Marc Siegel

The CDC scientists were at work until almost midnight last night trying to determine why people are getting so sick. They have discovered, Tauxe says, that the strain of E. coli in question is the very virulent Escherichia coli O157:H7. “This may be the worst actor we’ve seen,” he said, “in terms of all the kidney failure and hospitalizations.”

Tauxe said that studies being done in the laboratory at the CDC are not likely to show drug resistance because “less than 1 percent of E. coli is drug-resistant.”

E. coli O157 regularly resides in the intestines of animals including cows, deer and pigs, where it doesn’t cause disease. This bacterium makes the powerful “shiga” toxin, which breaks down the inside walls of blood vessels, causing bleeding and clotting. But cows and deer lack the receptors for this toxin on their blood cells, so they don’t bleed and they don’t clot. In fact, they have no symptoms at all. Cows and deer shed O157 into their stool, just as we humans shed thousands of kinds of bacteria that live in our intestines but don’t make us sick.

But very small amounts of the more virulent strains make us very sick. The current outbreak, with 159 cases across 24 states, includes 83 hospitalized individuals, of whom 27 (17 percent) developed the rare form of kidney failure known as hemolytic-uremic syndrome that is characteristic of the most severe form of this disease. This outbreak has included rates of kidney failure and hospitalization at three times the rate that is usually seen with this disease.

“There are many factors in addition to virulence that go into why this E. coli is making us sicker,” says Dr. Philip Tierno, director of Clinical Microbiological and Diagnostic Immunology at the New York Medical Center and author of “The Secret Life of Germs.” “A heavier concentration of the bacteria on spinach leads to more bacteria ingested. The more bacteria, the greater the amount of toxin, which leads to more complications, including the kidney failure we’re seeing.”

“Animals are incubators for E. coli O157,” Tierno says. Once it is shed in manure, it may spread to water and plants. A high concentration of this bacterium on the initial spinach crop leads to an explosion of bacterial growth within the harvested fields. Eleven outbreaks in salad foods have occurred in the United States since 1995.

Tauxe at the CDC is confident that the outbreak will be controlled. “We’re not seeing a lot of secondary spread,” he said. “Nothing among people who have touched those who are sick, no one in nursing homes. That’s a very good sign.”

But for those who have gotten the dreaded bug, a high percentage have gotten sick. This is a wakeup call for the food industry. Inadequate surveillance and easily contaminated crops — and, perhaps, an overabundance of deer — are factors that promote the creation and perpetuation of superbugs like the one that is currently riding our spinach.

This story has been corrected since it was originally published.