Multistate product traceforward investigation to link imported romaine lettuce to a US cyclosporiasis outbreak - Nebraska, Texas, and Florida, June-August 2013

Abstract

During June-August 2013, 25 US states reported 631 cyclosporiasis cases including Nebraska and Iowa where a regional investigation implicated common-source imported salad mix served in two chain restaurants. At least two common-origin growing fields were likely sources of contaminated romaine lettuce. Using producer- and distributor-provided data, we conducted a grower-specific traceforward investigation to reveal exposures of ill US residents elsewhere who reported symptom onset during 11 June-1 July 2013, the time period established in the Nebraska and Iowa investigation. Romaine lettuce shipped on 2-6 June from one of these Mexico-origin growing fields likely caused cyclosporiasis in 78 persons reporting illness onsets from 11 June to 1 July in Nebraska, Texas, and Florida. Nationwide, 97% (314/324) of persons confirmed with cyclosporiasis with symptom onset from 11 June to 1 July 2013 resided in 11 central and eastern US states receiving approximately two-thirds of romaine lettuce from this field. This grower's production practices should be investigated to determine potential sources of contamination and to develop recommendations to prevent future illnesses.

Keywords: Cyclospora; epidemiology; foodborne infections; outbreaks; parasites.

Fig. 1.

General overview of the possible distribution channels* by which potentially contaminated producer A romaine lettuce-containing salad products were supplied from Mexico-origin growers and fields (growing lots) through distributors X†, D, or P to locations reported by persons ill with cyclosporiasis – multiple US states, May–June, 2013. (* This flowchart is provided only as an overview of the various growers and distribution channels described herein. Accordingly, the channels depicted are not intended to demonstrate product movement from a specific grower to any specific final end point-of-service. † Distributor X did not have a single physical location. The product was delivered directly to this company's various distribution hubs by producer A.)

Fig. 2.

US distribution of producer A romaine or romaine-containing lettuce products from grower R/ranch R/lot R by proportion of overall total poundage*, product type, and production code†, 2–6 June 2013‡. [* ‘Overall total’ represents the entire volume (100%) of processed romaine lettuce products from grower R/ranch R/lot R including all production code T product shipped by producer A from 2 to 6 June 2013 to multiple distributors in 18 central and eastern US states. † Texas and Arkansas both received small volumes of production code T product included in the overall total which contained romaine lettuce not originating from grower R/ranch R/lot R (2·4% of ‘overall total’ in approximate equal volume in both states). However, this product did share the same production line and shift and potentially shared pre-processing wash water thus potential for cross-contamination existed. ‡ Product distribution as depicted herein is not intended to represent any specific geographical locations of either distributors or cities within the states in which they are located.]

Fig. 3.

US distribution of cyclosporiasis cases with 11 June–1 July 2013 illness onset by state (N = 324).

Fig. 4.

Confirmed cyclosporiasis cases by reported date of onset and location of residence (N = 270), Texas, June–August 2013. (* Three Texas cases depicted with illness onsets prior to 8 June 2013 are reported as residents of ‘All other areas in Texas’ and not the city 1 metropolitan area; this distinction potentially includes Houston metropolitan area residents. For Texas cases occurring after 8 June, residents of the Houston metropolitan area and ‘All other areas in Texas’ are enumerated and represented accordingly.)

Fig. 5.

Confirmed cyclosporiasis cases by US state of residence, reported date of onset, and reported exposures, June–August 2013. [* Restaurant chain C exposure. † Three Texas cases in residents of two metropolitan areas other than city 1 (not included in Texas city 1 case count, N = 150).]