See attached case study and answer 12 questions attached to case study.
Foodborne Disease Outbreak Investigation: Epidemiologic Case Study for theClassroom: A Multistate Outbreak of E. coli O157:H7 (Student’s Version) A Multistate Outbreak of E. coli O157:H7 Infection STUDENT’S VERSION Original investigators: Thomas Breuer,1 Denise H. Benkel,1,2 Roger L. Shapiro,1 William N. Hall,3 Mary M. Winnett,4 Mary Jean Linn,2 Jakob Neimann,1 Timothy Barrett,1 Stephen Dietrich,3 Francis P. Downes,3 Denise M. Toney,5 James L. Pearson,5 Henry Rolka,1 Laurence Slutsker,1 and Patricia M. Griffin1 1Centers for Disease Control and Prevention, 2Virginia Department of Health, 3Michigan Department of Community Health, 4Medical College of Virginia, 5Virginia Division of Consolidated Laboratory Services Case study and instructor’s guide created by: Jeanette K. Stehr-Green, MD NOT E: This c ase study is based o n two re al-life outb reak inve stigation s underta ken in Michigan and Virginia in 1997. Some aspects of the original outbreaks and investigations have been altered, however, to assist in meeting the desired teaching objective s and allo w com pletion o f the case stu dy in less th an 3 ho urs. Students should be aware that this case study describes and promotes one particular appro ach to fo odbor ne disea se outbre ak investig ation. Pr ocedur es and p olicies in outbreak investigations, however, can vary from country to country, state to state, and outbreak to outbreak. It is anticipa ted that the epidem iologist inv estigating a foodb orne dise ase outb reak will work w ithin the fra mewo rk of an “ investiga tion team ” which includes persons with expertise in epidemiology, microbiology, sanitation, food science, and environmental health. It is through the collaborative efforts of this team, with each member playing a critical role, th at outbre ak investig ations a re succes sfully com pleted. Please send us your comments on this case study by visiting our website at http://www.phppo.cdc.gov/phtn/casestudies . Please in clude the name of the case study w ith your commen ts. April 2002 U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES Public Health Service Centers for Disease Control and Prevention Atlanta, Georgia 30333 STUDENT’S VERSION A Multistate Outbreak of E. coli O157:H7 Infection Learning objectives: After completing this case study, the student should be able to: 1. describe the unique role the laboratory can play in the detection and investigation of a foodborne disease outbreak 2. perform in-depth interviews of selected case-patients to generate hypotheses about the source of an outbreak and mode of transmission 3. determine the most efficient epidemiologic study design to test a hypothesis (including the case definition and appropriate comparison group) 4. list three ways to select a comparison group for a study and the advantages and disadvantages of each method 5. list detailed product information that will facilitate a traceback procedure 6. discuss the relative merits of an intervention based on changes in product processing (or design) versus changes in consumer or producer behaviors PART I - OUTBREAK DETECTION Escherichia coli O157:H7 was first identified as a human pathogen in 1982 in the United States of America, following an outbreak of bloody diarrhea associated with contaminated hamburger meat. Sporadic infections and outbreaks have since been reported from many parts of the world, including North America, Western Europe, Australia, Asia, and Africa. Although other animals are capable of carrying and transmitting the infection, cattle are the primary reservoir for E. coli O157:H7. Implicated foods are typically those derived from cattle (e.g., beef, hamburger, raw milk); however, the 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. Most laboratories that culture stool do not routinely test for E. coli O157:H7, but require a special request from the health care provider. Only recently has E. coli O157:H7 infection become nationally notifiable in the U.S. Outside the U.S., reporting is limited to a few but increasing number of countries. In the last week of June 1997, the Michigan Department of Community Health (MDCH) noticed an increase in laboratory reports of E. coli O157:H7 infection. Fifty-two infections had been reported that month, compared with 18 in June of 1996. In preliminary investigations, no obvious epidemiologic linkages between the patients were found. The increase in cases continued into July. A Multistate Outbreak of E. coli O157:H7 Infection Studen t’s version - p. 2 Question 1A: What could account for the increase in cases reported to MDCH? Question 1B: What information might help determine which of these explanations is the most likely cause of the increased numbers? A Multistate Outbreak of E. coli O157:H7 Infection Studen t’s version - p. 3 Laboratory subtyping can help determine if an increased number of isolates of the same bacterial species results from a common source outbreak. Subtyping methods are based on selected biologic and/or genetic characteristics of bacteria that tend to differ between isolates of the same species. In a common source outbreak, however, isolates typically arise from the same parent organism. These isolates will be similar to each other with respect to these biologic and genetic characteristics and have similar subtyping results. One subtyping method is DNA "fingerprinting" by Pulsed Field Gel Electrophoresis (PFGE). In DNA fingerprinting, the bacterial DNA is cut into pieces. The pieces are separated by placing them in a jelly-like substance (i.e., the gel), acting as a sieve, to which a pulsing electric field is applied. The electric field drives the DNA pieces across the gel over a period of hours. The smaller pieces move through the gel more quickly and the larger pieces more slowly resulting in a separation of the DNA into distinct bands. The bands are made to fluoresce and are read under ultraviolet illumination. This DNA “fingerprint” resembles a bar code. (Figure 1) Figure 1. Typical DNA banding p attern resulting from PFGE. Different DNA composition will result in different PFGE banding patterns. Bacteria descended from the same original parent will have virtually identical DNA and their DNA fingerprints will be indistinguishable. Identification of a cluster of isolates with the same PFGE pattern suggests that they arose from the same parent and could be from the same source. Similar DNA fingerprints alone, however, are insufficient to establish a linkage between isolates and a common source outbreak. An epidemiologic investigation is necessary to demonstrate that there is a common source and to identify it. To be most useful, PFGE subtyping needs to be performed on a routine basis, in real time, so that results are available (and reviewed) soon after a case is first detected. A Multistate Outbreak of E. coli O157:H7 Infection Studen t’s version - p. 4 Question 2: Compare the DNA fingerprints in Figure 2 from seven of the Michigan E. coli O157:H7 cases. Each isolate has its own vertical lane (i.e., column). Controls appear in lanes #1, 5, and 10. Which Michigan isolates appear similar? Figure 2. PFGE results on E. coli O157:H7 isolates from Michigan, June-July 1997. A Multistate Outbreak of E. coli O157:H7 Infection Studen t’s version - p. 5 DNA fingerprinting, performed in the MDCH State Laboratory during the second week of July showed that 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. Based on the PFGE findings, MDCH suspected the cases of E. coli O157:H7 infection resulted from a common source. On July 15, MDCH initiated an investigation. The Centers for Disease Control and Prevention (CDC) was asked to join the investigation. A Multistate Outbreak of E. coli O157:H7 Infection Studen t’s version - p. 6 PART II - DESCRIPTIVE EPIDEMIOLOGY AND HYPOTHESIS GENERATION The 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 nonbloody diarrhea or result in no symptoms. In some persons, particularly children under 5 years of age and the elderly, the infection can cause a complication called hemolytic uremic syndrome, in which the red blood cells are destroyed and the kidneys fail. About 2-7% of infections lead to this complication. For the outbreak investigation in Michigan, a case was defined as diarrhea ($3 loose bowel movements a day) and/or abdominal cramps in a resident of Michigan with onset of symptoms between June 15 and July 15 and a stool culture yielding E. coli O157:H7 with the outbreak strain PFGE pattern. Question 3: What are the advantages and disadvantages of this case definition? How might you change it? A Multistate Outbreak of E. coli O157:H7 Infection Studen t’s version - p. 7 Of the initial 38 persons who met the case definition, 26 (68%) were female with a median age of 31 years. (Table 1) Table 1. Age group and gender distribution for persons with E. coli O157:H7 infection and the outbreak PFGE pattern, Michigan, June 15 - July 15, 1997. (N=38) Age group (years) Gender TOTAL 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. Question 4: Compare the age and gender distribution of E. coli O157:H7 cases from the Michigan outbreak and those reported from U.S. FoodNet sites in 1997. (see Appendix 1) A Multistate Outbreak of E. coli O157:H7 Infection Studen t’s version - p. 8 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. Onset of illness occurred from mid-June to mid-July, peaking on June 22. (Figure 3) Figure 3. Date of illness onset for persons with E. coli O157:H7 infection and the outbreak PFGE pattern, Michigan, June 15 - July 15, 1997. (N=38) From July 16 - 19, hypothesis-generating interviews were undertaken with