Deadly E. coli Outbreak in Canada

Lessons Learned in Fighting the Bacteria

In May, the worst E. coli outbreak in North American history struck Walkerton, a small farming community 90 miles west of Toronto, Canada. More than 1,000 people became ill and at least seven people died from the infection. Health officials believe that the harmful O157:H7 strain of E. coli bacteria most likely entered the town’s drinking water supply when animal excrement from nearby farms was washed into wells during a flood earlier in the month.

Five times earlier in the year, weekly water samples had shown the presence of coliform but not the deadly O157:H7 strain of E. coli. The Public Utilities Commission was taking corrective measures to treat the water, including increased chlorination and extra testing.

But at the time of the E. coli outbreak, the city’s chlorine disinfection system was malfunctioning. Canadian health officials expressed their confidence that if the system had been working properly, the bacteria would have been killed before it infected the people of Walkerton.

The E. coli bacteria

Since the first description of E. coli related illness in 1982, infections have been reported in 30 countries on six continents. An estimated 73,000 cases of E. coli O157:H7 infection occur annually in the U.S. Of these cases, 2,100 require hospitalization for Hemolytic Uremic Syndrome (HUS), a condition that often leads to kidney failure or neurological impairment. With a death rate of three to five percent, an estimated 61 people die of E. coli -related HUS every year in the U.S.

Sources of infection

According to the Centers for Disease Control and Preven-tion (CDC), the vast majority of E. coli outbreaks are traced to contaminated food. Inadequately chlorinated drinking water also can lead to infection. Roughly eight percent of the E. coli outbreaks in the U.S. are related to recreational water use including swimming in a polluted lake or pool.

A recent survey by the University of Arizona examined 100 home washing machines for the presence of coliform, including E. coli and staph bacteria. Sixty percent of the machines tested positive for at least one of them. The study concluded that much of the harmful bacteria is killed by the heat of a dryer, but that chlorinated bleach should also be used in the wash cycle to decrease the chances of infection.

A survey of the quality of water drawn from domestic wells by the CDC and the National Center for Environ-mental Health revealed that the 14 million households in the U.S. that rely on domestic well water to supply their drinking water are at risk of contamination from harmful strains of E. coli.

Following the Midwest flood in 1993, the CDC and state health and environmental departments in nine Mid-western states conducted an analysis of domestic wells in the flooded river basins. Water samples were collected from 5,520 households with domestic wells. E. coli was detected in 11 percent of the samples. The most probable contamination sources were septic tanks and lateral fields, structures that contain human fecal material. The study recommended building wells away from con-tamination sources, routinely testing for contaminants, and effectively disinfecting the water with chlorine.

Proper disinfection kills E. coli

Studies by the U.S. Environmental Protection Agency have concluded that properly administered chlorine water treatment practices are effective in inactivating harmful and deadly strains of E. coli. In such tests, multiple strains of E. coli were subjected to a chlorination treatment of 1.1 mg/L free chlorine and 1.2 mg/L total chlorine for one minute. E. coli populations were reduced by four orders of magnitude. Based on these findings, the authors of the study concluded that standard chlorine disinfection practices in the U.S. effectively kill harmful strains of E. coli bacteria.

For general information about E. coli, visit the Centers for Disease Control and Prevention web site at www.cdc.gov.

For specific information on the chlorine inactivation of E. coli, go to www.cdc.gov/ncidod/eid/vol5no3/rice.htm.