The outbreak, one of the largest this year, was identified at the Falkirk and District Royal Infirmary, the hospital which treated victims of last year's epidemic in which 19 people died. It will renew concern about Scotland's high rate of infection with E. coli, one of the nastiest food-poisoning bacteria, which remains unexplained.
Health officials said six of the 21 in the latest outbreak were ill with symptoms such as diarrhoea and the rest had been identified as carriers of the bacterium in tests but remained well. Thirteen of those infected were patients, seven were nurses and one was a domestic worker.
Central Scotland NHS Trust, which runs the unit, said doctors and nurses were taking action to contain the infection.
Dr Derek Sinclair, medical director for the trust, said: "We have taken all necessary measures to give care to those affected in this outbreak. We are trying to establish the source and samples have been taken from a wide range of people and places."
He added: We have no idea of the source at this stage and we are examining all options."
The outbreak occurred in three continuing-care wards at the hospital, which has about 70 elderly male and female patients, ranging in age from 70 to 90, and more than 100 staff, all of whom will be tested for the infection.
Dr Sinclair confirmed the hospital kitchens have been screened, but said: "We didn't check with a microscope, that's why everything has now been sent for analysis."
Last year's outbreak infected 496 people over five weeks in November, of whom 272 were confirmed in laboratory tests. The outbreak was linked with cooked meat and pies supplied by John M Barr and Son, a butcher in Wishaw, which supplied more than 60 outlets in Lanarkshire and the Forth Valley.
Until last year, previous annual totals of cases of E. coli had ranged from 200 to 250 but Scotland's rate of infection is now running at three to four times the English level. Officials privately believe Scottish methods of collecting data are more accurate, and there is greater awareness of the problem north of the border, but that this is unlikely to account for all the difference.
Dr John Cowden, consultant epidemiologist at the Scottish Centre for Infection and Environmental Health, said responsibility for preventing outbreaks lay with kitchen staff who prepared food.
The E. coli bug contaminates only the surface of the meat, through contact with cattle faeces, and is destroyed in cooking. Cooked meat can become contaminated, however, through contact with infected raw meat and must be kept separate from it.
Dr Cowden said: "Present controls at the farm and the slaughterhouse are insufficient to guarantee food free of bacteria. Therefore the final responsibility lies with the person preparing the food.
"This is a heat-sensitive bacterium. If you cook the food properly and ensure it is not cross contaminated subsequently, it is safe," he said.
Dr Cowden said a rare steak was safe provided it was well cooked on the outside. However, a hamburger made from minced meat, where what was on the outside was now on the inside, must be well cooked all through to be safe.
The E. coli bacterium is so virulent only a very small dose is required to cause infection and the frail elderly are the most vulnerable to infection from it.
Professor Hugh Pennington, chairman of the inquiry into last year's E. coli outbreak, said on BBC Radio Four earlier that Scotland was one of the worst hit countries in the world by the bacterium but it was not understood why.
He added that full implementation of the recommendations for improvements to food safety in his report should lead to a significant reduction in cases.
"The lesson is we have got to keep this organism out of the body. The thing is out there challenging the food safety system all the time," Professor Pennington said.
"We don't know how to eradicate it - we have to tackle the critical control points in the food chain, not contaminating ready-to-eat foods with raw meat and cooking at the proper temperature."Reuse content