In total, 240 people infected with the outbreak strains of E. coli O157:H7 were reported from 37 states. Illnesses started on dates ranging from March 13, 2018 to August 22, 2018. Ill people ranged in age from 1 to 93 years, with a median age of 26. Sixty-six percent of ill people were female. Of the more than 201 people with information available, 104 were hospitalized, including 28 people who developed hemolytic uremic syndrome, a type of kidney failure. Five deaths were reported from Arkansas, California, Minnesota (2), and New York.
In addition to this outbreak being unusually large, case-patient clinical course was unusually severe. The proportion of case-patients developing HUS (12.7%) was twice as high as previous outbreaks of Shiga toxin-producing E. coli O157 (6.3%). There were five deaths in this outbreak (2.2%), which is almost four times higher than expected (0.6%). This could be explained by the strain’s stx2a toxin subtype, which produces more virulent toxins than other types. Outbreaks with stx2 toxin are more likely to result in increased rates of HUS.
WGS analysis of isolates from 184 ill people identified antibiotic resistance to chloramphenicol, streptomycin, sulfisoxazole, tetracycline, and trimethoprim-sulfamethoxazole. Standard antibiotic resistance testing of eight clinical isolates by CDC’s National Antimicrobial Resistance Monitoring System (NARMS) laboratory confirmed these findings. Isolates from four of those ill people also contained genes for resistance to ampicillin and ceftriaxone. These findings do not affect treatment guidance since antibiotics are not recommended for patients with E. coli O157 infections. Epidemiologic, laboratory, and traceback evidence indicated that romaine lettuce from the Yuma growing region was the likely source of this outbreak.
The FDA and state and local regulatory officials traced the romaine lettuce to 23 farms and 36 fields in the Yuma growing region. The FDA, along with CDC and state partners, started an environmental assessment in the Yuma growing region and collected samples of water, soil, and manure. CDC laboratory testing identified the outbreak strain of E. coli O157:H7 in water samples taken from a canal in the Yuma growing region. WGS showed that the E. coli O157:H7 found in the canal water is closely related genetically to the E. coli O157:H7 from ill people. Laboratory testing for other environmental samples is continuing. FDA is continuing to investigate to learn more about how the E. coli bacteria could have entered the water and ways this water could have contaminated romaine lettuce in the region.
Certainly, as well cited above, leafy greens have been a source of E. coli-related illnesses for decades, and there have been concerns raised about lettuce grown in the Yuma region. The CDC reports as of May 20, 2010, a total of 26 confirmed and 7 probable cases related to an E. coli O145 outbreak have been reported from 5 states since March 1, 2010 linked to shredded romaine grown in Yuma. In the FDA’s “Environmental Assessment Report in December 2010,” the authors determined:
that the R.V. park is a reasonably likely potential source of the outbreak pathogen based upon the evidence of direct drainage into the lateral irrigation canal; the moist soil in this drainage area; the multiple sewage leach systems on the property; the presence of other STEC found in the lateral irrigation canal and in the growing fields of the suspect farm; and the fact that the section of the lateral canal downstream from the R.V. park supplies water to only one other farm in addition to the suspect farm.
Two pumps are located on the main Wellton canal near the lateral canal split that supplies water to fields of the suspect farm; one gasoline powered pump on a trailer and one permanent electric pump with an attached hose. The electric pump supplies canal water to an attached open-end hose. The site is not secured from vehicles and the hose pump is also unsecured. At the time of this investigation there were people living in recreational vehicles on undeveloped land within one mile of the hose pump. The fact that this area is open to vehicles and the pump and hose are unsecured make it possible for an R.V. owner to dump and rinse out their R.V. septic system into the main Wellton canal at the lateral canal split that supplies the farm. The ground near the hose pump shows erosion evidence of drainage into the Wellton canal. Soil collected from this erosion site tested positive for other Stx2-producing STEC but did not match the outbreak strain.
In a 2009 “Survey of Selected Bacteria in Irrigation Canal Water – Third Year” written by Jorge M. Fonseca, he correctly predicted the human and industry problems that were likely to plague the Yuma lettuce growers:
Despite the fact that no Arizona lettuce grower has been involved in any contaminated-lettuce outbreak, it is of paramount importance to determine the reasons why Arizona lettuce is regarded as safe. This can help lower possibilities of any emerging problem and prevent a catastrophic damage to the industry, as it has occurred in other regions when no control was taken to reduce risks of contaminated product.
A PowerPoint done by Dr. Fonseca again illustrated the varying risks of lettuce production in Yuma. An example of a few of his points of concern:
And, then the 2018 romaine lettuce E. coli outbreak struck, sickening hundreds in the United States and Canada with dozens suffering from acute kidney failure with five reported deaths. Once again, the Wellton Irrigation Canal was the focus of attention in the “Memorandum to File on the 2018 Environmental Assessment”:
During this EA, three samples of irrigation canal water collected by the team were found to contain E coli O157:H7 with the same rare molecular fingerprint (using whole genome sequencing (WGS)) as the strain that produced human illnesses (the outbreak strain). These samples were collected from an approximate 3.5-mile stretch of an irrigation canal in the Wellton area of Yuma County that delivers water to several of the farms identified in the traceback investigation as shipping romaine lettuce that was potentially contaminated with the outbreak strain. The outbreak strain was not identified in any of the other samples collected during this EA, although other pathogens of public health significance were detected.
Not surprisingly, the FDA in its full “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 was in fact a well-know and long-standing risk:
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.
Well-established reservoirs for E. coli O157:H7 are the intestinal tract of ruminant animals (e.g., cattle, goats, and deer) that are colonized with STEC and shed the organism in manure. Ruminant animals colonized with STEC typically have no symptoms. In contrast, human infection with E. coli O157:H7 usually produces symptomatic illness often marked by severe, often bloody, diarrhea; severe adverse health outcomes or even death can result. Humans shed E. coli O157:H7 in the stool while ill and sometimes for short periods after symptoms have gone away, but humans are not chronic carriers. Various fresh water sources, including municipal well, and recreational water, have been the source of E. coli O157:H7 infections in humans, as has contact with colonized animals at farms or petting zoos. However, most E. coli O157:H7 infections in humans occur from consuming contaminated food.
In its summary of its environmental findings (also summarized in a November 1, 2018 to public officials) 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 this 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.
 Lyndsay Bottichio, et al., Shiga Toxin-Producing E. coli Infections Associated with Romaine Lettuce – United States, 2018, Clinical Infectious Diseases (December 9, 2019), https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciz1182/5669965.