Another case of E. coli infection linked to Chicken & Rice Guys food outlets was identified Thursday, bringing to 15 the tally of people sickened in one of the bigger food-borne illness outbreaks to strike the state.
State and municipal health officials were testing food from the restaurants and samples from about 120 food handlers associated with the now-shuttered Chicken & Rice Guys restaurants and food trucks, in the hope of pinpointing the source of the illnesses.
Ten people in the outbreak required hospitalization, a high percentage. But Dr. Anita Barry, director of the Infectious Disease Bureau at the Boston Public Health Commission, said none of those infected had developed hemolytic uremic syndrome, the deadly side effect of E. coli infections that can shut down the kidneys.
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Instead, she said, they were most likely hospitalized because of bloody diarrhea, one of the symptoms of infection with the organism officially known as Shiga-toxin producing Escherichia coli 0157:H7. Other symptoms include abdominal cramps and dehydration. Officials had no information on the victims’ condition, but Barry said some had been discharged from hospitals.
Health officials expressed confidence that, with the restaurants temporarily closed, there is little continuing risk to public health — even though three of the victims did not recall eating at Chicken & Rice Guys, which has three restaurants in Boston and one in Medford, plus a fleet of food trucks. No other food establishment is under investigation.
None of those sickened are children or elderly people, the groups most vulnerable to the ill effects of this infection. Most are in their 20s, with a few in their 30s and 40s, according to Kevin Cranston, director of the Bureau of Infectious Disease at the state Department of Public Health, which is investigating the outbreak along with municipal health authorities.
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Nine of the 15 victims live in Boston, Barry said, and the rest in nearby cities and towns in Middlesex and Suffolk counties.
The first victim fell ill March 30 and the most recent on April 4.
“This is a pretty substantial outbreak” for one that thus far appears to be limited to Massachusetts, Cranston said.
“The reality is most people eat in restaurants and don’t get sick,” he added. “This is an exception to the rule.”
Based on past experience, the most likely scenario is that a sick employee contaminated the food, Cranston said. The employee could have touched food that was then taken to more than one Chicken & Rice Guys outlet.
Another possibility is that a food ingredient commonly used at Chicken & Rice Guys — but not common elsewhere — became contaminated.
But the source may never be identified.
“Most E. coli investigations do not determine a specific food item,” said Bill Marler, the Seattle food safety lawyer, “because people ate the evidence.”
Last month, a man from Middlesex County fell ill with E. coli amid a nationwide outbreak linked to I.M. Healthy SoyNut Butter. And in 2014, an 8-year-old Braintree boy died of E. coli infection linked to grass-fed beef purchased at Whole Foods.
But big outbreaks of E. coli are not common in the state. Last year seven people, including three from Massachusetts, were sickened with E. coli linked to meat from Adams Farm Slaughterhouse in Athol.
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Boston inspection officials could not recall another E. coli outbreak in the city. William “Buddy” Christopher, commissioner of Boston’s Inspectional Services Division, said the current outbreak is unique for involving a business with so many parts. Chicken & Rice Guys has four eat-in or take-out restaurants, five food trucks, and a kitchen commissary in Somerville that supplies the food it sells on its trucks.
“This is the first time we’ve seen E. coli in multiple locations,” Christopher said.
Christopher said there have been no significant health code violations at the chain in recent years. An inspection of the Allston restaurant Tuesday found six violations of city health and safety codes, although none was deemed critical. Violations included a failure to properly cool food at room temperature and greasy build-up on a grill, but inspectors found no risk factors for food-borne illness.
Chicken & Rice Guys can reopen its restaurants and trucks after all food handlers have tested negative for E. coli twice, and after local inspectors determine that each outlet complies with food codes.
Eric F. Nusbaum, who heads the Wheelwright Consultants restaurant and food safety advisory group in Greenfield, said while E. coli bacteria grow in animals and are traditionally found in meat products, most recent cases have been linked to contaminated produce. “You’re dealing with a natural product that has to be grown with exposure to animals and birds if they fly over and defecate on produce,” he said.
He added that Massachusetts food safety regulations are more stringent than federal oversight requires, and that the state has far fewer cases of food-borne illness as a result.
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“The good news is that food is safer than it’s ever been the US,” he said. “But there is still a risk.”
The first cases in the recent outbreak came to light April 5, when the Boston Public Health Commission received a report of a positive laboratory test for E. coli, followed quickly by additional cases. On Friday, the Department of Public Health and the Boston Public Health Commission alerted health care professionals to be vigilant for E. coli symptoms and to collect samples for testing.
Meanwhile, public health nurses interviewed victims, asking what and where they ate in the seven days before developing symptoms. A large majority recalled eating at a Chicken & Rice Guys outlet, enough to make the chain a prime suspect and prompt its closing Tuesday.
Barry, of the Boston health department, said three people did not mention eating at Chicken & Rice Guys. But people don’t always remember where they ate, or they eat food provided by someone else without knowing its source.
Felice J. Freyer can be reached at felice.freyer@globe.com.
Correction: Due to a reporting error, a previous version of this story provided an incorrect age for the Braintree boy who died of E. coli.