those days

    • Wore gloves. Therefore, believed that hand washing didn’t matter.

    • She wasn’t ill

    • BUT her infant had watery diarrhea from March 7 to 10.

    • The same Norwalk-like virus was found in her child’s stool

    • Transmitted by contamination, not through infection


  • What needs to be done to re-open deli bar?

    • Throw away all leftover deli bar food

    • Clean and disinfect all equipment and surfaces in deli bar

    • Require foodhandlers to submit stool sample prior to return to work?

    • Educate foodhandlers on proper procedures

    • Develop a sick foodhandler policy