The December 2017 Cluster
In late 2017, three cases of E. coli O157:H7 were reported in Wisconsin. These Wisconsin cases were found to have genetic fingerprints that matched each other, which was an essentially definitive signal to investigating epidemiologists that the cases of illness had a common source. Wisconsin state health officials determined that this cluster was likely sickened by a contaminated mixed green salad served at an unidentified school.
The three Wisconsin cases were classified as part of a CDC cluster (CDC Cluster Code: 1801MLEXH-1). This cluster soon grew to definitively include numerous cases from other states (Arizona, California, Maine, Michigan, New Jersey, Nevada, New York, Utah, Washington, and Wisconsin). Ultimately, the cluster included approximately 17 cases from ten states, all of whom had experienced an onset of symptoms from November 6, 2017, to January 9, 2018.
Genetic analysis conducted by the CDC on the “isolate” (i.e., the E. coli O157:H7 bacteria isolated from her stool sample) showed that her illness was a genetic match to the cluster that originated in Wisconsin. The client was, therefore, definitively included as a confirmed case in this ten-state cluster of E. coli O157:H7 illnesses, as is set forth in the CDC’s official “line list” (i.e., chart of all confirmed cases in a cluster), which is reproduced below:
The Spring 2018 E. coli O157:H7 Cluster
Not long after the 1801MLEXH-1 cluster occurred, 240 E. coli O157:H7 cases from 37 states were identified in one of the largest ever E. coli outbreaks linked to the consumption of contaminated romaine lettuce. Due to its sheer size, this outbreak investigation demanded significant federal resources, including thorough epidemiologic and environmental investigations by CDC and FDA. Almost all the confirmed cases in this outbreak, known by CDC Cluster Code 1804 MLEXH-1, occurred from March 13 through early May 2018.
The critical findings from the 2018 cluster were as follows:
· The 2018 cluster involved an identical strain, by genetic analysis, to the late 2017 cluster;
· CDC’s epidemiologic investigation conclusively identified romaine lettuce grown in the Yuma, Arizona growing region as the source of the outbreak;
· The E. coli strain involved in the 2018 cluster originated at an extremely large Centralized Animal Feeding Operation (CAFO) located in Wellton, Arizona (cattle are overwhelmingly the predominant animal source of E. coli O157:H7);
· The E. coli strain involved in the 2018 cluster was found in three different water samples from an irrigation canal that flows along the border of the CAFO in Wellton, Arizona.
The Yuma, Arizona Growing Region
In the past, lettuce from the Yuma growing region has been the source of several E. coli outbreaks.
Not surprisingly, the FDA, in its complete “Environmental Assessment of Factors Potentially Contributing to the Contamination of Romaine Lettuce Implicated in a Multi-State Outbreak of E. coli O157:H7” concluded that the risk of environmental contamination associated with the 1804MLEXH-1 outbreak was, in fact, a persistent risk likely associated with past outbreaks, as well:
Food safety problems related to raw whole and fresh-cut (e.g., bagged salad) leafy greens are a longstanding issue. As far back as 2004, FDA issued letters to the leafy greens industry to express concerns about continuing outbreaks associated with these commodities. FDA and our partners at CDC identified 28 foodborne illness outbreaks of Shiga-toxin producing E. coli (STEC) with a confirmed or suspected link to leafy greens in the United States between 2009 and 2017. This is a time frame that followed industry implementation of measures to address safety concerns after a large 2006 outbreak of E. coli O157:H7 caused by bagged spinach.
STEC contamination of leafy greens has been identified by traceback to most likely occur in the farm environment.
Contamination occurring in the farm environment may be amplified during fresh- cut produce manufacturing/processing if appropriate preventive controls are not in place. Unlike other foodborne pathogens, STEC, including E. coli O157:H7, is not considered to be an environmental contaminant in fresh-cut produce manufacturing/processing plants.
In its summary of its environmental findings, the “FDA (in part) identified the following factors and findings as those that most likely contributed to the contamination of romaine lettuce from the Yuma growing region with E. coli O157:H7 that caused [the 1804MLEXH-1 outbreak]”:
• FDA has concluded that the water from the irrigation canal where the outbreak strain was found most likely led to contamination of the romaine lettuce consumed during this outbreak.
• There are several ways that irrigation canal water may have come in contact with the implicated romaine lettuce, including direct application to the crop and/or use of irrigation canal water to dilute crop protection chemicals applied to the lettuce crop, either through aerial or ground-based spray applications.
• How and when the irrigation canal became contaminated with the outbreak strain is unknown. A large animal feeding operation is nearby, but no obvious route for contamination from this facility to the irrigation canal was identified. Other explanations are possible, although the EA team found no evidence to support them.