Challenges of Investigating Community Outbreaks of Cyclosporiasis, British Columbia, Canada Lena Shah, Laura MacDougall, Andrea Ellis, Corinne Ong, Sion Shyng, Linda LeBlanc, and the British Columbia Cyclospora Investigation Team 1 Investigations of community outbreaks of cyclosporia- sis are challenged by case-patients’ poor recall of exposure resulting from lags in detection and the stealthy nature of food vehicles. We combined multiple techniques, includ- ing early consultation with food regulators, traceback of suspected items, and grocery store loyalty card records, to identify a single vehicle for a cyclosporosis outbreak in Brit- ish Columbia, Canada, during 2007. C yclospora cayetanensis is an emerging coccidian para- site that causes outbreaks of protracted and relapsing gastroenteritis (1,2). Delays in clinical diagnosis caused by the waxing and waning symptoms of Cyclospora infection, coupled with a long incubation period (median 7 days) and concealed food vehicles (e.g., herbs), result in poor recall of food exposures. Therefore, outbreaks in which no com- mon meal is eaten are even more diffcult to solve. In 2007, the British Columbia Centre for Disease Control used early collaboration with the Canadian Food Inspection Agency (CFIA), grocery card shopping records, and product trace- back for several suspected items simultaneously to suc- cessfully implicate a vehicle in a community Cyclospora outbreak. The Study From May 1 through July 30, 2007, a total of 29 cases of locally acquired Cyclospora infection were reported in British Columbia (Figure 1; Table 1). An initial investi- gation was conducted around the 6 laboratory-confrmed case-patients reported in the last 2 weeks of May and the frst week of June (phase 1). No common exposure was re- ported, and case reports subsided. During the last week of June, case reports resumed, and phase 2 of the investigation was initiated. A total of 19 confrmed and 4 probable cases were identifed with symptom onsets during June 28–July 20, 2008. Fifty-three percent of these cases occurred in male patients. No case-patients were hospitalized. Average time from symptom onset to positive laboratory result was 17 days (range 6–31 days). During phase 2, a total of 17 confrmed case-patients were interviewed with hypothesis-generating question- naires about items eaten in the 2 weeks before symptom on- set. The instrument included questions about restaurant his- tory with meal details; grocery stores frequented; and yes/ no questions about >70 fruits and vegetables, 8 herbs, and 16 mixed foods (e.g., salsa, pesto) previously implicated in outbreaks of foodborne disease. No common restaurants or events were identifed. Frequently reported foods were compared with popu- lation controls from Canadian (Waterloo, Ontario) and American (Oregon; US Foodborne Diseases Active Sur- veillance Network [FoodNet]) published food consumption surveys (3–5). Although such measurements may be limited by the timing of questionnaire administration and the recall period considered, they can be useful comparators during the hypothesis-generating stages of an investigation. By the end of phase 2, strawberries, cilantro, and sweet basil were reported more often than expected by case-patients (Table 2). Garlic and red peppers also were commonly eaten by case-patients; however, population comparisons were un- available. Eighty-eight percent of case-patients reported having eaten romaine lettuce; 85% of controls in the Water- loo survey (4,5) had eaten lettuce of any type, and romaine lettuce consumption was much less commonly reported in the FoodNet survey (3) (Table 2). Other foods assessed were not eaten more often than expected. A formal case– control study was considered premature in the early stages of phase 2 because no strong hypothesis emerged from early interviews and comparisons to population controls. We further explored the plausibility of various hypotheses through a combination of methods described below that al- lowed room for additional hypotheses to emerge or existing hypotheses to strengthen as cases accrued. DISPATCHES 1286 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 15, No. 8, August 2009 1 British Columbia Cyclospora Investigation Team: Dan Moreau, Craig Nowakowski (Vancouver Island Health Authority); Mark Ritson, Arne Faremo (Vancouver Coastal Health Authority); Krista Wilson, Brian Gregory (Interior Health Authority); Susan Schleicher, Shendra Brisdon, Jason Stone (Fraser Health Authority); Julie Wong, Joe Fung, Linda Hoang (Provincial Health Services Authority Laboratories); and Lisa Wu (Canadian Food Inspection Agency, Burnaby). Author affliations: Public Health Agency of Canada, Ottawa, Ontar- io, Canada (L. Shah); British Columbia Centre for Disease Control, Vancouver, British Columbia, Canada (L. Shah, L. MacDougall, C. Ong, S. Shyng); Public Health Agency of Canada, Guelph, Ontario, Canada (A. Ellis); and Canadian Food Inspection Agency, Ottawa (L. LeBlanc) DOI: 10.3201/eid1508.081585