Federal officials discovered one source of a growing nationwide E.coli outbreak linked to romaine lettuce that has revived concerns over a leafy green industry that has long grappled with how to produce safe food. But questions still remained over the origins of most U.S. illnesses.
The U.S. Food and Drug Administration on Friday said it has traced the whole-head romaine lettuce that sickened eight inmates in an Alaska jail to Harrison Farms in Yuma, Ariz., but that it is still looking for the source of chopped lettuce that sickened dozens more Americans.
The update from regulators comes amid fresh concerns over food safety in the leafy greens industry, as the outbreak in romaine lettuce grows to the largest multistate E. coli outbreak since one in 2006 tied to spinach that killed three people and sickened more than 200.
On Friday, the Centers for Disease Control and Prevention added 14 more ill people to its national count, which now totals 98 people across 22 states. It said the strain of E. coli O157:H7 making people sick is particularly virulent, leading to a high hospitalization rate and large numbers of ill people who have developed kidney failure.
The FDA said it isn’t clear at what point in the supply chain the whole-head lettuce sent to Alaska was contaminated, and that investigators are examining the entire route from between the farm and the jail where it was served. Harrison Farms has finished its harvest and is no longer growing lettuce, the FDA said. Harrison Farms couldn’t immediately be reached for comment.
Stic Harris, who leads an FDA network devoted to investigating illness outbreaks, said the agency is examining two dozen other farms to identify where the chopped lettuce that sickened people in other parts of the country came from.
“In this case you’re looking at a more of a web trying to figure out where it came from,” he said.
For more than two weeks, the FDA and the CDC have been investigating an outbreak of E.coli O157:H7, tracing a variety of routes lettuce took to reach consumers across the country, collecting and analyzing hundreds of records.
Early in the search, health officials zeroed in on Yuma, Ariz., as the likely source of tainted lettuce, warning consumers not to eat any romaine lettuce grown in the desert region. Without a specific source, the agency didn’t announce a broad product recall.
But the FDA said it’s likely that any romaine sold in stores now is from California, not Arizona, as the growing season ends there and shifts north. The CDC is still advising consumers not to eat or buy romaine lettuce if they can’t tell where it is from.
The E.coli outbreak is the second one linked to leafy greens in less than six months. Officials weren’t able to identify a specific type of leafy green—or its source—in the earlier outbreak, which sickened 25 people in 15 states, including one who died.
Much of the difficulty in pinpointing an outbreak’s origin stems from the complexity of the U.S. supply chain, in which food is grown, packed, processed and shipped by multiple companies before it reaches grocery store shelves. Produce from different farms can be combined into a single shipment to buyers. Product labels often don’t indicate where lettuce was grown.
More commingling occurs at central processing facilities, where in the case of chopped romaine, the first item to be implicated in the outbreak, processors mix products from multiple farms into a single bag. Mr. Harris said pouring through records kept by farms and suppliers, including handwritten ones, is “extremely tough.”
“Tracking a product from the farm through to the point of sale isn’t easy,” said David Acheson, a food-safety consultant who was previously associate commissioner for foods at the FDA. “It’s not a new problem and it’s obviously not fixed.”
Produce is the top culprit in foodborne illnesses, according to a CDC analysis, which said fruits, nuts and vegetables were responsible for 46% of illnesses between 1998 and 2008. Leafy vegetables accounted for the most illnesses, the agency said.
Lettuce presents a particular risk because it is eaten raw rather than cooked, eliminating a process that can destroy harmful bacteria spread by animal droppings or contaminated water in fields. Additional processing, like washing, chopping and bagging, can spread bacteria onto other food or equipment, amplifying the impact of a single food safety breach.
Robert Tauxe, deputy director of the CDC’s division of foodborne, waterborne and environmental diseases, said the outbreak strain of E.coli, which is transmitted through animal fecal matter, can be spread through the environment via water, wild animals or people.
The FDA said most foodborne illness outbreaks tied to leafy greens occur during transition periods in the spring and fall as production shifts from Arizona to Florida. It said increased equipment sharing between farms, rainfall or field preparation activities during those times could be responsible for the spread of bacteria.
Food safety in the leafy greens industry underwent a major overhaul after the deadly E.coli outbreak in 2006. Industry members established a voluntary marketing agreement, under which participating handlers of leafy green vegetables agree to buy produce only from farms that follow specific safety procedures. Members in California and Arizona produce approximately 90% of the leafy greens grown in the U.S., the organization says, under practices verified through government audits.
The industry has made big strides in making leafy greens safer to eat, said Michele Jay-Russell, a microbiologist and manager of the Western Center for Food Safety at University of California-Davis who helped investigate the 2006 outbreak.
“It’s hard to completely eliminate these outbreaks when you have a raw product and wide distribution,” said Ms. Jay-Russell. “But it’s fairly disconcerting that we have this again 12 years later.”
Starting in January, large produce farms became subject to a new food safety rule, part of sweeping legislation passed by Congress in 2011, which for the first time set federal standards for how U.S. fruits and vegetables are grown, harvested, packed and stored. Enforcement of the rule is in its infancy, however, and a provision requiring farmers to test their water supplies for harmful bacteria doesn’t take hold for at least another two years.
Write to Jesse Newman at jesse.newman@wsj.com