How is this acceptable?

I have spent today reviewing this most interesting study and took the liberty of editing it down for reading “consumption.”

This outbreak was the largest multistate STEC O157 outbreak in several decades, eclipsing in magnitude a 2006 outbreak linked to fresh spinach. As there are an estimated 26 unreported illnesses for every STEC O157 case reported to PulseNet, the true size of this outbreak was likely much larger than the 240 illnesses reported through PulseNet, suggesting that thousands of people were actually sickened in this outbreak.

240 case-patients (not 210) from 37 states; 104 were hospitalized, 28 developed hemolytic uremic syndrome, and five died. Of 179 people who were interviewed, 152 reported consuming romaine lettuce in the week before illness onset. Twenty sub-clusters were identified.

Product traceback from sub-cluster restaurants identified numerous romaine lettuce distributors and growers; all lettuce originated from the Yuma growing region.

Water samples collected from an irrigation canal in the region yielded the outbreak strain of E. coli O157.

One case-patient was co-infected with E. coli O157 and E. coli O61, and two case-patients were infected with only E. coli O61. Case-patients were 1–93 years (median 26 years), and 66% (158/239) were female. All outbreak isolates were related genetically by WGS (0–40 SNP differences). Two distinct sub-clades that included the majority of outbreak isolates, which had isolates with 0–19 SNP differences.

In April 2018, the New Jersey Department of Health and the Pennsylvania Department of Health notified the Centers for Disease Control and Prevention of two E. coli infection clusters. Before becoming ill, people in both clusters reported eating salads at a national restaurant chain. Pulsed- field gel electrophoresis showed that the E. coli O157 isolated from ill people in both states had the same, rare PFGE pattern combination.

Product traceback efforts by local health jurisdictions, state health departments, and FDA could not identify a single product lot, processor, or farm as the source of the entire outbreak. In total, 36 fields on 23 farms were identified during the traceback investigation. Only the Alaska correctional facility sub-cluster could be traced to a single farm. All of the romaine lettuce identified was traced to farms in the Yuma growing region. FDA, CDC and state partners conducted an EA in the growing region, which consisted of farms in Imperial County, CA and Yuma County, AZ.

In June 2018, E. coli O157 with PFGE patterns indistinguishable from clinical outbreak isolates and closely related genetically to clinical isolates in both main clades were detected in irrigation water from the Yuma growing region. The outbreak strains were detected in three separate water samples from sites along a 3.5-mile section of the Wellton irrigation canal that runs adjacent to romaine lettuce farms identified during traceback and next to a concentrated animal feeding operation (CAFO) with approximately 105,000 cattle. No E. coli O61 was identified from the environmental assessment or water samples collected. No romaine lettuce was available for sampling during this outbreak since harvesting had ended in this region by the time the environmental assessment began.

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 STEC 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.

The ultimate source of E. coli O157 for this outbreak is unknown, but reasonable hypotheses exist that explain how romaine lettuce could have become contaminated. The E. coli O157 outbreak strain was detected at multiple points in a Yuma growing region irrigation canal, and the contaminated canal water may have subsequently contacted romaine lettuce in several ways, including direct application to the crop through aerial or land-based spray chemicals diluted with canal water.

However, how and when E. coli O157 was introduced into the irrigation canal is unknown, as well as why the majority of illnesses were associated with romaine when other types of lettuce were grown in the same area during the time of interest. A CAFO was located adjacent to the irrigation canal where the outbreak strain was found, but no obvious route for contamination from the facility was identified. Environmental contamination could have been caused by the adjacent CAFO, as cattle are a well-documented reservoir for pathogenic E. coli O157.


  1. Rangel JM, Sparling PH, Crowe C, Griffin PM, Swerdlow DL. Epidemiology of Escherichia coli O157:H7 outbreaks, United States, 1982-2002. Emerging Infectious Diseases, 2005; 11(4): 603-9.
  2. Dewey-Mattia D, Manikonda K, Hall AJ, Wise ME, Crowe SJ. Surveillance for Foodborne Disease Outbreaks – United States, 2009-2015. Morbidity and Mortality Weekly Report Surveillance Summaries, 2018; 67(10): 1-11.
  3. Sharapov UM, Wendel AM, Davis JP, et al. Multistate Outbreak of Escherichia coli O157:H7 Infections Associated with Consumption of Fresh Spinach: United States, 2006. Journal of Food Protection, 2016; (12): 2024.
  4. Food Bill Aims to Improve Safety. FDA Consumer Health Information 2010.
  5. Scallan E, Hoekstra RM, Angulo FJ, et al. Foodborne illness acquired in the United States– 
major pathogens. Emerging Infectious Diseases, 2011; 17(1): 7-15.
  6. Heiman KE, Mody RK, Johnson SD, Griffin PM, Gould LH. Escherichia coli O157 Outbreaks in the United States, 2003-2012. Emerging Infectious Diseases, 2015; 21(8): 1293-301.
  7. National Shiga toxin-producing Escherichia coli (STEC) Surveillance Overview. Atlanta, Georgia: CDC, 2012.
  8. General information: Escherichia coli. Available at:
  9. PulseNet home page. Available at:
  10. Katz L.S., Griswold T., Williams-Newkirk A.J., et al. . A Comparative Analysis of the Lyve-SET 
Phylogenomics Pipeline for Genomic Epidemiology of Foodborne Pathogens. Frontiers in 
Microbiology, 2017; 8:375.
  11. Foodborne Diseases Active Surveillance Network (FoodNet): population survey. Available at:
  12. Environmental Assessment; Yuma 2018 E. coli O157:H7 Outbreak Associated with Romaine Lettuce. US Food and Drug Administration, October 24, 2018.
  13. National Antimicrobial Resistance Monitoring System for Enteric Bacteria (NARMS). Available at:
  14. National Antimicrobial Resistance Monitoring System for Enteric Bacteria (NARMS): Human Isolates Surviellance Report for 2015 (Final Report). Atlanta, Georgia: CDC, 2018.
  15. National Outbreak Reporting System (NORS) Dashboard. Available at:
  16. Gould LH, Demma L, Jones TF, et al. Hemolytic Uremic Syndrome and Death in Persons with Escherichia coli O157:H7 Infection, Foodborne Diseases Active Surveillance Network Sites, 2000– 2006. Clinical Infectious Diseases, 2009; 49(10): 1480-5.
  17. Food Safety Modernization Act (FSMA) Final Rule on Produce Safety. Available at:
  18. Beauvais W, Gart EV, Bean M, et al. The prevalence of Escherichia coli O157:H7 fecal shedding in feedlot pens is affected by the water-to-cattle ratio: A randomized controlled trial. PloS One, 2018; 13(2): e0192149-e.
  19. Karmali MA. Factors in the emergence of serious human infections associated with highly pathogenic strains of shiga toxin-producing Escherichia coli. International Journal of Medical Microbiology, 2018; 308(8): 1067-72.
  20. Wilson D, Dolan G, Aird H, et al. Farm-to-fork investigation of an outbreak of Shiga toxin- producing Escherichia coli O157. Microbial Genomics, 2018; 4(3): e000160.