At least 53 people have been sickened by tainted, chopped romaine lettuce in an expanding E. coli outbreak that now spans 16 states, the Centers for Disease Control and Prevention said on Wednesday.
The contaminated greens have been traced to Yuma, Ariz., but investigators recommended abundant caution because they have not yet identified a specific source.
Restaurants and consumers should avoid and trash any pre-chopped romaine lettuce from that area, it said, even if some has already been consumed without consequence.
“If you do not know if the lettuce is romaine, do not eat it and throw it away,” the C.D.C. said in a statement.
The Wednesday report shows that the outbreak has expanded in recent days. On Friday, the C.D.C. had reported only 35 illnesses in 11 states.
The people infected in the outbreak range in age from 10 to 85. Seventy percent of them are female. At least 31 people were hospitalized and five had developed kidney failure, according to the agency. No one has died yet.
The first illness linked to the outbreak began on March 13 and the last was recorded as starting on April 6, but there may be more to come. Any infections that began after March 29 may have yet to be counted because it typically takes two to three weeks for an illness to be reported.
The agency was first alerted to the outbreak by health officials in New Jersey, who had noticed an increase in E. coli cases in the state, said Dr. Laura Gieraltowski, an epidemiologist at the C.D.C. After some discussion, it became clear that many of those infected had eaten chopped romaine lettuce at restaurants before getting sick.
Concerned, the agency looked for related cases by checking PulseNet, a national network of laboratories that catalog samples of harmful bacteria from infected patients.
“When we looked back into our PulseNet system we saw that there were other cases in other states with the same DNA fingerprint,” Dr. Gieraltowski said.
The C.D.C. learned that the others infected by that particular strain, E. coli O157:H7, had also eaten chopped romaine lettuce at restaurants before getting sick, she said. It turned over the information to the Food and Drug Administration, which helped trace the outbreak to Yuma, Ariz.
To pinpoint the exact source, though, investigators would need samples of the tainted lettuce. But because of the short shelf life of lettuce and the time it takes for an outbreak to be identified, obtaining such a sample may prove difficult.
“For leafy greens, not many people are going to have a bag of lettuce or a leftover salad or something like that in the refrigerator several weeks after they became ill,” Dr. Gieraltowski said.
The E. coli, which is transmitted through feces, may have come either from a person or an animal. The C.D.C. has said that the current E. coli outbreak does not appear to be related to another that began late last year and was traced more generally to “leafy greens.”
Pennsylvania was home to 12 people infected in the outbreak, more than any other state. Idaho was next, with 10 recorded illnesses. The other states involved were Alaska, Arizona, California, Connecticut, Illinois, Louisiana, Michigan, Missouri, Montana, New Jersey, New York, Ohio, Virginia and Washington.