Romaine Lettuce still sits on the shelves as a shopper walks through the produce area of an Albertsons market in Simi Valley, California, Nov. 20, 2018. Credit: Mark J. Terrill | AP

Federal health officials said Monday that only romaine lettuce from certain parts of California is unsafe to eat and 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 can’t 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 Food and Drug Commissioner Scott Gottlieb on Monday.

Federal health officials said the most likely source of contamination is from the Central Coastal 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- and greenhouse-grown romaine also does not appear to be affected in the outbreak. Romaine from these 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 different growing regions, including Florida and Arizona, is being restocked in grocery shelves.

The new warning from the Food and Drug Administration 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.

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.

Restaurants were told not to serve it, stores not to sell it, and people not to buy it, no matter where or when the lettuce was grown. It didn’t matter if it was chopped, whole head or part of a mix.

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 in 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, Arizona, growing region, but investigators never conclusively determined the precise source. The FDA’s Gottlieb has said the leading suspect is contaminated canal water used by multiple farms.

Illnesses in the current outbreak, which is not related to the one in Yuma, started in October. 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.

The current outbreak, 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.