【正文】 n (with PFGE pattern), Michigan, June 15 July 15, 1997. (N=38) APP E N DIX 1Ag e g r oup( y ea r s )Gende rT O T ALMal e F e m a l e0 1 5 ( 3% ) 5 ( 3% ) 10 ( 3% )1 9 77 ( 48% ) 77 ( 43% ) 154 ( 45% )10 19 36 ( 22% ) 18 ( 10% ) 54 ( 16% )20 29 10 ( 6% ) 20 ( 11% ) 30 ( 9% )30 39 6 ( 4% ) 12 ( 7% ) 18 ( 5% )40 49 7 ( 4% ) 5 ( 3% ) 12 ( 4% )50 59 7 ( 4% ) 17 ( 10% ) 24 ( 7% )60+ 14 ( 9% ) 24 ( 13% ) 38 ( 11% )TO TA L 162 ( 10 0% ) 178 ( 10 0% ) 340 ( 10 0% )).Ag e g r oup( y e a r s )G e nderTO TA LM al e F e m a l e0 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% )T O T AL 12 ( 101% ) 26 ( 101% ) 38 ( 100% ).MI Cases FoodNet Data Michigan counties 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. Epidemic Curve Figure 3. Date of illness onset for persons with E. coli O157:H7 infection and the outbreak PFGE pattern, MI, June 15 July 15, 1997. (N=38) Epidemic Curves ? How to set it up ? What it tells you ? Mode of transmission ? Propagated ? Common source ? Timing of exposure ? Course of exposure Epidemic Curves 0123456781 2 3 4 5 6 7 8 9 10 11 12W e e kCasesPropagated source: single exposure, no secondary cases (., measles) Epidemic Curves 0123456789101 2 3 4 5 6 7 8 9 10 11 12W e e kCasesPropagated source: secondary and tertiary cases (., hepatitis A) Epidemic Curves 0123456781 2 3 4 5 6 7 8 9 10 11 12D aysCasesCommon source: point exposure (., salmonella) Epidemic Curves 0123456781 2 3 4 5 6 7 8 9 10 11 12D aysCasesCommon source: Intermittent exposure (., contaminated blood product) Epidemic Curve Figure 3. Date of illness onset for persons with E. coli O157:H7 infection and the outbreak PFGE pattern, MI, June 15 July 15, 1997. (N=38) Steps in Outbreak Investigation ? Verify the diagnosis ? Confirm the outbreak ? Case definition ? Descriptive epidemiology Steps in Outbreak Investigation ? Verify the diagnosis ? Confirm the outbreak ? Case definition ? Descriptive Epidemiology ? Develop a hypothesis Developing a Hypothesis Ask questions!! But of whom… . And when... Determining the Probable Period of Exposure ? Mean/Median incubation period ? Minimum/maximum incubation period Estimating date of exposure 0123456789101 3 5 7 9 11 13 15 17 19 21 23 25 27 29D aysCasesPeak One incubation period Rubella = 18 days Probable time of exposure Estimating date of exposure 0123456789101 3 5 7 9 11 13 15 17 19 21 23 25 27 29D aysCasesMaximum incubation 21 days Minimum incubation 14 days Probable time of exposure E. Coli Epidemic Curve Figure 3. Average incubation period = 4 days ( range 38 days) Focus of Questions Focus of Questions ? demographic information ? clinical details of the illness with date of onset, duration, and severity of symptoms ? visits to health care providers or hospitals, and laboratory results ? a plete food history in the last 7 days ? water exposure in the last 7 days (., drinking water, exposure to recreational waters) ? exposure to other ill persons in the last 7 days ? exposure to children in day care in the last 7 days ? exposure to a farm or farm animals in the last 7 days ? travel outside the immediate area in the last 7 days Interview Results Variable Cases ( n=38) Fe m al e 26 (68% ) Med Age 31 Rec water e xposur e 13 (34% ) Other Ill person 6 (16% ) Day car e 18 (47% ) Farm 2 (5% ) Fair 18 (47% ) Trave l 9 (24% ) Ha m burge r 25 (66% ) Meat 22 (58% ) Mi lk 32 (84% ) Alfalfa sprout s 19 (50 %) Lett uce 24 (63% ) Findings Thus Far ? Cases are spread over 10 counties ? No uniform attendance at any mon event ? Onset of symptoms among known cases extends over approximately one month. ? The median age of patients is 31 years (range 276)。 68% of cases are among females. ? Factors present in over 50% of cases: ? Female, milk, hamburger, lettuce, alfalfa sprouts ? Role of fair attendance, water exposure? Hypothesis? Hypothesis of Investigators ? Lettuce and/or alfalfa sprout consumption is associated with E. coli infection Steps in Outbreak Investigation ? Verify the diagnosis ? Confirm the outbreak ? Case definition ? Descriptive epidemiology ? Develop a hypothesis Steps in Outbreak Investigation ? Verify the diagnosis ? Confirm the outbreak ? Case definition ? Descriptive epidemiology ? Develop a hypothesis ? Test the hypothesis Pick a Control Group Controls Selected ? 2 controls selected for every case ? Matched to the case by: ? Age group ?(02 years, 25 years, 512 years, 1218 years, 1860 years, and 60+ years) ? Sex Methods to Identify Controls ? Random digit dailing ? Neighborhood controls ? Other patients of same physician Selection of Controls ? The investigators identified controls for the study using random digit dialing. ? Exposure information among cases was collected for the 7 days before onset of illness. ? For controls, exposure information was collected for the 7 days before the interview and for the 7 days before the onset of illness in the matching case. ? Twentyseven casecontrol sets were interviewed。 the remaining casepatients could not be reached. Interview Results V