Academic journal article Canadian Journal of Public Health

A Descriptive Analysis of Giardiasis Cases Reported in Ontario, 1990-1998

Academic journal article Canadian Journal of Public Health

A Descriptive Analysis of Giardiasis Cases Reported in Ontario, 1990-1998

Article excerpt

Giardia is the most commonly reported human intestinal parasite in Ontario, and affects millions worldwide.1,2 Giardiasis was the third leading reported enteric disease in Canada and Ontario in 1998, after campylobacteriosis and salmonellosis.3,4 Drinking unfiltered water, direct fecal-oral transmission, or ingestion of contaminated food are the most probable sources of infection.1 The microbial quality of Canadian drinking water has been questioned since the Walkerton, Ontario outbreak of 2000, when municipal water was contaminated with Escherichia coli 0157:H7 and Campylobacter spp. following heavy spring rains.5 Canadian waterborne giardiasis outbreaks involving municipal water systems using only chlorination treatment have been reported in 100 Mile House, British Columbia,6,7 Edmonton, Alberta8,9 and Botwood/ Peterview, Newfoundland.10 Although infection is usually asymptomatic, manifestations may include abdominal cramps, bloating, frequent loose and/or pale greasy stools, fatigue, weight loss, chronic diarrhea, fever, rash and joint manifestations.11,12 It is important to identify groups at increased risk of infection because of the serious nature of symptoms and the presence of Giardia may indicate other pathogens in the water supply.

Routine analyses of surveillance data are useful to identify high risk groups using the geographic distribution of cases to target disease-control resources. Potential risk factors can be identified by examining the relationship between the spatial distribution of disease incidence and reported exposures. This study is part of a larger project to investigate water quality in Ontario. Its purpose is to describe the demographic, temporal and spatial distributions, and reported risk factors and symptoms of giardiasis in Ontario during the years 1990-1998 inclusive using notifiable disease data.


Data sources

The study's database consisted of all records of giardiasis cases reported to the Ontario Ministry of Health between January 1990 and December 1998 through the Reportable Disease Information System (RDIS). RDIS defines a case of giardiasis as an individual with clinically compatible signs and symptoms with either a) demonstration of trophozoites or cysts in stool or small bowel specimens, or b) an epidemiologic link to two or more laboratory-confirmed cases. Twelve fields were examined for each case (Table I).

For each case, the reported residential postal code was linked to geographic areas used in the 1996 Census of Population for Canada and to latitude/longitude co-ordinates using a commercial database (Enhanced Postal Code File, Desktop Mapping Technologies, Inc., Markham, Ontario). This associated each case to a census division and an urban/rural designation. Statistics Canada defines "urban" as an area with a minimum population of 1,000 and a population density of at least 400 per square kilometre. All other areas are considered rural.

The 1991 Canadian census population formed the reference population for the standardization of rates and the Ontario population from the 1996 Canadian census was used to calculate expected population proportions. Annual population estimates calculated by Statistics Canada were used to calculate factor-specific rates.

Data quality evaluation

Data completeness and internal consistency were assessed.13,14 Implausible values, "unknown", and "other" were replaced with "unspecified". Percentages of missing and unspecified values were calculated for each field. Duplication was checked by comparing the date of birth, episode date, sex and postal code of each case.

Descriptive analysis

Provincial age- and sex-adjusted rates (per 100,000) were calculated for each year using the direct standardization method.15 Data were aggregated by 5-year age groups, and annual age- and sex-adjusted rates were calculated for each census division using population estimates. …

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