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 investigation was conducted around the 6 laboratory - confirmed case - patients reported in the last 2 weeks of may and the first week of june ( phase 1 ) .
no common exposure was reported , 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 confirmed and 4 probable cases were identified with symptom onsets during june 28july 20 , 2008 .
average time from symptom onset to positive laboratory result was 17 days ( range 631 days ) . confirmed and probable cases of cyclosporiasis ( n = 29 ) , by date of onset , british columbia , canada , may  august 2007 . *
laboratory - confirmed cases were reported to public health by medical diagnostic laboratories and specimens forwarded for confirmation to the public health reference laboratory .
oocysts included shape ( spherical ) , size ( 810 m in diameter ) , oocyst wall ( well - defined ) , internal contents with refractile globules , autofluorescence , and modified acid - fast or safranin staining ( 1 ) . during phase 2 , a total of 17 confirmed case - patients were interviewed with hypothesis - generating questionnaires about items eaten in the 2 weeks before symptom onset .
the instrument included questions about restaurant history 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 .
frequently reported foods were compared with population controls from canadian ( waterloo , ontario ) and american ( oregon ; us foodborne diseases active surveillance network [ foodnet ] ) published food consumption surveys ( 35 ) .
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
garlic and red peppers also were commonly eaten by case - patients ; however , population comparisons were unavailable .
eighty - eight percent of case - patients reported having eaten romaine lettuce ; 85% of controls in the waterloo 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 ) .
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 allowed room for additional hypotheses to emerge or existing hypotheses to strengthen as cases accrued .
* case - patients were 17 persons with laboratory - confirmed cases interviewed during phase 2 ( june 24july 21 , 2007 ) .
us foodnet , foodborne diseases active surveillance network ; na , not applicable . detailed questionnaires asked whether foods were eaten in a restaurant or were store - bought and about type of packaging and method of preparation ( because c. cayetanensis is heat - sensitive ) ( 6 ) .
we reinterviewed early case - patients using the second questionnaire and interviewed later case - patients using both questionnaires . in phase 1 ,
garlic eaten at restaurants by all 4 persons with confirmed infections were traced back to different suppliers ; only 1 case - patient ate raw garlic in a restaurant .
three case - patients also reported eating cooked garlic at home ; cooking would have inactivated the pathogen . early and proactive collaboration with cfia involved a general assessment of the country of origin and distribution patterns for frequently eaten foods . according to cfia records ,
romaine lettuce and red peppers sold during the exposure period were not imported from a known cyclospora - endemic country and were widely distributed in canada and the united states .
comm . ) . because interviews , population control comparisons , and product distribution limited suspected foods to strawberries , cilantro , and basil , we began preliminary traceback of all 3 suspected items . environmental health officers and regional cfia staff
interviewed grocery store owners , restaurant managers , and distributors to trace produce to its supplier .
local strawberries eaten by case - patients from 3 small markets were traced back to 2 local farms in geographically separate regions of british columbia .
cilantro eaten by case - patients was traced to 2 suppliers ; both supplied home - grown rather than imported produce .
of 14 case - patients with confirmed basil exposures , 4 ( 57% ) ate only organic basil supplied by distributor a. additionally , 4 ( 29% ) reported multiple basil exposures , including exposure to organic basil from distributor a ( figure 2 ) . in british columbia , organic basil enjoys a smaller market share than the conventional product .
traceback of basil eaten by persons with confirmed cyclosporiasis ( n = 14 ) , british columbia , canada , may 
august 2007 . in phase 2 , 12 ( 71% ) of 17 case - patients reported shopping at grocery c. records of any grocery store purchases for the households of 8 consenting case - patients were obtained through grocery c s savings card program ; other case - patients were not cardholders .
all purchase histories were requested for 1 month before symptom onset to account for the typical incubation period plus product shelf life .
records from 3 ( 38% ) case - patients showed purchases of the same organic basil supplied by distributor a. two case - patients had bought organic basil on the same day at the same location .
of the remaining 5 case - patients who recalled purchasing organic basil but whose consumer card records did not confirm it , 2 had not used their cards for large portions of the incubation period .
we collected supplier information for organic basil during a visit to the distribution warehouse and local farm site of distributor a. the remaining 2 ( 14% ) case - patients with basil exposure previously unlinked to distributor a were confirmed through trace - forward from distributor a. the first had eaten organic basil at a smaller market supplied by distributor a under another trade name .
the second had eaten conventional basil from a grocery store supplied by distributor a. distributor a confirmed using organic basil to supplement conventional basil shipments when supply was low .
late summer outbreaks of cyclosporiasis in british columbia are unusual ; distributor a confirmed that imported product was used throughout the summer in 2007 because of a poor local growing season .
all case - patients in phase 2 who recalled basil exposure ( 82% ) could have been exposed to organic basil from distributor a. once this common vehicle was identified , cfia conducted a full traceback of organic basil by using formal documentation including invoices , shipment numbers , and airway bills . the suspected imported basil was no longer available for testing .
using distributor a invoices , we identified a specific shipment of organic basil imported from 1 of 2 mexican supplier farms , and cfia notified mexican authorities .
the mexican farm was located in a region previously linked to cyclosporiasis outbreaks ( r. cardinal , cfia , pers .
detailed interviews , modified traceback of several suspected items , and information about product distribution and market share led to organic basil as a primary hypothesis .
food regulators could pinpoint a specific shipment and trace it to its origin because consumer cards provided the exact purchase dates for basil that case - patients could not recall .
overall , the approach used in this investigation increased the work load typically requested of team members during foodborne outbreaks .
however , this combination of investigative methods successfully identified a single vehicle during a community cyclosporiasis outbreak where a common menu was not available .
 investigations of community outbreaks of cyclosporiasis 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 , including early consultation with food regulators , traceback of suspected items , and grocery store loyalty card records , to identify a single vehicle for a cyclosporiasis outbreak in british columbia , canada , in 2007 . 