Although cattle manure has been pinpointed as a likely source of contaminated spinach that sickened nearly 200 people, scientists and food safety experts are continuing to probe California's Salinas Valley's environment for clues as to how the lethal bacteria spread to the fields and tainted so much food.
On Thursday, federal and state investigators announced that they have matched E. coli O157:H7 in feces from an unidentified cattle ranch to the bacteria found in sick people as well as bagged spinach they ate. Spinach fields surround the sprawling ranch, which covers thousands of acres.
Food safety experts are not surprised by the genetic match to cattle, since they had long suspected they were the source. Yet many mysteries remain, and until they are solved the safety of the region's leafy greens cannot be assured.
Because spinach has a short growing cycle, about 30 days, the tainted August crop probably was not the first of the season from those fields. So, scientists want to know what was unusual about that crop.
How did the pathogen move from cows to crops? Large numbers of wild pigs frequent the ranch and might have fed on manure and excreted it in fields or tracked it there. Runoff or spring flooding could have transported feces from the ranch, but the suspect cattle pastures are downhill from spinach fields. How did it spread to so much spinach?
Other sources can't be ruled out, officials said, given the eight previous outbreaks that have been traced back to other Salinas Valley lettuce and spinach fields over the last decade.
Kevin Reilly, deputy director of the California Department of Health Services' prevention services division, said 13 people from his agency and the FDA were taking samples in four fields Friday, focusing on the cattle ranch and trying to find out "precisely how the contamination could have reached the actual spinach on the fields."
"We're talking about detective work ... . We don't have that definitive evidence yet but we have a number of clues," he said.
Food experts have long known that leafy vegetables are susceptible to pathogens, hosting more fecal bacteria than other produce. In laboratories, pathogens live for weeks on leafy greens, thriving even as the vegetables wilt. And in soil, they survive for months.
Given the right set of circumstances - particularly warm temperatures - pathogens thrive. If the host, the bacterium and a favorable environment - the "disease triangle" - coexist, the conditions are ripe for an epidemic.
"Something was out of balance, if you will," said University of California, Davis, food pathologist Trevor Suslow. "It could be egregious contamination but it is equally possible that it was some initial contamination and then the spinach was exposed to conditions that let it multiply. 0157:H7 grows very, very well on spinach, unfortunately, and the warmer the temperature, the faster it grows."
In a new study of 2,000 pre-harvest vegetables and fruits, lettuce, spinach and cabbage "had significantly higher E. coli prevalence than did all the other produce types," says a report by University of Minnesota food microbiologist Francisco Diez-Gonzalez and his colleagues.
For conventionally grown lettuce and leafy greens, 20 to 25 percent tested in 2003 and 2004 contained E. coli. None of the berries, broccoli, peppers and zucchini had E. coli; 5 percent of the squash did, according to the study, conducted on crops in Wisconsin and Minnesota and published in a scientific journal in August.
The crops were tested before harvest or cleansing. None contained the pathogenic E. coli or salmonella. But even though it was generic E. coli, which does not cause illness, it shows that feces had contaminated the lettuce and spinach crops.
Even on certified organic farms, lettuce more frequently contained E. coli than other organic crops. However, the organic ones harbored less than the traditionally grown ones.
"Anything that is leafy green had more E. coli positive samples," Diez-Gonzalez said.