The October 2020 issue of the CDC’s Emerging Infectious Diseases has a study about E. coli O157:H7 outbreaks from 2009 to 2018 and found that there were forty leafy greens STEC outbreaks during that time frame. STEC is Shiga toxin-producing E. coli bacteria, most often E. coli O157:H7.
Those outbreaks caused 1,212 illnesses, 77 cases of hemolytic uremic syndrome (HUS), and eight deaths. More of those outbreaks were linked to romaine lettuce than any other type of leafy green.
Forty-five percent of those outbreaks occurred in the fall, and 28% occurred in the spring. Leafy greens are the second most common source of foodborne STEC outbreaks, after ground beef. The connection between those two products is that cattle are considered the major reservoir for the pathogen, and STEC shed from cattle and wild animals can contaminate leafy greens, either directly in the fields, or through contaminated irrigation water, runoff, or dust containing feces.
The peak outbreak months, October and April, coincided with the time frame when growers use California Central Valley growing region to fill the gap between the Central California Coast and desert regions of California, Arizona, and Mexico.
Environmental assessments, which are crucial to determining the source of the pathogen, rarely occur during the outbreak investigation. Officials can only investigate the environment after epidemiological and traceback investigations identify where the contaminated product is grown, processed, or distributed.
These environmental assessments have suggested a possible link between product contamination and STEC contamination of nearby soil or water caused by cattle or wild pigs, dairy farms, or concentrated animal feeding operations (CAFOs). Some leafy green growing regions are home to large numbers of cattle, especially the California Central Valley growing region.
Investigators think that romaine may be more vulnerable to contamination because of its shape and physiology. Romaine lettuce grows tall, with loosely clumped leaves open at the top like a V, where water can collect, giving bacteria time to contaminate it.
The report recommends that traceback for leafy greens needs to be improved. For instance, lot information may be printed on the packaging for leafy greens, but points of sale may not record and track it after receiving the product. Complete, detailed records of transactions at each point on the fork-to-farm continuum are critical to trace greens during and outbreak investigation. Retails could require producers to be able to trace leafy greens and greens used in packaged mixes back to the farms where they were harvested.
While industry has made some changes, knowledge gaps still remain, including why outbreaks are so linked to romaine, and the drivers of the seasonality of the outbreaks. Filling these gaps could help growers, producers, and the government to craft prevention strategies to avoid or mitigate future outbreaks.
Marshall KE, Hexemer A, Seelman SL, Fatica MK, Blessington T, Hajmeer M, et al. Lessons Learned from a Decade of Investigations of Shiga Toxin–Producing Escherichia coli Outbreaks Linked to Leafy Greens, United States and Canada. Emerg Infect Dis. 2020;26(10):2319-2328. https://dx.doi.org/10.3201/eid2610.191418
