Outbreak Investigation Methods: From Mystery to Mastery icon

Outbreak Investigation Methods: From Mystery to Mastery


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Public Health Information Network Series II

Outbreak Investigation Methods: From Mystery to Mastery

Session I: Recognizing an Outbreak


Mock Case Study Beginning


It is October 29th in the city of Jonesville, USA. Emergency rooms, local public health clinics, and primary care private practices have been visited by a total of 42 individuals complaining of abdominal cramps and bloody diarrhea, with onset of symptoms in the past 48 hours. So far, all individuals have been females ranging in age from their 20s to their 60s.


Public health laboratory tests have identified the presence of Escherichia coli (E. coli 0157:H7) bacterium in case stool samples.


Activity 1: Case Report


Instructions: Now that you have a number of cases identified and you suspect a possible outbreak, you will need to conduct interviews. Based on the information provided in the mock case study paragraph, generate one question per case report category of Who, What, When, Where, and Why / How that you will want to ask each case. There will be no “right” answers here; the purpose of this exercise is to help you think through the types of questions that you would need to ask during an outbreak investigation.


You want the questions to yield information that will serve a two-fold purpose:

  1. The information helps guide you towards a leading hypothesis.

  2. The exploratory questions yield both qualitative (e.g., narrative) and quantitative (e.g., descriptive or numeric) information that you can analyze now and later in the hypothesis testing part of the investigation process [data analysis will be the focus of the November session in this series].


WHO: other ill persons – age, sex, symptoms, and whether they sought care


Your question: _____________________________________________________


WHAT: physical condition, symptoms, medication, and medical care sought


Your question: _____________________________________________________


WHEN: when did the affected become ill


Your question: _____________________________________________________


WHERE: city/school, address, telephone number of ill persons


Your question: _____________________________________________________


WHY / HOW: suspected cause of illness, risk factors, modes of transmission, hints from those who did not become ill – remember to collect a complete food history for a minimum of 72 hours, and possibly up to five days for diarrheal illnesses.


Your question: _____________________________________________________

^ Mock Case Study Beginning


It is October 29th in the city of Jonesville, USA. Emergency rooms, local public health clinics, and primary care private practices have been visited by a total of 42 individuals complaining of abdominal cramps and bloody diarrhea, with onset of symptoms in the past 48 hours. So far, all individuals have been females ranging in age from their 20s to their 60s.


Public health laboratory tests have identified the presence of Escherichia coli (E. coli 0157:H7) bacterium in case stool samples.


Activity 2: Develop a Case Definition


Instructions: Use only the mock case study information provided above to develop a case definition that could be used in an outbreak investigation.


Remember to consider four things:


1. What are the clinical criteria (signs, symptoms, laboratory tests) provided in the case study information?

____________________________________________________________


2. Who is sick (e.g., males or females only? Limited to a certain race, ethnicity, or age group)?


_______________________________________________________________________


  1. Do not include any part of a potential hypothesis in the case definition (e.g., if you suspect a certain food establishment as the source of illness, you cannot state, “. . . who ate at X food establishment. . .” in the case definition.


4. Your case definition should be a short, simple statement that includes only key information.


^ Your Case Definition


Activity 3: Develop a Leading Hypothesis


In this final activity for the “Recognizing an Outbreak” session, you are going to develop a leading hypothesis based on the information provided. In addition to new case study details, you will need to read information about the bacteria E. coli. The information provided here is an abridged version of that posted online by the Centers for Disease Control and Prevention’s Foodborne Outbreak Response and Surveillance Unit, Foodborne and Diarrheal Diseases Branch at: http://www.cdc.gov/ncidod/dbmd/diseaseinfo/escherichiacoli_g.htm.


^ Mock Case Study Continued . . .


It is October 29th in the city of Jonesville, USA. Emergency rooms, local public health clinics, and primary care private practices have been visited by a total of 42 individuals complaining of abdominal cramps and bloody diarrhea, with onset of symptoms in the past 48 hours. So far, all individuals have been females ranging in age from their 20s to their 60s.


Public health laboratory tests have identified the presence of Escherichia coli (E. coli 0157:H7) bacterium in case stool samples.


Investigators have descriptively analyzed case interview data, and now know that 35 of the 42 cases worked out at the same local gym from the dates of October 21st and October 23rd. Cases who did not work out at the gym have reported sharing living quarters with cases who did work out at the gym.


Case report data have revealed that in addition to two public water fountains, the gym has a water cooler with bottled water from a supplier, and a complementary juice bar that serves one seasonal juice each month. Five-day case food histories indicate that from October 21st and 23rd, eleven (25%) of the 42 cases consumed water cooler water, and 33 (79%) of the 42 cases consumed locally bottled apple cider at the juice bar. Three (7%) cases ate at the same local restaurant—a franchise that serves hamburgers and chicken sandwiches.





^ Escherichia coli O157:H7 Symptoms and Complications


The combination of letters and numbers in the name of the bacterium refers to the specific markers found on its surface and distinguishes it from other types of E. coli. E. coli O157:H7 infection often causes little or no fever, but severe bloody diarrhea and abdominal cramps; sometimes the infection causes non-bloody diarrhea or no symptoms. It has an incubation period of 1 – 10 days, averaging 3 – 4 days. Most persons recover without antibiotics or other specific treatment in 5-10 days. In some persons, particularly children under 5 years of age and the elderly, the infection can also cause a complication called hemolytic uremic syndrome, in which the red blood cells are destroyed and the kidneys fail.


How is E. coli O157:H7 spread?


The organism can be found on a small number of cattle farms and can live in the intestines of healthy cattle. Meat can become contaminated during slaughter, and organisms can be thoroughly mixed into beef when it is ground. Bacteria present on the cow's udders or on equipment may get into raw milk.

1. Eating meat, especially ground beef that has not been cooked sufficiently to kill E. coli O157:H7 can cause infection. Although the number of organisms required to cause disease is not known, it is suspected to be very small.


2. Other known sources of infection are consumption of sprouts, lettuce, salami, unpasteurized milk and juice, and swimming in or drinking sewage-contaminated water.


3. Bacteria in diarrheal stools of infected persons can be passed from one person to another if hygiene or hand washing habits are inadequate.
----------------------------------------------------------------------------


Instructions: Answer each question below based on the information provided. You will then develop your own leading hypothesis statement.


Pathogen:


What pathogen has been confirmed in public health laboratory samples?


_________________________


Source:


Do cases appear to share a common exposure in a geographic location or structure?

Yes No


If Yes, where? _____________________________________________________


Do cases appear to share a common exposure to a food or beverage?


Yes No


If yes, what food or beverage? __________________________________________


^ Mode of Transmission:


How is E. coli O157:H7 infection transmitted?


____________________________________________________________


Possible mode(s) of transmission in the case study:


____________________________________________________________


^ Period of Interest:


If a common exposure is suspected, on what date [or range of dates] did that exposure take place?


Date(s): _________________________________________


Given the average incubation period of 3 – 4 days for the bacteria and the date of onset of illness of October 27th or 28th most cases, the most likely exposure date would have been

October ________.


Your Leading Hypothesis


Listed below are three possible leading hypotheses. Only one is correct. Try to identify the best one.

    1. Exposure to gastrointestinal illness occurred at the local gym in October

    2. Exposure to E. coli O157:H7 occurred through juice served at the local gym in late October

    3. Adult women who worked out at the local gym from Oct 21-23 are becoming infected with E. coli O157:H7 through drinking unpastuerized apple cider contaminated at the orchard and served at the gym’s juice bar




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