Outbreak of cyclosporiasis associated with basil in Missouri in 1999.

During the summer of 1999, an outbreak of cyclosporiasis occurred among attendees of 2 events held on 24 July in different counties in Missouri. We conducted retrospective cohort studies of the 2 clusters of cases, which comprised 62 case patients. The chicken pasta salad served at one event (relative risk [RR], 4.25; 95% confidence interval [CI], 1.80-10.01) and the tomato basil salad served at the other event (RR, 2.95; 95% CI, 1.72-5.07) were most strongly associated with illness. The most likely vehicle of infection was fresh basil, which was included in both salads and could have been grown either in Mexico or the United States. Leftover chicken pasta salad was found to be positive for Cyclospora DNA by means of polymerase chain reaction analysis, and 1 sporulated Cyclospora oocyst was found by use of microscopy. This is the second documented outbreak of cyclosporiasis in the United States linked to fresh basil and the first US outbreak for which Cyclospora has been detected in an epidemiologically implicated food item.