ARCHIVED - Canadian Integrated Surveillance Report: Salmonella, Campylobacter, verotoxigenic E. coli and Shigella, from 2000 to 2004

Human Verotoxigenic E. coli Cases

An overall decline in reported cases of verotoxigenic E. coli cases was observed after 2000 (Table 19). A slight increase in 2004 was related to an increase in reported cases in British Columbia and Alberta (Figure 29). A large number of cases were reported to NDRS in 2000 due to a waterborne outbreak of E. coli O157:H7 in Walkerton, Ontario. As a result, the province reported both lab-confirmed and epidemiologically-linked cases.

The rates derived from the NDRS and NESP data were similar (Figure 29). Although most provincial/ territorial rates were fairly stable between 2000 and 2004, a slight decrease was noted in the eastern provinces, including: Ontario, Québec, New Brunswick, Nova Scotia, and Prince Edward Island. The large peak in Prince Edward Island in 2002 was attributable to E. coli outbreaks that occurred at a psychiatric hospital⁽¹⁹⁾ and a daycare.

Figure 29: Rates of verotoxigenic E. coli infections (per 100,000 population) as reported to the National Notifiable Disease Summary program (NDRS) and the National Enteric Surveillance Program (NESP) by province/territory, 2000 to 2004*

* Note the different scale used for Nunavat and the Northwest Territories.

Verotoxigenic E. coli Serotypes

The majority (94%) of verotoxigenic E. coli infections reported to the NESP between 2000 and 2004 were serotype O157. The number of cases reported by serotype and year is listed in Table 20.

Long-term Trends

The national reporting rate of verotoxigenic E. coli infection steadily declined between 1995 and 2004, with the exception of a peak in 2000 that corresponded with a large waterborne outbreak in Walkerton, Ontario (Figure 30).

Figure 30: Reported rates of verotoxigenic E. coli infections (per 100,000 population), 1995 to 2004, NDRS and NML/NESP*

* NML data (1995-1997) include only E. coli O157 isolates received; NESP data (1998-2004) include E. coli O157 and other toxin-producing isolates reported.

Monthly and Provincial/Territorial Trends

The seasonal trend of verotoxigenic E. coli cases shows a clear increase beginning in the spring, peaking in the summer and declining in the fall each year (Figure 31). In May 2000, the peak was due to the waterborne outbreak of E. coli O157:H7 in Walkerton, Ontario. Increased reporting observed in December 2001 was likely due to an outbreak related to catered events in Saskatchewan where more than 70 people were reported ill and 15 cases of E. coli O157:H7 PT 21 were confirmed. In May 2002, a cluster of more than 80 cases across Canada of E. coli O157:H7 PT 14a, was suspected to be linked to ground beef.

Figure 31: Reported cases of verotoxigenic E. coli by month, 2000 to 2004, NDRS and NESP

The seasonal variation in verotoxigenic E. coli rates by province/territory is shown in Figure 32. Summer peaks remained apparent across most provinces and territories, while the spring peak in Ontario and the fall peak in Prince Edward Island were largely due to specific outbreaks. The high rate in Nunavut in the summer reflects the 26 cases reported in 2000, several of them associated with the consumption of contaminated ground beef.

Figure 32: Average reported rate of verotoxigenic E. coli infections (per 100,000 population per season*) by province/territory, 2000 to 2004, NDRS

* Winter includes December, January and February; Spring includes March, April and May; Summer includes June, July and August; Fall includes September, October and November.

Figure 32 - table equivalent

Age and Gender Distribution

As with Salmonella and Campylobacter, infants and young children had the highest rate of infections and hospitalizations due to verotoxigenic E. coli (Figure 33). The rate progressively declines with increasing age, although it begins to increase again among the elderly.

The gender distribution for verotoxigenic E. coli infections remained fairly consistent over the fiveyear period, with a higher rate being reported in females (females: 5.48 per 100,000 population; males: 4.47 per 100,000 population). This trend is supported by findings reported in studies investigating the incidence of gastrointestinal illness in Canadian populations (20,21).

Figure 33: Reported rate of verotoxigenic E. coli infections and hospitalizations (per 100,000 population) by age group, 2000 to 2004 combined, NDRS and CIHI

Exposure Settings for Outbreaks and Case Clusters

There were 129 outbreaks and case clusters related to verotoxigenic E. coli and 1196 outbreak-related laboratory-confirmed cases reported to the NML and NESP between 2000 and 2004. Outbreaks and case clusters of E. coli O157 by exposure setting are shown in Table 21. Household settings represented the largest number of reported outbreaks, while community settings resulted in higher outbreakrelated case counts. The largest community outbreak occurred in May 2000, when the drinking water system in Walkerton, Ontario became contaminated with E.coli O157:H7 and Campylobacter. Seven deaths were associated with the outbreak, in four cases, the sole cause of death was E.coli infection as a result of the outbreak⁽²²⁾. The investigation into the outbreak identified 1346 people that met the outbreak case definition which included both E.coli and Campylobacter⁽²²⁾. Several outbreaks occurred in non-residential institutions including daycare settings. Person-to-person transmission and other risk factors unique to daycare settings may increase the potential for transmission and infection⁽²³⁾.

* R - residential and NR - non-residential

Travel-acquired Infections

Only 10 cases of verotoxigenic E. coli reported to the NESP between 2000 and 2004 were identified as travel-related. A history of travel was provided for 10/6505 of E. coli infections reported to the NESP over the five-year period. Although foreign travel is one of the main risk factors for gastrointestinal illness, this information is rarely captured or reported and is therefore greatly under-represented in the NESP. Travel to Mexico and Caribbean countries accounted for half of the travel-acquired verotoxigenic E. coli infections recorded (Table 22).