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BACKGROUND: Cyclosporiasis has historically been rare in Nebraska. On June 28, 2013, two seemingly unrelated cases of cyclosporiasis were reported with onset dates within 10 days of each other. Concurrently, Iowa reported several persons with cyclosporiasis. An investigation was initiated to define the nature and extent of this outbreak.
METHODS: We defined a confirmed case as cyclosporiasis in a Nebraska resident with laboratory-confirmed illness and a probable case as clinically compatible illness in a dining partner with onset ≤14 days following a meal shared with a confirmed case. We defined the outbreak period as June 1–August 31, 2013; persons with foreign travel ≤14 days prior to symptom onset were excluded. Using a questionnaire focusing on exposure to known risk factors for cyclosporiasis ≤14 days prior to symptom onset, we interviewed all persons with laboratory-confirmed illness. On July 15, a cluster investigation and cohort study were initiated comparing patients with confirmed and probable illness with non-ill dining partners. Traceback was conducted to ascertain product lot codes served in implicated restaurants and to identify a common producer and distributor of implicated produce.
RESULTS: A total of 87 confirmed cyclosporiasis cases were identified in Nebraska. Of these, 53 (61%) reported exposure to restaurant chain A or B; 92% (49/53) of whom reported consuming salad. Chains A and B are owned by the same parent company sharing common food items from the same distribution supply chain. The cohort study compared 27 persons with confirmed (n=17) and probable (n=10) illness with 15 non-ill dining partners. House salad exposure was significantly associated with higher illness risk among confirmed/probable ill persons compared with their non-ill dining partners (26/27, 96% vs. 7/15, 47% [risk ratio, 7.09; 95% confidence interval, 1.10–45.38]). A single production code (Lot T) of bagged, pre-washed salad mix (iceberg, romaine, red cabbage, and carrots) supplied by a common producer and distributor was served as house salad at all Chain A/B restaurants linked to ill-persons with confirmed cases. Iowa and Wisconsin officials similarly identified Lot T distribution to restaurant A/B outlets where ill persons reported dining in those states.
CONCLUSIONS: An outbreak of cyclosporiasis was associated with exposure to single lot (Lot T) bagged, pre-washed salad at restaurant chains A/B. Further study is needed to identify a common grower for an ingredient in the salad mix and to determine if ill persons not associated with restaurants A/B share a common product exposure with those having restaurant A/B exposure.