It's unlikely that anyone currently has edible romaine lettuce that is contaminated with the toxic strain of E. coli bacteria sickening people nationwide since March. That was the message Wednesday from the Centers for Disease Control and Prevention, and if not exactly a thundering signal of “all clear,” it was pretty close.

Romaine lettuce has a shelf life of about 21 days. The current outbreak has been traced to the Yuma, Ariz., growing region, the source of virtually all lettuce sold in this country during the winter months. The CDC said Wednesday that April 16 was the last day romaine lettuce was harvested in the Yuma area. The leafy-greens industry has shifted to California over the past two months.

“Romaine lettuce from the Yuma growing region is past its shelf life and is probably no longer being sold in stores or served in restaurants,” the CDC said in a news release. In the latest official update, the CDC noted that new cases of E. coli-related food poisoning came from the period when contaminated lettuce might still be in circulation or in home refrigerators.

“It takes two to three weeks between when a person becomes ill with E. coli and when the illness is reported to CDC,” the agency said. “The most recent illnesses reported to CDC started when romaine lettuce from the Yuma growing region was probably still available in stores, restaurants, and in peoples’ homes.”

The CDC has stopped advising consumers to throw away romaine lettuce if they cannot confirm where it's from.

As of Tuesday, 172 people in 32 states had been sickened in the outbreak, an increase of 23 people and three states since the last update, a week earlier. One death has been reported. Of those who became ill, 75 have been hospitalized, including 20 with kidney failure.

In scale, this outbreak is approaching that of the 2006 baby-spinach E. coli outbreak that sickened more than 200 people and killed five. The strain of E. coli, known as O157:H7, produces a Shiga toxin that can affect people seriously, causing diarrhea, vomiting and, in severe cases, kidney failure.

The Food and Drug Administration has been trying to discover exactly where and when the romaine involved in this latest outbreak was contaminated. One farm in Yuma has been identified as the source of the lettuce that sickened eight prisoners in Alaska, but FDA Commissioner Scott Gottlieb signaled Wednesday evening just how complicated the case is.

The FDA, he tweeted, “ruled out that the contamination was caused by just one farm suggesting it was a complex problem and will take further time to investigate.”

 Lena H. Sun contributed to this report.

Read more: