Chipotle Mexican Grill’s confirmed on Sunday, Nov 01, it had closed all its restaurants in Washington state and Oregon due to a reported outbreak of E. coli bacteria. The issue is being investigated by the company and health authorities. Meanwhile, scared diners are keeping away from the popular burrito chain.

“After being notified by health department officials in the Seattle (Wash.) and Portland, Ore. areas that they were investigating approximately 20 cases of E. coli, including people who ate at six of our restaurants in those areas, we immediately closed all of our restaurants in the area out of an abundance of caution” Chipotle said in a earlier statement.

Health officials are trying to identify the cause of the E. coli food poisoning that has now sickened over 35 people who ate at eight Chipotle restaurants in the greater Seattle and Portland areas.

The company said the vast majority of its stores in the area had reported no problems. “We offer our deepest sympathies to those that have been affected by this situation.”

It is the third outbreak of food contamination at Chipotle restaurants since August. Those earlier cases involved salmonella and the highly infectious virus norovirus.

“Even after the company sounds the ‘all-clear,’ we believe that it will take some time for traffic to return,” Maxim Group restaurant analyst Stephen Anderson said in a research note. He estimated that the same-restaurant sales hit could be as much as 75 basis points this quarter and 25 basis points in the first quarter of next year.

The 1,700-outlet chain has grown quickly since it opened in 1993 with a single location, distinguishing itself from typical fast-food restaurants by touting its use of healthy and high-quality fresh ingredients in its menu of burritos, tacos and salads.

There is a growing trend among restaurants, as with Chipotle, to use more fresh, unprocessed food. While that may be good for nutrition, experts say it raises the risk of foodborne illness because cooking kills pathogens that cause illness.

​​”Health officials want people who have eaten at a Chipotle between October 14 and 23, and become ill with vomiting and bloody diarrhea, to see their health care provider and mention this outbreak,” the agency said in a notice on its website.

E. coli is among a vast array of bacteria that live in the human gut and which cause no problems. But some strains can cause serious symptoms and even be life-threatening, and are spread by oral contact with fecal matter.

Shares in Chipotle, which has had two other food safety lapses this year, were down 0.3 per cent at US$621.36 ($944.13) in midday trading on Wednesday. The stock closed at just over US$750 on October 13.