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How Did E. coli O145 Contaminate Lettuce? Part III

A look at how E. coli O145 could have contaminated romaine lettuce on a farm in Yuma – Part III



As state and federal public health officials continue to investigate the E. coli O145 outbreak tied to bagged Freshway Foods romaine lettuce, which has sickened at least 23 people in 4 states, many questions remain.



The supply chain from the field to the supermarket is a long one, with many potential points along the way for contamination to occur. Where did the lettuce pick up E. coli O145, a pathogen found primarily in cattle and wildlife feces? According to the latest out of the U.S. Food and Drug Administration (FDA), investigators are focusing on an undisclosed farm in Yuma, Arizona which could be linked to the outbreak. If the contamination did occur on the farm, how could it have happened?



Unlike Salinas Valley, America’s salad bowl, which has been the source of several E. coli outbreaks, including the Dole spinach outbreak in 2006, Yuma-grown leafy greens have never been implicated.



Food Safety News paid a visit to the Yuma area and talked with epidemiological experts to explore a number of hypotheses. This series will look at three ways the E. coli O145-contaminated lettuce–if it was grown in Yuma–could have picked up the bug. Part I explored dust and mud as possible modes of contamination. Yesterday, Part II looked at wildlife intrusions, and today, Part III will discuss irrigation water.



Part III – Irrigation Water



It takes between 40 and 50 inches of water per acre to produce a desirable lettuce crop, according to the University of Arizona’s Cooperative Extension. The fields in Yuma depend on a network of open canals that channel water from the Colorado River to provide adequate water for growing greens in the desert.

yumawater2.jpgAs we noted yesterday, one of the largest cattle feedlots in the country is located in Wellton, AZ, around 20 miles, as the crow flies, (and downstream) from the heart of leafy green production in the Gila and Dome Valleys near Yuma. (At left: irrigation canal with Wellton feedlot in the backdrop).

Naturally, the high concentration of cattle and manure produces a fecal, muddy mix in which E. coli bacteria can thrive. As we discussed in Part I and Part II, that mud or mud-turned-dust can travel via wind, vehicle, person, or wild animal. It is possible that one of these modes of transmission could contaminate irrigation water headed for a leafy green field.

Irrigation water has long been recognized by food safety scientists as one of the most plausible and probable sources of fresh produce pathogenic contamination, especially in states like Arizona that use irrigation to manage essentially all crop production.
 
In 1996, cattle in an adjacent field were implicated as the source of E. coli O157:H7 during a multi-state outbreak associated with the consumption of lettuce. Investigators speculated that contaminated water was used to irrigate the lettuce fields.

A 2002 study done by the Department of Food Sciences at Rutgers University concluded that “E. coli can survive for extended periods in water, and lettuce irrigated with contaminated water results in contamination of the edible portion of the lettuce plant.”
  
Trevor Suslow, who is an Extension Research Specialist for the Department of Plant Sciences at the University of California, Davis, issued a report for the Produce Safety Project in 2009 detailing the potential risk of using irrigation water to treat produce fields.
   
For one, there are no federal standards in the United States for irrigation quality.
“FDA provides no specifics, critical limits, or metrics based on indicators or pathogen prevalence in a standardized sample volume of any size,” he wrote. “Producers are held to self-determination of the broadly applicable position that water should be ‘of appropriate quality for its intended source, or treating and testing water on a regular basis and as needed to ensure appropriate quality.’”

“In most cases,” he continued, “the microbiological quality of surface water used for irrigation is not known because it is not tested in any meaningful frequency.”

The problem, according to Suslow, is compounded by a lack of existing research on irrigation quality. And of the existing studies on irrigation safety, most are concerned primarily with chemical rather than microbiological questions.
 
yumawater1.jpg“As a result,” he concluded, “the knowledge gap regarding sanitary quality of irrigation waters is nationwide.”

During the 2006 E. coli O157:H7 outbreak traced to contaminated spinach, irrigation water was considered as a possible source of contamination.

The leafy greens industry is still feeling the repercussions of 2006 spinach outbreak, as sales of packaged salad remain impacted, hovering around 20 percent below prior periods. The current E. coli O145 outbreak linked to romaine lettuce is certainly not improving the industry’s prospects, and is at risk of further alienating consumers by withholding the plant facility where the outbreak originated.
 
The 2006 spinach outbreak, though, accelerated efforts of produce industry leaders to define practical and meaningful prevention practices and standardized safety criteria.

Leafy green marketing agreements aimed at food safety have been established in California and Arizona, and the industry is seeking to expand nationwide.
 
As Arnott Duncan, a grower and committee member for the Arizona Leafy Green Marketing Agreement (AZLGMA) explained to Food Safety News in an interview, water testing is a key ingredient in the program.
 
“Everyone tests their water,” said Duncan. “It’s a big concern.”

“If you pull a positive, it could mean holding product on the farm,” he added.
 
States like California and Arizona have also begun to test more regularly for irrigation quality. However, the overwhelming majority of this database is privately and tightly held. Food Safety News was largely unsuccessful in its attempts to contact water officials in the Yuma, AZ area.

“There is a need for a bit of in-depth reporting on surface irrigation water,” said Dale Hancock, an epidemiologist and field disease investigator at Washington State University. “Irrigation water and dust would be among the usual suspects.”

Helena Bottemiller co-wrote and contributed to the research for this article. Pictured: Top: Open irrigation canal with the feed towers for the feedlot in Wellton in the background. Bottom: Irrigation canal in Dome Valley. Photos by Bottemiller.
 

© Food Safety News
  • Paul Nunes

    Great series.