Federal and state health officials are investigating a multistate Escherichia coli O157:H7 outbreak that has sickened 17 people in 13 states, and preliminary tests by the US Centers for Disease Control and Prevention (CDC) show that the outbreak strain is closely related to one in Canada that has been associated with romaine lettuce.

The CDC said illness onsets range from Nov 15 through Dec 8, according to a press release today sent to journalists. Affected states include California, Connecticut, Illinois, Indiana, Michigan, Nebraska, New Hampshire, New York, Ohio, Pennsylvania, Virginia, Vermont, and Washington.

State and local authorities are interviewing sick people to see what they ate in the week before they became ill. Because a source of the US infections hasn’t been identified, the CDC said it is unable to recommend if US residents should avoid a particular food. “This investigation is ongoing, and more information will be released as it becomes available,” it said.

The Public Health Agency of Canada (PHAC) issued its first announcement about an E coli outbreak linked to romaine lettuce on Dec 11. In a Dec 21 update, it said it is so far investigating 40 cases from five provinces: Ontario, Quebec, New Brunswick, Nova Scotia, and Newfoundland and Labrador. It urged the public to avoid eating romaine lettuce until more is known about the contamination.