Cases of the bacterial illness were traced to a few of the fast-casual Mexican food restaurants, but the company voluntarily closed down all 43 of its locations in the two states as a precaution.
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 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, said Marisa D'Angeli, medical epidemiologist with the Washington State Department of Health.
The company is not planning to close any other restaurants in other states because there is no evidence of a link to other locations, Arnold said. Reopening the shuttered locations in Oregon and Washington will depend on the investigation, company spokesman Chris Arnold said.
"Right now, that is the priority," he said.
Health officials expect the number of illnesses to rise as more people hear about the outbreak and go to the doctor.
D'Angeli 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.
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.
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.
Symptoms of E. coli infection include diarrhea, abdominal cramps, nausea and vomiting. Health officials say the best defence against the bacterial illness is to thoroughly wash hands with soap and water.
Shares of Chipotle tumbled almost 3 per cent in early trading Monday.
Donna Gordon Blankinship, The Associated Press