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

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?

lettuce-field1-featured.jpgUnlike 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. Yesterday, Part I explored dust and mud as possible modes of contamination. Today Part II looks at wildlife intrusions, and tomorrow Part III will discuss irrigation water.

Part II – Wildlife Intrusions

As we noted yesterday, one of the largest cattle feedlots in the country is located 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. That mud, or mud-turned dust, could be picked up by birds or other animals stopping through the area.

With the exception of migratory birds and desert rodents, Yuma has very few wild animals that could venture onto leafy green farmland carrying E. coli bacteria. There are no forests in close proximity to the fields, unlike the Salinas Valley, where feral swine and other wildlife could excrete or externally carry E. coli onto a greens field. The large, multistate E. coli spinach outbreak in 2006 linked to Salinas is thought to have been caused by a wildlife intrusion, possibly by a feral pig, but a mode of transmission was never found.

“The role of wildlife as a source of foodborne microbial contamination along the farm-to-fork continuum is a long-standing concern among public health and food safety agencies,” according to a 2008 report by Edward Atwill at the Western Institute for Food Safety and Security at the University of California at Davis. “[The 2006 spinach outbreak] heightened concerns about the ability of wildlife to forage within or to transit through the produce production environment, and what biosecurity measures are in place to prevent wildlife access to foods that are minimally processed and often consumed raw.”

Wild birds, in particular, are considered potential vehicles for carrying pathogens from farm to farm. A study conducted by Canadian researchers in 2001 found indistinguishable E. coli O157 subtypes at two different feedlots approximately 100 km apart, determining that wild birds were among the only potential common vehicles shared between the two feedlots.

Another study performed by Ohio State University researchers in 2008 yielded similar results.

“The patterns of bird movement . . . along with the isolation of the pathogen E. coli O157 from both birds and cattle, further support the hypothesis that birds play a critical role in the dissemination of important foodborne bacteria among farms.”    

When Food Safety News visited Yuma, we were struck by how many birds there were. It was easy to imagine the possibility of birds playing a role in contamination in a lettuce field (All leafy greens commercially grown in Yuma have been harvested, production has moved back to Salinas).

“It’s one of the things we’re concerned about,” says Arnott Duncan, a grower who serves on the Arizona Leafy Greens Marketing Agreement committee. “But we aren’t going to be shooting the birds. There are a number of ways greens could become contaminated.”

Zach Mallove co-wrote and contributed to the research for this article.