Federal health officials said Monday that only romaine lettuce from certain parts of California is unsafe to eat and that romaine lettuce entering the market will now be labeled to give consumers information about when and where it was harvested.

If consumers, retailers and food service facilities cannot determine whether the romaine was grown outside California, they should not eat it and should throw it away, even if some of it was eaten and no one got sick, according to a lengthy statement from Scott Gottlieb, commissioner of the Food and Drug Administration.

FDA officials said the most likely source of contamination is from the Central Coast growing regions in northern and central California. Romaine lettuce harvested outside those regions “does not appear to be related to the current outbreak,” the FDA said. Hydroponically grown and greenhouse-grown romaine also does not appear to be affected in the outbreak. Romaine from those sources is safe to eat, the FDA said.

No common grower, supplier, distributor or brand of romaine lettuce has been identified in the outbreak. Several major romaine lettuce producers have agreed to label products with a harvest date by region, and new romaine from other growing regions, including Florida and Arizona, is being restocked in grocery shelves.

The new warning from the FDA and the Centers for Disease Control and Prevention came as the number of people sickened by the outbreak grew to 43 people in 12 states. Sixteen of those people have been hospitalized, including one person with severe kidney failure. No deaths have been reported.

The updated information follows an unusually broad warning that federal health officials issued two days before Thanksgiving, telling consumers to throw away any romaine lettuce they may already have purchased. At the time of the outbreak, the majority of romaine on the market was being grown in the Central Coast region of California. Since then, harvesting of romaine has ended and shifted to the winter growing regions, which include the California desert region of the Imperial Valley, the desert region of Arizona in and around Yuma, and Florida, Gottlieb’s statement said.

Romaine lettuce grown in Mexico is exported to the United States during the winter months. Smaller quantities are grown in other states.

“At this time, the FDA has no information to suggest any of these growing areas are involved in the current outbreak, which began well before any romaine lettuce from these winter growing locations was available for harvest,” Gottlieb’s statement said. Hydroponically grown and greenhouse-grown romaine also does not appear to be related to the outbreak, he said.

“There is no recommendation for consumers or retailers to avoid using romaine harvested from these sources,” the statement said.

In a separate update and statement, the CDC said “it may take some time” before the new labels on romaine products are available. If the romaine lettuce is not labeled with a harvest growing region, do not buy, serve, sell or eat it, the CDC said. This includes all types or uses of romaine, such as whole heads of romaine, hearts of romaine, and bags and boxes of pre-cut lettuce and salad mixes that contain romaine, including baby romaine, spring mix and Caesar salad.

Restaurants and retailers should check the label on bags or boxes or ask suppliers about the source of their romaine lettuce, the CDC said.

Investigators have been tracing back the romaine eaten by people sickened in the outbreak. U.S. officials are also coordinating with the Public Health Agency of Canada, which is also investigating a similar outbreak. The Canadian agency reported 22 confirmed cases in three provinces: Ontario, Quebec and New Brunswick.

E. coli are bacteria that live naturally in the intestines of cattle, poultry and other animals. The bacteria can contaminate a wide variety of agricultural products through contact with feces from infected animals. Most E. coli strains are harmless to humans, but some varieties can cause severe illness.

Five people died in the most recent major outbreak from contaminated romaine, which lasted from March to June of this year and led to 210 cases in 36 states. That outbreak was traced to the Yuma growing region in Arizona, but investigators never conclusively determined the precise source. Gottlieb has said the leading suspect is contaminated canal water used by multiple farms.

Illnesses in the current outbreak started in October and are not related to the Yuma outbreak. The strain in this one has the same genetic fingerprint as the one that caused illnesses late last year in the United States and Canada. Canada linked its cases to romaine lettuce specifically, but U.S. investigators said only that the origin was in leafy greens. That outbreak was declared over in January. The precise origin was never determined.

All three outbreaks — the current one, the one from Yuma and the one from last year — were caused by contamination of an E. coli strain known as O157:H7. It produces a Shiga toxin that in severe cases can lead to hemolytic uremic syndrome, a type of kidney failure.

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