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

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 19 in 3 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 (pdf), 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 will explore dust and mud contamination, Part II will look at wildlife intrusions, and Part III will discuss irrigation water.

Part I – Dust, Mud, and E. coli

One of the largest cattle feedlots in the country is found in Wellton, AZ, around 20 miles, as the crow flies, from the heart of leafy green production in the Gila and Dome Valleys near Yuma. Naturally, the high concentration of cattle and manure produces a fecal, muddy mix in which E. coli bacteria can thrive. In a dry, windy environment like Yuma, mud-turned-dust can carry E. coli.

Most food safety experts agree that dust is a prime suspect in pathogenic produce contamination.

A study conducted in 2002 by researchers at Texas Tech University found that some common feeding practices were leading to a higher likelihood of dust-borne bacteria.

cattle yuma.jpg“A standard feeding practice in some western feedlots is to feed at sunrise. This protocol results in cattle that have digested their food by the evening, which coincides with the active, dust-generating period,” the study discovered, leading the researchers to conclude that “airborne transmission could be a primary route of infection.”
History supports the dust-borne hypothesis.

In September 2001, Ohio health officials identified a cluster of E. coli O157 infections at a fair in Lorain County, Ohio. A series of studies linked the outbreak to a large, open building that was used throughout the fair for animal shows. The building’s floor was covered with sawdust, and on the last night of the fair, a large dance was held in the same building. Investigators concluded that an animal from an earlier show, most likely a cow, defecated on the ground, and the sawdust kicked up by other animals and humans contaminated nearly the entire building.

One report found this: “E. coli O157 survived and possibly multiplied in the sawdust. The sawdust may have become airborne during a large event such as the dance. Individuals who touched contaminated surfaces in the building became infected when they ate or drank without adequately washing their hands. It is possible that some may have swallowed bacteria that landed directly into their mouths or onto their food or drink.”

Again, one year later, air-borne E. coli was cited as the probable origin of an outbreak.

In 2002, an outbreak of E. coli O157:H7 sickened 82 people at an Oregon county fair, the largest E. coli outbreak in Oregon state history. Although not confirmed, health officials postulated that possible exposures leading to the outbreak occurred at animal enclosures, including the cattle tent, horse barn, and exposition halls that housed goats, sheep, rabbits, chickens, ducks, and guinea pigs.

Investigators eventually traced the transmission path of the E. coli O157:H7 bacteria to pipes 15 feet above goat pens in a fair exhibition hall, where about 75 people, including 12 children, were believed to have been infected, leading them to believe the toxins were spread through the air.

Judging from these examples, it is clear how fecal bacteria from a cattle farm’s manure pit could spread from feedlot to produce farm, especially in a dusty, highly windy environment like that of Yuma.

However, Michele Jay-Russell, a veterinarian and food safety specialist with the Western Institute for Food Safety and Security at the University of California, Davis, sounded doubtful about air-borne transmission in the Freshway E. coli O145 outbreak, citing the distance between feedlots and produce farms as a potential barrier.

“With that distance, it would be a stretch biologically,” she said. “It would probably be more than just wind.” Instead, she mentioned vehicles and clothing as possible carriers.

“But,” she added, “It’s certainly on the list of possibilities.”

Helena Bottemiller co-wrote and contributed to the research for this article. Pictured: Cattle feedlot outside of Wellton, AZ. Photo by Bottemiller.