ARCHIVED - Descriptive Epidemiology of Enteric Illness for Selected Reportable Diseases in Ontario, 2003

1 December 2006

Volume 32

Number 23

Z Rajda, MD, BSc, CPHI(C), MHSc (1), D Middleton, BSc, DVM MSc (2)

1. Senior Consultant, Food Safety and Safe Water Unit, Infectious Diseases Branch, Ministry of Health and Long-Term Care, Toronto, Ontario

2. Senior Veterinary Consultant, Enteric and Zoonotic Diseases Unit, Infectious Diseases Branch, Ministry of Health and Long-Term Care, Toronto, Ontario

In Ontario, health units are responsible for the investigation of enteric diseases, designated as reportable under the Health Protection and Promotion Act. Health units are also responsible for reporting this information to the Ontario Ministry of Health and Long-Term Care (MOHLTC) through a public health monitoring system called the Reportable Diseases Information System (RDIS)¹.

The objective of this article is to provide descriptive epidemiologic findings on the occurrence of 10 reportable enteric diseases for the calendar year 2003 in Ontario. To better describe the epidemiologic findings pertaining to enteric illnesses acquired in Ontario, travel-associated cases were removed from the analysis so that only illnesses acquired in Ontario were considered.

Methods

The records of 10 enteric diseases for the year 2003 were obtained on 22 March, 2005, from the RDIS database. The diseases chosen for analysis included botulism, campylobacteriosis, hepatitis A, listeriosis, paratyphoid fever, salmonellosis, shigellosis, typhoid fever, verotoxin-producing E. coli (VTEC) and yersiniosis.

Cases identified as illness acquired during travel outside the province were removed from the dataset. A case was identified as travel-associated if the individual had travelled to a destination outside of Ontario and became ill no more than 2 days after travel for Clostridium botulinum, 10 days for Campylobacter, 50 days for hepatitis A, 70 days for Listeria, 3 months for paratyphoid and typhoid fever, and 7 days for Salmonella, Shigella, VTEC and Yersinia.

The remaining records were considered for analysis if the Episode Date was in the year 2003. Data were analyzed using SPSS (version 13.0). The 2003 Ontario population estimates derived from the 2001 Canadian census data were used to calculate incidence rates of enteric disease.

Some of the selections within the variable Risk Setting were regrouped to include “institutions” (day care, hospital, medical office, nursing home and school), “local travel” (animal zoo, camping, encounter followed major event, farm, lake/river/ stream/pond, local travel, local camping, local vacation property, travel within Ontario, pool/spa, rendezvous outside usual domicile) and “other” (facility for the developmentally disabled, shelter/rooming house).

The percentage of missing and unspecified values was calculated for each variable and is shown in Table 1. The findings presented in the Results section of the report do not include missing data. Data completeness and internal consistency were assessed. Duplicate cases were checked by comparing the Episode Date, Episode Date Type, Disease, Organism/Agent, Subtype, Date of Birth and Gender.

Table 1. Number and percentage of missing and unspecified values by variable for cases of enteric disease reported in Ontario, 2003 (n = 6,088)

Results

A total of 7,400 cases of enteric disease were identified in RDIS for the year 2003. Twenty-seven cases were identified as duplicates and were removed from the dataset. Of the remaining 7,373 cases of enteric disease, 1,285 (17.4%) were identified as travel-associated and were removed from further analysis. The analysis was performed on the remaining 6,088 cases.

Campylobacter species were the most frequently reported enteric pathogen (Figure 1). The number of reported cases by pathogen and species/serotype for the 10 selected enteric diseases is shown in Table 2. The remaining part of the report does not include any discussion of typhoid, paratyphoid or botulism because of the small number of cases reported in Ontario for 2003.

Table 2. Enteric pathogens by number of cases and serotype for selected reportable diseases in Ontario, 2003 (n = 6,088)

Figure 1. Number of reported cases of selected enteric reportable diseases in Ontario, 2003 (n = 6,088)

Figure 2. Incidence rates of selected reportable enteric diseases by sex in Ontario, 2003 (n = 6,086)

Males accounted for 53% of the cases overall (Figure 2). Incidence rates varied by age group, children between 0 and 4 years of age having the highest incidence rate of enteric illness for most of the selected reportable diseases (Figure 3).

Figure 3. Incidence of selected reportable enteric diseases in Ontario, 2003 (n = 5,836)

In Ontario, the incidence rates of enteric illnesses varied among the health units (Table 3).

Forty-one percent of the reported cases occurred in the 3 months from July to September. The dates of symptom onset for 44.0% of campylobacteriosis, 38.7% of salmonellosis, 36.5% of yersiniosis and 33.7% of VTEC occurred between July and September (Figure 4).

Table 3. Incidence rates (per 100,000 population) for selected reportable enteric diseases in Ontario, 2003 (n = 6,086)

* Rates are per 100,000 population.
** Populations are mathematical estimates for 2003 and are based on 2001 Census data.

Figure 4. Number of cases of selected reportable enteric diseases by month in Ontario, 2003 (n = 6,079)

click on the image to enlarge

Seventeen deaths were reported in 2003, with an overall case-fatality rate of 0.28%. The overall in-patient hospitalization rate was 5.2% (Table 4). Overall, the most frequently reported mode of transmission was foodborne, at 73.1%, followed by other (14.9%), person-toperson (7.5%), waterborne (3.5%) and animal contact (1.0%) (Figure 5).

Figure 5. Enteric pathogens by mode of transmission in Ontario, 2003 (n = 2,280)

click on the image to enlarge

Table 4. Enteric pathogens by hospitalization (n = 453) and death (n = 17) for selected reportable diseases in Ontario, 2003

Overall, the most frequently reported risk setting was the private home, at 58.5%, followed by food service sector (20.7%), workplace (7.0%), other (4.6%), institutions (4.6%) and local travel (4.6%) (Figure 6)

Figure 6. Enteric pathogens by risk setting in Ontario, 2003 (n = 2,692)

click on the image to enlarge

Discussion

It is known that passive surveillance processes capture only a portion of enteric disease episodes occurring in the community². The 7,373 cases reported in Ontario in 2003 likely represent under-reporting of the true number of enteric disease episodes in Ontario. The true incidence is difficult to calculate, as valid estimates of under-reporting are not available³. The potential for bias should be considered when interpreting the results presented in this report because for a number of variables in the database there was a large percentage of missing or unspecified data.

The 7,373 cases of enteric illness reported in 2003 is the lowest number of cases reported since 1991, the first year that valid data were available from RDIS. From 1991 to 2003, the annual number of cases ranged from 12,697 in 1991 (RDIS data 1991-1997) to 7,373 in 2003^4,5. The reason for the decrease is not known, but it may be a combination of safer food and better food safety practices.

Of the 7,373 cases of enteric disease, 1,285 (17.4%) were identified as travel-associated cases and, for the purposes of this report, removed from further analysis. The proportion of travel associated cases identified in 2003 was similar to that (16.3%) identified in 2002⁵ but less than the 24.6% identified in the period from 1997 to 2001⁴.

The 6,088 cases of enteric illness acquired in Ontario remain a significant burden of illness, especially when under-reporting is considered. Five of every 100 cases required hospital in-patient care. Further, the enteric pathogen was linked as a cause of or contributor to at least five of the 17 reported deaths.

The incidence rates for many of these enteric illnesses varied considerably among Ontario's 37 health units. The reason for the regional differences in incidence rates is not known.

The number of cases increased in the spring, peaking in July through September. The 3 months from July to September accounted for 41% of all cases, whereas 25% would be expected if there was an equal seasonal distribution. This finding is consistent with the accepted knowledge that enteric illnesses, consisting of predominantly foodborne illnesses, have an increase in seasonal distribution in the summer.

The most frequently reported mode of transmission in 2003 was foodborne (73.1%). This proportion was consistent with the 75.9% reported for Ontario in 2002 (with travel-associated cases removed)⁵. The remaining 27% in 2003 consisted of other (14.9%), person-to-person (7.5%), waterborne (3.5%) and animal contact (1.0%).

The most frequently reported risk setting was the private home (58.5%). This was slightly lower than the 64.3% reported for Ontario in 2002 (travel-associated cases removed)⁵. Other identified risk settings in 2003 were the food service sector (20.7%), workplace (7.0%), other (4.6%), institutions (4.6%) and local travel (4.6%).

Conclusion

The 7,373 enteric illnesses reported in 2003 is the lowest total number of cases reported since 1991. The main epidemiologic findings in this report are the foodborne transmission in the private home setting, occurring more frequently during the summer months.

Acknowledgements

The authors would like to thank the staff at the public health units as well as the staff at laboratories in Ontario for their work in investigating and reporting enteric illnesses. A special thank you to Steven Johnson from the Infectious Diseases Branch, Ontario Ministry of Health and Long-Term Care for his help and effort in the production of this article.

References

1. Ministry of Health. Mandatory health programs and services guideline. Ontario: Government Printing Office, 1999.

2. Centers for Disease Control and Prevention. The FoodNet surveillance report for 2001. Atlanta, GA: CDC, 2002. URL:<http://www.cdc.gov/foodnet/annual/2001/2001annualreport _pdf.pdf>.

3. Mead PS, Slutsker L, Dietz V et al. Food-related illness and death in the United States. Emerg Infect Dis 2002;5:607-25.

4. Lee M, Middleton D. Enteric illness in Ontario, Canada, from 1997 to 2001. J Food Prot 2003;66:953-61.

5. Rajda Z, Middleton D. Descriptive epidemiology of enteric diseases in Ontario, 2002. Public Health Epidemiology Report Ontario 2004;15(4).