E. coli O157:H7 is a potentially deadly bacterium that can cause bloody diarrhea, dehydration and, in severe cases, kidney failure. The very young, seniors and people with weak immune systems are most at risk. A recent lawsuit against National Steak and JBS noted that there are an estimated 73,480 illnesses linked to E. coli O157:H7 infections each year in the United States, leading to 2,168 hospitalizations and 61 deaths.

USDA data analyzed by The Star show that large plants have had higher rates of positive E. coli tests than smaller plants. Federal meat safety officials said the latest data show those differences are disappearing.

But they acknowledged that the volume of meat a plant produces is a key issue. A USDA study published in March showed that from 2007 through 2011, E. coli positives at very small plants resulted in only 465,000 pounds of contaminated beef. A slightly lower rate of positive tests at large plants, however, produced more than 51 million pounds of contaminated beef.

Regardless, USDA officials and other experts agree that most E. coli generally originates at larger slaughter plants, where pathogen-laden manure is a bigger problem because that’s where cattle are coming in from the feedlots.

Federal inspection records obtained by The Star under the federal Freedom of Information Act include hundreds of references to fecal contamination problems at four of the largest beef slaughter plants in Kansas, Nebraska and Colorado. For example, at Tyson’s Dakota City, Neb., beef plant, inspectors noted: “massive fecal contamination; multiple carcasses with varying degrees of fecal contamination; periods of very significant fecal, ingesta and abscess contamination.”

Another federal inspector at Tyson found “a piece of trimmed fat approximately 14 inches long with feces the length of it,” and another noted, “fecal contamination . . . was so great . . . couldn’t keep up.”

But Tyson officials said such reports only provide a “snapshot of beef production.” The company said it has added two full-time safety technicians at the plant, as well as additional workers, to assess carcasses and make sure fecal contamination is eliminated.

USDA and beef industry officials point out that E. coli illnesses have dropped dramatically in recent years, although the Food Safety and Inspection Service cautioned that no “consistent trend” has emerged in recalls of contaminated beef.

“A miracle has occurred in the beef industry,” said Janet Riley, senior vice president for public affairs at the American Meat Institute. “Beef is safer, more affordable and more plentiful than it ever has been.”

James Marsden, a food safety professor at Kansas State University, told The Star that processors and the USDA could do better with mechanically tenderized steaks.

“E. coli is impossible to eradicate from beef cattle,” he said. But a key to eliminating it in mechanically tenderized steaks is to use “interventions” such as spraying lactic acid on the meat to reduce or eliminate surface contamination. Some companies do that, he said, but the USDA does not require it.

None of that, however, prevented Lamkin’s illness after eating beef that had been mechanically tenderized, according to a lawsuit she filed last year.

“I was amazed to learn how these steaks are processed,” Lamkin said. “I never dreamed of anything happening like this.”