Calgary seeks help to identify E. coli source

The Calgary Health Region has called in an epidemiologist with the Public Health Agency of Canada to try to identify the reason for an unusually high number of cases of Escherichia coli O157:H7 this summer.

In July and the first part of August, 46 people in the Calgary region were diagnosed with E. coli O157:H7. Normally, over that period of time the region would see 5–10 cases resulting from an infection with that strain of the bacteria, says Dr. Brent Friesen, the medical officer of health.

Eleven people were hospitalized as a result of the summer infections, and 6 had developed hemolytic uremic syndrome as of Aug. 10.

The Region asked for and received help from a field epidemiologist with the Canadian Public Health Agency because the medical officer and his staff were unable to trace the E. coli to a single source. They were also puzzled because the E. coli infections do not appear to be resulting from a specific source, such as the one in Walkerton, Ont., in 2000 that left 2600 people ill and killed 7.

Although the Calgary incidents are not classified as an outbreak, “this is higher than the number of cases we would normally see,” Friesen told CMAJ. “We have not been able to link the increase in cases to 1 particular source, or to explain it.”

A laboratory analysis of the E. coli showed several different strains and clusters, he says. Some patterns were clustered in families, which Friesen would expect if family members shared a single source of infection, but others do not appear to have any commonality.

As a result, the Region is increasing its surveillance.

“We're collecting more detailed information on the cases to try to generate some hypotheses about what may be potential sources, and are working with the provincial lab to do further review and analysis of the genetic pattern, to see if there's some similarity,” Friesen says.

One cluster of cases in early July that involved 4 family members was traced to raw ground beef, Friesen says. Another cluster of 4 cases shared a link to a restaurant. But the Region remains concerned that there may be a broader exposure to a contaminated food product or some other source that it has not yet identified, Friesen says.

So far, there is no indication that the E. coli is waterborne, as was the case in Walkerton.

As a whole, southern Alberta has higher rates of sporadic E. coli that cannot be explained or linked to a particular source, Friesen says.

“Past efforts to identify particular sources through measures such as case controls or even to examine potential sources that have been suggested in the past, such as feed lots or irrigation canals have not been successful in identifying the source or reason for the higher number of cases that we have experienced,” he says.

“Clearly, we are challenged by these situations.”

In order to investigate the E. coli cases, the Region has suspended its follow-up investigations of other enteric infections, to concentrate staff resources. “We've attached a higher priority to it, but it has meant that the follow-up on other resources and illnesses is delayed,” Friesen says.