A major waterborne outbreak of E. coli O157 has struck the small town of Walkerton, Ontario, about 180km north of Toronto. As of 7 June, seven fatalities had been attributed to the outbreak, with four more deaths under investigation, and several people still in serious condition. The deaths have occurred mainly in the elderly, but also included a 30 year old man and a 2 year old child. More than 1,000 people are believed to have been ill, with over 400 attending the emergency department of the local hospital with severe gastroenteritis symptoms. Schools, childcare centres and many businesses in the town have been closed for the duration of the outbreak.

In contrast to the harmless E. coli strains normally found in the intestine of humans and warm blooded animals, the O157 strain produces a potent toxin (verotoxin) which causes cell damage, leading to the development of bloody diarrhoea (haemorrhagic colitis). The infection may also result in severe dehydration and kidney damage requiring dialysis treatment (Haemolytic Uremic Syndrome, HUS). Fatalities may occur particularly among young children and the elderly, and some survivors suffer permanent kidney damage which may require life-long dialysis or a transplant.

Toxigenic E. coli (including O157 and other related strains) are carried by 10-15% of healthy ruminants (including cattle, sheep, goats and deer). The bacteria may be transmitted to humans by consumption of raw or undercooked meats, or by contamination of foodstuffs or water supplies with faeces from infected humans or animals. Outbreaks have also been associated with recreational water bodies, and direct contact with animals. The infectious dose may be as low as 5 to 10 organisms, and the incubation period ranges from 2 to 8 days. The bacteria are readily killed by chlorination of drinking water.

No specific treatment is yet available to combat the infection, and recent research suggests that antibiotics and anti-diarrhoeal medication may increase the risk of HUS developing. The Canadian government has granted special permission for victims of this outbreak to be treated with an experimental drug called Synsorb Pk developed by a Canadian biotechnology company. The drug binds the verotoxin produced by E.coli O157 infection in the gut, preventing it from reaching the bloodstream and thus reducing the risk of organ damage. The drug is presently completing the final stages of a 4 year clinical trial and is scheduled for release later this year.

The Walkerton community of 4,800 residents is served by a groundwater supply drawn from several wells. It is suspected that one of the wells became contaminated with animal waste after heavy rainfall in the area on May 12. The chlorination system for the water supply had reportedly been performing unreliably for some time and new equipment had been ordered.

Water samples taken on Monday 15 May tested positive for E. coli and the results were reported to the local water utility on Thursday 18 May. A program of flushing and chlorination was commenced to clear the contamination. The first signs of the outbreak occurred on 19 May when a local paediatrician reported two cases of bloody diarrhoea to the public health authority. By Sunday 21 May cultures of faecal specimens had confirmed E. coli O157 as the cause of the illness. Meanwhile more cases of severe gastroenteritis were reported by doctors and the local hospital, and on Sunday 21 May a boil water notice was issued by health authorities.

According to reports in Canadian newspapers, a police investigation is now underway into the actions of the Walkerton Public Utilities Commission which supplies water and electricity to the town. The utility has only nine employees, with two individuals primarily responsible for managing the water supply.

Allegations have been made that the utility did not report either the E. coli findings or the problems with chlorination equipment to public health officials even when directly questioned about the safety of the water supply. The Health Officer for the region has been quoted as saying he contacted the utility when the first cases of illness were reported on Friday 19 May and was assured that the water supply was safe and secure. These assurances were repeated when the utility was again contacted the following day.

Health investigators initially concentrated their investigation on the possibility of foodborne transmission, but by Sunday 21 May they had become convinced that the municipal water supply was the most likely source. A boil water order was issued and the Public Health Unit collected water samples for analysis. These samples were reported positive for E. coli on Tuesday 23 May. It is reported that only then did the Public Utilities Commission personnel inform health officials of the previous E. coli test results and the chlorination problems. Control of the town's water supply has now been handed over to the Ontario Clean Water Agency for a six month period while the investigation continues.

The outbreak is the first waterborne E. coli outbreak from a municipal supply reported in Canada, and has attracted intense national media attention. A number of legal actions on behalf of victims have been lodged with the courts, citing negligence on the part of various parties. The outbreak has also triggered alarm in other towns and among homeowners with private wells, and testing laboratories have reportedly been overwhelmed with requests for water tests.

Some commentators have charged that the Ontario governments cutbacks in funding for environment protection and the devolution of responsibilities to regional and municipal authorities led to a failure of public health protection. The Ministry of the Environment (MOE) budget has been reduced by 40% since 1995, and it has been alleged the ministry has adopted a "soft" approach to enforcement and prosecution. The closure of government owned water testing laboratories in 1996 has also drawn criticism, after it emerged that the private testing laboratory had no legal obligation to report E. coli detections to government officials.

The MOE has launched an internal investigation after admitting it failed to follow protocols to notify the regional Medical Officer of Health of several recent detections of total coliforms or E. coli in the Walkerton supply. These test results were reported voluntarily to the MOE by a different private laboratory which was performing routine tests for the Walkerton water utility. This laboratory ceased working for the utility only weeks before the current outbreak, and testing was taken over by a different private contractor.

In the face of mounting public pressure, Ontario’s Premier announced a full inquiry into the outbreak on 31 April. Opposition parties are calling for broad ranging terms of inquiry to cover environmental regulation and enforcement in relation to water quality protection, as well as an investigation of the Walkerton outbreak.