The E. coli outbreak linked to romaine lettuce has sickened 43 people in 12 states, the US Food and Drug Administration said Monday.

The FDA said that the ongoing outbreak is linked to the "end of season" harvest in some parts of California -- but the agency still says people should not eat any romaine lettuce.

People have become sick in California, Connecticut, Illinois, Massachusetts, Maryland, Michigan, New Hampshire, New Jersey, New York, Ohio and Wisconsin.

An additional 22 people in Canada are also ill, so the FDA is coordinating its investigation with the Canadian health and food safety authorities, the agency said.

When the outbreak was announced last week, the FDA and the Centers for Disease Control and Prevention warned consumers to stay away from all romaine lettuce, but the FDA said the investigation was focused on California and Mexico.

"Over the Thanksgiving holiday, the FDA continued to investigate the outbreak," according to a statement from FDA Commissioner Dr. Scott Gottlieb. "Our investigation at this point suggests that romaine lettuce associated with the outbreak comes from areas of California that grow romaine lettuce over the summer months, and that the outbreak appears to be related to 'end of season' romaine lettuce harvested from these areas. The involved areas include the Central Coast growing regions of central and northern California."

Lettuce growing and harvesting in the winter months is taking place in California and Arizona's desert regions and Florida, as well as Mexico. Currently, the FDA investigation does not implicate lettuce from any of these areas.

While the romaine supply undergoes a "clean break" to ensure all the contaminated lettuce is effectively gone from the market, the FDA has asked producers and distributors to provide clear labeling with the lettuce's date and origin in the future.

A task force within the lettuce industry has also been established to determine better solutions for labeling long-term in order to help with tracing.

"Based on discussions with major producers and distributors, romaine lettuce entering the market will now be labeled with a harvest location and a harvest date," Gottlieb said. "Romaine lettuce entering the market can also be labeled as being hydroponically or greenhouse grown. If it does not have this information, you should not eat or use it.

"If consumers, retailers and food service facilities are unable to identify that romaine lettuce products are not affected -- which means determining that the products were grown outside the California regions that appear to be implicated in the current outbreak investigation -- we urge that these products not be purchased, or if purchased, be discarded or returned to the place of purchase."

Symptoms of E. coli infection, which usually begin about three or four days after consuming the bacteria, can include watery or bloody diarrhea, fever, abdominal cramps, nausea and vomiting, according to the CDC. Most people infected by the bacteria get better within five to seven days, though this particular strain of E. coli tends to cause more severe illness.

People of all ages are at risk of becoming infected with Shiga toxin-producing E. coli, according to the FDA. Children under 5, adults older than 65 and people with weakened immune systems, such as people with chronic diseases, are more likely to develop severe illness, but even healthy children and adults can become seriously ill.