WASHINGTON — In an alert with little precedent, the U.S. Centers for Disease Control and Prevention, the U.S. Food and Drug Administration and the Canadian Public Health Agency are warning consumers in both countries to avoid eating romaine lettuce. The agency issued the warning after genetic tests linked strains from a current E. coli O157:H7 outbreak to strains associated with an E. coli outbreak that occurred in the fall of 2017.

“Currently, the F.D.A. does not have enough traceback information to identify the source of the contamination that would allow us to request a targeted recall from specific suppliers,” the F.D.A. said. “At this stage in the investigation, the most efficient way to ensure that contaminated romaine is off the market would be for industry to voluntarily withdraw product from the market, and to withhold distribution of romaine until public health authorities can ensure the outbreak is over and/or until F.D.A. can identify a specific source of contamination.

“Until then, the F.D.A. advises that consumers should not eat and discard romaine, or any mixed salads containing romaine, until more information on the source of the contamination and the status of the outbreak can be determined.”

In the current outbreak, 32 people in the U.S. have become infected due to E. coli O157:H7. The illnesses started appearing in early October. In Canada, 18 people have been infected with the same DNA fingerprint of E. coli.

Industry groups such as the Produce Marketing Association, United Fresh Produce Association and Western Growers are urging processors to voluntarily and proactively prepare traceback and trace forward data in the event investigators contact them.

“Food safety is our top priority,” the groups said. “We must take swift action to protect consumers by stopping shipment of romaine lettuce, withdrawing any product that has been shipped to retail stores or restaurants and aiding the outbreak investigation.

“We believe a withdrawal of romaine lettuce is the fastest way to clear up the supply chain of any romaine that could be responsible for illnesses. Additionally, we are calling on handlers to clean and sanitize any harvest or packing equipment that may have been used in recent weeks to prevent cross-contamination of produce during future harvest and processing activities.”