COLUMBUS — In December 2019, the U.S. Centers for Disease Control (CDC) issued a food safety alert for romaine lettuce originating from Salinas, California, asking retailers not to sell it to the public, regardless of the date on the product. This followed a Nov. 22, 2019 warning issued by the USDA Food Safety and Inspection Service (FSIS).
Across 25 states, 138 people were infected with this strain so far in 2019. At least 36 new cases were reported in December. In its latest report, issued on Dec. 19, the CDC announced 72 people were hospitalized, and of these, 13 developed kidney failure. Ohio has the third largest number of cases in the nation.
According to Melanie Amato, press secretary for the Ohio Department of Health (ODH), the onset dates for the infections in Ohio ranged from Oct. 30 to Nov. 14 and were reported by three people in Franklin County, three people in Lake County, and one person each in the following counties: Auglaize, Cuyahoga, Delaware, Lorain, Medina and Stark. They ranged in age from 14 to 89 years old and eight of the 12 reported cases were women.
Oliver Fisher, health commissioner at the Auglaize County Health Department, provided additional details regarding the case occurring in the Sidney Daily News readership area.
“The case is an 89-year-old female that presented with symptoms to a local hospital at the end of October. ODH’s Food Core conducted follow up investigations with the case and was informed that she purchased different pre-packaged salads at the time prior to onset of symptoms. Testing did confirm the link [to] the E. coli O157:H7 outbreak,” Fisher said.
To get to the “root” of these lettuce-based E. coli outbreaks, the U.S. Department of Agriculture (USDA) spent $1.86M on a randomized control study. It was part of a larger, multi-state project funded in excess of $10M during that same period by the USDA/FIFA/SCRI. The study’s results were published as an article titled “Experimental In-Field Transfer and Survival of Escherichia coli from Animal Feces to Romaine Lettuce in Salinas Valley, California,” which appeared just prior to the national outbreak beginning Sept. 24, 2019, according to the CDC. Robert L. Buchanan, director of the University of Maryland’s Center for Food Safety and Security Systems, who served as principal investigator over the multi-state grant, stated, “this project extended over 5 years, and this was one of the subprojects that was likely started late in the course of the project.” Despite the lag time to journal publication, the conclusions of this study appear to have been available to the USDA much earlier through his grant report, which is available online.
The study’s results pointed to water as a likely source of contamination. The grant report suggested “The Salinas Valley trials” revealed “irrigation just prior to harvest could ‘splash’ bacteria from fecal material into furrows beyond [the standard 5 feet barrier] thus requiring an extended buffer or prevention by removal of fecal material before final overhead irrigation prior to harvest.” One of the co-authors on the Salinas Valley field trial publication is Dr. Rob Atwill, professor of Veterinary Medicine at UC Davis and director of the Western Institute for Food Safety & Security, who performed the role of statistician and epidemiologist on the project.
Atwill indicated the Cornell study about cattle and cattle water troughs does not apply to Salinas Valley since there are few to no cattle near most of the lettuce fields in this region.
“We tested 2,700 cattle in the vicinity of Salinas Valley over a period of three years and it was only found infrequently,” Atwill said. “We don’t think cattle have anything to do with many of these E. coli O157:H7 outbreaks traced back to central coastal California except maybe when it comes to beef.”
Instead, Atwill believes the role of wild animals defecating in the Salinas Valley growing fields is a more likely the source of E. coli O157:H7 contamination on the heads of romaine lettuce leading to outbreaks and recalls. To this end, his team conducted field tests with harmless E. coli that were genetically unique so the researchers could easily identify them in nature. They gathered scat from lab animals—for example, chickens to simulate wild birds—which did not have any pathogenic strains of E. coli, and mixed it with a commercially available unique E. coli strain.
Even when chemical fertilizers are used on crop fields, in theory, people working on farms who travel from a nearby cattle area to a crop field, for example, could transfer the E. coli O157:H7 human pathogen by stepping in cow manure and then tracking it into crop fields on their shoes. However, most field pickers are employed as part of “specialized teams” and do not perform both functions in the Salinas Valley, observed Atwill.
For this recent outbreak, the FDA says it was unable to pinpoint a specific “grower, supplier, distributor, or brand of romaine lettuce” responsible. But recently conveyed that the Wisconsin Department of Health Services found “E. coli O157:H7 in an unopened bag of Fresh Express® brand Leafy Green Romaine” lettuce.
The FDA, like the USDA, are helping long-term investigations into the problem, but the FDA’s time and human resources are limited so they often collaborate on in-depth studies with academic scientists, Atwill explained. Atwill is the Director of the Western Center for Food Safety and Security. The FDA has a cooperative agreement with them, and FDA employees contributed to the article Atwill co-authored. During the study, the FDA suggested to Atwill, despite the fact E. coli itself must come from fecal matter, water is one likely vector spreading the bacteria.
“This dialogue happens with the FDA happily,” said Atwill. “To give you a concrete example in this paper, when you irrigate a field of produce, water can accumulate in furrow. This was where my eyes opened a little in terms of listening to the FDA scientists about other ways contamination can occur.”
The FDA suggested to Atwill that under heavy irrigation, the water in the furrow might carry the E. coli downstream where it ponds on the surface, Atwill said. By placing modified scat into furrows between rows of lettuce, Atwill’s study found that contaminated surface water, when it ponds from heavy rain or irrigation, can splash up onto the lettuce heads. However, the window of time is short for these routes of contamination given that irrigation often lasts only two to three hours. This is because Salinas Valley soil is “granular,” which means surface water usually seeps quickly and easily into the ground. But this same soil type makes it less likely to serve as a kind of “filter,” and so “it could contaminate,” said Atwill.
One question that arises, then, is whether E. coli from surface scat could also potentially cause groundwater contamination allowing this dangerous pathogen to spread—affecting wells, water sources for people and animals, and field irrigation systems. Atwill and a colleague at UC Davis, Thomas Harter, a groundwater hydrologist, previously investigated this lead by digging wells into the ground and performing multiple microbiological studies on that groundwater under California dairies. E. coli they found did not transport well there below 100 ft, where an irrigation well would be located.
“The numbers were insufficient to cause, in my opinion, a multi-state E. coli outbreak,” Atwill said. “You may find others who have a different opinion, but I don’t consider groundwater a prime suspect in this region of California. I would vigorously say the science shows that’s not the cause.”
Instead, Atwill’s conclusion was the contamination of lettuce was most likely due to scat from wild animals, such as elk, feral pigs, ground squirrels, crows, or mice found in the region, which were deposited by the animals in the crop fields, dried and dispersed by wind to contaminate more lettuce, and worsened by irrigation or heavy rain just prior to harvest which facilitates surface water splashing onto lettuce heads. Handling contaminated lettuce, which then can transfer from worker’s hands to previously uncontaminated lettuce, during harvest or processing is another reason for increased the number of lettuce heads affected and the likelihood they would end up in a grocery store or restaurant to be eaten by a customer, Atwill explained.
Testing prior to harvest is performed by many companies since “farmers don’t want recalls,” Atwill said. So, some large farms hire a Ph.D. food scientist and voluntarily test their lettuce fields for dangerous pathogens. He encouraged visualizing the enormous Salinas Valley lettuce fields that “stretch to the horizon” to realize the farms are simply unable to test enough samples.
“This is the challenge … They maybe test less than 1 percent of the millions of heads of lettuce they grow, and that is not even the entire head of lettuce. It is just a leaf, or part of a leaf. You can’t prove every head of lettuce is safe. Sample testing is good agricultural practice, but it is just dealing with the tip of the iceberg,” Atwill said, throwing in a lettuce pun.