North Bay's outbreak was the largest of its kind in Ontario since seven people died and about 2,500 others fell ill from water tainted with the same strain in Walkerton, Ont., in May 2000.

Contaminated onions were likely behind an outbreak of E. coli at a fast-food restaurant in North Bay, Ont., that sickened 235 people last fall, health officials announced Monday.

A Harvey's restaurant was shut down from Oct. 12 to Nov. 12 after health officials linked it to the outbreak of E. coli O157:H7, which causes severe abdominal cramping and sometimes bloody diarrhea.

On Monday, health officials with the North Bay Parry Sound District Health Unit released the findings of their investigation, which concluded "inconsistent" cleaning could be to blame.

"The epidemiological investigation indicated that this outbreak was caused by a point source at the Harvey’s restaurant in North Bay, most likely contaminated onions," the report's authors wrote.

"Although the initial source of the contamination was not identified, the risk of exposure lingered on-site for about a week. Inconsistent cleaning of the onion dicer may have perpetuated the contamination for several days."

The suspected onion could not be confirmed through environmental testing, food sampling and tests of staff and contacts, said Dr. Jim Chirico, acting medical officer of health for the North Bay Parry Sound District Health Unit.

Onion was considered the most likely source of contamination, based on a statistical analysis comparing what those who fell ill ate versus controls who did not fall ill.

It is very common that the source is never identified in such investigations since E. coli symptoms show up one to 10 days (and an average of three to four days) after the contaminated food is eaten, and the source is often gone by then. 

It's unlikely that the original source of the E. coli will ever be found, agreed food science Prof. Keith Warriner, an expert in food safety at the University of Guelph.

"At some point, the E. coli must have gone from the meat or from … [a] contaminated person or an infected person, found its way on to the onions, and it sounds to me like the dicer must have [become a] continuous source of contamination," Warriner said.

In this outbreak, exposure to the onions peaked Oct. 5-10 — several days before people sought medical attention and the health unit learned of the cases.

No deaths occurred during the outbreak.

Of the 235 people sickened, 93 reported bloody diarrhea, 26 were hospitalized and one case of hemolytic uremic syndrome (HUS) in a child was reported.

HUS is a life-threatening condition that is treated in hospital intensive-care units. It kills three to five per cent of those afflicted.

Some people who recover may have to contend with lifelong complications that can include blindness, paralysis and kidney failure.

The investigation spanned 10 health units in Ontario and one other province, with local, provincial, national and international experts participating.   

Chirico noted public awareness of the outbreak seemed to pay off, since the rate of secondary infections (people infected by others) was only five per cent, compared with an average of 10 per cent in previous such outbreaks.

Consistent cleaning key

For future restaurant-based outbreaks, the report's authors recommended investigating cases among patrons and employees simultaneously, with stool samples collected from employees as soon the restaurant is closed to increase the likelihood of identifying infected staff.

They also recommended that:

  • Food-processing equipment should be thoroughly cleaned regularly to prevent buildup of bacteria.
  • Kevlar safety gloves, which were used at Harvey's, should be avoided or covered with single-use latex gloves.
  • Since steel-mesh gloves can harbour bacteria, they should be cleaned and sanitized regularly.
  • Equipment with multiple parts should be completely disassembled prior to cleaning.
  • Starting a cleaning schedule for the dicer may focus employees' attention on its proper cleaning.

Consistent application of cleaning policies and procedures is the issue, said Peter Jekel, the health unit's director of environmental health, noting Harvey's has been compliant and diligent in adhering to sanitation practices since the incident.

Neither the restaurant nor the parent company, Cara Operations. were available for comment.

As for lessons learned from the outbreak, Chirico said it showed why it's so important for food handlers to stay home when sick and to be diligent about hygiene and hand washing.

"When you think of how many millions and millions of products are consumed on a daily basis, and we have not had real explosion of outbreaks, I think that speaks to the safety system that we do have," Chirico told a news conference.

North Bay had one confirmed case of E. coli O157:H7 each year between 2005 and 2007.

The report was produced in collaboration with the Public Health Agency of Canada's Canadian Field Epidemiology Program.