SEATTLE -- An E. coli outbreak linked to Chipotle restaurants in Washington state and Oregon has sickened nearly two dozen people in the third outbreak of foodborne illness at the popular chain this year.
Cases of the bacterial illness were traced to six of the fast-casual Mexican food restaurants, but the company voluntarily closed down 43 of its locations in the two states as a precaution - all those in Washington state and in the Portland, Oregon, area.
Three people in the Portland area and 19 people in western Washington have gotten sick with E. coli as of Friday. Seventeen of them had eaten at a Chipotle restaurant during the past few weeks. Eight people have been hospitalized but no deaths have been reported.
Health officials said they expect the number of cases to rise as more people hear about the outbreak and go to the doctor.
Marisa D'Angeli, medical epidemiologist with the Washington State Department of Health, encouraged anyone who has been sick with intestinal symptoms and has eaten at Chipotle since mid-October to go see their doctor and get tested. She also said anyone with bloody diarrhea should go to the doctor whether they have eaten at Chipotle or not.
"Certainly a lot of people who have diarrheal illnesses don't go to the doctor, they say at home, so we don't really know that they might be cases," CBS News medical contributor Dr. Tara Narula told "CBS This Morning." "Or if they go they don't get their stool tested, which is the way to diagnose this. And if they do get tested, it sometimes takes several days for that lab result to be confirmed."
Health officials are investigating the cause of the outbreak. The source was most likely a fresh food product because it probably could not be traced to one sick individual or one instance of cross-contamination of food since the cases are connected with various restaurants, D'Angeli said.
Finding the origin of the outbreak could take days or weeks, Narula said. "The CDC has investigated over 20 E. coli outbreaks since 2007. They go through a group of steps to try to isolate the source and then do source trace-back. We think in this case it's something in the food supply chain because it's infected multiple restaurants," she said.
The company is not planning to close any other restaurants in other states because there is no evidence of a link to other locations, company spokesman Chris Arnold said. Reopening the shuttered locations in Oregon and Washington will depend on the investigation, he said.
"Right now, that is the priority," Arnold said.
Chipotle has faced other recent foodborne outbreaks. A salmonella outbreak linked to tomatoes sickened dozens of people in Minnesota beginning in August, according to state health officials. In California, health workers said norovirus sickened nearly 100 customers and employees at a Chipotle restaurant in Simi Valley in mid-August.
A food safety lawyer who is involved in other lawsuits against Chipotle says people should not assume a company that focuses on local and fresh ingredients is going to be immune from food safety issues.
"People shouldn't have a false sense of security that local means safer," said Bill Marler of Seattle law firm Marler Clark.
Marler, who built his national reputation with the 1993 E. coli outbreak at Seattle Jack in the Box restaurants, said, "Having three problems in a couple of months means that Chipotle is not paying attention to food safety like it should."
In the Northwest outbreak, the investigation started with talking to everyone diagnosed with E. coli and finding out what they ate and where. Test samples from those individuals will go to state labs in Washington and Oregon.
Then, samples of food from the restaurants will be tested at a U.S. Food and Drug Administration laboratory to see if bacteria from the food matches the human cases.
People have reported symptoms of E. coli infection in Clackamas and Washington counties in Oregon, and Clark, King, Skagit and Cowlitz counties in Washington.
There are hundreds of E. coli and similar bacteria strains in the intestines of humans. Most are harmless, but a few can cause serious problems, especially in children or older adults. Symptoms of E. coli infection include diarrhea, abdominal cramps, nausea and vomiting.
In about 5 to 10 percent of cases, Narula said patients may suffer a serious complication called hemolytic uremic syndrome, which could lead to kidney failure.
Health officials say the best defense against the bacterial illness is to thoroughly wash hands with soap and water.
Shares of Chipotle tumbled almost 3 percent in early trading Monday.