E. Coli’s Spread Is Still a Mystery

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 the 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 state 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 UC 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. O157:H7 grows very, very well on spinach, unfortunately, and the warmer the temperature, the faster it grows.”

Alejandro Castillo, a Texas A&M; University associate professor of food microbiology, said the virulence of the epidemic has puzzled him. Far more people were hospitalized, many with kidney failure, than during any previous O157:H7 outbreak. Three people died.

“How in heaven can you get so much produce contaminated?” Castillo said. “We need to figure out the mechanism of transport to the spinach fields. If it’s a short distance, wind can transport it, floods, rain, whatever. Even people walking from one side to another.”

Whether it was animal hoofs, human feet, creeks or something else that brought it to the fields, Castillo and Suslow suspect that water -- irrigation water, floodwaters, runoff or underground flow -- spread it.

“One way or another I imagine that water could ultimately play a role -- if not causing the initial contamination, then spreading it,” Suslow said.

Castillo said that “something, such as the irrigation system, magnified the effect,” spreading it from leaf to leaf during growth, harvesting or processing.

In the Salinas Valley, sprinklers are used on spinach, which Castillo calls risky. “They are smaller plants, close to the soil; the water can transport it,” he said. “Spray irrigation should be eliminated from the crops when the plants are already growing.”

Suslow, who has long studied the region’s crops, began tracing back the chronology to uncover what may have caused the pathogen to multiply. He noticed that in mid-July, as green rosettes sprouted in spinach fields, record-breaking temperatures smothered the normally cool coastal valley for days.

In Hollister, temperatures exceeded 100 degrees and broke records from July 22-25, according to National Weather Service data. A normal July day there peaks at 81 degrees. Gilroy was even worse, with 10 consecutive days reaching 99 degrees or more, peaking at 112 -- 23 degrees above its normal high.

Less than two months later, those crops were responsible for sickening people in 26 states.

The heat wave that gripped the state could have triggered explosive growth of pathogens in the fields. Infected bags had a “use-before” date of Aug. 30 so the plants probably were mid-growth then, Suslow said. Temperatures were normal during harvest and processing.

“It’s not a smoking gun,” Suslow said, but it is likely to have raised bacteria counts.

Robert Mandrell, head of the U.S. Department of Agriculture’s produce safety and microbiology research unit in Albany, Calif., said pathogens, especially intestinal ones like E. coli, like warm temperatures, so the heat wave is “an interesting factor that we have thought about.” He said, however, that it is impossible to re-create the field conditions to know for sure what happened.

Dean Cliver, food safety professor at the UC Davis School of Veterinary Medicine, said the heat “may partly explain why the outbreak was more extensive, but it doesn’t tell how the contamination keeps happening.”

Although the investigation in the latest outbreak is narrowed to four fields in Monterey and San Benito counties, including the unidentified one near the cattle ranch, FDA and state officials emphasize that “the history of E. coli O157:H7 outbreaks linked to leafy greens indicates an ongoing problem.”

Since 1995, 20 outbreaks from lettuce and spinach, including at least nine in the Salinas Valley, have accounted for 608 illnesses and five deaths in the United States, the FDA says.

Reilly said investigators have not figured out “what’s different about the Salinas Valley.”

Illnesses traced to vegetables have been a concern since the early 1990s. About 76 million people in the United States are sickened by food-borne illnesses each year, and at least 12% of outbreaks in the 1990s were linked to produce.

Just a few grams of manure holds millions of E. coli organisms and it takes only a few to sicken people.

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% tested in 2003 and 2004 contained E. coli. None of the berries, broccoli, peppers and zucchini had E. coli; 5% 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.

Microbiologists theorize that a variety of characteristics make leafy plants vulnerable.

Greens are grown on the ground and they spread out, creating a lot of surface area in contact with soil. The leaves are porous and prone to holding particles. Baby spinach is harvested with mowers, which leave cuts where bacteria clusters can attach, sometimes tenaciously.

“Because leaves are obliged to breathe to carry out photosynthesis, a certain amount of liquid winds up in that spongy middle,” Cliver said.

When scientists grow O157:H7 and salmonella on leafy plants, the pathogens “do very well,” Mandrell said. “They can find a niche on the plant surface and survive.” He cautions, however, that little is known about persistence in the environment, rather than in labs.

Herbs also are germ factories -- at least in laboratories -- and some have caused food poisoning outbreaks, including one in Florida last year from basil.

In a University of Florida lab, Amarat Simonne inoculated cilantro, parsley and other herbs with high doses of salmonella and O157:H7. She found that “both bacteria are extremely persistent on all test herbs” even after 24 days of refrigeration.

That suggests that herbs and other leafy plants can sicken people weeks after they are harvested and purchased, said Simonne, an associate professor of food safety and quality.

“On these types of surfaces, they can survive pretty long,” she said. “We actually had to terminate some of the study because the herbs started looking bad.”

marla.cone@latimes.com