The operator of Eurest Dining Services said while so far there have been no reported listeriosis cases in Ontario prisons, random surveillance samples of products from its food-production facility indicated traces of Listeria monocytogenes, the Canadian Broadcasting Corp. reported Saturday.

Dr. David Williams, Ontario's acting chief medical officer of health, said following Friday's report to the Halton Regional Health Department, all potentially infected prisoners should be tested immediately.

"Although there have been no cases, I am asking individuals who were present at these seven correctional institutions between the dates of Nov. 13 and 16, 2008, to seek medical attention if they develop symptoms of listeriosis," he said in a statement Saturday.

The bacterium can be found in certain raw or processed foods and could potentially cause death in those infected if not treated.

The CBC said infection symptoms include diarrhea, nausea, fever, severe headaches and a stiff neck.