Two examples of case-controls studies from Finland

1. Two examples of case-controls studies from Finland Markku Kuusi National Public Health Institute (KTL) Department of Infectious Disease Epidemiology

2. Investigation of waterborne outbreaks In Finland 4 � 8 waterborne outbreaks reported/year Norovirus and campylobacter most common etiologic agents Investigation is challenging collaboration important: epidemiology, microbiology and inspection of water system

3. Methods in investigating waterborne outbreaks In principle investigation follows same steps as any other outbreak Descriptive epidemiology important Problems when conducting analytical studies nearly all may have been exposed to same water publicity recall bias: often cases think more about exposure cases more active respondents ? selection bias

4. Outbreak August 3, 2000: 3 persons with gastroenteritis contacted local health centre Next days: more persons with GID to health centre, stool samples collected August 9: Campylobacter jejuni isolated from stool samples August 10: KTL notified of the outbreak

5. Preliminary investigation and control measures Interviews of individual patients suggested that water might be the source Water samples collected on August 4 and August 10 Boil-water notice on August 10 Chlorination on August 11

6. Methods Epidemiology descriptive: line-list of all persons who contacted the health centre case-control study Microbiology 74 stool samples collected water samples typing of campylobacter: PFGE, serotyping Environmental thorough inspection of the water supply system (non-chlorinated ground well water)

7. Case-control study Case definition resident of the municipality diarrhoea (?3 loose stools/24 hours) AND/OR vomiting AND/OR abdominal pain onset: between July 31 and August 20, 2000 Only one case/household (earliest onset) Of eligible 250 cases 50% randomly selected to the study Exclusion: travelled abroad within 10 days before illness onset

8. Controls 3 controls/case selected randomly from population registry Matched design age (+/- 1 year) sex postal code of residency

9. Data collection Postal questionnaire symptoms onset of illness treatments consumption of water tap water well water bottled water consumption of poultry, unpasteurised milk, etc. two-week period before onset of symptoms in the case mailed on September 15

10. Cases of gastroenteritis, Asikkala municipality, August 2000

11. Characteristics of cases Median age 46 (range 5 � 89 years) 65% women Symptoms diarrhoea 95% abdominal pain 89% nausea 76% fever 63% vomiting 36% Median duration 6 days

15. Environmental investigation Two ground water wells and two reservoirs both in proximity of a large lake vacation houses close to well B no connection to municipal sewage system compost 15 meters from the well wells and reservoirs not fenced lock of one reservoir broken

16. Conclusions Strong epidemiological and microbiological evidence of waterborne etiology Water system probably contaminated for several days Exact mechanism of contamination unknown several risk sites for contamination Safety of water system should be improved UV-disinfection chlorination new ground water wells?

17. Yersinia pseudotuberculosis Zoonosis occurring especially in the northern hemisphere typical symptoms: fever and severe abdominal pain incubation period: 1-10 days Postinfectious complications reactive arthritis, erythema nodosum Mode of transmission contaminated food or water direct contact with animals Vehicle rarely identified

18. Surveillance in Finland Clinical laboratories report cases with Y. pseudotuberculosis isolated from stool samples to National Infectious Disease Register and the strains are sent to NPHI for typing 30-60 cases/year reported Most common serotypes O:1 and O:3 Seven outbreak investigations 1997, 1998 (two), 1999, 2001, 2003 and 2004 Domestic iceberg lettuce identified as vehicle in a case-control study in 1998 1) Bacteria not isolated from foodstuffs prior to 2003 1) Nuorti JP et al. J Infect Dis, 2004

19. Outbreak May 23, 2003: NPHI notified of a cluster of Y. pseudotuberculosis infections Ill persons were school and day-care children in Kotka in southeaster Finland (population 55,000) Ill persons had eaten lunches prepared by one institutional kitchen Produces 7,000 meals/day to local schools and day-care centres

20. Preliminary investigations Case finding Routine notifications to NIR Active case finding at all health care centres and hospitals of the area Lunch menus Two-week period (from April 28 to May 9) before the first cases Inspection of the kitchen Food samples not available anymore Review and assessment of food preparation procedures Finding out the origin of all foodstuffs

21. Investigations I Epidemiological investigation � case-control tudy II Food and environmental investigations � based on results of epidemiological investigation � investigation of production � environmental samples III Laboratory investigation Subtyping of Y. pseudotuberculosis strains (serotyping ja pulsed-field gel electrophoresis; PFGE)

22. Methods � case-control study Population at risk 7392 persons: SCHOOLS DAY-CARE CENTRES 6,223 school children - 495 day-care children 534 members of staff - 140 members of staff Case definition Y. pseudotuberculosis O:1 isolated from stool samples, OR erythema nodosum or reactive arthritis diagnosed clinically by a physician Onset of symptoms between May 9 and 28 Controls selected randomly from the population at risk; frequence matching according to class

23. Cases 103 persons met the case definition 77 (74%) recruited for the study 44 (58%) had erythema nodosum and one had reactive arthritis 40 (53%) men 26 cases identified after the study was begun Main symptoms fever (92%), abdominal pain (86%), arthralgia (44%), diarrhoea (24%), vomiting (17%)

24.

25. Case-control study - results 194 (3 %) controls selected Excluded controls: 22 with fever and abdominal pain 76 cases and 172 healthy controls Response rate in telephone interviews cases: 99% controls: 96%

26. Odds ratios for fresh products

27. Quantity of grated carrots eaten and Odds ratio (dose-response)

28. Environmental investigation Carrots traced to the production farm Growing and storage conditions assessed Farmer produced and processed washed and peeled carrots to two institutional kitchens Carrots from previous year�s crop Stored during winter at 0 �C No animal faecal contamination could be identified

29. Environmental samples Samples from the production line: 12 samples of soil mixed with carrot residues Y. pseudotuberculosis isolated from 5 (42%) samples Serotype O:1 PFGE pattern indistinguishable from strains isolated from patients Carrots provided also to schools in Tampere Y. pseudotuberculosis O:1 clusters detected in May Strains isolated from patients indistinguishable from the strains isolated in Kotka by PFGE

30. Conclusions One of the largest foodborne outbreaks in Finland For the first time Y. pseudotuberculosis isolated from the production line of the epidemiologically implicated foodstuff Y. pseudotuberculosis is an emerging pathogen The bacteria are not always detected in routine stool cultures Animal reservoirs and contamination mechanisms need to be studied further

31. Recommendations Contamination of fresh products is causing an increasing number of outbreaks Carrots are an unusual vehicle The practices of handling fresh products in institutional kitchens should be reevaluated Sell of previous year�s carrots in April-May should be discontinued Surveillance of this emerging pathogen in other European countries should be strengthened